Passive driving forces of proximal tubular fluid and bicarbonate transport: gradient dependence of H+ secretion.
The effect of oncotic pressure changes on fluid (Jv) and net bicarbonate transport (JHCO-3) and the transepithelial bicarbonate permeability (PHCO-3) were measured by an improved luminal and capillary microperfusion method that allows paired experiments on the same tubule. Rat proximal tubules were pump-perfused and Jv and [HCO-3] measured with [14C]inulin and a pH glass electrode. Raising peritubular protein (0-8-15 g/100 ml bovine serum albumin) stimulated Jv and HCO-3 reabsorption. The response to oncotic pressure changes was asymmetrical since changes of the luminal protein concentration had no significant effects. Whereas transepithelial solvent drag effects on HCO-3 must be minimal, peritubular protein most likely stimulates translocation of fluid and bicarbonate from intercellular spaces into peritubular capillaries. PHCO-3 was measured from HCO-3 net flux along a lumen-to-capillary-directed electrochemical potential gradient. In these experiments active H+ transport and Jv were minimized by 10(-4) M acetazolamide and luminal raffinose. PHCO-3 was 1.77 X 10(-5) cm X s-1 and was unaffected by increasing luminal flow rate from 10 to 45 nl X min-1. Since bicarbonate backflux is only a small fraction of physiological rates of JHCO-3, net transport alterations at varying [HCO-3] in the lumen must be due to changes in active HCO-3 (H+) transport. Thus, active H+ ion secretion across the luminal membrane of the proximal tubule is gradient dependent.
D000086:Acetazolamide / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000652:urine*; D002196:Capillaries / Q000502:physiology; D004848:Epithelium / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007687:Kidney Tubules, Proximal / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D008297:Male; D009997:Osmotic Pressure; D010477:Perfusion; D051381:Rats; D011919:Rats, Inbred Strains; D012079:Renal Circulation; D012710:Serum Albumin, Bovine
D001639:Bicarbonates; D012710:Serum Albumin, Bovine; D000086:Acetazolamide