Apamin-sensitive, non-nitric oxide (NO) endothelium-dependent relaxations to bradykinin in the bovine isolated coronary artery: no role for cytochrome P450 and K+.

Since cytochrome P(450)-derived metabolites of arachidonic acid and K(+) have been implicated in endothelium-derived hyperpolarizing factor (EDHF)-dependent responses, the aim of this study was to determine whether such factors contribute to non-nitric oxide (NO), endothelium-dependent relaxation to bradykinin (BK) in bovine isolated coronary artery. In rings of artery contracted with U46619 and treated with indomethacin (3 microM) and N(G)-nitro-L-arginine (L-NOARG; 100 microM), relaxation to BK (0.01 nM-0.3 microM) was blocked by approximately 60% after inhibition of K(+) channels with either high extracellular K(+) (high [K(+)](o); 15 - 67 mM) or apamin (0.3 microM). Ouabain (1 microM), an inhibitor of Na(+)/K(+)-ATPase, decreased the sensitivity to BK without affecting the maximum response. In L-NOARG-treated rings, ouabain had no further effect on the relaxation to BK. An inhibitor of inward-rectifying K(+) channels, Ba(2+) (30 microM), had no effect on relaxations to BK in the absence or presence of either L-NOARG or ouabain. KCl (2.5 - 10 mM) elicited small relaxations ( approximately 20%) that were abolished by nifedipine (0.3 microM) and ouabain. Both the high [K(+)](o)/apamin-sensitive relaxation to BK, and the relaxation to the K(ATP) channel-opener, levcromakalim (0.6 microM), were unaffected by the cytochrome P(450) inhibitor, 7-ethoxyresorufin (10 microM), or by co-treatment with a phospholipase A(2) inhibitor, arachidonyl trifluoromethyl ketone (AACOCF(3); 3 microM) and a diacylglycerol (DAG)-lipase inhibitor, 1, 6-bis-(cyclohexyloximinocarbonylamino)-hexane (RHC 80267; 30 microM). The non-NO/high [K(+)](o)-insensitive, approximately 40% relaxation to BK was, however, abolished by these treatments. Therefore, neither cytochrome P(450)-derived metabolites of arachidonic acid 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. Cytochrome P(450)-derived metabolites may be released at higher BK concentrations to act in parallel with NO and the high [K(+)](o)/apamin-sensitive mechanism.