Endothelial-smooth muscle cell interactions in the regulation of vascular tone in skeletal muscle.
The SMCs of skeletal muscle arterioles are intricately sensitive to changes in membrane potential. Upon increasing luminal pressure, the SMCs depolarize, thereby opening VDCCs, which leads to contraction. Mechanisms that oppose this myogenic tone can involve voltage-dependent and independent dilator pathways, and can be endothelium-dependent or independent. Of particular interest are the pathways leading to hyperpolarization of SMCs, as these can potentially evoke both local and conducted dilation. This review focuses on three agonists that cause local and conducted dilation in skeletal muscle: ACh, ATP, and KCl. The mechanisms for the release of these agonists during motor nerve stimulation and/or hypoxia, and their actions to open either Ca(2+) -activated K(+) channels (KCa ) or inwardly rectifying K(+) channels (KIR ) are described. By causing local and conducted dilation, each agonist has the ability to improve skeletal muscle blood flow during exercise and ischemia.