The nitroxyl anion (NO): a novel regulator of sGC and vascular tone

Traditionally the vascular effects of nitric oxide (NO) have been attributed to the free radical form of NO (NO^•) yet the reduced form of NO (NO^-) is also produced endogenously and vasodilates both large conduit and small resistance-like arteries. Interestingly, NO^• and NO^- have been shown to have distinct mechanisms of action in the cardiovascular system, particularly in the heart. This study aimed to determine if the vasorelaxant effects of NO^- differed to those of NO^• in rat small mesenteric resistance arteries.

Mesenteric arteries (~350 μm diameter) were obtained from male Sprague-Dawley rats, mounted in small vessel myographs and isometric force and intracellular membrane potential measured simultaneously. Vessels were precontracted to ~50% of maximum (determined by K⁺(124 mM)) with methoxamine. Cumulative concentration-response curves to NO^• (NO gas), the NO^- donor, Angeli's salt and the NO-independent soluble guanylate cyclase (sGC) activator, YC-1 were examined.

Vasorelaxation to Angeli's salt (pEC₅₀ = 7.02 ± 0.67 -log M; R_max = 96.0 ± 2.2%, n = 4) was accompanied by simultaneous vascular smooth muscle cell hyperpolarisation (pEC₅₀ = 6.82 ± 0.32, 10 μM AS -17.8 ± 4.4 mV, n = 4). In contrast, maximal vasorelaxation to NO^• (pEC₅₀ = 6.82 ± 0.39, 92.1 ± 1.3%) was achieved before a small hyperpolarization response was observed at 1 μM NO^• (-4.9 ± 2.3 mV, n = 5). Both relaxation and hyperpolarisation responses to Angeli's salt were significantly attenuated (P < 0.05, n = 5) by the NO^- scavenger, L-cysteine (3 mM) and abolished by the sGC inhibitor, ODQ (10 μM; P < 0.05, n = 4). The K_v channel inhibitor, 4-aminopyridine (1 mM) caused a 4-fold (P < 0.05, n = 4) decrease in sensitivity to Angeli's salt and abolished the hyperpolarisation response (P < 0.05). In contrast glibenclamide (K_ATP channel inhibitor) and charybdotoxin (BK_Ca/IK_Ca channel inhibitor) were without effect. YC-1 also induced vasorelaxation and hyperpolarisation in rat small mesenteric arteries.

In conclusion, in rat small mesenteric arteries, NO^- mediates relaxation in part via cGMP-dependent activation of K_v channels. In contrast, NO^•-mediated vasorelaxation occurs independently of vascular smooth muscle hyperpolarisation. Thus, the redox siblings NO^• and NO^- have distinct mechanisms of vasorelaxation in resistance-like arteries.