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.

Acute Effects of Synthetic Porcine Calcitonins on the Renal Excretion of Magnesium, Inorganic Phosphate, Sodium and Potassium

“…It is possible that the CT-induced fall in plasma phosphate concentration in para¬ thyroidectomized animals is a result of the phosphaturic action of CT, since it has been shown to be abolished by previous nephrectomy (Robinson, Martin, Matthews & Maclntyre, 1967). a-Calcitonin administered to our parathyroidectomized animals, in which we assume bone résorption to be minimal, produced not only a fall in plasma phosphate concentration but also a small fall in plasma calcium concentration.…”

Calcitonin Activity of Particulate Fractions of the Ovine Thyroid Gland

“…However, it has been clarified that the hormone's hypocalcemic effect is not dependent upon direct inhibition of PTH. This view was well documented by the studies of Tashjian (1965) and Robinson et al (1967) Chase and Aurbach (1970) and Murad et al (1970) found that porcine CT caused small increases (about two-fold) in concen--J6-tration of cyclic AMP in bone and in kidney homogenates in vitro (Murad et al, 1970). These investigators demonstrated that there was little or no effect of dose, whereas PTH showed a log-dose response in this system over a ten to twenty-fold range of cyclic AMP Their studies indicated that pyrophosphate-coated crystals dissolved more slowly.…”

Regulation of Plasma Calcium in Man: The Influence of Parathyroid Hormone and Calcitonin

“…These effects are due to inhibition of bone resorption as has been shown in many ways in numerous animal experiments [1,5,28,39,44,47,49,57,59].…”