Myoendothelial signal transmission through gap junctions in arterioles

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Segments of guinea-pig mesenteric arterioles (diameter : 50 - 100 μm) have been investigated In electron micrographs preparations were found to have a single layer of smooth muscle cells. Typical gap junctions were readily observed between endothelial cells only. While immunoreactivity to connexin 40 was strongly expressed on the membranes of endothelial cells only, that to connexin 43 was expressed on both smooth muscle and endothelial cell membranes. Neurobiotin injected into smooth muscle cell diffused into several neighboring smooth muscle cells while that injected into an endothelial cell diffused into many endothelial cells. The diffusion between two layers could also be observed.Using the conventional or perforated whole-cell clamp method and the video-edge detection, the electrical and mechanical activities of the everted or adventitia-removed arterioles have been observed. The endothelial cells were well coupled to each other. The coupling resistance between adjacent smooth muscle cells was estimated to be at least 90 MΩ and that between a smooth muscle cell and the whole endothelial layer to be 0.9 GΩ.The arteriole segments maintained some basal tone under the control condition, since they dilated when the external Ca^<2+> was removed. The arteriolar diameter was randomly fluctuating due to small and local contractions which were not accompanied by the membrane potential fluctuations. A gap junction blocker 18 β-glycyrrhetinic acid hyperpolarized the smooth muscle membrane and decreased the arteriolar basal tone. As high K^+ solution induced comparable contractions in the absence and the presence of 18 β-glycyrrhetinic acid, this substance did not seem to interfere with the smooth muscle contractility. The endothelium might depolarize the smooth muscle membrane and induce the basal tone when these two layers are coupled with intact myoendothelial gap junctions.