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 relaxation of isolated endothelium-intact arterial rings precontracted with norepinephrine (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 inhibitor of NO synthase, partially inhibited 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 shifting the concentration-response curve and abolished the response when combined 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^-5 M), and almost completely blocked by tetraethylammnonium (TEA, 10^-3 M), iberiotoxin (10^-7 M) and 4-aminopyridine (4-AP, 5 × 10^-3 M). The cytochrome P450 inhibitors, 7-ethoxyresorufin (7-ER, 10^-5 M) and miconazole (10^-5 M) also significantly inhibited ACh-induced relaxation. Ouabain (10^-6 M), an inhibitor of Na⁺ , K⁺ -ATPase, or K⁺ -free solution, also significantly inhibited ACh-induced relaxation. ACh-induced relaxation was not significantly inhibited by 18- α-glycyrrhetinic acid (18 α-GA, 10^-4 M). These results of this study indicate that ACh-induced endothelium-dependent 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 (K_v) and the large-conductance, Ca²⁺ -activated K⁺ channel (BK_Ca). The results further suggest that EDHF released by ACh may be a cytochrome P450 product.