Phosphate transport in the proximal convoluted tubule: effect of intraluminal pH

The effect of intraluminal hydrogen ion concentration on phosphate absorption was evaluated in the rat superficial proximal tubule. Early proximal convoluted tubules were perfused in vivo with buffered equilibrium solutions, and the saturation kinetics for phosphate transport was determined by altering intraluminal phosphate concentration at pH values of 7.65 and 6.5. The apparent Jmax and Km parameters of phosphate transport were about twofold greater with intraluminal pH 7.65 compared with pH 6.5 values. Accordingly, intraluminal hydrogen ion had a direct effect on tubular phosphate transport. Endogenous or exogenously administered parathyroid hormone inhibited phosphate absorption at both pH values, indicating that the phosphaturic action of parathyroid hormone is independent of the intraluminal pH value. Elevation of plasma phosphate inhibited phosphate absorption unrelated to the intraluminal phosphate concentration at both pH values. These data suggest that elevation of plasma phosphate may decrease phosphate absorption by inhibiting efflux from the cell into the peritubular capillary. In conclusion, proximal phosphate absorption is determined by the intraluminal pH value, circulating parathyroid hormone level, and plasma phosphate concentration acting through separate cellular and membrane mechanisms.