Effects of estradiol treatment on rabbit luteal adenylyl cyclase: loss of luteinizing hormone receptors and attenuation of the regulatory N component activity.

We have reported previously that exogenously administered estradiol (E2) results in attenuation of the LH response of rabbit luteal adenylyl cyclase (approximately 50% less activity than control). This was accompanied by a much lesser reduction in the response of the system to the beta-adrenergic agonist isoproterenol (approximately 35% less activity than control). The purpose of the study reported here was to determine if the decreased responsiveness of adenylyl cyclase was the result of altered hormone receptor levels. To this end, hormone receptors were assessed by Scatchard analysis of specific binding. We confirmed that 4-day E2 treatments, which elevated serum E2 levels from 5 to 21 pg/ml. resulted in decreases in LH- and isoproterenol-stimulated adenylyl cyclase activities by 52% and 20%, respectively. In addition, we found that NaF-stimulated activity was also decreased by 20%. Basal adenylyl cyclase activity was unaffected. Upon assessment of the LH and beta-adrenergic receptor levels in luteal membranes, we found that E2 treatment resulted in marked reduction in LH receptor levels to 28% of the control value without changes in the levels of beta-adrenergic receptors. In view of the concomitant changes in the responsiveness of luteal membranes to isoproterenol and NaF, we determined whether E2 treatment affected luteal membrane levels of the stimulatory nucleotide-binding regulatory component (N) of adenylyl cyclase. N component activity was measured using a reconstitution assay that employs the stimulatory N component-deficient cyc- variant of the S49 mouse lymphoma cell line as an acceptor for luteal N component. Using this assay, we found that luteal membrane N component activity was reduced by 20-25% in E2-treated rabbits compared to that in control rabbits. All of the changes noted above were statistically significant. The results uncovered two heretofore unrecognized effects of E2 treatment: 1) loss of LH receptors, and 2) modification of the membrane component responsible for coupling of stimulatory receptors to the catalytic component of adenylyl cyclase.