Relaxation by bradykinin in porcine ciliary artery. Role of nitric oxide and K(+)-channels.

PURPOSE To assess the effects of K(+)-channel blockers on bradykinin-induced relaxations in porcine ciliary artery. METHODS Vascular isometric forces were measured with a myograph system. Ciliary vascular rings were precontracted with thromboxane A2 analog (U 46619, 10(-7) M) to assess dose-dependent (10(-10)-3 x 10(-6) M) bradykinin-induced relaxation after addition of one of the following: the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or inactive enantiomer (D-NAME, 10(-4) M); the nonspecific K(+)-channel blocker tetra-ethylammonium (TEA, 10(-2) M); or the ATP-sensitive K(+)-channel blocker glibenclamide (10(-5) M). The effect of TEA on relaxations to the NO donor, sodium nitroprusside (SNP, 10(-10)-10(-4) M) was investigated. The membrane potential of vascular smooth muscle cells (VSMC) was recorded after exposure to bradykinin (2.5 x 10(-7) M). RESULTS Endothelium-dependent relaxations to bradykinin (maximal [max], 99% +/- 3%) were strongly inhibited by L-NAME (max, 39% +/- 4%, P < 0.01) and partially by TEA (max, 62% +/- 3%, P < 0.01) or glibenclamide (max, 77% +/- 4%, P < 0.01). Administration of glibenclamide plus L-NAME further suppressed bradykinin-induced relaxation (max, 23% +/- 6%; P < 0.01), whereas TEA and L-NAME (max, 6% +/- 2%; P < 0.01) abolished the relaxation. SNP relaxations were unaffected by TEA. Bradykinin had no effect on the membrane potential of VSMC. CONCLUSIONS In porcine ciliary artery, the endothelium-dependent relaxations to bradykinin are primarily mediated by NO and involve K(+)-channels. As only relaxations to bradykinin, but not those mediated by SNP, were inhibited by TEA, this implies that K(+)-channel blockers most likely affect the bradykinin-evoked NO production or release by the endothelium.