EFFECTS OF OVARIAN-STEROID HORMONES AND THYROXINE ON CALCITONIN SECRETION IN PREGNANT RATS

In the present study, the roles of ovarian steroid hormones and thyrox ine (T-4) in regulating the secretion of calcitonin (CT) in pregnant r ats were examined. The levels of plasma progesterone, pre-and post-CaC l2 plasma CT, and recovery time of plasma CT and calcium after calcium challenge were greatest in midterm pregnant rats. The levels of basal plasma progesterone, CT, calcium, and recovery time of plasma CT afte r calcium challenge were less in late pregnant rats, but basal plasma estradiol was highest in late pregnancy. The concentrations of plasma T-4 were gradually decreased in rats during pregnancy. Regardless of t he presence of estradiol, administration of progesterone in ovariectom ized (Ovx) rats resulted in an increase of plasma T-4 as well as the b asal and calcium-induced secretion of CT. Administration of estradiol alone did not alter the CaCl2-induced levels but decreased the post-Ca Cl2 levels of plasma calcium in Ovx rats. The basal levels of plasma C T were decreased in Ovx rats treated with T-4. These results suggest t hat the hypercalcitoninemia in midterm pregnant rats is due to an incr eased secretion of progesterone. Hypocalcitoninemia in late pregnant r ats, however, is due in part to lower plasma calcium.