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Questions and Answers
What is the primary purpose of a drug's chemical name?
Which type of drug name is recognized by competent scientific bodies?
Which pharmacokinetic process is necessary for a drug to have its effects?
What is a characteristic of the vascular endothelium?
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What major consequence do tight junctions in certain organs have on drug distribution?
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Who is considered the first to attempt a theoretical background of pharmacology?
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What was Paracelsus known for rejecting?
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What philosophical stance did Galen take regarding the sources of pharmacological knowledge?
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Which of the following statements reflects Wepfer's sentiments on poisons?
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What is the primary basis of traditional medicine according to the history of pharmacology?
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What was a significant advancement made by Johann Jakob Wepfer in pharmacology?
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Why is the historical aspect of pharmacology important?
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Which approach did early pharmacologists primarily rely on before scientific methods were established?
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What facilitates the transfer of hormones or other molecules in endocrine glands?
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Which mechanism is the major pathway for drug absorption?
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Fick's first law of diffusion states that drug molecules move from areas of:
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What primarily influences the absorption of weak acids?
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Which type of drugs are poorly absorbed in the gastrointestinal tract?
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Which factor is NOT part of the pH partition hypothesis affecting absorption?
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Why do very weak bases with pKa < 5 have rapid absorption?
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How does the pKa of a weak acid influence its absorption in the stomach?
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What primarily influences the rate of loss of gases during pulmonary excretion?
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What factor influences the quantity of a drug excreted into sweat?
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Which statement about drug excretion in milk is true?
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Which factor does NOT influence the route of drug administration?
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What is a characteristic of oral ingestion as a route of drug administration?
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What is a characteristic of carrier mediated transport?
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Which process involves the movement of drugs along a concentration gradient at an accelerated rate?
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Which factor does NOT modify drug absorption?
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What type of substances cannot permeate membranes by passive diffusion?
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Which mechanism primarily governs drug absorption in the gastrointestinal tract?
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What is the primary function of drug distribution?
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Which condition favors the filtration or pore transport mechanism?
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Which factor does NOT affect the solubility of a drug at the site of absorption?
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Study Notes
Fenestrated Endothelium
- Found in endocrine glands
- Facilitates transfer of hormones and other molecules through pores in the endothelium
Movement of Small Molecules Across Cell Membranes
- Four main ways:
- Diffusion through lipids
- Diffusion through aqueous pores
- Transmembrane carrier proteins
- Endocytosis
Passive Diffusion
- Major mechanism for drug absorption
- Driven by concentration or electrochemical gradient
- Follows Fick's first law of diffusion: molecules diffuse from a region of higher concentration to lower concentration until equilibrium is reached.
Influence of pH on Passive Diffusion
- Governed by:
- Dissociation constant (pKa) of the drug
- Lipid solubility of the unionized drug
- pH of the absorption site
- Most drugs are weak acids or bases that exist in both ionized and non-ionized forms, with non-ionized molecules being more lipid soluble and diffusing more easily.
- Transmembrane distribution of a weak electrolyte is determined by its pKa and the pH gradient across the membrane.
Weak Acids, Bases and Absorption
- Weak acids:
- Very weak acids (pKa > 8) are mostly unionized at all pH values, leading to rapid absorption independent of GI pH.
- Acids with pKa between 2.5 and 7.5 are best absorbed in acidic conditions of the stomach (pH < pKa) where they are predominantly unionized.
- Stronger acids with pKa < 2.5 are ionized across the entire GIT pH range and are poorly absorbed.
- Weak bases:
- Very weak bases (pKa < 5) are mostly unionized at all pH values, leading to rapid absorption independent of pH.
- Bases with pKa between 5 and 11 are best absorbed in the more alkaline conditions of the intestine (where they are predominantly unionized).
- Stronger bases with pKa > 11 are ionized across the entire GIT pH range and are poorly absorbed.
Brief History of Pharmacology
- Medicaments have been used for treating diseases for a long time.
- Traditional knowledge, empirical information, formed the basis of early medicine.
- Claudius Galen (AD 129–200): First attempted to consider the theoretical background of pharmacology.
- Theophrastus von Hohenheim (1493–1541), aka Paracelsus: Rejected irrational concoctions and mixtures of medieval medicine.
- Johann Jakob Wepfer (1620–1695): First to verify pharmacological and toxicological actions through animal testing.
Drug Nomenclature
- Drugs have three categories of names:
- Chemical name: describes the substance chemically
- Non-proprietary (generic) name: name accepted by a scientific body
- Proprietary (Brand) name: name assigned by the manufacturer
Pharmacokinetics
- Deals with absorption, distribution, metabolism, and excretion of drugs.
- Drug concentration at the site of action depends on all these processes.
- All pharmacokinetic processes involve drug passage across membranes.
Movement of Drug Molecules Across Cell Barriers
- Cell membranes act as barriers between the body's aqueous compartments.
- For drugs to pass from one side of a cell to the other, they need to cross at least two cell membranes.
- Vascular endothelium: Gaps between cells are packed with proteins that act as filters.
- Some organs (e.g. CNS, placenta) have tight junctions between cells, preventing harmful molecules from entering.
- Other organs (e.g. liver, spleen) have discontinuous endothelium, allowing free passage between cells.
Carrier Mediated Transport
- Involves binding of the drug to specific receptors on the membrane for translocation into the cytoplasm.
- Characteristics:
- Selectivity
- Competitive inhibition by chemical congeners
- Requires energy
- Saturability
- Movement against electrochemical gradient
- Active transport: movement against the concentration gradient
- Facilitated diffusion: movement along the concentration gradient but at a faster rate than simple diffusion
Filtration or Pore Transport
- Passage through the water-filled pores in membranes.
- Accelerated by the hydrodynamic flow of water under hydrostatic or osmotic pressure gradient.
- Requires water solubility and a smaller molecular size than the pore diameter.
Drug Absorption
- Rate at which a drug leaves its administration site and the extent to which this occurs.
- Bioavailability: extent to which a drug reaches its site of action or a biological fluid from which the drug has access to its site of action.
- Absorption occurs through one or a combination of the processes described earlier.
Factors That Modify Absorption
- Drug solubility at the site of absorption
- Dosage form
- Local conditions at the site of action
- Drug concentration at the site of absorption
- Circulation to the site of absorption
- Area of the absorbing surface
- Route of administration
Drug Distribution
- Process by which absorbed drug or drug directly introduced into the circulation is carried to various interstitial and cellular fluids.
Pulmonary Excretion
- Any volatile material can be excreted through the lungs, regardless of the route of administration.
- Rate of excretion depends on respiration rate, pulmonary blood flow, and gas solubility in blood.
Sweat and Saliva Excretion
- Depends on diffusion of un-ionized lipid-soluble forms across gland epithelial cells.
- Quantity excreted is determined by the drug's pKa and the pH of the secretions.
- Excretion in sweat can cause skin reactions.
- Substances excreted in saliva are usually swallowed.
Excretion in Milk
- Concentration of drugs in milk depends on:
- Amount of drug in maternal blood
- Lipid solubility of the drug
- Drug's degree of ionization
- Extent of its active excretion
- Milk is more acidic (pH 6.5) than plasma, meaning basic drugs are more concentrated in this fluid.
Clinical Significance of Drug Excretion
- Excretion determines how long a drug remains in the body, influencing its duration of action.
Routes of Drug Administration
- Drugs are administered for local or systemic effects.
- The choice of route depends on:
- Target tissue
- Physico-chemical properties of the drug
- Disease state
Systemic Routes of Administration
- Oral ingestion: Administration of drugs through the mouth, mostly for systemic effects.
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Description
This quiz covers the mechanisms of movement of small molecules across cell membranes, focusing on passive diffusion and its dependence on factors like pH and drug characteristics. You will explore the role of fenestrated endothelium in hormone transfer and the principles governing drug absorption through diffusion. Test your understanding of these essential concepts in cellular physiology.
