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

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Abstract
The effect of intraluminal H+ concentration on PO4 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 PO4 transport was determined by altering intraluminal PO4 concentration at pH values of 7.65 and 6.5. The apparent Jmax and Km parameters of PO4 transport were about 2-fold greater with intraluminal pH 7.65 compared with pH 6.5 values. Intraluminal H+ had a direct effect on tubular PO4 transport. Endogenous or exogenously administered parathyroid hormone inhibited PO4 absorption at both pH values, indicating that the phosphaturic action of parathyroid hormone is independent of the intraluminal pH value. Elevation of plasma PO4 inhibited PO4 absorption unrelated to the intraluminal PO4 concentration at both pH values. Elevation of plasma PO4 may decrease PO4 absorption by inhibiting efflux from the cell into the peritubular capillary. Proximal PO4 absorption seems to be determined by the intraluminal pH value, circulating parathyroid hormone level and plasma PO4 concentration acting through separate cellular and membrane mechanisms.

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