Endothelial Ion Channels and Cell-Cell Communication in the Microcirculation

Endothelial cells in resistance arteries, arterioles, and capillaries express a diverse array of ion channels that contribute to Cell-Cell communication in the microcirculation. Endothelial cells are tightly electrically coupled to their neighboring endothelial cells by gap junctions allowing ion channel-induced changes in membrane potential to be conducted for considerable distances along the endothelial cell tube that lines arterioles and forms capillaries. In addition, endothelial cells may be electrically coupled to overlying smooth muscle cells in arterioles and to pericytes in capillaries via heterocellular gap junctions allowing electrical signals generated by endothelial cell ion channels to be transmitted to overlying mural cells to affect smooth muscle or pericyte contractile activity. Arteriolar endothelial cells express inositol 1,4,5 trisphosphate receptors (IP(3)Rs) and transient receptor vanilloid family member 4 (TRPV4) channels that contribute to agonist-induced endothelial Ca(2+) signals. These Ca(2+) signals then activate intermediate and small conductance Ca(2+)-activated K(+) (IK(Ca) and SK(Ca)) channels causing vasodilator-induced endothelial hyperpolarization. This hyperpolarization can be conducted along the endothelium via homocellular gap junctions and transmitted to overlying smooth muscle cells through heterocellular gap junctions to control the activity of voltage-gated Ca(2+) channels and smooth muscle or pericyte contraction. The IK(Ca)- and SK(Ca)-induced hyperpolarization may be amplified by activation of inward rectifier K(+) (K(IR)) channels. Endothelial cell IP(3)R- and TRPV4-mediated Ca(2+) signals also control the production of endothelial cell vasodilator autacoids, such as NO, PGI(2), and epoxides of arachidonic acid contributing to control of overlying vascular smooth muscle contractile activity. Cerebral capillary endothelial cells lack IK(Ca) and SK(Ca) but express K(IR) channels, IP(3)R, TRPV4, and other Ca(2+) permeable channels allowing capillary-to-arteriole signaling via hyperpolarization and Ca(2+). This allows parenchymal cell signals to be detected in capillaries and signaled to upstream arterioles to control blood flow to capillaries by active parenchymal cells. Thus, endothelial cell ion channels importantly participate in several forms of Cell-Cell communication in the microcirculation that contribute to microcirculatory function and homeostasis.