Cellular mechanisms of nitric oxide-induced relaxation of corporeal smooth muscle in the guinea-pig.

The cellular mechanism of nitric oxide (NO)-induced relaxation in corporeal smooth muscle (CSM) of the guinea-pig was investigated. Changes in the intracellular concentration of calcium ions (Ca(2+)), membrane potential and isometric tension were measured. CSM cells exhibited spontaneous depolarizations and transient increases in Ca(2+) (Ca(2+) transients) which were accompanied by contractions. This spontaneous activity was abolished by nifedipine (10 microM). NO released by 3-morpholino-sydnonimine (SIN-1, 10 microM) hyperpolarized the membrane and prevented the generation of spontaneous depolarizations. SIN-1 also abolished Ca(2+) transients and associated contractions. These effects of SIN-1 were blocked by 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ, 10 microM), an inhibitor of guanylate cyclase. Noradrenaline (NA, 1 microM) increased Ca(2+) to levels similar to those produced by high potassium-containing solution (high K(+) solution, K(+) = 40 mM), however, NA-induced contractions were three times greater in amplitude than those induced by high K(+) solution. In NA precontracted preparations, SIN-1 inhibited 80 % of the contraction and decreased Ca(2+) by 20 %. In contrast, nifedipine reduced Ca(2+) by 80 %, while the level of contraction was decreased by only 20 %. SIN-1-induced reduction in Ca(2+) but not the tension effect, was abolished by pretreatment with cyclopiazonic acid (CPA, 10 microM). In high K(+) precontracted preparations, SIN-1 inhibited 80 % of the contraction and reduced Ca(2+) by 20 %. Nifedipine, however, largely abolished increases in both Ca(2+) and tension under these circumstances. These results suggest that decreasing the sensitivity of contractile proteins to Ca(2+) is probably the key mechanism of NO-induced relaxation in CSM of the guinea-pig.