G Protein Expression in Human Feto-placental Vascularization. Functional Evidence for Gsαand GiαSubunits,

GTP-binding proteins are key elements in coupling receptors to various effector systems. Using ADP-ribosylation by cholera (CTX) and pertussis (PTX) toxins and an immunodetection technique, we investigated the G protein expression profile in smooth muscle of stem villi vessels obtained from human term placentae. In placental vascular smooth muscle, we report the presence of two CTX-protein substrates of 42 and 45 kDa recognized by Gsαantibodies, and three Giαisoforms, substrates of PTX, identified as Gi1α, Gi3α(two proteins of 41 kDa) and Gi2α(a 40-kDa protein). We also characterized another target of PTX, a 40-kDa Goα-immunoreactive protein and detected the PTX-insensitive Gq-G11αproteins. To assess the functional significance of the Gαproteins identified in this tissue, we measured the adenylyl cyclase activity in the presence of guanyl nucleotides alone or with increasing concentrations of vasoactive intestinal peptide (VIP), and examined whether VIP-bound sites, in the presence of GTPγS, promote the release of Gαproteins from the membranes of vascular smooth muscle. At low concentrations (0.1 n to 0.01μm), guanyl nucleotides stimulated adenylyl cyclase activity in a dose-dependent manner, while at higher concentrations (10μ to 1 m ) the stimulation rate of cAMP production by guanyl nucleotides decreased. In a dose-dependent manner, VIP in the presence of GTPγS increased adenylyl cyclase activity and specifically promoted the release of both Gsαisoforms. In contrast, the release of Gi1and Gi2αisoforms was not significantly increased in the presence of VIP, while GTPγS alone stimulated their release. Our data show physical evidence of the activation of Gsproteins by VIP-bound membrane receptors, resulting in dissociation and release of Gsαsubunits in the soluble fraction. They assess the specific coupling of the two Gsαisoforms to VIP receptors in smooth muscle wall of placental stem villi vessels. It would be of interest to investigate whether changes in Gsαexpression and/or function are associated with the placental angiogenesis process during pregnancy.