The involvement of K+ channels and the possible pathway of EDHF in the rabbit femoral artery

Experiments were designed to characterize the cellular mechanisms of action of endothelium-derived vasodilator substances in the rabbit femoral artery . Acetylcholine (ACh, 10(-8)-10(-5) M) induced a concentration-dependent re laxation of isolated endothelium-intact arterial rings precontracted with n orepinephrine (NE, 10(-6) M). The ACh-induced response was abolished by the removal of endothelium. N-G-nitro-L-arginine (L-NAME, 10(-4) M), an inhibi tor of NO synthase, partially inhibit-ed ACh-induced endothelium-dependent relaxation, whereas indomethacin (10(-5) M) showed no effect on ACh-induced relaxation. 25 mM KCl partially inhibited ACh-induced relaxation by shifti ng the concentration-response curve and abolished the response when combine d with L-NAME and NE. In the presence of L-NAME, ACh-induced relaxation was unaffected by glibenclamide (10(-5) M) but significantly reduced by apamin (10(-6) M), and almost completely blocked by tetraethylammonium (TEA, 10(- 3) M), iberiotoxin (10(-7) M) and 4-aminopyridine (4-AP, 5 x 10(-3) M). The cytochrome P450 inhibitors, 7-ethoxyresorufin (7-ER, 10(-5) M) and miconaz ole (10(-5) M) also significantly inhibited ACh-induced relaxation. Ouabain (10(-6) M), an inhibitor of Na+, K+-ATPase, or K+-free solution, also sign ificantly inhibited ACh-induced relaxation. ACh-induced relaxation was not significantly inhibited by 18-alpha-glycyrrhetinic acid (18 alpha-GA, 10(-4 ) M). These results of this study indicate that ACh-induced endothelium-dep endent relaxation of the rabbit femoral artery occurs via a mechanism that involves activation of Na+, K+-ATPase and/or activation of both the voltage -gated K+ channel (Kv) and the large-conductance, Ca2+-activated K+ channel (BKCa). The results further suggest that EDHF released by ACh may be a cyt ochrome P450 product.