Disappearance of beta-adrenergic response of human myometrial adenylate cyclase at the end of pregnancy.

The density of beta-adrenergic receptors in both the outer and inner layers of the human myometrium decreases during the last weeks of pregnancy. Although in preterm myometrium (32-35 weeks of pregnancy) beta-adrenergic receptors are positively coupled to adenylate cyclase, we found that isoproterenol, epinephrine, and norepinephrine did not stimulate the enzyme in the inner or outer myometrial layer at term (39-40 weeks of pregnancy). At this stage, the addition of 10(-4) mol/L guanyl-5'-imidodiphosphate increased (from 10(-8) to 10(-4) mol/L) basal adenylate cyclase activity in a dose-dependent manner, indicating that the catalytic component of the enzyme remains linked to the stimulatory guanyl nucleotide binding protein (Gs). Compared to the preterm response, at term the myometrial adenylate cyclase response to 10(-4) mol/L guanyl-5'-imidodiphosphate was decreased, which may reflect a decrease in the amount of functional Gs. Altogether these changes are consistent with reduced Gs coupling to the catalytic component. However, the similarity of the responses of preterm and term myometrium to forskolin excluded the possibility of an alteration of the catalytic component of adenylate cyclase during the last weeks of pregnancy. The fact that a stimulatory effect of isoproterenol on adenylate cyclase was found after islet-activating protein pretreatment indicates that human term myometrium contains a functional inhibitory guanyl nucleotide binding protein which is involved in the modulation of the beta-adrenergic adenylate cyclase response. Our data suggest that modifications in the coupling mechanisms between receptors and the catalytic component are implicated in the loss of beta-adrenergic adenylate cyclase stimulation in the myometrium at the end of pregnancy.