Apmin-sensitive, non-nitric oxide (NO) endothelium-dependent relaxations to bradykinin in the bovine isolated coronary artery: no role for cytochromeP-450 and K+

1 Since cytochrome P-450-derived metabolites of arachidonic acid and K+ hav e been implicated in endothelium-derived hyperpolarizing factor (EDHF)-depe ndent responses, the aim of this study was to determine whether such factor s contribute to non-nitric oxide (NO), endothelium-dependent relaxation to bradykinin (BK) in bovine isolated coronary artery. 2 In rings of artery contracted with U46619 and treated with indomethacin ( 3 mu M) and N-G-nitro-L-arginine (L-NOARG; 100 mu M), relaxation to BK (0.0 1 nM-0.3 mu M) was blocked by similar to 60% after inhibition of K+ channel s with either high extracellular K+ (high [K+](o); 15-67 mM) or apamin (0.3 mu M). 3 Ouabain (1 mu M), an inhibitor of Na+/K+-ATPase, decreased the sensitivit y to BK without affecting the maximum response. In L-NOARG-treated rings, o uabain had no further effect on the relaxation to BK. An inhibitor of inwar d-rectifying K+ channels, Ba2+ (30 mu M), had no effect on relaxations to B K in the absence or presence of either L-NOARG or ouabain. 4 KCl (2.5 - 10 mM) elicited small relaxations (similar to 20%) that were a bolished by nifedipine (0.3 mu M) and ouabain. 5 Both the high [K+](o)/apamin-sensitive relaxation to BK, and the relaxati on to the K-ATP channel opener, levcromakalim (0.6 mu M), were unaffected b y the cytochrome P-450 inhibitor, 7-ethoxyresorufin (10 mu M), or by co-tre atment with a phospholipase A(2) inhibitor, arachidonyl trifluoromethyl ket one (AACOCF(3); 3 mu M) and a diacylglycerol (DAG)-lipase inhibitor, 1,6-bi s-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267; 30 mu M). The non-NO/ high [K+](o)-insensitive, similar to 40% relaxation to BK was, however, abo lished by these treatments. 6 Therefore, neither cytochrome P-450-derived metabolites of arachidonic ac id nor K+ appear to mediate the EDHF-like relaxation to BK (i.e the non-NO, high [K+](o)/apamin-sensitive component) in bovine coronary arteries. Cyto chrome P-450-derived metabolites may be released at higher BK concentration s to act in parallel with NO and the high [K+](o)/apamin-sensitive mechanis m.