Maximal reabsorptive capacity for inorganic phosphate (TmPi) in the absence of parathyroid hormone in the rat: decrease of the TmPi during prolonged administration of phosphate and the role of calcium.

Clearance titration studies were performed to examine the maximal reabsorptive capacity for inorganic phosphate (TmPi) in acutely parathyroidectomized (PTX) rats. Under control conditions, the reabsorption of Pi was 6.01 +/- 0.92mumol/min (mean +/- S.D.), and during the administration of phosphate the reabsorption of Pi increased initially to 11.62 +/- 2.20 and decreased finally to 7.97 +/- 2.02mumol/min. The Reabs/GFR values were 2.12 +/- 0.23 (control), 3.57 +/- 0.21 and 2.49 +/- 0.42mumol/ml (experimental), respectively. Thus, the TmPi decreased significantly during phosphate loading. Because this drop of the TmPi was accompanied by a significant decrease of the calcium concentration in plasma in these PTX animals, another series of experiments was performed in which the calcium losses in plasma were replaced. Under these conditions the decrease of the TMPi was significantly smaller than in hypocalcemia. It is concluded that calcium may regulate the renal Pi transport in the absence of parathyroid hormones, at least in hypo- and normocalcemic states.