Calcitonin decreases the renal tubular capacity for phosphate reabsorption

The effects of pharmacologic doses of synthetic salmon calcitonin on the renal tubular capacity of phosphate (Pi) transport were determined in the presence and absence of maximally phosphaturic doses of parathyroid hormone (PTH). Thyroparathyroidectomized rats were given graded infusions of Pi (1, 2, and 3 mumol/min) to prevent the hypophosphatemic effects of calcitonin and to determine the maximum transport of Pi for the kidney (TmPi/GFR). The maximum transport of Pi for the rats treated with calcitonin was 2.46 +/- 0.27 mumol/ml. This value was significantly less than that of 3.88 +/- 0.32 mumol/ml (P less than 0.05) for the control animals but was significantly greater than the maximum transport of Pi of 1.16 +/- 0.05 mumol/ml (P less than 0.05) for the rats treated with PTH. Furthermore, there was no significant difference between the maximum transport of Pi for the rats treated with PTH and that of 1.04 +/- 0.05 mumol/ml for the rats treated with PTH plus calcitonin. We conclude that pharmacologic doses of calcitonin decrease the tubular capacity for Pi reabsorption of the kidney and that the effect is significantly smaller than that of maximally phosphaturic doses of PTH.