Integrated Ca 2+ Signaling Between Smooth Muscle and Endothelium of Resistance Vessels

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Abstract
—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 endothelial cell [Ca2+]i in modulating phenylephrine (PE)-induced contractions in 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 elicited 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 contractile response. In addition, the biphasic conducted response was converted into a strong conducted vasoconstriction. PE caused an initial rise in smooth muscle [Ca2+]i at the stimulated site, which was followed by a rapid decrease below baseline. Endothelial cell loading of BAPTA had minimal effect on the initial [Ca2+]i peak but eliminated the secondary decrease in smooth muscle [Ca2+]i. Intraluminal application of charybdotoxin plus apamin mimicked the change in vasomotor state induced by BAPTA. These data lead us to hypothesize that, after smooth muscle stimulation, intercellular Ca2+ signaling between smooth muscle and endothelium causes a secondary rise in endothelial cell Ca2+, which triggers a hyperpolarizing event and initiates a conducted vasodilation. We conclude that smooth muscle and endothelium operate as a functional unit in these vessels.

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