Mediation of EDHF-induced reduction of smooth muscle [Ca(2+)](i) and 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 (18alpha-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.