Inhibition by calcitonin of hypercalcaemia induced by 1,25-dihydroxycholecalciferol

The influence of injections of calcitonin on the hypercalcaemia and hyperhophosphataemia induced by 1 alpha-hydroxycholecalciferol (1alpha-OHD3) and 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) was studied in growing lambs and lactating cows respectively. In lambs, the hypercalcaemic and hyperphosphataemic effect of 0.1 microgram 1 alpha-OHD3/kg body weight (i.v.) was completely inhibited by 20 units calcitonin/kg body weight, injected subcutaneously in four equal doses at intervals of 12 h, the first injection of calcitonin occurring immediately after injection of 1 alpha-OHD3. Moreover, the same dose of calcitonin partly inhibited the hypercalcaemia and hyperphosphataemia observed after injection of 0.25 microgram 1 alpha-OHD3/kg body weight. In lactating cows, four injections of calcitonin (5 u./kg body weight each, at intervals of 12 h) prevented the rise of plasma calcium and phosphate levels occurring after injection of 1,25-(OH)2D3 (0.01 microgram/kg body weight). The results indicated that high plasma calcitonin levels can inhibit the hypercalcaemic and hyperphosphataemic effects of 1,25-(OH)2D3 in ruminants. Thus, hypocalcaemia might occur in parturient cows in spite of raised concentrations of 1,25-(OH)2D3 in plasma.