Na Homeostasis Flashcards
Na Reabsorption in Prox Tubule
65%
- Apical membrane - Na moves down gradient (use this to couple w/ other molecules)
- Na/H exchanger
- Na/glucose co-transporter
- Na/Phos co-transport
- Na/AA co-transport
- Na/SO4- - Basolateral membrane - Na/K pump (ATP) which maintains Na+ gradient OR Na/bicarb co-transporter (transports 3 bicarb out w/ 1 Na b/c electrochem gradient of pump)
- Anions (Cl-) also reabsorbed to maintain neutral charge
- Iso-osmotic so reabsorb water along w/ Na+
How is Cl- moved into proximal tubule?
Reabsorbed to maintain neutral charge
Transcellular (on apical membrane)
- Cl/bicarb exchanger
- Cl/formate exchanger
- SO4-/oxalate exchanger then…Cl/oxalate exchanger
- Also some paracellular (b/n cells) movement of Cl-
What happens to Na once it exits basolateral membrane of tubule?
- Na then reabsorbed into peritubular capillary
- Driven by high oncotic pressure/low hydrostatic pressure in capillary
Na Reabsorption in Loop (thin v thick)
25-30%
- Thin loops only inactive transport
- Thick loop uses active transport
- Half of Na+ enters paracellularly
- Half of Na+ enters via apical Na-K-2Cl co-transporter then basolateral Na-K pump
What other ion movement occurs across TAL?
- Cl- then exits on basolateral via own channel
- K+ recycled back out apical via ROM K Channel (maintain gradient)
- Paracellular Ca++ and Mg++ (b/c electrochemical gradient from K+ pumped out)
Na Reabsorption in Distal Tubules and Collecting Ducts
8%
- Early distal - Na/Cl co-transporter
- Act by aldosterone
- Inhibited by thiazide diuretics - Later distal/collecting - ENaC (epithelial sodium channels)
- Act by aldosterone and vasopressin
- Inhibited by K sparing diuretics and aldosterone antagonists - In intercalated cells of distal nephron … acid/base regulation
- Apical Na-dep Cl-/Bicarb exchanger paired w/ another parallel Cl-/bicarb exchanger (called pendrin) - Once again, have basolateral Na/K pump to maintain Na+ gradient (except in these intercalated cells)
How does angio II affect glomerulus?
- It constricts afferent and efferent arterioles of glomerular capillary (but afferent counteracted by PGEs)
- Inc hydrostatic pressure in glomerular capillary but dec flow through this capillary so oncotic pressure rises more quickly which dec GFR (SO NET EFFECT IS MAINTAIN GFR)
How does angio II affect prox tubule?
- (downstream of constriction of efferent)
- Dec hydrostatic pressure in peritubular capillary and inc oncotic peritubular capillary to enhance reabsorption of Na+ and water (+urea and uric acid) in proximal tubule
- Also stimulates Na/H exchanger in proximal tubule
What stimulates vasopressin? What effect does it have on Na in tubules?
- Act if volume depletion or high osm
- Main job = inc water reabsorption in collecting duct
- ALSO… Act Na-K-Cl co-transporter of TAL & Act ENaCs
What stimulates aldosterone? What effects does it have on Na in tubules?
- Secreted in response to angio II and inc K+ in plasma
- Works on distal/collecting tubules
- Act Na-Cl co-transporter
- Act Na-K pump
- Inc # and opening of ENaCs
- Inc K+ and H+ secretion to get rid of K+ into urine
What effect does ANF have on Na in tubules?
Inc GFR and inhibit Na-K pump so no longer have Na gradient for apical transporters
What effect does dopamine have on Na in tubules?
- Inhibits Na/H exchanger and Na-K pump of proximal
What effect does endothelin have on Na in tubules?
- Inhibits ENaC
What effect does ouabain and its analogues have on Na in tubules?
- Inhibits Na/K pump so no longer have Na gradient for apical transporters
- Ouabain synthesized by adrenal
- Analogues made by hypothalamus
How do prostaglandins affect Na in tubules?
- Some enhance renin secretion by JG cells
- Some dilate afferent or efferent arterioles
- Inhibit Na reabsorption in TAL and collecting duct
