title: Effects of intraluminal pH and dietary phosphate on phosphate transport in the proximal convoluted tubule.
abstract: The proximal tubule cell adjusts its phosphate absorption appropriately to meet the needs of the organism. Studies were performed to characterize some of the cellular changes involved with dietary phosphate adaptation. First, early proximal convoluted tubules were perfused with equilibrium Ringer solutions buffered to pH 7.65 or 6.5. Saturation kinetics for phosphate transport were determined at each pH value. Rats maintained on a diet of normal phosphate composition demonstrated the apparent Jmax and Km parameters about twofold greater with intraluminal pH 7.65 vs. pH 6.5. The Jmax values increased to 53.47 +/- 3.71 and 42.73 +/- 5.48 pmol X min-1 X mm-1, respectively, when the rats were placed on a phosphate-restricted diet for 5 days. By contrast, adaptation to a high dietary phosphate content resulted in diminished phosphate absorption, 8.53 +/- 1.80 and 12.87 +/- 1.61 pmol X min-1 X mm-1, for the respective pH 7.65 and 6.5 values. Second, the effect of peritubule phosphate concentration was evaluated at constant intraluminal phosphate concentrations. Unidirectional lumen-to-blood phosphate efflux was inhibited at all plasma phosphate concentrations in animals maintained on normal dietary phosphate. By contrast, rats adapted to a low phosphate diet exhibited an increase in phosphate absorption: pH 7.65, 86.21 +/- 2.63, and pH 6.5, 140.84 +/- 86.76 pmol X min-1 X mm-1 when plasma concentrations were elevated two- or three-fold from 2.4 to 6.4 mM. This was attributed to enhanced phosphate exchange on the basolateral membrane. Further hyperphosphatemic levels, above 6.4 mM, inhibited phosphate absorption. These data suggest that net phosphate absorption is determined, in part, by factors other than sodium-dependent uptake of phosphate by the brush border membrane including intracellular pH and peritubular phosphate that act in concert to control renal phosphate absorption.
mesh_terms: D000042:Absorption; D000818:Animals; D001692:Biological Transport; D004032:Diet; D006863:Hydrogen-Ion Concentration; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D009994:Osmolar Concentration; D010280:Parathyroid Glands / Q000502:physiology; D010477:Perfusion; D010710:Phosphates / Q000008:administration & dosage* / Q000097:blood / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013965:Thyroidectomy
chemical_list: D010710:Phosphates