Signaling across myoendothelial gap junctions--fact or fiction?

Gap junctions interconnect vascular cells homocellularly, thereby allowing the spread of signals along the vessel wall, which serve to coordinate vessel behavior. In addition, gap junctions provide heterocellular coupling between endothelial and vascular smooth muscle cells, creating so-called myoendothelial gap junctions (MEGJs). Endothelial cells control vascular tone by the release of factors that relax vascular smooth muscle. Endothelial factors include nitric oxide, prostaglandins, and an additional dilator principle, which acts by smooth muscle hyperpolarization and is therefore named endothelium-derived hyperpolarizing factor (EDHF). Whether this principle indeed relies on a factor or on intact MEGJs, which allow direct current transfer from endothelial to smooth muscle cells, has recently been questioned. Careful studies revealed the presence of vascular cell projections that make contact through the internal elastic lamina, exhibit the typical GJ morphology, and express connexins in many vessels. The functional study of the physiological role of MEGJs is confined by the difficulty of selectively blocking these channels. However, in different vessels studied in vitro, the dilation related to EDHF was sensitive to experimental interventions that block MEGJs more or less specifically. Additionally, bidirectional electrical coupling between endothelial and smooth muscle cells was demonstrated in isolated small vessels. In marked contrast, similar approaches used in conjunction with intravital microscopy, which allows examination of vascular behavior in the intact animal, did not verify electrical or dye-coupling in different models investigated. The discrepancy between in vitro and in vivo investigations may be due to size and origin of the vessels studied using these distinct experimental approaches. Additionally, MEGJ coupling is possibly tightly controlled in vivo by yet unknown mechanisms that prevent unrestricted direct signaling between endothelial and smooth muscle cells.