Endothelium-dependent hyperpolarization and relaxation resistance to N-G-nitro-L-arginine and indomethacin in coronary circulation

Objective: It is controversial whether endothelium-dependent relaxation res istance to inhibitors of nitric oxide (NO) and prostacyclin synthases is co mpletely attributed to endothelium-derived hyperpolarizing factor (EDHF). T his study examined NO release and K+ channels involved in endothelium-depen dent relaxation and hyperpolarization resistance to N-G-nitro-L-arginine (L -NNA) and indomethacin in coronary arteries with emphasis on the microarter ies. Methods: NO release, isometric force, and membrane potential of porcin e coronary arteries were measured using a NO-specific electrode, wire myogr aph, and microelectrode, respectively. Results: In large arteries pretreate d with indomethacin, bradykinin (BK) evoked a rise in [NO] from 5.5+/-2.4 n M to 105.0+/-19.6 nM and hyperpolarization. L-NNA treatment significantly r educed the BK-stimulated rise in [NO] to 32.1+/-11.3 nM but did not affect the hyperpolarization. In the presence of indomethacin and L-NNA, U-46619 c ontracted and depolarized (from -51+/-3 mV to -30+/-4 mV) vascular smooth m uscle in microarteries. The addition of BK produced dose-dependent relaxati on (maximal: 70.2+/-5.7%) and repolarization (membrane potential: -50+/-4 m V). Oxyhemoglobin eliminated indomethacin and L-NNA-resistance rise in [NO] but not relaxation (42.3+/-4.4%) and repolarization (-40+/-2 mV) by BK. Te traethylammonium, charybdotoxin, and iberiotoxin partially decreased the BK -induced responses. Apamin alone did not affect the relaxation by BK; howev er, in combination with charybdotoxin it almost completely abolished the BK -induced relaxation and hyperpolarization. Conclusions: In porcine coronary arteries, both EDHF and NO contribute to BK-induced relaxation resistance to indomethacin and L-NNA. Large conductance Ca2+-activated K+ channels (BK Ca) may play an important role in mediating the BK-induced responses and sm all conductance Ca2+-activated K+ channels might function as 'backup' mecha nisms when BKCa is curtailed. (C) 2000 Elsevier Science B.V. All rights res erved.