Programme Contents

K_v as a target for nitroxyl anion (NO⁻)-mediated vasodilatation

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 (Irvine et al., 2003). Interestingly, NO^• and NO⁻ have been shown to have distinct mechanisms of action in the cardiovascular system, particularly in the heart (Paolocci et al., 2003). This study aimed to determine if the vasorelaxant effects of NO⁻ differed to those of NO^• in rat small mesenteric resistance arteries. Male Sprague-Dawley rats were killed via CO₂ sedation and cervical dislocation. Mesenteric arteries (∼350μm diameter) were isolated, mounted in small vessel myographs and isometric force and intracellular membrane potential measured simultaneously. 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, 10μM 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 hyperpolarisation response was observed at 1μM 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 (3mM) and virtually abolished by the sGC inhibitor, ODQ (10μM; P<0.05, n=4). In contrast, ODQ only decreased the sensitivity of NO^•-mediated vasorelaxation approximately 10-fold (P<0.05, n=4) and failed to affect NO^•-mediated hyperpolarisation. The K_v channel inhibitor, 4-aminopyridine (1mM) caused a 4-fold (P<0.05, n=4) decrease in sensitivity to Angeli's salt and abolished the hyperpolarisation response (P<0.05). Glibenclamide (K_ATP channel inhibitor) and charybdotoxin (BK_Ca/IK_Ca channel inhibitor) were without effect. YC-1 also induced smooth muscle hyperpolarisation (10μM YC-1 -43.0±6.3 mV, n=3) which was attenuated by 4-aminopyridine (10μM YC-1 -23.5±2.3 mV, P<0.05, n=3). 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 and in part via cGMP-independent pathways. Thus, the redox siblings NO^• and NO⁻ have distinct mechanisms of vasorelaxation in resistance-like arteries.

Irvine, J.C., Favaloro, J.L. & Kemp-Harper, B.K. (2003) Hypertension 41, 1301-1307.

Paolocci, N., Katori, T., Champion, H.C., St John, M.E., Miranda, K.M., Fukuto, J.M., Wink, D.A. & Kass, D. (2003) Proceedings of the National Academy of Science 100, 5537-5542.