1. Renal tubular reabsorption Stephen P. DiBartola Department of Veterinary Clinical Sciences College of Veterinary Medicine Ohio State University Columbus OH 43210

2. What do the kidneys do? The glomeruli “non-discriminantly” filter the blood, and the tubules take back what the body needs leaving the rest as waste to be excreted. Some wastes also can be actively added to the tubular fluid.

3. Renal tubular reabsorption Excretion refers to the removal of solutes and water from the body in urineExcretion refers to the removal of solutes and water from the body in urine Reabsorption (movement from tubular fluid to peritubular blood) and secretion (movement from peritubular blood to tubular fluid) refer to direction of movement of solutes and water across the renal tubular epitheliumReabsorption (movement from tubular fluid to peritubular blood) and secretion (movement from peritubular blood to tubular fluid) refer to direction of movement of solutes and water across the renal tubular epithelium

4. Renal tubular reabsorption The luminal cell membranes are those that face the tubular lumen (“urine” side)The luminal cell membranes are those that face the tubular lumen (“urine” side) The basolateral cell membranes are those are in contact with the lateral intercellular spaces and peritubular interstitium (“blood” side)The basolateral cell membranes are those are in contact with the lateral intercellular spaces and peritubular interstitium (“blood” side)

5. Renal tubular reabsorption The transmembrane potential difference is the electrical potential difference between the inside and outside of the cellThe transmembrane potential difference is the electrical potential difference between the inside and outside of the cell The transepithelial potential difference is the electrical potential difference between the tubular lumen and the peritubular interstitiumThe transepithelial potential difference is the electrical potential difference between the tubular lumen and the peritubular interstitium

6. Renal tubular reabsorption The term transcellular refers to movement of solutes and water through cellsThe term transcellular refers to movement of solutes and water through cells The term paracellular refers to movement of solutes and water between cellsThe term paracellular refers to movement of solutes and water between cells Epithelial cell junctions can be “leaky” (proximal tubule) or “tight” (distal convoluted tubule, collecting duct)Epithelial cell junctions can be “leaky” (proximal tubule) or “tight” (distal convoluted tubule, collecting duct)

7. Terminology Transepithelial versus transmembrane potential difference Luminal versus basolateral membranes Transcellular versus paracellular transport

8. Renal tubular reabsorption Leaky epithelia (proximal)Leaky epithelia (proximal) Small transepithelial concentration differenceSmall transepithelial concentration difference Small TEPDSmall TEPD High water permeabilityHigh water permeability Tight epithelial (distal) Large transepithelial concentration difference Large TEPD Low water permeability

9. Nephro-man says … Luminal surface Basolateral surface Epithelial tight junctions Just think of it as a six-pack

10. Renal tubular reabsorption That renal tubular reabsorption must occur is intuitively obvious because … The fluid filtered into Bowman’s space is an ultrafiltrate of plasma containing many vital small molecular weight solutes (e.g., glucose, amino acids, bicarbonate) but these solutes do not normally appear in urine

11. Renal tubular reabsorption Solute reaborption in the proximal tubule is isosmotic (water follows solute osmotically and tubular fluid osmolality remains similar to that of plasma)Solute reaborption in the proximal tubule is isosmotic (water follows solute osmotically and tubular fluid osmolality remains similar to that of plasma) 65% of water and solute reabsorption occurs in the proximal tubule65% of water and solute reabsorption occurs in the proximal tubule 90% of bicarbonate90% of bicarbonate 99% of glucose & amino acids99% of glucose & amino acids Proximal tubules: coarse adjustmentProximal tubules: coarse adjustment Distal tubules: fine adjustmentDistal tubules: fine adjustment

12. Cl - goes up because Na + is reabsorbed with glucose, amino acids, P i and HCO 3 - Glucose, amino acids, P i and HCO 3 - go down due to reabsorption with Na + Unchanged due to isosmotic reabsorption

13. Secondary active co-transport (glucose, amino acids, phosphate) LUMINAL BASOLATERAL Glucose, P i amino acids Na + H+H+H+H+ 3 Na + 2 K + K+K+K+K+ HCO 3 - + H 2 CO 3

14. Types of transport processes Passive transport (simple diffusion)Passive transport (simple diffusion) Facilitated diffusionFacilitated diffusion Primary active transportPrimary active transport Secondary active transportSecondary active transport PinocytosisPinocytosis Solvent dragSolvent drag

15. Passive transport (simple diffusion): Definition Movement of a substance across a membrane as a result of random molecular motionMovement of a substance across a membrane as a result of random molecular motion

16. Passive transport (simple diffusion): Characteristics No metabolic energy required Rate of transfer dependent on electrochemical gradient across membrane and membrane permeability characteristics Rate of transfer linearly related to concentration of diffusion substance (no V max )

17. Facilitated diffusion: Definition Movement of a substance across a membrane down its electrochemical gradient after binding with a specific carrier protein in the membraneMovement of a substance across a membrane down its electrochemical gradient after binding with a specific carrier protein in the membrane

18. Facilitated diffusion: Characteristics Saturable (has a V max )Saturable (has a V max ) Structural specificity and affinity of carrier for substance transportedStructural specificity and affinity of carrier for substance transported Transfer may occur in either direction across membraneTransfer may occur in either direction across membrane Does not directly require metabolic energyDoes not directly require metabolic energy

19. Facilitated diffusion: Examples Glucose, amino acids: Basolateral membranes of proximal tubulesGlucose, amino acids: Basolateral membranes of proximal tubules Sodium: luminal membranes of proximal tubulesSodium: luminal membranes of proximal tubules

20. Primary active transport: Definition Movement of a substance across a membrane in combination with a carrier protein but against an electrochemical gradientMovement of a substance across a membrane in combination with a carrier protein but against an electrochemical gradient

21. Primary active transport: Characteristics Directly requires metabolic energy (i.e. hydrolysis of ATP)Directly requires metabolic energy (i.e. hydrolysis of ATP) Saturable (has a V max )Saturable (has a V max ) Structural specificity and affinity of the carrier for the substance transportedStructural specificity and affinity of the carrier for the substance transported

22. Primary active transport: Examples Na + -K + ATPaseNa + -K + ATPase H + ATPaseH + ATPase H + -K + ATPaseH + -K + ATPase Ca +2 ATPaseCa +2 ATPase

23. Secondary active transport: Definition Two substances interact with one specific carrier in the cell membrane and both substances are translocated across the membraneTwo substances interact with one specific carrier in the cell membrane and both substances are translocated across the membrane Co-transport Transported substances move in the same direction across the membraneCo-transport Transported substances move in the same direction across the membrane Counter-transport Transported substances move in opposite directions across the membraneCounter-transport Transported substances move in opposite directions across the membrane

24. Secondary active transport: Characteristics “Uphill” transport of one substance is linked to “downhill” transport of another substance“Uphill” transport of one substance is linked to “downhill” transport of another substance Carrier must be occupied by both substances (or be unoccupied) to be mobile in the membraneCarrier must be occupied by both substances (or be unoccupied) to be mobile in the membrane Saturable (has a V max )Saturable (has a V max ) Demonstrates specificity and affinity of carrier for substance transportedDemonstrates specificity and affinity of carrier for substance transported “Uphill” transport occurs without direct input of metabolic energy“Uphill” transport occurs without direct input of metabolic energy

25. Secondary active transport: Examples Glucose, amino acids, or phosphate with sodium in luminal membranes of proximal tubulesGlucose, amino acids, or phosphate with sodium in luminal membranes of proximal tubules Sodium and hydrogen ions in luminal membranes of proximal tubulesSodium and hydrogen ions in luminal membranes of proximal tubules

26. Secondary active transport The metabolic energy for secondary active transport of Na + at the luminal membrane in the proximal tubule comes from Na + -K + ATPase which transports Na + out of the cell across the basolateral membrane and maintains a favorable electrochemical gradient for the entry of Na + at the luminal membraneThe metabolic energy for secondary active transport of Na + at the luminal membrane in the proximal tubule comes from Na + -K + ATPase which transports Na + out of the cell across the basolateral membrane and maintains a favorable electrochemical gradient for the entry of Na + at the luminal membrane

27. Secondary active co-transport (glucose, amino acids, phosphate) LUMINAL BASOLATERAL Glucose, P i amino acids Na + H+H+H+H+ 3 Na + 2 K + K+K+K+K+ HCO 3 - + H 2 CO 3

28. PinocytosisPinocytosis Definition: Uptake by cells of particles too large to diffuse through the cell membraneDefinition: Uptake by cells of particles too large to diffuse through the cell membrane Example: Reabsorption of filtered proteins in the proximal tubulesExample: Reabsorption of filtered proteins in the proximal tubules

29. Solvent drag: Definition A solvent such as water moving across an epithelium by osmosis can drag dissolved solutes with itA solvent such as water moving across an epithelium by osmosis can drag dissolved solutes with it

30. Morphologic features of proximal tubular cells Large surface area for reabsorption of water and solutes (brush border, lateral cellular interdigitations)Large surface area for reabsorption of water and solutes (brush border, lateral cellular interdigitations) Large numbers of mitochondria to provide ATPLarge numbers of mitochondria to provide ATP Leaky epithelial junctionsLeaky epithelial junctions

31. Routes of transport across proximal tubular epithelium ParacellularParacellular 1% of surface area1% of surface area 5-10% of water transfer5-10% of water transfer Passive diffusion or solvent drag onlyPassive diffusion or solvent drag only Requires favorable electrochemical gradientRequires favorable electrochemical gradient Passive diffusion of ions and large non- polar solutesPassive diffusion of ions and large non- polar solutes Transcellular 99% of surface area 90-95% of water transfer Passive or active transport All active transport occurs by this route

32. Intrasegmental axial heterogeneity of proximal tubule P1: sodium, water, bicarbonate, amino acids, glucose, and phosphate reabsorbed P2: sodium, water and chloride reabsorbed P3: Organic acids and bases transported

33. Secondary active transport Glucose, Amino acidsGlucose, Amino acids T max high and constant (kidney not a regulator of plasma glucose and amino acid concentrations)T max high and constant (kidney not a regulator of plasma glucose and amino acid concentrations) PhosphatePhosphate T max low and altered by PTH (kidney is a regulator of plasma phosphtate concentration)T max low and altered by PTH (kidney is a regulator of plasma phosphtate concentration)

34. Secondary active transport: Glucose

35. Secondary active transport: Phosphate

36. Na + -K + ATPase In renal tubular cells found only in basolateral membraneIn renal tubular cells found only in basolateral membrane When ATP is hydrolyzed, 2 K + ions are pumped into the cell and 3 Na + ions are pumped outWhen ATP is hydrolyzed, 2 K + ions are pumped into the cell and 3 Na + ions are pumped out Maintains favorable electrochemical gradient for Na + entry at luminal membraneMaintains favorable electrochemical gradient for Na + entry at luminal membrane Maintains cell membrane potential difference and intracellular osmolalityMaintains cell membrane potential difference and intracellular osmolality

37. PinocytosisPinocytosis Endocytosis: Filtered proteins adsorbed to sites on luminal membranes that are internalized to form endosomes. Fusion with lysosomes forms endolysosomes in which digestion of proteins occursEndocytosis: Filtered proteins adsorbed to sites on luminal membranes that are internalized to form endosomes. Fusion with lysosomes forms endolysosomes in which digestion of proteins occurs Hydrolysis of filtered proteins to constituent amino acids by enzymes in brush border of proximal tubular cellsHydrolysis of filtered proteins to constituent amino acids by enzymes in brush border of proximal tubular cells

38. Urea: Passive diffusion Urea is passively reabsorbed in the proximal tubuleUrea is passively reabsorbed in the proximal tubule More urea is reabsorbed at low tubular flow rates than at high tubular flow ratesMore urea is reabsorbed at low tubular flow rates than at high tubular flow rates Contributes to BUN increasing out of proportion to creatinine in dehydrated patients even before GFR decreasesContributes to BUN increasing out of proportion to creatinine in dehydrated patients even before GFR decreases

39. Calcium homeostasis 99% of Ca +2 in bone, < 1% intracellular, 0.1% extracellular99% of Ca +2 in bone, < 1% intracellular, 0.1% extracellular Much homeostasis achieved by altering GI absorption via calcitriolMuch homeostasis achieved by altering GI absorption via calcitriol Only 60% of plasma Ca +2 (ionized and complexed) is available for glomerular filtrationOnly 60% of plasma Ca +2 (ionized and complexed) is available for glomerular filtration

40. Renal handling of Ca +2 Filtered by glomeruli and reabsorbed by tubulesFiltered by glomeruli and reabsorbed by tubules 99% of filtered Ca +2 is reabsorbed (exception: horse)99% of filtered Ca +2 is reabsorbed (exception: horse) Proximal tubule: 60-65%Proximal tubule: 60-65% Loop of Henle: 25-30%Loop of Henle: 25-30% Distal tubule & collecting duct: 4-9%Distal tubule & collecting duct: 4-9%

41. Renal reabsorption of Ca +2 Proximal tubule, medullary thick ascending loop of Henle: passive and paracellular (favorable electrochemical gradient)Proximal tubule, medullary thick ascending loop of Henle: passive and paracellular (favorable electrochemical gradient) Distal nephron: active and transcellularDistal nephron: active and transcellular Ca +2 diffuses down electrochemical gradient at luminal membraneCa +2 diffuses down electrochemical gradient at luminal membrane Transported across basolateral membrane by Na + -Ca +2 antiporter and Ca +2 ATPaseTransported across basolateral membrane by Na + -Ca +2 antiporter and Ca +2 ATPase

42. Factors affecting renal Ca +2 reabsorption Proximal tubule: Ca +2 reabsorption parallels Na + and water reabsorptionProximal tubule: Ca +2 reabsorption parallels Na + and water reabsorption Increased by volume depletionIncreased by volume depletion Decreased by volume expansionDecreased by volume expansion

43. Factors affecting renal Ca +2 reabsorption Increased serum P i stimulates PTH release (via decreased serum Ca +2 )Increased serum P i stimulates PTH release (via decreased serum Ca +2 ) PTH increases Ca +2 reabsorption and decreases P i reabsorption in kidneyPTH increases Ca +2 reabsorption and decreases P i reabsorption in kidney Allows retention of Ca +2 but excretion of P i mobilized from bone by PTH and absorbed from gut via calcitriolAllows retention of Ca +2 but excretion of P i mobilized from bone by PTH and absorbed from gut via calcitriol

44. Factors affecting renal Ca +2 reabsorption Metabolic acidosis stimulates Ca +2 reabsorption in distal tubulesMetabolic acidosis stimulates Ca +2 reabsorption in distal tubules Metabolic alkalosis inhibits Ca +2 reabsorption in distal tubulesMetabolic alkalosis inhibits Ca +2 reabsorption in distal tubules

45. Renal handling of phosphate Filtered by glomeruli and reabsorbed by tubules but not secretedFiltered by glomeruli and reabsorbed by tubules but not secreted 75-95% of the filtered load of P i is reabsorbed in the proximal tubule by co-transport with Na +75-95% of the filtered load of P i is reabsorbed in the proximal tubule by co-transport with Na +

46. Renal handling of phosphate P i -rich meal will increase serum P i and filtered load with consequent increase in urinary P i excretionP i -rich meal will increase serum P i and filtered load with consequent increase in urinary P i excretion Increased serum P i will increase PTH (via decreased serum Ca +2 ) which will decrease T max for P i reabsorption in proximal tubule and increase urinary P i excretionIncreased serum P i will increase PTH (via decreased serum Ca +2 ) which will decrease T max for P i reabsorption in proximal tubule and increase urinary P i excretion