Ability of Thyrocalcitonin to Protect Against Hypercalcemia in Adult Rats

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Abstract
The protective action of thyrocalcitonin (TC) against hypercalcemia, well established by others in young rats, was studied in adult nongrowing (“plateaued”) 4–7-month-old female rats in comparison with young, growing 6–7-week old female rats. Confirming previous work by others, injected TC produced much smaller decreases in serum calcium and phosphate in adults than in young rats. However, the hypocalcemic effect of TC in adult females was enhanced by concurrent injection of phosphate 83 μmoles/100 g body wt, just as it is in young rats. In the presence of the thyroid gland, the adult rats, like young rats, were protected against hypercalcemia after intragastric or iv calcium. The serum calcium was normal in sham-operated rats and elevated in thyroparathyroidectomized (TPTX) rats of both age groups at 1 and 2 hr after intragastric calcium (10 mg/100 g body wt). Two hr after iv calcium (2.75 mg/100 g body wt), serum calcium had returned almost to normal in sham-operated adult rats; it was still extremely high (12.8 mg/100 g body wt) in TPTX adults. The protective effect of the thyroid gland after iv calcium was evident earlier in the young rats. One hr after calcium the serum calcium of shamoperated young rats was normal, whereas in TPTX young rats it was markedly elevated; serum calcium was normal in both intact and TPTX young rats 2 hr after iv calcium. Injection of porcine TC counteracted the hypercalcemia 2 hr after intragastric calcium in both young and adult TPTX rats. Serum inorganic phosphate was significantly higher in TPTX than in sham-operated rats of both age groups 2 hr after intragastric calcium. The results provide further evidence that TC may play an important role, that of protection against hypercalcemia, in the normal physiology of adult nongrowing, as well as young growing, animals.(Endocrinology93: 1354, 1973)

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