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

1. 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.
2. BK produced an endothelium-dependent relaxation of arteries pre-contracted with the thromboxane A₂ agonist, U46619. The B₂ receptor antagonist, Hoe 140 (0.1, 1 and 10 μM), produced a parallel shift to the right of the concentration-response curve to BK with a pA₂ of 7.75.
3. Neither the cyclo-oxygenase inhibitor, indomethacin (10 μM) alone, the nitric oxide synthase inhibitor, N^ω-nitro-L-arginine methyl ester (L-NAME, 300 μM) alone, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 μM) 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 Ca²⁺ ionophore, calcimycin ({"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187), and by the inhibitor of sarcoplasmic reticulum Ca²⁺-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.
4. The indomethacin plus L-NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 mM KCl and vice versa. However, in the presence of KCl 40 mM, indomethacin plus L-NAME did not affect the nitric oxide donor, S-N-acetylpenicillamine-induced relaxation.
5. The indomethacin plus L-NAME resistant component of the relaxation to BK was significantly attenuated by the K⁺ channel blocker tetrabutylammonium (TBA, 1 mM). However, it was not affected by other K⁺ channel blockers such as apamin (10 μM), 4-aminopyridine (100 μM), glibenclamide (10 μM), tetraethylammonium (10 mM) and charybdotoxin (50 nM).
6. In the presence of indomethacin plus L-NAME, the relaxation produced by BK was not affected by the phospholipase A₂ inhibitor, quinacrine (10 μM) or by the inhibitor of cytochrome P450, SKF 525a (10 μM). Another cytochrome P450 inhibitor, clotrimazole (10 μM) which 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 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.