Gap junctions in vascular smooth muscle.

Gap junction channels are the only class of channels which span two closely apposed plasma membranes. There are over a dozen identified subunit proteins or connexins. Three generic types of gap junction channels are possible. They are the homotypic, heterotypic and heteromeric types, each of which has a distinct connexin distribution. The major connexin found in vascular smooth muscle is connexin43. Connexin40 has also been found in vascular smooth muscle. The biosynthesis, trafficking and degradation of connexin43 has only recently been studied but the data reveal some interesting features such as a short half-life of only 1.5-3.5 h. The gating characteristics of connexin43 reveal mode shifting behaviours which result in prolonged closure of the majority of the gap junction channels. The channel also displays weak voltage dependence and is poorly selective. These features suggest that dynamical regulation of cell-to-cell coupling is best designed to reduce cell-to-cell coupling, rather than enhance it, by down regulation of channel activity through the biosynthesis, trafficking, degradation pathways and gating via mode shifting.