Integrated Ca2+ signaling between smooth muscle and endothelium of resistance vessels

Cell-cell communication in the arteriolar wall was examined using the Ca2+- sensitive indicator fura-2 and the Ca2+ buffer BAPTA as means of measuring and buffering cellular Ca2+. The experiments focused on the role of endothe lial cell [Ca2+](i) in modulating phenylephrine (PE)-induced contractions i n in vitro arterioles of the hamster cremaster. Fura-2-AM and BAPTA-AM were applied intraluminally to accomplish endothelium-specific loading. PE was applied to short segments of arterioles using pressure-pulse ejection from a micropipette. Under control conditions at the site of stimulation, PE eli cited a strong vasoconstriction preceded by an increase in endothelial cell [Ca2+](i). A very small biphasic conducted response was observed at sites upstream from the stimulation site. BAPTA sharply reduced the measured Ca2 response in the endothelium. This was associated with an enhanced local co ntractile response. In addition, the biphasic conducted response was conver ted into a strong conducted vasoconstriction. PE caused an initial rise in smooth muscle [Ca2+](i) at the stimulated site, which was followed by a rap id decrease below baseline. Endothelial cell loading of BAPTA had minimal e ffect on the initial [Ca2+](i) peak but eliminated the secondary decrease i n smooth muscle [Ca2+](i). Intraluminal application of charybdotoxin plus a pamin mimicked the change in vasomotor state induced by BAPTA. These data l ead us to hypothesize that, after smooth muscle stimulation, intercellular Ca2+ signaling between smooth muscle and endothelium causes a secondary ris e in endothelial cell Ca2+, which triggers a hyperpolarizing event and init iates a conducted vasodilation. We conclude that smooth muscle and endothel ium operate as a functional unit in these vessels.