Phosphate transport in the proximal convolution of the rat kidney II. Effect of extracellular Ca2+ and application of the Ca2+ ionophore A 23187 in chronic PTX animals.

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