Mediation of EDHF‐Induced Reduction of Smooth Muscle [Ca²⁺]iand Arteriolar Dilation by K⁺Channels, 5,6‐EET, and Gap Junctions

Objective : To characterize the role of K + channels, the cytochrome P‐450 (CYP) metabolite 5,6‐EET, and gap junctions in modulation of arteriolar myogenic tone by a non‐nitric oxide nonprostaglandin mediator, termed “endothelium‐dependent hyperpolarizing factor” (EDHF), released to acetylcholine (ACh) in skeletal muscle arterioles. Methods : In isolated rat gracilis arterioles, simultaneous changes in smooth muscle (aSM) [Ca 2+ ] i (assessed by changes in fura‐2 ratiometric signal, R Ca ) and diameter were measured in response to ACh in the presence of indomethacin and l ‐NAME. Results : ACh, the K ATP channel opener pinacidil, and the Ca 2+ channel inhibitor verapamil elicited comparable decreases in aSM [Ca 2+ ] i (max.: −32 ± 3%, 29 ± 3%, and −30 ± 3%, respectively) and arteriolar dilations (max.: 90 ± 4%, 96 ± 2%, and 95 ± 2%, respectively). ACh‐induced responses were inhibited by KCl‐depolarization, K Ca channel blockers (TEA, charybdotoxin), or gap junction inhibitors (18α‐glycyrrhetinic acid, hyperosmolar sucrose). The K ATP channel inhibitor glibenclamide, the K IR channel inhibitor barium chloride, or the CYP inhibitor 17‐octadecynoic acid (ODYA) were without effect. The putative EDHF analogue 5,6‐EET elicited constrictions in the presence of the endothelium that could be prevented by indomethacin or a TxA 2 receptor antagonist, whereas in the absence of the endothelium, EDHF elicited only small, charybdotoxin‐insensitive decreases in aSM R Ca and dilations (max.: −8 ± 2% and 27 ± 4%, respectively). Conclusions : In skeletal muscle arterioles, EDHF 1) substantially and rapidly reduces myogenic tone by decreasing aSM [Ca 2+ ] i via opening K Ca channels, 2) it is unlikely to be 5,6‐EET or other CYP metabolites, but 3) requires functional gap junctions.