CHARACTERIZATION OF ENDOTHELIUM-DERIVED RELAXING FACTORS RELEASED BY BRADYKININ IN HUMAN RESISTANCE ARTERIES

1 Relaxing factors released by the endothelium and their relative cont ribution to the endothelium-dependent relaxation produced by bradykini n (BK) in comparison with different vasodilator agents were investigat ed in human omental resistance arteries. 2 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 mu M), produced a parallel shift to the right of the concentration- response curve to BK with a pA(2) of 7.75. 3 Neither the cyclo-oxygena se inhibitor, indomethacin (10 mu M) alone, the nitric oxide synthase inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME, 300 mu M) al one, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 mu M) alone, n or the combination of L-NAME plus Hb affected the concentration-respon se curve to BK. Conversely, the combination of indomethacin with eithe r L-NAME or Hb attenuated but did not abolish the BK-induced relaxatio n. By contrast, the relaxations produced by the Ca2+ ionophore, calcim ycin (A23187), and by the inhibitor of sarcoplasmic reticulum Ca2+-ATP ase, thapsigargin (THAPS), were abolished in the presence of indometha cin plus L-NAME. Also, the presence of indomethacin plus L-NAME produc ed contraction of arteries with functional endothelium. 4 The indometh acin plus L-NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 mM KCl and vice versa. How ever, in the presence of KCl 40 mM, indomethacin plus L-NAME did not a ffect the nitric oxide donor, S-N-acetylpenicillamine-induced relaxati on. 5 The indomethacin plus L-NAME resistant component of the relaxati on to BK was significantly attenuated by the K+ channel blocker tetrab utylammonium (TBA, 1 mM). However, it was not affected by other K+ cha nnel blockers such as apamin (10 mu M), 4-aminopyridine (100 mu M), gl ibenclamide (10 mu M), tetraethylammonium (10 mM) and charybdotoxin (5 0 nM). 6 In the presence of indomethacin plus L-NAME, the relaxation p roduced by BK was not affected by the phospholipase A(2) inhibitor, qu inacrine (10 mu M) or by the inhibitor of cytochrome P450, SKF 525a (1 0 mu M). Another cytochrome P450 inhibitor, clotrimazole (10 mu M) whi ch also inhibits K+ channels, inhibited the relaxation to BK. 7 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 a n endothelium-derived hyperpolarizing factor). They indicate that nitr ic oxide and cyclo-oxygenase derivative(s) can substitute for each oth er in producing relaxation and that the third component is not a metab olite of arachidonic acid, formed through the cytochrome P-450 pathway , in these arteries.