Endothelial Ca2+ wavelets and the induction of myoendothelial feedback

Abstract

When arteries constrict to agonists, the endothelium inversely responds, attenuating the initial vasomotor response. The basis of this feedback mechanism remains uncertain, although past studies suggest a key role for myoendothelial communication in the signaling process. The present study examined whether second messenger flux through myoendothelial gap junctions initiates a negative-feedback response in hamster retractor muscle feed arteries. We specifically hypothesized that when agonists elicit depolarization and a rise in second messenger concentration, inositol trisphosphate (IP₃) flux activates a discrete pool of IP₃ receptors (IP₃Rs), elicits localized endothelial Ca²⁺ transients, and activates downstream effectors to moderate constriction. With use of integrated experimental techniques, this study provided three sets of supporting observations. Beginning at the functional level, we showed that blocking intermediate-conductance Ca²⁺-activated K⁺ channels (IK) and Ca²⁺ mobilization from the endoplasmic reticulum (ER) enhanced the contractile/electrical responsiveness of feed arteries to phenylephrine. Next, structural analysis confirmed that endothelial projections make contact with the overlying smooth muscle. These projections retained membranous ER networks, and IP₃Rs and IK channels localized in or near this structure. Finally, Ca²⁺ imaging revealed that phenylephrine induced discrete endothelial Ca²⁺ events through IP₃R activation. These events were termed recruitable Ca²⁺ wavelets on the basis of their spatiotemporal characteristics. From these findings, we conclude that IP₃ flux across myoendothelial gap junctions is sufficient to induce focal Ca²⁺ release from IP₃Rs and activate a discrete pool of IK channels within or near endothelial projections. The resulting hyperpolarization feeds back on smooth muscle to moderate agonist-induced depolarization and constriction.