Maximal reabsorptive capacity for inorganic phosphate (Tm Pi) in the absence of parathyroid hormone in the rat: Decrease of theTm Pi during prolonged administration of phosphate and the role of calcium

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Abstract
Clearance titration studies were performed to examine the maximal reabsorptive capacity for inorganic phosphate (Tm Pi) in acutely parathyroidectomized (PTX) rats. Under control conditions, the reabsorption of Pi was 6.01±0.92 μmol/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±2.02 μmol/min. The Reabs./GFR values were 2.12±0.23 (control), 3.57±0.21 and 2.49±0.42 μmol/ml (experimental), respectively. Thus, theTm Pi decreased significantly during phosphate loading. Because this drop of theTm Pi 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 theTm Pi was significantly smaller than in hypocalcemia. It is concluded that calcium may regulate the renal Pi transport in the absence of parathyroid hormone, at least in hypo- and normocalcemic states.