FUNCTIONAL COUPLING OF THE ALPHA-2-ADRENERGIC RECEPTOR-ADENYLATE CYCLASE COMPLEX IN THE PREGNANT HUMAN MYOMETRIUM

We previously reported that in the pregnant human myometrium the binding sites labeled with [3H]idazoxan have the pharmacological characteristics of α2-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 α2-adrenergic mechanism in this tissue was further approached by measuring adenylate cyclase responses and examining the different pertussis toxin-sensitive G-proteins. The two α2-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 α2-adrenergic antag-onists such as yohimbine and idazoxan. Pretreatment with pertussis toxin completely suppresses the inhibition of adenylate cyclase mediated by α2-adrenergic receptors, suggesting that an inhibitory protein of the G, type is involved. Pertussis toxin, known to catalyze the ADP ribosyla-tion of the α-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 α-subunits of Gi-like proteins, whereas that at 39 kDa might correspond to a Go α-like substrate. These results indicate the presence of α2-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. © 1990 by The Endocrine Society.