Abstract
It has been observed that elevated concentrations of estradiol, the principle luteotropin in the rabbit, reduce LH-stimulated adenylyl cyclase activity in corpora lutea during midpseudopregnancy without suppressing serum and tissue progesterone concentrations. If LH modulates intraluteal cAMP levels in this species, this suggests that in the presence of exogenous estradiol, progesterone synthesis may be independent of cAMP. To test this possibility and to investigate the physiological significance of LH in regulating the rabbit corpus luteum, the concentrations of cAMP and progesterone and the activity of adenylyl cyclase were measured in luteal homogenates, the numbers of LH/hCG receptors were estimated in crude membrane preparations, and the concentrations of progesterone and estradiol were measured in serum on days 2-12 of pseudopregnancy in rabbits treated with or without estradiol. Throughout days 2-12 of pseudopregnancy, estradiol treatment increased LH-stimulated adenylyl cyclase activity on day 2, but reduced its activity after day 4 by up to 50%, reduced the number of LH/hCG receptors after day 2 by up to 50%, and had no effect on the activities of basal, epinephrine-stimulated, or NaF-stimulated adenylyl cyclase. Tissue cAMP levels were not altered by estradiol treatment, nor were serum progesterone concentrations (except for an increase on day 2). Since LH receptors and LH-stimulated adenylyl cyclase were both reduced by day 5 of pseudopregnancy without a concomitant decrease in luteal cAMP or serum progesterone concentrations, our data suggest that LH is not a physiological regulator of luteal cAMP or serum progesterone during days 4-12 of pseudopregnancy. We propose that basal adenylyl cyclase activity, which is not reduced by estradiol treatment, may play a more significant role than LH-stimulated adenylyl cyclase activity in regulating tissue cAMP levels and the progesterone synthetic capacity of the rabbit corpus luteum.
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