Characterization of endothelium‐derived relaxing factors released by bradykinin in human resistance arteries

Relaxing factors released by the endothelium and their relative contribution to the endothelium‐dependent relaxation produced by bradykinin (BK) in comparison with different vasodilator agents were investigated in human omental resistance arteries. BK produced an endothelium‐dependent relaxation of arteries pre‐contracted with the thromboxane A 2 agonist, U46619. The B 2 receptor antagonist, Hoe 140 (0.1, 1 and 10 μ M ), produced a parallel shift to the right of the concentration‐response curve to BK with a pA 2 of 7.75. Neither the cyclo‐oxygenase inhibitor, indomethacin (10 μ M ) alone, the nitric oxide synthase inhibitor, N ω ‐nitro‐ L ‐arginine methyl ester ( L ‐NAME, 300 μ M ) alone, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 μ M ) alone, nor the combination of L ‐NAME plus Hb affected the concentration‐response curve to BK. Conversely, the combination of indomethacin with either L ‐NAME or Hb attenuated but did not abolish the BK‐induced relaxation. By contrast, the relaxations produced by the Ca 2+ ionophore, calcimycin (A23187), and by the inhibitor of sarcoplasmic reticulum Ca 2+ ‐ATPase, thapsigargin (THAPS), were abolished in the presence of indomethacin plus L ‐NAME. Also, the presence of indomethacin plus L ‐NAME produced contraction of arteries with functional endothelium. The indomethacin plus L ‐NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 m M KCl and vice versa . However, in the presence of KCl 40 m M , indomethacin plus L ‐NAME did not affect the nitric oxide donor, S‐N‐acetylpenicillamine‐induced relaxation. The indomethacin plus L ‐NAME resistant component of the relaxation to BK was significantly attenuated by the K + channel blocker tetrabutylammonium (TBA, 1 m M ). However, it was not affected by other K + channel blockers such as apamin (10 μ M ), 4‐aminopyridine (100 μ M ), glibenclamide (10 μ M ), tetraethylammonium (10 m M ) and charybdotoxin (50 n M ). In the presence of indomethacin plus L ‐NAME, the relaxation produced by BK was not affected by the phospholipase A 2 inhibitor, quinacrine (10 μ M ) or by the inhibitor of cytochrome P450, SKF 525a (10 μ M ). Another cytochrome P450 inhibitor, clotrimazole (10 μ M ) which also inhibits K + channels, inhibited the relaxation to BK. These results show that BK induces endothelium‐dependent relaxation in human small omental arteries via multiple mechanisms involving nitric oxide, cyclo‐oxygenase derived prostanoid(s) and another factor (probably an endothelium‐derived hyperpolarizing factor). They indicate that nitric oxide and cyclo‐oxygenase derivative(s) can substitute for each other in producing relaxation and that the third component is not a metabolite of arachidonic acid, formed through the cytochrome P‐450 pathway, in these arteries.British Journal of Pharmacology (1997) 121 , 657–664; doi: 10.1038/sj.bjp.0701169