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.

We previously reported that in the pregnant human myometrium the binding sites labeled with [3H]idazoxan have the pharmacological characteristics of alpha 2-adrenergic receptors. Competitive experiments have also revealed that the stable guanine nucleotide analog guanyl-5'-imidodiphosphate decreases the apparent affinity of norepinephrine and clonidine for myometrial [3H]idazoxan-binding sites. In the present study, the alpha 2-adrenergic mechanism in this tissue was further approached by measuring adenylate cyclase responses and examining the different pertussis toxin-sensitive G-proteins. The two alpha 2-adrenergic agonists norepinephrine and clonidine inhibited adenylate cyclase activity in both the outer and inner layers of the pregnant human myometrium. The inhibitory effect of these agonists is completely reversed by alpha 2-adrenergic antagonists such as yohimbine and idazoxan. Pretreatment with pertussis toxin completely suppresses the inhibition of adenylate cyclase mediated by alpha 2-adrenergic receptors, suggesting that an inhibitory protein of the Gi type is involved. Pertussis toxin, known to catalyze the ADP ribosylation of the alpha-subunit of several G-proteins, labels three substrates at 39, 40, and 41 kDa. The more intense labeling occurring on the 40- to 41-kDa components are assigned to alpha-subunits of Gi-like proteins, whereas that at 39 kDa might correspond to a Go alpha-like substrate. These results indicate the presence of alpha 2-adrenergic receptors in the human myometrium at the end of pregnancy that are functionally linked to inhibition of adenylate cyclase activity via the Gi protein.