Differential action of parathyroid and thyroid hormones on effective intestinal absorption of calcium. In vivo studies with 47calcium in rats.

Abstract

ABSTRACT The effects of parathyroidectomy (PTX) and radio-thyroidectomy (131I-TX) as well as the actions of parathyroid extract (PTE), dibutyryl cyclic adenosine monophosphate (DBcAMP), calcitonin (CT) and thyroid extract (thyreoidea sicca) on effective intestinal calcium absorption in intact, PTX and 131I-TX rats were evaluated by a new, physiological in vivo method. Ten hours after the administration of 47calcium labelled food the animals were killed and the entire intestine was removed. Whole body retention of 47calcium was measured allowing the calculation of the effective intestinal absorption of calcium (true absorption minus excretion within ten hours). The known actions of PTE and DBcAMP were confirmed by this method. CT did not exert a direct effect in any of the experimental groups. The absence of thyroid hormones (TX rats) remarkably reduced effective calcium absorption. In the presence of endogenous parathyroid hormone (PTH) (TX rats) the administration of minimal amounts of thyroid hormones was sufficient to increase effective calcium absorption, whereas PTE and DBcAMP did not have any effect. In the absence of PTH (TX-PTX rats) thyroid hormones did not enhance effective absorption, indicating, that thyroid hormones alone do not stimulate the effective absorption of calcium. It is concluded, that the thyroid hormones act indirectly, as a permissive agent, enabling PTH to exert its active stimulating effect on the effective intestinal absorption of calcium.