Roles of calcium-activated and voltage-gated delayed rectifier potassium channels in endothelium-dependent vasorelaxation of the rabbit middle cerebral artery

1 The cellular mechanism(s) of action of endothelium-derived vasodilator substances in the rabbit middle cerebral artery (RMCA) were investigated. Specifically, the subtypes of potassium channels involved in the effects of endothelium-derived relaxing factors (EDRFs) in acetylcholine (ACh)-induced endothelium-dependent vasorelaxation in this vessel were systematically compared. 2 In the endothelium-intact RMCA precontracted with histamine (3 mu M), ACh induced a concentration-dependent vasorelaxation, which was sensitive to indomethacin (10 mu M) or N-G-nitro-L-arginine (L-NOARG; 100 mu M); pot values 8.36 vs 7.40 and 6.38, P<0.01 for both, n=6 and abolished by a combination of both agents. ACh caused relaxation in the presence of high K+ PSS (40 mM KCl), which was not affected by indomethacin, but abolished by L-NOARG and a combination of indomethacin and L-NOARG. 3 In the presence of indomethacin, relaxation to ACh in the endothelium-intact RMCA precontracted with histamine was unaffected by either glibenclamide (10 mu M), an ATP-sensitive K+ channel (K-ATP) blocker, 4-aminopyridine (4-AP, 1 mM) or dendrotoxin (DTX, 0.1 mu M), delayed rectifier K+ channel (K-v) blockers. However, relaxation responses to ACh were significantly inhibited by either LY83583 (10 mu M) and 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 mu M), guanylyl cyclase inhibitors, or charybdotoxin (CTX; 0.1 mu M), iberiotoxin (ITX, 0.1 mu M) and apamin (APA, 0.1 mu M), large conductance Ca2+-activated K+ channels (BKCa) blocker and small conductance Ca2+-activated K+ channel. (SKCa) blocker, respectively. 4 In the presence of L-NOARG, relaxation to ACh was unaffected by glibenclamide or the cytochrome P450 mono-oxygenase inhibitor, clotrimazole (1 mu M), but was significantly inhibited by either 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 10 mu M) and 2',3'-dideoxyadenosine (2',3'-DDA, 30 mu M), adenylyl cyclase inhibitors, or 4-AP, DTX, CTX, ITX and APA. 5 In the endothelium-denuded RMCA precontracted with histamine, authentic NO-induced relaxation was unaffected by glibenclamide, 4-AP and DTX, but significantly reduced by ODQ, ITX and APA. Authentic prostaglandin I-2 (PGI(2))-induced relaxation was unaffected by glibenclamide, but significantly reduced by 2',3'-DDA, 4-AP, DTX, ITX and APA. Forskolin-induced relaxation was significantly inhibited by high K+, CTX and 4-AP. 6 These results indicate that: (1) in the RMCA the EDRFs released by ACh are NO and a prostanoid (presumably PGI(2)), and there is no evidence for the release of a non-NO/PGI(2) endothelium-derived hyperpolarizing factor (EDHF), (2) K-Ca channels are involved in NO-mediated relaxation of the RMCA but both K-Ca and K-v channels are involved in PGI(2)-mediated relaxation.