Elimination PDF

Elimination Via Urinary System o Urinary system is composed of 4 pars: kidneys, ureter, bladder, urethra The Kidney o 2 main functions: 1) removes metabolic water from the body (excretion) 2) Regulates water and ion content in the blood...

Elimination Via Urinary System o Urinary system is composed of 4 pars: kidneys, ureter, bladder, urethra The Kidney o 2 main functions: 1) removes metabolic water from the body (excretion) 2) Regulates water and ion content in the blood o It functions this way through excretion of dilute solution (urine): which contains urea, mineral ions, water, and xenobiotics from the blood o The two kidneys have extensive blood supply, and the blood supply passes through the kidneys every 5 minutes to ensure waste material don’t build up o Plays major role in regulating blood volume b/c it controls amount of water to be excreted and amount of water to be reabsorbed § Renal damage can impact blood pressure o Regulates electrolytes in the blood by controlling secretion and reabsorption of Na+ and K+ ions o Regulated pH of blood by controlling secretion and reabsorption of hydrogen ions § More hydrogen ions excreted from blood à makes blood less acidic (more alkaline) § Hydrogen ions retained in blood à blood more acidic (less alkaline) o Regulates blood pressure by regulating amount of water excreted and amount of water reabsorbed back into blood o Plays a role in regulation of red blood cell production. When the number of red blood cells decrease, the level of oxygen in the blood also decreases. This causes kidney to secrete a substance called erythropoietin. § Erythropoietin travels to the bone marrow and causes it to produce more red blood cells. When enough red blood cells have been produced, this process is shut down via a negative feedback mechanism o The renal artery à carries blood to the kidneys o the renal vein à carries blood away from the kidneys o Blood flows to the kidneys of the adult human at a rate of roughly 1 L/min. o The most important part of the kidneys is a tubule called a nephron. o The adult human kidney contains approximately 1 million nephrons. Kidney Nephron System - Glomerular ultrafiltration o about 1/3 of blood plasma is filtered by the glomerulus to form dilute urine (ultrafiltrate) - Tubular secretion o Proximal Convoluted Tubule selectively secretes certain xenobiotics into urine - Tubular reabsorption o Distal Convoluted Tubule and Collecting Duct may reabsorb many xenobiotics into blood by passive di1usion Four Primary Mechanism in Urinary Excretion: 1) Filtration – from the blood to the glomerulus § Blood is filtered at the beginning of renal tubule § Filtration membrane contains large pores so small molecules readily pass into the tubule. About 99% of the filtrate is reabsorbed into blood, the remaining 1% is excreted as urine § The primary factors that influence filtration are size of the molecule and glomerular filtration, as determined by blood flow and pressure. § Proteins and blood cells are too large to pass through the pores § Depending upon the toxic agent and/or disease state, glomerular filtration may increase or decrease § Therefore, glomerular filtration rate may be used as an indicator for clearance of a compound that is excreted by renal filtration 2) Passive DiAusion – from the blood to the renal tubules § refers to the movement of substances across a biological membrane without requiring energy (ATP) § Simple diLusion: small, uncharged, lipophilic molecules  § Facilitated diLusion requires specific membrane proteins to move substances across membranes § This is an important route for lipid soluble compounds. § If the compound is non-ionized passive diLusion is high. § However, if it remains non-ionized in the tubule, reabsorption may take place just as easily. § Thus, retention in the tubule (and passive diLusion as a means of renal excretion) requires ionization of the compound in the tubule, which may be altered by pH of the tubular fluid. § The primary determinant of diLusion is the concentration gradient of the compound. 3) Active transport – of chemicals from the blood to the tubular lumen by energy-dependent carrier proteins § These protein transport systems are specific for weak acids or bases and may become saturated, setting a limit on the amount of material excreted in a given time period. 4) Facilitated diAusion – a process that essentially resembles active transport, except that it does not require energy Other factors aLecting urinary excretion include: - Plasma protein binding: highly bound compound = decreased ability to undergo glomerular filtration, but its ability to be actively transported NOT aLected - pH: if toxicant is a weak acid, alkalinization of the urine will increase its excretion by maintaining it in the ionised state. This is true for weak bases as well. Ultrafiltration produces the initial filtrate that contains both waste and essential solutes. Driving Forces:  Hydrostatic pressure: Pushes water and solutes out of the blood. Oncotic pressure: Opposes filtration due to the plasma proteins in the blood. Filtration pressure: Net pressure favoring filtration. Selective tubular reabsorption ensures the reclamation of vital components, preventing their loss in urine, while concentrating the waste products for excretion. Substances Reabsorbed: Proximal Tubule: Reabsorbs most of the filtrate, including glucose, amino acids, sodium, chloride, and water. Loop of Henle: Reabsorbs water in the descending limb and ions (sodium, potassium, chloride) in the ascending limb. Distal Tubule and Collecting Duct: Fine-tune reabsorption of ions (e.g., sodium under aldosterone control) and water (under antidiuretic hormone (ADH) regulation). Low Log Kow (Hydrophilic Compounds): These compounds are highly water-soluble and less likely to diLuse across the lipid membranes of renal tubular cells. They tend to remain in the tubular lumen after filtration and are excreted in urine. High renal clearance is observed due to minimal tubular reabsorption. High Log Kow (Lipophilic Compounds): o Highly lipophilic substances are reabsorbed efficiently by passive diffusion in the renal tubules. o These compounds tend to have low renal clearance, as they are more likely to re-enter the bloodstream. o Lipophilic drugs or toxins may require hepatic metabolism for further clearance. Failure of Homeostasis: Impair function of kidneys, impacting body’s ability to excrete xenobiotics - Kidney disease – caused by long term diabetes, infections, and chemical poisoning - Bladder & Kidney infections – caused by enteric bacteria entering urethra - Kidney stones – crystallization of mineral salts and uric acid that block passage of urine. Kidneys and First-Pass Cardiac Output The kidneys receive ~25% of cardiac output in the first pass because they play a crucial role in filtering blood and maintaining homeostasis. ~20% is filtered through the glomeruli (25 g/day of urea is filtered and excreted) The glomerular capillaries have very large pores (70 nm) and allow compounds up to a molecular weight of 60 kDa (< albumin) to be filtered Tubular Secretion: - Active Transport for acids, bases, neutrals into renal tubules o OCT: organic cation transporter o OAT: organic anion transporter  o MDR/MRP: multidrug resistant transporters Tubular Reabsorption - Passive: depends on ionization of xenobiotic; lipophilic substances will be reabsorbed from tubules more than hydrophilic substances: o Under high urinary pH, excretion of acids is increased o Under low urinary pH, excretion of bases is increased - Active: OCT’s, peptide transporters (PEP), MRPs Xenobiotic EAlux Pumps (e.g., P-gp/MDR, MRP) - Specialized proteins that actively transport substances, particularly hydrophobic compounds, across cellular membranes - Key role in detoxification and drug resistance by removing potentially harmful substances - All belong to ABS transporter superfamily: o Pumps that belong to ATP-binding cassette (ABC) transporter superfamily, characterized by their ability to hydrolyze ATP to power substrate transport - P-gp is a major player in multidrug resistance, transports a broad range of structurally diverse xenobiotics. o Substrates: Primarily xenobiotics; no endogenous substrates have been definitively identified. o Often described as a "hydrophobic vacuum" that extracts substrates from the inner leaflet of the lipid bilayer. o Structure: pore-forming protein with transmembrane domains and an ATP-binding cassette (ABC) region  - MRP1-7 (Multidrug Resistance-Associated Proteins) o contribute to drug resistance and xenobiotic transport o ELective in handling organic anions o Overlap in substrates with MDR/P-gp Elimination of Xenobiotics - Urine via Renal System: filtration, excretion - Feces via GI tract and liver: metabolism, excretion - Expired air via lungs: exhalation - Others: secretions in breast milk, sweat, saliva Fecal Excretion - Second major pathway for elimination of xenobiotics - Complex process - Excretion results from: o Direct elimination of non-absorbed compounds in the GI tract  o Delivery to the GI tract via the bile o Secretion into the intestinal luminal contents from enterocytes Biliary Elimination ▪ Biliary elimination is a significant source contributing to the fecal excretion of xenobiotics and is even more important for the excretion of metabolites. ▪ The liver plays an important role in removing xenobiotics from the blood after gastrointestinal absorption because blood from the GI tract passes via the portal circulation before reaching the general circulation. - First Pass E+ect ▪ The liver is also the main site for biotransformation of xenobiotics, and the resulting metabolites (typically now more hydrophilic) may be excreted directly into the bile. First-pass eAect by the liver - Xenobiotics in food and water carried from gut to liver via hepatic portal vein - Xenobiotics in systemic circulation are carried to liver via hepatic artery - Liver filters out a fraction of lipophiles into hepatocytes - Remaining lipophiles enter systemic blood via hepatic vein - Hepatocytes secrete their intracellular lipophiles to bile - Biliary excretion carries lipophiles to gut à feces Biliary Elimination  - Xenobiotics and/or their metabolites excreted into the bile enter the intestine and may be excreted with feces. - However, if the physicochemical properties favour reabsorption, an enterohepatic circulation may result. - The factors determining biliary excretion are not well understood, but in general the following is true: o Xenobiotics bound to plasma proteins are fully available for active biliary excretion. o Low molecular-weight compounds are poorly excreted into bile, while larger compounds or their conjugates (with MWs < 325) can be excreted in large quantities. o Glutathione and glucuronide conjugates have a high probability of being excreted ▪ Substances excreted into the bile typically are divided into three classes based on the ratio of their concentration in bile compared to that in plasma. ▪ Class A: Ratio of nearly 1. Include sodium, potassium, glucose, mercury, thallium, cesium, and cobalt. ▪ Class B: Ratio of bile to plasma usually between 10 and 1000. Include bile acids, bilirubin, lead, arsenic, manganese, and many other xenobiotics. ▪ Class C: Ratio below 1. Include inulin, albumin, zinc, iron, gold, and chromium. ▪ Class B substances are typically rapidly excreted into bile. Biliary excretion is regulated primarily by xenobiotic transporters present on the canalicular membrane, including: MRP2: Transport of organic anions including glucuronide and glutathione conjugates of many xenobiotics BCRP: ALinity for sulfated conjugates of xenobiotics MDR1: Primarily transports a variety of substrates into bile MATE1: Transport of organic cations BSEP: Secretion of bile salts and the regulation of bile flow Amanitin - Oligopeptide toxin produced by Amanita phalloides (toadstool) - Lethal human poisoning after ingestion (8-24 h post exposure) - High oral bioavailability - Inhibits RNA polymerase II → blocks mRNA synthesis - Follows bile salt uptake transporter (facilitated diLusion) o OATP1B3 – Humans (Liver, hepatocyte basolateral membrane) o NCTP – Rats - So, treatment for this toxin is to provide an OAT1PB3 transporter substrate (competitive inhibitor), such as rifampicin or cyclosporin. Hepatotoxicity: Cholestasis - Cholestasis is reduction or stoppage of bile flow - It can be caused by disorders of the liver, bile duct, or pancreas - The skin and whites of the eyes look yellow, the skin itches, urine is dark, and stools may become light-coloured and smell foul - It can be noted by the presence of bile plugs in canaliculi; accumulated bile in hepatocytes - Results in hepatocyte retention of compounds usually eliminated in bile - It can be caused by an impairment of transporters, such as their expression being down-regulated.  - The resulting build up of endobiotics and xenobiotics in hepatocytes can lead to liver toxicity Enterohepatic Cycle: Lipophilic Xenobiotics Excreted into Bile May be Reabsorbed in the Gut - Enterohepatic circulation causes increased retention of xenobiotics conjugated by glucuronic acid because they are deconjugated in the intestine and reabsorbed. - Liver excretes both lipophiles and conjugated lipophiles into the bile via eLlux pumps (biliary excretion) - lipophiles travel in bile and empty into small intestine at the duodenum - Some fraction of the lipophile is excreted in feces - Remaining fraction is reabsorbed from the gut into blood via passive diLusion. - Hepatic portal vein carries reabsorbed lipophile back to the liver - Enterohepatic cycle slows the clearance of many highly lipophilic xenobiotics - Liver metabolism can often biotransform a lipophile to a conjugated lipophile to prevent enterohepatic cycling --> faster clearance - Lipophile conjugation groups may be hydrolyzed in the gut by microbial enzymes (beta-glucuronidases) Example: Diethylstilbestrol (DES) - Classified as endocrine disruptor – a synthetic nonsteroidal estrogen - Exposure to DES occurs through ditary ingestion from supplemented cattle feed - From 1940s-1970s, was used in pregnant women to reduce complications, but caused vaginal cancer in daughters of women who used DES during pregnancy  - Undergoes enterohepatic circulation and is retained in the body by sequential conjugation and deconjugation Elimination of Lipophilic Xenobiotics Lipophiles are often diLicult to excrete (kidney and liver) ▪ Tend to be persistent in biota ▪ Degree of persistence is measured by the elimination half-life - Elimination half-life (t1/2) - measures persistence in body ▪ Rate of elimination of a xenobiotic from body by passive excretion is proportional to blood concentration (first-order kinetics) ▪ t1/2 = time taken to eliminate one-half of remaining xenobiotic ▪ Amount of chemical remaining at each half-life interval ▪ 1-->1/2 --> 1/4 --> 1/8 -->etc. Excretion of Lipophilic Xenobiotics Lipophiles often have a prolonged elimination half-life ▪ Poorly excreted through the kidney due to tubular reabsorption ▪ Enterohepatic circulation limits biliary excretion ▪ This results in bioaccumulation in to create high body burden Body burden ▪ Concentration of a chemical (or chemicals) in body tissues ▪ Most tissues cannot be directly sampled ▪ Biomonitoring studies rely on biomarkers in urine, blood, saliva, etc. ▪ Can “back-extrapolate” from blood concentrations → estimated tissue concentrations → health eLects Lipophile elimination speeded by biliary excretion via the liver ▪ Active transport systems -- not dependent on passive diLusion ▪ Conjugation reactions metabolize lipophiles ▪ Several active transport systems secrete conjugated lipophiles into the bile for excretion Persistent Organic Pollutants (POPs) ▪ Highly lipophilic organochlorine (OC) compounds ▪‘Dirty Dozen’ - persistent, bioaccumulative, toxic (PBT criteria) ▪ Very slow or non-existent conjugation in liver --> hard to excrete in bile