EVIDENCE THAT POTASSIUM CHANNELS MAKE A MAJOR CONTRIBUTION TO SIN-1-EVOKED RELAXATION OF RAT ISOLATED MESENTERIC-ARTERY

1 The NO donor 3-morpholino-sydnonimine (SIN-1; 0.01-10 mu M) evoked c oncentration-dependent relaxation of rat isolated mesenteric arteries pre-constricted with phenylephrine (1-3 mu M). The relaxation to SIN-1 was not significantly different between endothelium-intact or denuded arterial segments or segments in which basal nitric oxide (NO) synthe sis was inhibited (n=8; P>0.05). In contrast, the membrane permeable a nalogue of guanosine 3':5'-cyclic monophosphate (cyclic GMP), 8-Br-cyc lic GMP (0.01-1 mM), was much less effective in relaxing intact than d enuded arterial segments or intact arterial segments pre-incubated wit h NO synthase blockers (n=4; P<0.01). 2 1H-[1,2,4]oxadiazolo[4,3-a]qui noxalin-1-one (ODQ; 10 mu M; 10 min) alone, did not alter SIN-1-evoked relaxation in any tissues (n=5; P>0.05). However, in parallel experim ents, ODQ almost completely inhibited both basal and SIN-1-stimulated production of cyclic GMP in both the presence and absence of NO syntha se 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 o f endothelium-intact arterial segments to the potassium channel blocke r charybdotoxin (50 nM; 10 min), significantly inhibited SIN-l-evoked relaxation, reducing the maximum response by around 90% (n=5; P<0.01). In contrast, in arterial segments in which either the endothelial cel l layer had been removed or basal NO synthesis inhibited, relaxation t o 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 mu M) together, charybdotoxin did significantly inhibit SIN-l-evoked relaxation to a similar extent as intact tissues (maximum response red uced by around 80%; n=4; P<0.01). 4 Pre-incubation with apamin (30 nM; 10 min) or glibenclamide (10 mu M; 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 apa min, or ODQ and glibenclamide, SIN-I-evoked relaxation was significant ly attenuated in intact arterial segments and segments in which NO syn thesis was blocked. 5 Exposure of intact arterial segments to charybdo toxin and apamin, in the presence of NO synthase blockers, also signif icantly inhibited SIN-l-evoked relaxation, reducing the maximum respon se by around 80% (n=4; P<0.01). 6 Addition of superoxide dismutase (SO D; 30 u ml(-1)), potentiated relaxations to SIN-1 in all tissues, but did not alter the effects of charybdotoxin and ODQ on SIN-l-evoked rel axation. 7 These data show that although relaxation to the NO-donor SI N-1 is not significantly different between endothelium-intact and denu ded arterial segments, the mechanisms which mediate SIN-1-evoked relax ation in the rat isolated mesenteric artery appear to be modulated by the basal release of endothelium-derived NO. In the presence of an int act endothelial cell layer, the major mechanism for SIN-l-evoked relax ation appears to be the activation of charybdotoxin-sensitive potassiu m channels. In contrast, when basal NO synthesis is inhibited, SIN-1 a ppears to cause full relaxation by both the activation of a charybdoto xin-sensitive pathway and the stimulation of soluble guanylyl cyclase.