MODE OF ACTION OF THYROCALCITONIN

Abstract

SUMMARY The effect of nephrectomy on the response to thyrocalcitonin was studied in three groups of rats: intact animals on stock diet; parathyroidectomized rats on stock diet; and parathyroidectomized rats on high calcium, low phosphorus diet. In intact rats and in parathyroidectomized rats on a high calcium, low phosphorus diet, nephrectomy did not alter the fall in plasma calcium and phosphate produced by thyrocalcitonin. In parathyroidectomized rats on stock diet, however, plasma calcium was not changed by the hormone, but there was an isolated fall in phosphate which was prevented by previous nephrectomy. Experiments with 45Ca showed that when plasma calcium was lowered by thyrocalcitonin this was caused by inhibition of bone resorption. Since the hormone also causes phosphaturia, it was concluded that there are two sites of action: bone and kidney. The skeletal effect is usually dominant and the renal effect is the major one only when the rate of bone resorption is low.