REGULATION BY PROGESTERONE OF THE HIGH-AFFINITY STATE OF MYOMETRIAL BETA-ADRENERGIC-RECEPTOR AND OF ADENYLATE-CYCLASE ACTIVITY IN THE PREGNANT RAT

The effects of pregnancy or progesterone dominance on the beta-adrenergic responsiveness of the uterus were studied in myometrial membranes from mid- and late-pregnant rats (day 15 and on the 16th h of day 22 of pregnancy respectively) or 24 h after administration of progesterone. Levels of the high (R(H))- and low (R(L))-affinity states of the beta-adrenergic receptor were determined by competition experiments between I-125-labelled cyanopindolol binding and the selective beta-agonist isoproterenol. The ratio K(L)/K(H) (respective dissociation constants) was determined since it also reflects the degree of formation of the high-affinity state of the beta-adrenergic receptor. From day 15 to the 10th h of day 22 of pregnancy, two distinct affinity states were apparent: 80-55% R(H) (K(H) = 0.31-0.21-mu-M) and 45-20% R(L) (K(L) = 14-5-mu-M) with a ratio of K(L)/K(H) of 55-34. In the last 6 h before birth, beta-adrenergic receptors underwent uncoupling which was paralleled by decreased responsiveness of myometrial adenylate cyclase to isoproterenol (maximum velocity (V(max)) = 17 +/- 3 vs 44 +/- 3 fmol cyclic AMP/10 min per mg protein on day 15). At this stage of pregnancy, previous exposure to progesterone resulted in a 1.8-fold increase in I-125-labelled cyanopindolol-binding sites (B(max)) and the reappearance of the high-affinity state (67% R(H), K(H) = 0.19 +/- 0.04 (S.E.M.)-mu-M, ratio K(L)/K(H) = 81.1 +/- 16.9). These results were reversed in the presence of the antiprogestin RU486 (100% R(L), K(L) = 24.6 +/- 4.1-mu-M, 41% reduction of B(max)). Moreover, after progesterone, adenylate cyclase activity was strongly stimulated by isoproterenol (V(max) = 60 +/- 12 fmol cyclic AMP/10 min per mg protein vs 17 +/- 3 in controls). The data suggest (1) that progesterone may exert a permissive effect on beta-adrenergic responsiveness of the pregnant rat myometrium and (2) that at term, both a desensitization mechanism involving uncoupling of beta-adrenergic receptors and a decrease in activation of adenylate cyclase lead to a loss of myometrial response to beta-agonists.