Phosphate transport in the proximal convolution of the rat kidney

Abstract

Proximal inorganic phosphate (P _i ) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular blood capillaries. In chronic parathyroidectomized (PTX) rats addition of 3 μM of the Ca²⁺ ionophore A 23187 to the luminal perfusate had no effect on the P _i transport, although the isotonic fluid reabsorption was reduced by 20%. When the Ca²⁺ concentration in the perfusates was raised from 1.5 mM to 3.0 mM the reabsorption did not change significantly. But when Ca²⁺ was omitted from the perfusates the P _i reabsorption dropped by 19%, and when 2 mM EDTA were added to the perfusates P _i transport decreased by 35%. The influx of P _i from the interstitial space and from the cell into the phosphate-free luminal perfusate did not change, when the perfusates were Ca²⁺-free, but it increased by 23% in the presence of 2 mM EDTA. The data indicate that 1. a rise in intracellular Ca²⁺ above normal is not a factor which modifies “basal” P _i transport i.e. when P _i transport is independent of the action of parathyroid hormone. 2. A reduction of extracellular Ca²⁺ concentration from normal toward zero reduces P _i transport without changing the paracellular leak permeability for P _i . 3. With EDTA the paracellular leak permeability for P _i is increased, thus causing an even greater reduction in net P _i transport than with Ca²⁺-free solutions alone.