OVARIAN AND ADRENAL STEROIDS DURING PREGNANCY AND THE OESTROUS CYCLE IN THE RAT

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Abstract
Changes in adrenocortical function of the rat during the 4-day oestrous cycle and pregnancy were studied in vivo and in vitro. Regulation of adrenal production of progesterone and corticosterone was of particular interest. Morning (09.00 h) plasma concentrations of corticosterone were similar at each phase of the cycle, but a higher concentration was found in the afternoon (15.00 h pro-oestrus). However, the rate of progesterone utilization for corticosterone production by adrenal homogenates was essentially unchanged at all periods of the cycle. Therefore, the activities of the 21- and 11β-hydroxylating systems appeared not to be important determinants of plasma corticosterone levels (coefficient of determination (r2) = 0·06). These results indicate that adrenal secretion of progesterone is not specifically regulated during the cycle. On the other hand, resting plasma levels of corticosterone declined by 80–85% from the value on the morning of pro-oestrus between days 5 and 12 of pregnancy and then increased on day 22 and after parturition. These fluctuations did not result from alterations in the rate of metabolic clearance but rather, were closely coupled to the capacity of adrenal tissue to convert progesterone to corticosterone (r2 = 0·87). These findings suggest that progesterone utilization may be specifically curtailed at the expense of corticosterone production at days 5 and 12 of pregnancy. Concurrent measurements of adrenal venous progesterone and ovarian venous progesterone (under stressful conditions) showed that the adrenal gland secretes 2–10 times more progesterone than the ovary at all stages of the oestrous cycle, on day 22 of pregnancy, and on the first day of lactation. Thus, the adrenal gland is capable of secreting large amounts of progesterone which, if present at the appropriate time, may inhibit, delay or facilitate ovulation and onset of mating behaviour as described by other workers. Under such circumstances, however, progesterone secretion appears not to be specifically regulated but seems instead to be due to precursor spillover, since the proportionate secretion of corticosterone does not change.