Relaxation to authentic nitric oxide and SIN-1 in rat isolated mesenteric arteries: variable role for smooth muscle hyperpolarization

1 Authentic nitric oxide (NO; 0.1-10 mu moles) caused transient, dose-dependent relaxation of phenylephrine-induced tone without changing membrane potential in mesenteric arteries. Larger doses, above 10 mu moles, did not evoke more relaxation (maximal relaxation to 150 mu moles NO in denuded arteries, 69 +/- 17%, n=8) but stimulated muscle hyperpolarization (maximum 19 +/- 3 mV, n=5). 2 The soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1- one (ODQ; 10 muM), abolished relaxation to low doses of NO (n=4), but did not modify hyperpolarization with higher doses of NO (n=4). The potassium channel blocker charybdotoxin (ChTX; 50 nM) abolished hyperpolarization to high doses of NO and significantly reduced the maximal relaxation (to 43 +/- 6%, n=4; P < 0.01). ODQ and ChTX together abolished tension and membrane potential change to all doses of NO (n=4). 3 All relaxations to 3-morpholino-sydnonimine (SIN-1; 0.01-10 muM) were associated with hyperpolarization. When the endothelium was intact, ChTX inhibited hyperpolarization and relaxation to SIN-1 (n=5), while iberiotoxin (IbTX; 50 nM) or 4-aminopyridine (4-AP; 500 muM) reduced relaxation by 40% and 20%, respectively and by 80% in combination (n=6 in each case). 4 In denuded arteries, relaxation to SIN-1 was unaffected by either ChTX or ODQ alone, but abolished by the inhibitors together (n=6). Alone, 4-AP did not alter relaxation, but in the presence of ODQ it reduced the maximal response by around 45% (n=6; P < 0.01). 4-AP, ODQ and IbTX together inhibited relaxation to SIN-1 by 75% (n=6; P < 0.01). 5 Therefore, cyclic guanosine 3 ' ,5 ' -monophosphate (cyclic GMP)-independent smooth muscle hyperpolarization, possibly involving direct activation of calcium-activated and voltage-sensitive potassium channels, contributes to relaxation evoked by authentic NO and SIN-1. However, the importance of each pathway depends on the source of NO and with SIN-1 the relative contribution from each pathway is modified by the endothelium.