Biob34 Module 6 - Osmoregulation Lecture Slides PDF

Osmoregulation Osmotic pressure: pressure needed to offset movement of pure solvent (e.g. H20) across semipermeable membrane Hydrostatic pressure: pressure fluid physically exerts on surroundings Due to pull of gravity Pressure of blood on vessel wall...

Osmoregulation Osmotic pressure: pressure needed to offset movement of pure solvent (e.g. H20) across semipermeable membrane Hydrostatic pressure: pressure fluid physically exerts on surroundings Due to pull of gravity Pressure of blood on vessel wall © K. Welch – Do Not Distribute 1 Concepts you should recall Ionic concentration -tonic solutions © K. Welch – Do Not Distribute 2 Electrolyte composition: serum ≈ ECF © K. Welch – Do Not Distribute 3 Challenges in maintaining homeostasis (maintenance of extracellular fluid matrix) Ionic constituency Osmotic balance Depending on environment water/ions could tend to flow in or out of animal © K. Welch - Do Not Distribute 4  Challenges in maintaining homeostasis Humans, like terrestrial birds, usually produce slightly acidic urine (pH 5-7) of variable concentration (depending on water/salt intake) © K. Welch - Do Not Distribute 5 © K. Welch - Do Not Distribute 6 (Respiratory) Evaporative Water Loss © K. Welch – Do Not Distribute 7 Mammalian kidney © K. Welch - Do Not Distribute 8  Adaptation of mammalian kidney to environment Relative urine concentrating ability of kidney reflective of environment animal finds itself in (and osmoregulatory challenges it does/does not face © K. Welch - Do Not Distribute 9  Kangaroo rat Found in desert southwest US Very hot & arid environment Two factors that each ↑ water loss to environment Kangaroo rat must balance water loss with water intake With little liquid water intake Instead of “Intake,” relies heavily on “metabolic water” Water that is product of substrate oxidation Loss through urine is minimized © K. Welch - Do Not Distribute 10  Arid adapted birds limit (some) water loss Saudi Arabian (from arid environment) sparrows have less cutaneous water loss (CWL) than Ohio, USA (from “wet” environment) sparrows But not less respiratory water loss (RWL) © K. Welch - Do Not Distribute 11  Arid adapted birds limit (some) water loss Rate of CWL correlates with lipid amount and content in skin of the birds More free fatty acids  more CWL Ceramides and cerebrosides correlate with less CWL © K. Welch - Do Not Distribute 12 Metabolic water But what about water associated with tissue stores? Carb stores (glycogen) are hydrophilic Lipid is hydrophobic © K. Welch - Do Not Distribute 13  Fat stores as a hidden source of water storage Migrating birds don’t eat OR DRINK while flying Need for energy AND water Energy: fat is good because it is dense energy source Water: fat is good because it liberates 1.5 X as much water for each ATP made Not counting water hydrating glycogen stores © K. Welch - Do Not Distribute 14 Protein stores as a hidden source of water storage! Protein-for-water hypothesis Liberates bound water Also replenishes metabolic intermediates (anapleurosis) Reduces “unnecessary’” weight of extra-large flight muscles © K. Welch - Do Not Distribute 15 Hummingbirds: Dehydration is not the problem (during the day) Lots of water intake during the day (copious dilute nectar intake) Must produce dilute urine and maximize glucose reabsorption But at night, dilute urine production while fasting imposes dehydration danger © K. Welch - Do Not Distribute 16 Hummingbirds: Dehydration is not the problem (during the day) Graph showing amount of tracer in urine During day (and following morning) Tracer is washing out of system During night Little urine production or tracer dilution Evidence hummingbirds dramatically reduce GFR during night © K. Welch - Do Not Distribute 17 Hummingbirds: Dehydration is not the problem (during the day) Graph showing amount of tracer in urine During day (and following morning) Question, why is “L-glucose” used? Tracer is washing out of system During night What is “L”-glucose? No urine production or tracer dilution Evidence hummingbirds dramatically reduce GFR during night © K. Welch - Do Not Distribute 18 Remember: Carb absorption in intestine © K. Welch - Do Not Distribute 19  Glucose (or amino acid) reabsorption Look familiar? 1. Active glucose (or AA)/Na+ co-transport (via sep. transporters) Normally, ~100% of the 2.Facilitated diffusion filtered glucose is GLUT2 (for glucose, not AA) reabsorbed. 3. Na+ / K+ ATPase pump SGLT2 (~90%)/ SGLT1 (~10%) glucose transporter protein or AA (via other Na+-linked transporters) Na+/K+ ATPase pump Tubular epithelial cell Apical Basolateral Peritubular Capillary membrane membrane fluid endothelial cell © K. Welch - Do Not Distribute 20 Type II diabetes Common symptom: hyperglycemia Historical diagnosis: sweet tasting urine Why? © K. Welch - Do Not Distribute 21  Blood pH regulation Blood maintained with 0.025 M HCO3- and 0.00000004 M H+ There must be net H+ excretion and HCO3- reabsorption from filtrate CA catalyzes CO2 + H20 → HCO3- + H+ HCO3- exchanged for Cl- H+ pumped into lumen Where some can combine with HCO3- to form CO2 + H2O(which then diff. into cell → blood, etc.) Acidified lumen favors more Na+ diff. to cell Na+/K+ ATPase helps maintain Na+ gradient © K. Welch - Do Not Distribute 22  Capacity for urine acidification H+ pump stops working when urine pH < 4.5 Buffering of urine necessary for further H+ secretion Ammonia (and phosphates) can act as buffers Diffuses from blood or originally included in filtrate Since ammonia is breakdown product of AA (from proteins), more protein-filled diet will generally cause more acidic urine © K. Welch - Do Not Distribute 23  Vasopressin (and BP regulation) Also called antidiuretic hormone (ADH) Peptide hormone Produced in hypothalamus and released by posterior pituitary gland Increases water reabsorption from the collecting duct by increasing number of aquaporins © K. Welch - Do Not Distribute 24  Vasopressin (and BP regulation) Also called antidiuretic hormone (ADH) Release stimulated by increasing plasma osmolarity detected by osmoreceptors in the hypothalamus Release is inhibited by increasing blood pressure detected by stretch receptors in atria and baroreceptors in carotid and aortic bodies Alcohol inhibits release of ADH by pituitary cells Thus, acts as diuretic (promotes greater volume of dilute urine production) Alcohol © K. Welch - Do Not Distribute 25 Vasopressin Increases Cell Permeability © K. Welch - Do Not Distribute 26  Hypertension Body defends blood osmotic pressure Eating too much Na+ causes increase in plasma [Na+] Disrupting kidneys ability to allow enough Na+ excretion To defend osmotic pressure, more water is retained in plasma Dilutes Na+ and restores osmotic pressure balance But this causes increase in hydrosta c pressure (↑BP) © K. Welch - Do Not Distribute 27  Antihypertensive drugs Two (of many) classes: ACE inhibitors E.g. Lotensin®, Vasotec®, Monopril® Angiotensin II receptor blockers E.g. Atacand®, Teveten® © K. Welch - Do Not Distribute 28 Renin-Angiotensin-Aldosterone (RAA) Pathway Granular (juxtaglomerular) Juxtaglomerular cells secrete cells enzyme renin Secretion of renin controlled in three ways Baroreceptors in juxtaglomerular cells release renin in response to low blood pressure Sympathetic neurons in cardiovascular control center of medulla oblongata trigger renin secretion in response to low BP Macula densa cells in distal tubule respond to decreases in flow by releasing a paracrine signal that induces juxtaglomerular cells to release renin Thus, renin secreted when blood pressure or GFR are lower than normal © K. Welch - Do Not Distribute 29  Renin-Angiotensin-Aldosterone (RAA) Pathway RAA pathway helps regulate blood pressure Angiotensin II a vasoconstrictor Raises blood pressure by increasing resistance Aldosterone increases Na+ (and water) retention Raises blood pressure by increasing blood volume © K. Welch - Do Not Distribute 30  Renin-Angiotensin-Aldosterone (RAA) Pathway Renin converts angiotensinogen to angiotensin I Angiotensinogen an inactive protein in plasma Angiotensin converting enzyme (ACE) on epithelia of blood vessels converts angiotensin I to angiotensin II ACE Inhibitor © K. Welch - Do Not Distribute 31  Renin-Angiotensin-Aldosterone (RAA) Pathway Angiotensin II causes synthesis and release of aldosterone from adrenal cortex Steroid hormone Targets cells in distal tubule and collecting duct Stimulates Na+ and water reabsorption from filtrate Enhances K+ excretion Also stimulated by increases in circulating K+ Angiotensin II receptor blocker © K. Welch - Do Not Distribute 32