The NO donor 3-morpholino-sydnonimine (SIN-1; 0.01-10 microM) evoked concentration-dependent relaxation of rat isolated mesenteric arteries pre-constricted with phenylephrine (1-3 microM). The relaxation to SIN-1 was not significantly different between endothelium-intact or denuded arterial segments or segments in which basal nitric oxide (NO) synthesis was inhibited (n = 8; P > 0.05). In contrast, the membrane permeable analogue of guanosine 3':5'-cyclic monophosphate (cyclic GMP), 8-Br-cyclic GMP (0.01-1 mM), was much less effective in relaxing intact than denuded arterial segments or intact arterial segments pre-incubated with NO synthase blockers (n = 4; P < 0.01). 2. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM; 10 min) alone, did not alter SIN-1-evoked relaxation in any tissues (n = 5; P > 0.05). However, in parallel experiments, ODQ almost completely inhibited both basal and SIN-1-stimulated production of cyclic GMP in both the presence and absence of NO synthase blockers (n = 6; P < 0.01) indicating that full relaxation to SIN-1 can be achieved in the absence of an increase in cyclic GMP. 3. Exposure of endothelium-intact arterial segments to the potassium channel blocker charybdotoxin (50 nM; 10 min), significantly inhibited SIN-1-evoked relaxation, reducing the maximum response by around 90% (n = 5; P < 0.01). In contrast, in arterial segments in which either the endothelial cell layer had been removed or basal NO synthesis inhibited, relaxation to SIN-1 was not reduced in the presence of charybdotoxin (n = 6; P > 0.05). However, in the presence of NO synthase blockers and L-arginine (300 microM) together, charybdotoxin did significantly inhibit SIN-1-evoked relaxation to a similar extent as intact tissues (maximum response induced by around 80%; n = 4; P < 0.01). 4. Pre-incubation with apamin (30 nM; 10 min) or glibenclamide (10 microM; 10 min) did not alter SIN-1-evoked relaxation of phenylephrine-induced tone in any tissues (n = 4 and n = 6, respectively; P > 0.05). However, in the presence of either ODQ and apamin, or ODQ and glibenclamide, SIN-1-evoked relaxation was significantly attenuated in intact arterial segments and segments in which NO synthesis was blocked. 5. Exposure of intact arterial segments to charybdotoxin and apamin, in the presence of NO synthase blockers, also significantly inhibited SIN-1-evoked relaxation, reducing the maximum response by around 80% (n = 4; P < 0.01). 6. Addition of superoxide dismutase (SOD; 30 u ml-1), potentiated relaxations to SIN-1 in all tissues, but did not alter the effects of charybdotoxin and ODQ and SIN-1-evoked relaxation. 7. These data show that although relaxation to the NO-donor SIN-1 is not significantly different between endothelium-intact and denuded arterial segments, the mechanisms which mediate SIN-1-evoked relaxation in the rat isolated mesenteric artery appear to be modulated by the basal release of endothelium-derived NO. In the presence of an intact endothelial cell layer, the major mechanism for SIN-1-evoked relaxation appears to be the activation of charybdotoxin-sensitive potassium channels. In contrast, when basal NO synthesis is inhibited, SIN-1 appears to cause full relaxation by both the activation of a charybdotoxin-sensitive pathway and the stimulation of soluble guanylyl cyclase.
