General Pharmacology Notes PDF

General pharmacology Introduction This course will describe how drugs and toxins enter the body, how they can distribute throughout the body, how the body terminates their actions and excretes them and finally how drugs and toxin interact with cells to achieve their biological effects. This will be...

General pharmacology Introduction This course will describe how drugs and toxins enter the body, how they can distribute throughout the body, how the body terminates their actions and excretes them and finally how drugs and toxin interact with cells to achieve their biological effects. This will be followed by two lectures on autonomic pharmacology. EC50 Definitions Pharmacology is the science of drugs including their chemical properties and their effects on the body Pharmacokinetics is the study of how the body absorbs, distributes, metabolises then excretes drugs. Pharmacokinetics is a quantitative study. Pharmacodynamics is the study of how drugs have their effect on cells, tissues and the body. Pharmacodynamics can also be a quantitative science. Xenobiotic is any foreign chemical substance. For the purposes of these lectures it can be a drug or a toxin. Toxicology is the study of xenobiotics that have a detrimental effect on the body and the pathologies they cause. Pharmacodynamics Pharmacodynamics is the study of the mechanism of action of drugs and the biochemical and physiological effects they cause. You wish to surgically castrate a male cat and you want to render the animal insensitive to pain and stop it moving during the operation. You therefore need to administer a drug, or drugs, that will depress the parts of the nervous system that regulate pain sensation and motor movement. Agents that do this are termed analgesics and general anaesthetics respectively. The mechanisms by which these drugs achieve this clinical outcome – “anaesthesia”- are the pharmacodynamics of the drugs. Action of the drug is the initial drug-receptor interaction. In the case of our anaesthetics it will be the binding of the agent to the specific agent receptor on the neuron in the central nervous system. The subsequent events are termed drug effects. In our example the drug 1 causes influx of chloride ions into the neuron to reduce its excitability and function leading to a loss of consciousness. Drug effects are quantitative and are an alteration of pre-existing cell function. Some anaesthetic drugs work by depressing excitatory neurons, others work by stimulating inhibitory neurons, whilst some depress all neurons. Mechanisms of drug action There are a number of different mechanisms, apart from the drug-receptor interaction, by which drugs may have a therapeutic effect. Non‐cellular mechanisms 1. Physical effect (protectant, adsorbant). e.g. activated charcoal is used to adsorb ingested toxin from the gastrointestinal tract.” Aluminium hydroxide is used as a protectant for the stomach mucosa and is found in many preparations (e.g. “Mylanta”, “Peptosil”). 2. Chemical reactions (chelating agents). e.g. sodium EDTA is used to react with heavy metal toxins so they become soluble and can be excreted by the kidney. These agents are used in the treatment of heavy metal poisoning in animals (lead, arsenic). 3. Physicochemical (surfactants). These agents can be used to reduce surface tension within the alveoli of the lung 4. Modification of body fluid composition (e.g. mannitol - used as an osmotic diuretic and useful for pulling fluid out of tissues into the plasma e.g. to shrink the brain before neurosurgery). Cellular mechanisms 1. Physicochemical and biophysical (e.g. magnesium sulphate acts as an osmotic cathartic to relieve constipation - see later). 2. Modification of cell membrane structure and function (e.g. local anaesthetics bind to Na channels) 3. Neurohumoral transmission - interfere with synthesis, release and uptake of neurotransmitters (e.g. the indirect acting sympathomimetics like reserpine - see later) 4. Enzyme inhibitors – competitive - noncompetitive - reversible - irreversible An example of this drug action is the inhibition of cyclooxygenase enzymes (COX) by the nonsteroidal anti-inflammatory drugs. 5. Regulatory molecule activation or inhibition through receptor-mediated effects (receptors on membrane or in cytosol). A common example is the glucocorticoids. Receptors The most numerous drug receptors are the cells’ proteins (e.g. proteins that act as receptors for endogenous regulatory ligands, enzymes, transport proteins etc.). Some properties of receptors include: 2 1. Specificity - the receptor type will only bind a certain type of ligand (ligands are molecules that act at a receptor e.g. a hormone, a neurotransmitter etc.) 2. Saturable kinetics 3. Action vs. effect a. action - drug initiates chain of events b. effect - observable result of events c. affinity - degree to which drug is attracted to receptor d. amplification - response is caused by cascade response ≥ amplifies each step e. spare receptors - only a portion of receptors need be occupied to ≥ max effect. Results of receptor activation 1. Induction of synthesis of specific protein 2. Regulation of gated ion channels in plasma membrane 3. Regulation of plasma membrane enzymes Agonist: An agonist combines with the receptor and stimulates a reaction in the cell. Affinity is the tendency of a drug to combine with a particular type of receptor (see above). The level of activity of an agonist is its potency. Antagonist: An antagonist interferes with a receptor or part of the effector mechanism, but does not elicit a reaction. This usually means that the ligand or an agonist drug/toxin is prevented from binding to the receptor and causing an effect. Bonding between drug and receptor is normally reversible and increasing concentrations of ligand can displace the antagonist. Some antagonists bind irreversibly to their receptor (or enzyme) and cannot be displaced by increasing the concentration of ligand. Initially drug is attached by ionic bonding (charged drug and oppositely charged receptor). Drug is then attached by hydrogen bonds and van der Waal’s forces -> conformational change in receptor. The number of drug-receptor complexes determines the level of response up to a maximum response. As more receptors are occupied the response increases. The number of receptors occupied is proportional to drug concentration. Pharmacokinetics There are a few basic principles that underlie the science of pharmacology. The first is that the drug effects on the patient are proportional to the concentration of drug in the body and for many drugs this will correlate with the concentration of agent in the plasma. For most drugs and toxins that actually get into the body, increasing the levels of xenobiotic in blood increases the biological effect and the effect will persist as long as the blood levels 3 remain. Therefore, for most drugs and toxins measuring blood levels gives a simple measure of drug activity. There are certain exceptions to this, but they are rare. Examples of this group include those drugs that have a biological effect long after the blood levels have become undetectable. Some drugs/toxins may have effects even when they are not absorbed into the body – e.g. topical antiseptics applied to the skin to kill bacteria on the skin. There are a number of processes that will determine the concentration of xenobiotic that is found in the plasma and how long the compound remains there. These processes include absorption, distribution, metabolism and excretion (ADME) (Fig 1) Figure 1: The processes involved in regulating the biological effects of xenobiotics. Administration It is probably fair to say that all EFFECTIVE drugs have some potential adverse side effects – that is they cause some toxicity if given at high enough doses. There are parallels between pharmacology and toxicology – in the former a foreign chemical causes a desirable effect on the body, whilst in the latter the effects are deleterious. If you keep increasing the dose of a drug it will become a toxin! The aim of drug therapy is to administer a suitable dose to achieve a concentration of drug in the plasma that gives the desired biological response without causing toxicity. 4 Figure 2: A company has developed a new antibacterial drug and wants to know what dose rate and dosage interval to recommend. Dose rate A is too high (toxic effects) and C is too low (never reaches minimum effective concentration) whilst B is OK. The interval of clinical effect is that during which drug concentration exceeds MEC - this, in part, will determine the dosage interval. Many drugs and toxins have their effect by interacting with a receptor on a cell membrane or penetrating through the cell membrane to alter a metabolic process. Before the xenobiotic can have a biological effect it must enter the body. The body has processes that can allow the entry of useful substances (e.g. food stuffs) and prevent the entry of others. Drugs and toxins must have physicochemical properties that will allow them to enter the blood stream so they can have a systemic effect in the patient. The process of drug movement in the body is termed pharmacokinetics. Of course not all drugs and toxin have to enter the blood stream to stimulate a biological effect e.g. topically applied drugs used to treat superficial skin diseases, drugs that stay in the intestine to kill helminths and contact irritants like stinging nettle can have biological effects without entering the blood stream. You can sometimes use drugs for different purposes by changing the route of administration. Streptomycin is an antibiotic that is not absorbed from the gastrointestinal tract. If you had an animal with a bacterial pneumonia susceptible to streptomycin you would need to administer the drug by injecting it into a muscle where it can be absorbed into the blood stream for distribution to the diseased lung. If you gave the drug by mouth to this patient it would not have the desired effect. On the other hand, if you wanted to reduce the number of bacteria in the gut before an animal had intestinal surgery you can administer streptomycin by mouth. It will remain in the intestinal tract and reduce the level of bacterial growth. Streptomycin can be a toxic substance – causes deafness and damage to the vestibular nerve as well as kidney damage. Which route of administration is likely to be associated with this toxicity? 5 The movement of xenobiotics around the body can be described by pharmacokinetic models. The simplest is the 2 compartment model. Drug is absorbed into the central compartment, distributed to the peripheral compartment where the drug has its effect, returned to the central compartment from where the drug is excreted. Drug is always excreted from the central compartment. Figure 3: Compartments do not correspond to any physiological body space. The central compartment is probably equivalent to the blood and highly perfused organs. The peripheral compartment is equivalent to less well perfused tissues e.g. rumen, large intestinal contents, muscle etc. k12 and k21 are first order rate constants. Routes of administration The route of administration of a drug may influence both the effectiveness of drug action as well as owner compliance in giving the drug. If the owner cannot easily administer the drug he/she is unlikely to follow the dosing instructions accurately. An elderly owner brings her new dog to your surgery for vaccination and advice on heartworm preventative treatment. You can offer her heartworm prevention treatment that is either given daily, monthly or yearly. The treatments may be given orally, topically on the skin or as injections. The oral drugs can be given as a liquid, a tablet or as a chewable dosage form. What benefits and disadvantages does each form of medication have for this owner? 6 Important factors affecting stratum corneum barrier      molecular weight (ideally < 500 D) lipophilicity (Log P between 1-3 (ideal < 2.6)) epidermis actively metabolizes drugs effects of vehicle or formulation Regional differences / site of application Enteral route Drugs and toxins taken by mouth are said to be administered by the enteral route. This is a very convenient method of giving drugs. In the case of toxins it is the most common route of entry into the body, especially toxins associated with plants. The advantages of using the enteral route include:1. Oral drugs are convenient for the owner to give at home. 2. They do not need to be sterile (hence may be cheaper to produce) and do not require sterile needles and syringes to administer 3. They can be in the form of liquids (solutions, suspensions), pastes and solids. 4. Drugs given by mouth generally are safer for the patient than those given by injection because the blood levels do not rise as quickly. 5. The drug may be given with food to alter absorption (food generally slows absorption). 6. The dosage form (liquid, solid) can determine absorption rate so you can have ‘slow release’ preparations. This route of administration has some disadvantages which include:1. Some drugs can be degraded in the hostile environment of the gastrointestinal tract (low pH in the stomach, enzymes in the small intestine and bacteria in the lower bowel). Drug must be chemically stable when dissolved in intestinal fluids. 2. Drug action may be affected by food. This can be particularly important in ruminants and grazing horses. 3. Some drugs are rapidly metabolised by the liver as they are absorbed from the gut and carried by the portal vein directly to the liver. This is called the first pass effect. The first pass effect also includes metabolism by intestinal enzymes in the intestinal mucosal cells. 4. Drugs that do not dissolve in the aqueous environment of the intestinal contents may not be absorbed. Drugs usually have to be dissolved in the intestinal liquor to be absorbed. 5. Drug action can be fairly slow compared to injectable routes of administration. 6. In some species it is inconvenient to administer drugs by mouth (e.g. cattle, pigs, savage dogs and most cats). A farmer has a herd of Brahman cows and a number of them have a bacterial infection of the kidney. You plan to administer a suitable antibiotic that is given once daily. Outline the problems likely to be faced by the farmer if you give him an oral antibiotic to treat the cows. 7 In human medicine some drugs are taken by mouth and allowed to dissolve in the mouth and NOT swallowed – sublingual administration. In this case the drug is absorbed across the mucous membrane of the mouth and there is no first pass effect in the liver. This is NOT an enteral route of administration. Rectal administration Drugs can be delivered into the rectum. Absorption from the rectum can be erratic, but 50% of the absorbed drug will bypass the liver (reduced first pass effect) making it useful for administering drugs that are metabolised rapidly by the liver when taken by mouth. Per rectum administration is also useful in vomiting animals and for the treatment of colo-rectal diseases because you get higher doses of drug in contact with the lesion. The colo-rectal mucosa is fragile and is not suitable for the administration of irritant drugs. A 14 week old pup is presented with vomiting and diarrhoea suggestive of gastroenteritis. The dog is slightly dehydrated and you wish to give it fluids. The owners cannot afford hospitalisation and intravenous rehydration. You suggest that they may be able to manage the dog at home with oral fluids if the dog can retain the fluid. Do you think that rectal administration of fluids would be satisfactory? What other methods could you use, apart from IV administration, to give fluids to this pup? Protected oral dosage forms There are some pharmaceutical technologies that can selectively release drugs to certain parts of the gastrointestinal tract to either improve drug absorption or reduce side effects of the drug. “Enteric coatings” are resistant coverings on tablets and capsules that prevent the drug from being released in the stomach. This may be to prevent the drug irritating the gastric mucosa, to prevent destruction of the drug by stomach acid or to direct absorption to the small intestine where there is an enormous surface area (microvilli). Special coatings are available to release drugs into the colon. These are used for peptidic (large molecular weight) drugs that are broken down by enzymes in the small intestine. This technology is likely to become more popular as “designer” drugs (usually high molecular weight) come onto the market. Oral drug dosing in ruminant Some factors to consider:1. Drugs diluted in rumen fluid. 2. Drugs can be inactivated by rumen microflora. In some cases rumen microflora can increase the toxicity of ingested xenobiotics (e.g. cyanogenic glycoside poisoning). 3. Drugs may adversely affect the resident microflora causing ruminal indigestion (e.g. antibiotics). 4. You can bypass the rumen by pre-administering an agent that closes the oesophageal groove (e.g. copper sulphate). 8 Parenteral routes of administration When a drug is administered by any route that does not involve absorption from the intestinal tract, it is by a parenteral route. Examples of parenteral administration include:        intravenous (IV)* and intra-arterial (IA)* intramuscular (IM)* subcutaneous (SC)* intradermal (ID)* topical percutaneous intrathecal*, intraarticular* intraperitoneal* (IP) (commonly used in laboratory animals instead of IV) inhalation * requires a syringe and needle (which must be sterile!). Sometimes a catheter is placed in the space (vein, artery, dural space, etc.) and the drug is given slowly over an extended period of time. If the dose is measured per unit time (e.g. with an infusion pump) it is called a “constant rate infusion (CRI)”. The dose is expressed as mg/kg/hr. Generally speaking injectable dosage forms give:1. More rapid and higher peaks in blood drug concentrations. This will depend on the dosage form. Some drugs are administered in “slow release” forms for maintaining blood levels over a long period (e.g. sex hormones are given as intramuscular injection of drug in an oil base or slow release into the blood stream). 2. Protection of drug from the hostile environment of the intestine 3. No first pass effect. 4. The dosage form can be liquid or solid (I.M. and S.C.) but I.V. dosage forms MUST be an aqueous solution. Solid dosage forms are inserted with a special applicator (e.g. pellets of growth promotant drugs) 5. Suitable for fractious animals – e.g. dart guns. Cattle, sheep, pigs and unbroken horses can be handled safely in a crush or race. The disadvantages of injectable drugs are:1. They must be sterile and pyogen free (pyogens cause a rise in body temperature and are usually bacterial products) 2. They are less convenient because they require sterile apparatus to administer. 3. Multiple dosing apparatus (e.g. treating sheep with an injectable vaccine with a “repeater” syringe) can spread disease. 4. Injections may injure muscles leading to serious pathology – e.g. blackleg Intravenous administration Bioavailability is defined as the proportion of drug that reaches the blood stream unchanged. In the case of IV administration, all the drug enters the circulation unchanged. This is 100% bioavailability. Intravenous injections can be given as rapid boluses or as a slow infusion. Giving the drug slowly to effect is “titrating” the dose against an effect as is commonly used when inducing general anaesthesia in many animals, but not horses usually. 9 You are going to anaesthetise a horse with the short acting barbiturate thiopentone. This drug must be given IV because it is very irritant. What is likely to happen if you attempt to titrate the dose of thiopentone in this horse? If you decide to use a bolus injection of thiopentone instead, what problems do you foresee? Intravenous administration of a drug is indicated when:1. The drug is irritant. It will be rapidly diluted in the blood. Use a large blood vessel for the injection. When irritant drugs are injected IM, SC, IP etc they cause pain and tissue damage. 2. You wish to titrate the dose against an effect (e.g. inducing anaesthesia). 3. If you want a rapid response. High blood levels are reached within a very short time, but you have to watch for toxicity!. 4. You wish to inject a large volume, or deliver the drug slowly in a controlled way (titration), or you want to maintain a certain blood level for a prolonged period. Intra-arterial injection is sometimes used to get high concentrations of a drug in a specific site in the body. Sometimes used for cytotoxic (anti-cancer) drugs – inject into artery supplying tumour - so you get high levels of the drug in the tumour with less systemic toxicity. Intramuscular and subcutaneous injections These parenteral routes are commonly used in veterinary practice. Animals with loose skin can tolerate fairly large volumes injected under the skin without much pain. Intramuscular injections place the drug in a tissue that has a greater blood supply than the subcutaneous issues so absorption may be marginally faster. With SC and IM administration peak blood levels occur in about 30 minutes provided it is not a slow release preparation of the drug. These routes are particularly useful for depot drug dosage forms that release the drug slowly (e.g. hormones, growth promoters, long-acting corticosteroids, long-acting antibiotics). Blood flow to the site of injection impacts on the rate of absorption. A 6 week old kitten has a bacterial infection and is not nursing properly. It is dehydrated with a cold skin suggestive of shock. You decide to give the kitten fluids to rehydrate it. Because giving an intravenous infusion would be difficult, you decide to give it the fluids under the skin. What problems do you envisage with administering fluids this way and suggest a better way of administering fluids to this case 10 Inhalation Drugs that are inhaled reach the blood stream rapidly. This is why inhalation of anaesthetic agents is commonly used. In the case of a gaseous or volatile anaesthetic the drug enters the blood through the alveoli and is transported to the brain. Anaesthetics are generally highly lipophilic drugs so they penetrate membranes rapidly and easily get into the brain through the blood brain barrier. Because of their physicochemical properties these anaesthetics can leave the blood rapidly also which allows the animal to wake up quickly. What structures make up the blood brain barrier? When would you expect the functions of the blood brain barrier are compromised? In human asthmatics, treatment is often by inhalation “puffers”. In this case it is desirable for the drug not to enter the plasma, thus reducing undesirable side effects, but having a local effect on the inflamed airways. The physicochemical properties of the drugs are adjusted accordingly. You are asked to design a new dosage form of a steroid anti-inflammatory drug for inhalation in horses suffering allergic rhinitis. What physicochemical properties are required in the drug? Absorption Absorption entails the processes involved in getting the drug from the site of administration (e.g. intestinal lumen, muscle, subcutaneous tissue etc.) to the plasma. For IV injections there is no absorption. For all other routes of administration there will be barriers to absorption including cell membranes. The ease with which a drug can pass through cell membranes can affect its rate of absorption and hence its biological activity. You have a horse that is lame in a forelimb and you diagnose inflammatory changes in the carpus. You will treat the horse with an anti-inflammatory drug (phenylbutazone) administered in the feed. Track the course of the drug from the mouth to the inside of the joint and note which cells the drug must pass through. A group of calves is sprayed with an organophosphate dip to control ectoparasites. The calves are found convulsing, drooling and passing diarrhoea. They have been poisoned by the organophosphate. Follow the toxin from the outside of the skin to the brain and note which cells the toxin must pass through. 11 Passage of drugs across a membrane The physicochemical properties of a xenobiotic will, in part, determine the speed it crosses a cell membrane (e.g. molecular weight, lipid solubility, polarity, etc.). There are other factors that will also determine the speed and extent of penetration across a membrane:1. Concentration gradient of xenobiotic across the membrane – the greater the concentration of drug on one side of the membrane the faster it will move across the barrier. 2. Surface area of the membrane – the small intestine has a large surface area compared with the stomach and colon so most drug is absorbed in the small intestine. 3. Thickness of the membrane – thickened membranes (e.g. oedema) pose a greater barrier to movement of a xenobiotic. 4. Time molecules are in contact with the membrane – if molecules are only briefly in contact with the membrane absorption can be reduced – e.g. diarrhoea and oral medications. 5. Permeability of the membrane to the molecule – molecules that are small in size and highly lipophilic will be absorbed more rapidly. Molecules with an electrical charge are also poorly absorbed across biological membranes. The mechanisms xenobiotics use to cross a membrane include: simple diffusion  facilitated diffusion (carrier mediated)  active transport (carrier mediated)  filtration via membrane pores  bulk flow (intercellular pores and capillaries)  endocytosis Diffusion Most drugs cross membranes by simple diffusion by dissolving in the lipid of the cell membrane. Molecules must be non-ionised (i.e. not charged) to be lipophilic. Many drugs are either weak acids or weak bases and depending on the pH of the environment some of the molecules are charged (i.e. ionised). A weakly acidic drug like aspirin (pKa = 3.5) will be unionised in the stomach (pH = 1.0) but will be relatively more ionised in plasma (pH = 7.4). Molecules that are lipophilic will also readily cross membranes. This is of particular importance for some toxins and environmental contaminants (like dieldrin and DDT). Carrier mediated transport Transport proteins can assist the passage of xenobiotics across membranes. Active transport requires energy and will move drug against a concentration gradient (like pushing something uphill). Facilitated diffusion does not use energy but can move drug against a concentration gradient. 12 Filtration Small molecules (M. Wt. <100) that are also non-ionised can diffuse through pores in the cell membrane. Very few drugs are this small. Urea (used as non-protein nitrogen source in ruminant feeds) and ethylene glycol (antifreeze) are small M. Wt. toxins. Bulk flow Drugs pass through the spaces between cells. This is how drugs enter and leave capillaries. Inflammation causes separation of capillary endothelial cells and increases egress of large molecules into and out of the blood stream. Endocytosis Phagocytosis and pinocytosis usually occurs when the drug has been bound to a large carrier molecule. Barriers to drug movement Some tissues are protected from xenobiotics in the blood by special cellular barriers. The brain has the blood-brain barrier, the eye has the blood-ocular barriers. The capillary endothelial cell junctions are normally tight so only lipophilic drugs can easily exit the capillary. This is particularly important in tissues like the brain, eye and prostate. Distribution This phase takes the drug from the point of absorption to the site of drug action. Distribution also involves the return of drug from the site of drug action to the point of metabolism or excretion. Tissue levels of drug or toxin will be determined by: 1. 2. 3. 4. Blood flow to the tissue. Rate of drug penetration of the capillary wall Lipid solubility and degree of ionisation of the molecule Degree of protein binding Xenobiotics bound to protein are not biologically active, but the free molecules are. There will be an equilibrium between free and bound drug and this ratio will be constant, so as free drug is removed into a tissue some drug will split from the protein carrier and become available for biological activity. Drugs bound to protein mostly stay in the vascular compartment and their movement into the extracellular fluid is restricted. Protein-bound drugs are not filtered by glomerular filtration, although bound drugs can enter the hepatocytes because hepatic capillaries are fenestrated to allow egress of proteins. This assists with metabolism of protein-bound xenobiotics. The liver normally synthesises proteins that are secreted into the circulation – hence the fenestrated capillaries. Protein binding may also occur in tissues. 13 A dog is being treated with the drug digoxin because it has heart failure. Digoxin is approximately 90% protein bound and is potentially toxic at higher doses. The dog also has liver damage and has a hypoproteinaemia (low plasma protein). How would you adjust the dose of digoxin for this dog? The fact that drugs are protein bound does not diminish their potential clinical efficacy. The dose needs to be such that there is enough FREE drug to have the desired clinical effect. Figure 4: Movement of free and bound drug between body compartments. Acidic drugs bind to albumin and basic drugs bind to 1-glycoprotein. Alterations in pharmacokinetics of protein-bound drugs MAY occur when:1. There is liver disease. Reduced liver function can result in reduced synthesis of protein – hypoproteinaemia – so there will be more free (un-bound) drug. This can lead to increased drug efficacy and/or increased toxicity. 2. There is starvation or catabolism (as occurs in many diseases). Again hypoproteinaemia may occur. 3. There is renal disease with increased glomerular filtration – i.e. the glomerular vasculature “leaks” protein into the tubules. Protein-bound drug may be lost from the circulation due to the loss of protein through the damaged glomerular endothelium. Normally, the glomerulus will not pass proteins (>MWt 20,000) into the urine. Protein binding (summary) 1. Most acidic drugs bind to albumin and basic drugs to α1-glycoprotein. 2. Reversible. 3. Restricts diffusion into ECF, glomerular filtrate (drugs excreted by tubular secretion not so much affected) as well as joints, CSF and ocular compartments. 4. Protein bound drugs can still be very effective provided appropriate dose is used. 5. Protein binding also occurs in tissues -only free drug has effect. 6. Important for drug residues. 14 Metabolism (of xenobiotics) There are essentially 3 ways the body can deal with a drug or toxin:1. It can be excreted from the body unchanged. 2. It can be retained in the body. 3. It can be metabolised to facilitate excretion. Physicochemical properties that facilitate biological activity of a drug or toxin (i.e. lipid solubility) tend to make them difficult to remove from the body. Highly lipophilic compounds get into cells easily to stimulate some biological activity. If these compounds were excreted in the bile or urine unchanged they could easily be reabsorbed through the intestine or distal renal tubule respectively. The body needs to render these drugs less lipophilic and more hydrophilic so they can be excreted in the bile or urine (these are aqueous media). The body has the enzymatic machinery to metabolise many xenobiotics – why is this? The metabolic pathways are there to inactivate hormones, natural steroids and vitamins required for normal homeostasis. It just happens that the chemical reactions are common to natural substances and xenobiotics – mostly. There are some xenobiotics that cannot be metabolised because they are very non-reactive or the body lacks the chemical processes to degrade them. These substances pose a threat of drug residues in food when they get into animals used for food. Examples include dieldrin and DDT. Heavy metals like lead tend to be insoluble and collect in bone. Xenobiotic metabolism     enzymes for biotransformation are located in smooth endoplasmic reticulum (microsomal fraction) major site of drug metabolism is liver metabolism can also occur in most tissue especially lung, intestinal mucosa, kidney. bacteria in gut may also alter drug activity (increase or decrease). Liver metabolism The liver is an important site for xenobiotic metabolism because: it receives blood directly from the gut via the portal vein.  has a high concentration of enzymes.  it is a large organ with a high perfusion rate (approximately 27% of cardiac output). Hepatic metabolism of a xenobiotic usually yields: a more water soluble compound  reduced biological activity (sometimes there is increased activity)  increased excretability There may be individual and species differences in the handling of xenobiotics e.g. phenytoin toxicity occurs in about 1 in 10,000 humans because they have an aberrant P-450 isoenzyme. The study of this facet of pharmacology is pharmacogenomics. 15 Summary of hepatic metabolism  Reactions occur mainly in hepatocyte endoplasmic reticulum.  Drug conjugates are polar derivatives with increased water solubility.  Drugs that are highly ionised and/or very unreactive (provided they are not highly lipophilic) are excreted unchanged. Very few drugs are completely metabolised and some native drug is excreted intact (detectable for drug testing). The degradation pathway Phase I reactions generally lead to termination of drug action. In some cases of pro-drugs they can actually create the active drug (inactive prednisone converted to active prednisolone). There are metabolising enzymes for oxidation, reduction and hydrolysis. These reactions tend to have the xenobiotic molecule gain, or unmask, chemically reactive groups: hydroxyl (-OH)}  carboxyl (-COOH) } altered drugs become substrates for phase II reactions.  amine (-NH2)}  sulphydryl (-SH)}  Drugs maybe metabolised through more than 1 pathway to give a number of different  metabolites. Oxidation (insert an oxygen atom)  most frequent Phase I reaction.  mixed function oxidases which require NADH or NADPH, molecular O2 and cytochrome P-450 often give a highly reactive by-product.  microsomal P-450 is located in smooth endoplasmic reticulum. There are a large number of P-450 isoenzymes (= mixed function oxidase) and they vary between species. NADPH + A + H+ → AH2 + NADP+ AH2 + O2 → “active oxygen complex” “active oxygen complex” + drug → oxidised drug + A + H2O “A” represents the oxidised form and “AH2” represents the reduced form of cytochrome P-450. Hydroxylation (insert OH- group) is the most common oxidative reaction. Hydrolysis There are non-specific esterases (non-microsomal) in the liver, plasma, GI tract and other tissues (e.g. proteases and peptidases). These enzymes tend to degrade drugs in the tissues 16 without them having to return to the liver. A common example is the ester group of local anaesthetics which are metabolized by cholinesterases.  drugs containing ester - C - O - or amide - C - N - groups |||| | OO H an example of these enzymes include the acetylcholinesterases Reduction Occurs less frequently - in drugs containing disulphide (S:S), azo (N:N) or nitro (NO2) groups. Reduction reactions occurs in hepatic microsomes and entail a loss of oxygen and addition of hydrogen (requires anaerobic conditions and NADPH). The reactions can occur in SER, in the cytosol, by bacteria in the rumen and in the large intestine. Phase II reactions allow the reactive intermediate molecule generated by the phase I reaction to form a water-soluble, non-reactive metabolite that can be easily excreted.    drug is coupled with endogenous substances - conjugation or synthesis. can take place in liver, kidney and gut wall. conjugates = glucuronidation (UDP-glucuronosyl transferases) - quantitatively most important, especially for larger molecules. - glucuronidation in hepatocyte SER is very common (except all felines which are deficient in the enzyme glucuronyl transferase). - glucuronides eliminated mainly by the kidney (low M.W.) or bile (M.W. > 300) - acetylation (N-acetyltransferases) - sulphonation (sulphotransferases) - most excreted in urine -main substrates phenolic and alcoholic compounds. - glutathione conjugation (glutathione S-transferases) – for chemically reactive molecules. Species variability  Dog has little ability to acetylate amino groups.  Pig has low levels of sulphate conjugation.  Cats lack glucuronyl transferase. It is important to know this because it can influence which species can be given the drug, the possible half-life of the drug and even the drug’s effectiveness and toxicity. Factors affecting drug metabolism (biotransformation) Individuals react differently to a standard dose of a particular drug, which makes drug prescribing a challenge. One of the contributing factors to differences in responses is individual variations in the activity of metabolic enzymes. 17 The causes of this are: Genetic – gene polymorphism  Physiological (e.g. age, pregnancy, gender, species)  Environmental – induction and inhibition of drug metabolism enzymes  Disease status Specific causes of altered biotransformation 1. Age The capacity for hepatic drug metabolism is low at birth. Newborn humans can metabolise substrate efficiently, but do it at a slower rate than adults. Neonates have a poorly developed blood brain barrier and poor mechanisms for excretion so they are prone to toxicity (e.g. there is an increased risk when administering chloramphenicol and opioids). Age can affect duration of drug action and toxicity. In older animals there are a number of physiological and pathological states that have the potential to alter the pharmacokinetics of a drug. These can include altered proportion of body fat to muscle, reduced renal function, alterations in gastrointestinal absorption, heart disease, etc. You are presented with an old cat that has reduced renal function that was previously diagnosed and treated. You plan to treat the animal with a drug that is excreted by the kidney. Would a drug that is excreted by the kidney be contraindicated? What information is required to determine if it is? 2. Diet   Drug metabolising enzyme activity is reduced with starvation (decreased protein = hypoproteinaemia) Chemicals in diet can induce or inhibit enzymes (i.e. alter rate of metabolism) Enzyme induction and suppression Approximately 200 drugs and environmental chemicals can affect enzyme activity, particularly P-450 enzymes and transferase enzymes. Examples include:- Enzyme inducers    phenobarbital (frequently used anticonvulsant) chlordane, DDT (insecticides and environmental contaminants) eucalypts -> induction in sheep -> CCl4 toxicity 18 (historical) Enzyme inhibitors    chloramphenicol (antibiotic) organophosphates (insecticides, acaracides etc.) cimetidine (H2 receptor antagonist) and ethanol 3. Hormones Corticosteroids, thyroxine, insulin and sex hormones can alter metabolic rate etc. Pregnancy → reduced metabolism of some drugs Testosterone → induction 4. Disease Liver disease is potentially important to the pharmacokinetics of a drug – e.g. first pass effect, protein synthesis, drug metabolism, etc. If biotransformation is normally very rapid, reduction in hepatic blood flow may significantly alter biotransformation. Reduced protein binding of drug can result in increased metabolism with reduced duration of effect. In the case of some toxins it may increase toxicity also because of reduced clearance of toxin from the body. You are going to treat a sheep with Lantana poisoning (a liver toxin) with an analgesic drug. If the drug you were using is metabolised in the liver how might the pathology affect your dosing schedule? Drug Excretion and Pharmacokinetics The major routes of xenobiotic excretion are urine and bile. Volatile compounds can be excreted by the lung and some compounds are also excreted through saliva (swabbing horses), sweat (swabbing horses) and milk (public health issues, nursing animals). Note: the following discussion of kinetics applies to any route of excretion. 19 First order kinetics A constant proportion of drug excreted over time Zero order kinetics Drug is excreted at a constant rate Fig.5 a: A drug excreted by first order kinetics will give a straight line with plasma concentration in log form Fig.5 b: A drug excreted by zero order kinetics are removed from the plasma at a constant rate Most drugs are excreted as first order kinetics: the higher the plasma concentration the faster the excretion rate. Drugs excreted by zero order kinetics are often excreted through a transport mechanism that is saturable. Once the transporter is saturated the drug is removed from the plasma at a constant rate. This is seen in the elimination of phenylbutazone and explains why the dose must be reduced after the drug has been administered for a period of time. Renal excretion     drugs can be excreted into the urine unchanged or as metabolites drugs can pass into the glomerular filtrate (of course molecule must be water soluble) drugs can be actively secreted into the proximal tubule some drugs may be reabsorbed in distal part of renal tubule. Extent of filtration depends on:1. concentration of drug in plasma 2. degree of protein binding (> 80% binding) 3. GFR (dependent on arterial BP and renal vasoconstriction). Active secretion  occurs in proximal convoluted tubule and is not affected by protein binding.  fairly nonspecific carriers - a carrier system for organic anions (acids) and for organic cations (bases and quaternary ammonium compounds).  competition between compounds for carriers.  active transport sites on basal membrane - actively transported into cell then diffuses into lumen. 20 Reabsorption Active reabsorption in the proximal tubule is uncommon (e.g. PBZ), but passive reabsorption in distal tubules is common and depends on:1. Drug concentration (reabsorption of water in the tubule leads to increasing concentration of drug in lumen of the distal tubule and collecting duct). 2. Degree of ionisation of drug (only non-ionised drugs can be reabsorbed) 3. Urine pH (influences degree of ionisation – depends on pKa of drug). Biliary excretion     erythromycin, digoxin, chloramphenicol, morphine are examples of drugs (or their metabolites) excreted in bile usually M.W of drug is > 500. enterohepatic circulation (tetracyclines and human contraceptive mini-pill). Drug excreted into the bowel via the bile is reabsorbed. May require the bacterial cleavage of the conjugated drug. Enterohepatic cycling can effectively increase the half-life of the drug. bacterial -glucuronidase can metabolise drug glucuronides (chloramphenicol, morphine) -> reabsorption. See above. Other routes of excretion Saliva - levels may approach plasma levels. Can cause deranged taste sensations. Sweat Milk - slightly more acid than plasma so tend to have higher concentrations of basic drugs, less of acidic drugs. Half‐life (t½) This is the time it takes for the plasma concentration of a xenobiotic to fall by 50%. It is useful to know this so you know how often to give the drug to maintain the plasma concentration between MEC (MIC) and MTD. Bioequivalence Quite often in practice you will have the choice of different brands for a particular drug - e.g. amoxicillin 200 mg tablets are made by SmithKline Beecham, Heriot AgVet, Apex, Delta etc. You presume that when you give each of these preparations you will get the same biological response - i.e. the preparations are “bioequivalent”. The tablets will have the same quantities of the active drug - in this case amoxicillin - but the other bits and pieces that make up the dosage form (called excipients - include things like bulking agents, flavours, stabilisers etc) may vary between brands. This may alter the biological properties of the drug. Bioequivalence is determined by comparing different pharmacokinetic parameters of a new preparation with a standard preparation. The parameters measured are discussed below. 21 Sometimes a drug is prepared with different excipients to alter its pharmacokinetics (e.g. aspirin is available in a soluble form, an enteric coated form and as a tablet that disperses in the stomach. The physical form of the active ingredient (e.g. the crystal size) may dramatically affect solubility and hence bioavailability of an oral tablet, capsule etc. You have set up a drug company to produce another brand of amoxicillin 200 mg tablet. What information would you need to give the registering authority Figure 6: Graph of plasma concentration vs. time. The area under the curve *(AUC) is the total drug available. Systemic availability The ratio of areas under the curve (AUC’s) for a drug given by a non-IV route compared with IV administration. Example: an antibiotic injected into the vein has a systemic availability of 100%, but when given by mouth some maybe lost to bacterial breakdown, failure to cross from the lumen to the plasma etc so the systemic availability is less than 100%. Affected by:     rate and extent of absorption inactivation in gut fluids metabolism in gut wall and/or liver physicochemical properties of drug dosage form (e.g. long acting formulation). 22 There may be species variability in bioavailability due to factors like dilution or inactivation of drug in the rumen or lack of hepatic enzymes leading to reduced first pass effect and increased oral bioavailability. Apparent volume of distribution (Vd) The (apparent) volume of distribution is the volume of fluid that would contain the drug at the concentration found in the plasma after the drug is given. It is not related to any physiological spaces; it relates the plasma concentration to the dose given indicating where, and to what extent, the drug is distributed. Total body water Intracellular water Extracellular water Plasma A Vd of: 0.05 L/kg 0.2 L/kg 0.6 L/kg >1 L/kg = 60% body wt. = 40% body wt = 20% body wt = approx 5% body wt = approx 0.6 L/kg = approx 0.4 L/kg = approx 0.2 L/kg = approx 0.05 L/kg - highly protein-bound in plasma - distributed throughout ECF - distributed into cells - selectively bound in some tissues Vd is useful in determining drug doses – if the drug has a high Vd you may have to increase dose to maintain adequate plasma levels. Body clearance (Clβ) This is the volume of biological fluid from which drug would be removed per unit time. Body clearance is the sum of hepatic biotransformation and renal excretion. Clearance is a good estimate of elimination - better than t½ in drugs not administered by IV injection (IV drugs are not affected by absorption). Clearance determines dosage interval. When a drug is administered there is a period when blood levels are increasing yet the drug can be simultaneously eliminated – this is when t½ is less useful. When drugs are given IV distribution is very rapid and t½ is more useful. Steady state Is when the plasma drug concentration is relatively constant and this is achieved by multiple dosing. Steady state is reached in approx 5 x t½ of the drug when given orally. There are strategies you can use to hasten the time it takes to reach steady state - e.g. start the treatment with an injectable dosage form or increase the oral dose rate initially (very common with oral antibiotics in humans). 23 Autonomic pharmacology In animals primary disease of the autonomic nervous system (ANS) is rare although involvement of the ANS may contribute to the signs and pathology of the disease. The ANS can play a crucial role in maintaining the function of diseased organs (e.g. an animal with a failing heart will have increased sympathetic outflow [efferent] to maintain cardiac output) so interfering with ANS function may have adverse consequences for the patient. If you then give propranolol, which blocks this response, the animal may die suddenly. ANS innervates  Smooth muscle  cardiac muscle  exocrine glands Central integration of autonomic activity Autonomic reflex arc Afferent fibres (non-myelinated) transmit information about organ function to central nuclei to cause a change in efferent discharge. Some drugs work principally by altering afferent receptor areas and afferent impulse traffic, but most affect peripheral efferent or CNS nerve pathways. The hypothalamus is the principal supraspinal site modulating sympathetic and parasympathetic outflow. Posterior and lateral nuclei are mainly sympathetic, while the medial and anterior nuclei are mainly parasympathetic. Hypothalamus controls: blood pressure  body temperature  carbohydrate metabolism  water-electrolyte balance  sexual responses  emotions  sleep Medulla oblongata nuclei control: blood pressure  expiratory-inspiratory respiratory phases (often interact with hypothalamic regions) Cerebral cortical foci may influence autonomic activity (Pavlovian responses). Some drugs have pronounced central effects in addition to peripheral effects (e.g. amphetamine [CNS stimulation], phenothiazines [lower blood pressure + tranquillizer]). Almost all drugs in this section have primary action by altering some step(s) in the process of neurohumoral transmission. 24 Physiological events in neurotransmission 1. Axonal conduction – not affected by most drugs (local anaesthetics must be administered at high dose in the vicinity of the axon to block neural transmission) You administer a large volume of the local anaesthetic lignocaine into the spinal canal of a dog and it soon collapses with low blood pressure, reduced cardiac output - as seen by weak thready pulse - and pale oral mucous membranes. Explain how blockade of autonomic transmissions leads to the clinical signs. 2. Neurotransmitter release – an axonal action potential arrives at the nerve terminal where the neurotransmitter is stored in vesicles. The stored neurotransmitter is then released into the synaptic cleft. Ca++ is required for exocytotic discharge of the neurotransmitter from the vesicles. 3. Receptor events - the transmitter substance moves across the cleft and forms a bond with the receptor on postsynaptic membrane. Receptor events may be excitatory (increase in permeability to all ions -> depolarization-repolarization) or inhibitory (selective increase in permeability to a smaller ion only - e.g. K+, Cl- → hyperpolarisation). 4. Catabolism of neurotransmitter – there are both extraneural and intraneural enzymes to break down transmitter. Reuptake and dispersal away from receptor sites terminates the effect of the neurotransmitter. Adrenergic pharmacology Administration of a drug designed to stimulate, or mimic, the sympathetic nervous system may work by either activating adrenergic receptors on effector cells innervated by the sympathetic NS or by stimulating cells with receptors, but no sympathetic innervation. Release of adrenal gland hormones cause this latter effect. Adrenergic receptors are found on, or in, the cell membrane and the agonist is either noradrenaline (NA = neurotransmitter) or adrenaline (from adrenal gland). In USA adrenaline is called epinephrine and noradrenaline is norepinephrine. Knowledge of the location and function of all autonomic receptors is a prerequisite for the study of autonomic pharmacology. They are summarized here, but consult a suitable textbook. 25 Receptors α1 receptors Location Blood vessels, iris, GI tract, hepatocytes, sweat glands, piloerector muscles, bladder sphincter, male and female reproductive tract. Stimulation causes vasoconstriction and increased blood pressure. mydriasis (dilatation of pupil) contraction of gut sphincters and reduced gut motility urinary retention (contraction of sphincter) increased bile secretion and increased glycogenolysis uterine contraction and ejaculation generalised sweating piloerection Clinical use stimulation include treatment of hypotension, to reduce nasal congestion (vasoconstriction) and use with local anaesthetics (vasoconstriction) to increase duration of anaesthetic effect. α 2 receptors Location -presynaptic on all adrenergic nerve terminals (negative feedback control) -postsynaptic on some effectors (pancreas) and in some vascular beds Stimulation causes Clinical use stimulation include tranquillisation (veterinary) and to treat hypertension in humans* *The drug clonidine is used to treat hypertension in humans. Its mode of action is the stimulation of central α 2 receptors near the vasomotor and cardiac centres leading to increased vagal outflow, bradycardia and reduced cardiac output. In veterinary practice this is an undesirable side effect of α 2 agonist tranquillisers (xylazine, detomidine, medetomidine). β1 receptors Location Myocardium, adipocytes, GI tract, renal arterioles. Stimulation causes positive chronotropic and inotropic effects* increased lipolysis giving increased blood lipids reduced GI motility releases renin - angiotensin II to cause vasoconstriction Clinical use positive inotropic actions in circulatory shock, hypotension and cardiac arrest. 26 β2 receptors Location Bronchial smooth muscle; skeletal muscle; blood vessels in brain, heart, kidneys, skeletal muscle; mast cells; uterus and hepatocytes Stimulation causes bronchodilatation increased skeletal muscle excitability vasodilatation in above tissues relaxation of the pregnant uterus, increased contraction of the non-pregnant uterus. decreased bile secretion and increased glycogenolysis stabilise mast cell membranes. Clinical use bronchodilatation and prevention of premature labour. The stabilisation of mast cell membranes is also useful in agonist therapy for allergic asthma. * Inotrope – increases force of cardiac contraction; chronotrope – increases frequency of cardiac contraction) Specific drugs 1. Agonists (sympathomimetics) Catecholamines Adrenaline Noradrenaline Dopamine α and β, greater potency at β α and β, greater potency at β α and β + dopaminergic receptors The catecholamines have a short duration of action because of the ubiquitous distribution of metabolizing enzymes - catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). They will have to be infused continuously if you want a prolonged action. Non-catecholamine (adrenomimetic) Ephidrine α and β (non-selective) – pseudoephidrine (“Sudafed”) is a stereoisomer. Phenylephrine α Isoprenaline β1 and β2 (non-selective) – has been used as a bronchodilator Dobutamine β1 – commonly used to increase blood pressure in anaesthetised patients. It is given as an infusion (CRI). Salbutamol β2 – used as a bronchodilator Xylazine α2 27 Mechanisms of action Direct acting - drug attaches to, and activates receptor (all the drugs listed above are direct acting) Indirect-acting sympathomimetics Tyramine, amphetamine - must be taken up into the neuron to release stores of NA in the terminal neuron. Repeat administration leads to reduction in stores leading to a state of tachyphylaxis. Desipramine (a tricyclic antidepressant - TCA) and cocaine can block the reuptake transporters so they act as indirect sympathomimetics. Amphetamine is very lipophilic and can enter the neuron directly (bypass uptake mechanism). Reserpine will deplete stores of NA (used to treat hypertension). Adrenomimetic drugs can be metabolised by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). Dopamine Dopamine is a naturally occurring catecholamine (NA and adrenaline synthesised from dopamine) and is found in adrenergic nerves and the adrenal medulla. It is a neurotransmitter. Dopamine has cardiovascular effects by:1. releasing NA from adrenergic neurones 2. acts on α and β1 receptors 3. acts on dopamine receptors. D1 receptors are found on blood vessels and mediate vasodilatation, natriuresis and diuresis. Dopamine has a more specific action than the other catecholamines. D2 receptors occur on ganglia, adrenal cortex and cardiovascular centres in CNS and mediate hypotension, bradycardia and regional vasodilatation. 2. Adrenergic antagonists Phenoxybenzamine Phentolamine Prazosin Propranolol Atenolol Labetalol α1 and α2 (non-selective) α1 and α2 (non-selective) α1 β1 and β2 (non-selective) β1 β1 with some α α blocking agents (non‐selective antagonists)  phenoxybenzamine  phentolamine Clinical uses of these agents include the treatment of peripheral vasospasm, hypertension, phaeochromocytoma and circulatory shock. 28 α2 receptors mediate feedback inhibition of noradrenaline release and when blocked there will be increased sympathomimetic activity (not seen at α1 and α2 receptors but will occur at β receptors). Selective α1 blockers Prazosin has less reflex sympathomimetic activity than the non-selective drugs because α2 receptors are not affected. Extrasynaptic α2 receptors in blood vessels remain functioning and maintain vasomotor tone in resistance and capacitance beds. Selective α2 blockers Yohimbine can be used as an antidote for α2 agonist tranquillizers. It has been used as an aphrodisiac (probably to no effect as it has no effect on libido, but does stimulate mood) and for treating erectile problems in men. blockers (nonselective) Propranolol is non-selective for β1 and β2. Clinical uses of this class include the treatment of hypertension, cardiac dysrhythmias (AV conduction abnormalities), hypertrophic cardiomyopathies, anxiety. The long term antihypertensive mechanisms of action involve central regulation mainly. β1 selective blockers β2 inhibition can lead to bronchoconstriction (caution using propranolol in patients with airway disease) so selective 1 blockers were developed to treat hypertension and cardiac patients. All blockers must be used with care in patients with pre-existing heart disease because loss of sympathetic activity may cause reduced cardiac output and sudden lessening of cardiac output. Intrinsic sympathetic activity Some blockers are partial agonists, as well as antagonists, so they maintain some basal stimulation (forestall cardiac depression and bronchoconstriction). Adrenergic neuron‐blocking and catecholamine‐depleting agents These agents act pre-synaptically at nerve terminals and prevent release of NA and do not block the postsynaptic receptors. They do not prevent the action of direct-acting sympathometics (receptor not occupied).  Reserpine works by depleting catecholamines (no veterinary use - psychic disorders?). 29   Bretylium is an adrenergic neuron-blocking agent. (local anaesthetic–like in action). Cocaine inhibits amine uptake pump – indirect acting sympathomimetic. Cholinergic pharmacology Cholinomimetics (parasympathomimetics) Mechanisms of action Direct – acting These drugs work by binding to, then stimulating, the receptor. Acetylcholine does not find many clinical indications because it is rapidly degraded by the ubiquitous acetylcholinesterase and pseudocholinesterases. It can be used in intraocular surgery for rapid miosis (dribbled into anterior chamber to rapidly constrict pupil).    Pilocarpine (muscarinic agonist) can be used to facilitate pupillary constriction. Arecoline (muscarinic agonist) has been used as a taenicide in dogs (historical only). It can cause severe cramping and colic. Bethanecol (agonist with preference for muscarinic receptors) is not degraded by cholinesterases. Importantly, it has little effect on muscarinic receptors in cardiac and vascular smooth muscle and is used to cause micturition and GI peristalsis. Indirect action cholinomimetics Drugs tha