Phosphorus and Calcium Homeostasis in Chronic Subtotally Nephrectomized Parathyroidectomized Rats

Calcium and phosphorus balance studies were carried out in subtotally nephrectomized rats (NX), with or without parathyroidectomy (NX-PTX; NX sham-PTX) in order to determine the ability of the remnant kidney to regulate excretion of these elements in the absence of parathyroid hormone. The rats were fed three different phosphorus diets, and calcium intake was also varied. We found that the NX-PTX rats adapted to the three different phosphorus diets in a manner indistinguishable from the NX sham-PTX rats. The per cent of ingested phosphorus excreted in the urine increased as dietary phosphorus increased. When supplementary calcium was added to the diet urinary phosphorus excretion fell and fecal phosphorus increased, in an identical fashion in the NX-PTX and NX sham-PTX animals. Urinary calcium excretion decreased as dietary phosphorus increased, and U_CaV increased when supplementary calcium was provided in the diet. Total body calcium and phosphorus balance (intake-(urine+feces)) varied with intake, but was not significantly different between the NX-PTX and NX sham-PTX rats. These experiments demonstrate that subtotally nephrectomized rat have a parathyroid-independent mechanism(s) for regulating both urinary and fecal calcium and phosphorus excretion. The mechanism is not revealed by the present study, but may relate to changes in serum calcium and/or phosphorus which occur following parathyroidectomy.