On the mode of the hypocalcemic action of thyrocalcitonin and its enhancement by phosphate in rats.

The hypocalcemic action of sc porcine thyrocalcitonin (TC) administered alone or together with sodium phosphate was studied in rats. The dose of TC required to produce hypocalcemia was much lower in young male rats (27–47 days old) after an 18-hr fast than after feeding a diet with a high calcium to phosphate ratio (Ca/P = 5, Ca = 1.5%) for one week. However, the maximum 75-min response of serum calcium in the rats fed the high Ca/P diet was as great as that in the fasted rats. In older male rats (about 1 yr old), serum calcium fell 10% 2½ hr after TC alone, and 15% when phosphate (not hypocalcemic by itself) was given with TC. These effects in the older rats and in the young rats fed the high Ca/P diet seemed too large to be due solely to inhibition of the rate of bone resorption, which, in both situations, probably was lower than in the young fasted rats. Instead, the results suggested that TC, both alone and with phosphate, must cause hypocalcemia at least in part by another mechanism. In other studies...