G protein expression in human fetoplacental vascularization. Functional evidence for Gs alpha and Gi alpha subunits

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 Gs alpha antibodies, and three Gi alpha isoforms, substrates of PTX, identified as Gi1 alpha, Gi3 alpha (two proteins of 41 kDa) and Gi2 alpha (a 40-kDa protein). We also characterized another target of PTX, a 40-kDa Go alpha-immunoreactive protein and detected the PTX-insensitive Gq-Gi1 alpha proteins. To assess the functional significance of the G alpha proteins 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 GTP gamma S, promote the release of G alpha proteins from the membranes of vascular smooth muscle. At low concentrations (0.1 nM to 0.01 microM), guanyl nucleotides stimulated adenylyl cyclase activity in a dose-dependent manner, while at higher concentrations (10 microM to 1 mM) the stimulation rate of cAMP production by guanyl nucleotides decreased. In a dose-dependent manner, VIP in the presence of GTP gamma S increased adenylyl cyclase activity and specifically promoted the release of both Gs alpha isoforms. In contrast, the release of Gi1 and Gi2 alpha isoforms was not significantly increased in the presence of VIP, while GTP gamma S alone stimulated their release. Our data show physical evidence of the activation of Gs proteins by VIP-bound membrane receptors, resulting in dissociation and release of Gs alpha subunits in the soluble fraction. They assess the specific coupling of the two Gi alpha isoforms to VIP receptors in smooth muscle wall of placental stem villi vessels. It would be of interest to investigate whether changes in Gs alpha expression and/or function are associated with the placental angiogenesis process during pregnancy.