Renal transport of phosphate during phosphate loading in the fresh-water turtle

Abstract

The net renal transport of phosphate was measured in fresh-water turtlesChrysemys picta orC. scripta elegans before and during acute elevation of plasma phosphate by intravenous phosphate infusion. In parathyroid-intact (sham PTX) animals, net secretion or net reabsorption of phosphate could be observed during control infusions. With increasing plasma phosphate net secretion was no longer seen, and net reabsorption increased, eventually reaching a reabsorptive maximal transport rate (Tmax) of −1.5 μmoles/ml GFR at plasma concentrations of 4.0–5.0 mmoles/l. A similar pattern was observed in chronically parathyroidectomized (chronic PTX, 7–14 days) animals, but net reabsorption in this group was higher at all plasma levels, and net secretion was never observed. Tmax in chronic PTX animals was estimated at −2.0 μmoles/ml GFR. Chronic PTX animals treated with mammalian parathyroid extract (100 IU, intramuscularly, three days prior to infusion experiments) showed a reversal of the effects of PTX on net phosphate transport. Net reabsorption was reduced and net secretion was observed at both low and high plasma phosphate concentrations. These results demonstrate that net renal phosphate transport in the intact fresh-water turtle is characterized by a reabsorptive Tmax. The demonstration of net phosphate secretion in these animals requires the presence of parathyroid hormone. The secretory component of phosphate transport may be either a transepithelial flux of phosphate which is saturated at very low plasma phosphate concentrations, or a process distinct from a peritubular-to-luminal transport, and thus independent of plasma phosphate concentration.