Regulation by progesterone of the high-affinity state of myometrial β-adrenergic receptor and of adenylate cyclase activity in the pregnant rat

The effects of pregnancy or progesterone dominance on the β-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 β-adrenergic receptor were determined by competition experiments between 125 I-labelled cyanopindolol binding and the selective β-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 β-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 μ m ) and 45–20% R L ( K L =14–5 μ m ) with a ratio of K L / K H of 55–34. In the last 6 h before birth, β-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 125 I-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. ) μ 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 μ 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 β-adrenergic responsiveness of the pregnant rat myometrium and (2) that at term, both a desensitization mechanism involving uncoupling of β-adrenergic receptors and a decrease in activation of adenylate cyclase lead to a loss of myometrial response to β-agonists.