Abstract

The present study was designed to investigate whether nitric oxide (NO) could interfere with intracellular Ca⁺⁺ release through different pathways in vascular smooth muscle. Phasic contractions of rat aorta induced by phenylephrine or caffeine in Ca⁺⁺-free solution were used as an indicator of intracellular Ca⁺⁺release through the inositol 1,4,5-triphosphate receptor pathway and the ryanodine receptor pathway, respectively. In addition, cytoplasmic Ca⁺⁺ concentration ([Ca⁺⁺]_i) in vascular smooth muscle cells was determined by fluorescence measurement. Acetylcholine (ACh) inhibited the phenylephrine-evoked phasic contractions in Ca⁺⁺-free solution in endothelium-intact but not -denuded aortic rings in a dose-dependent manner. However, ACh did not affect the action of caffeine. The inhibition by ACh was blocked completely by the NO synthase inhibitor N^ω-nitro-l-arginine, which could be reversed totally by l-arginine but not d-arginine. Methylene blue, a soluble guanylate cyclase inhibitor, also abolished the inhibition by ACh. Sodium nitroprusside, an NO donor, attenuated the phenylephrine- but not caffeine-induced phasic contractions in denuded aortic rings in Ca⁺⁺-free solution. The effect of sodium nitroprusside was reversed substantially by methylene blue. Furthermore, sodium nitroprusside inhibited the elevation of [Ca⁺⁺]_i induced by phenylephrine in vascular smooth muscle cells isolated from rat aorta in the absence of extracellular Ca⁺⁺, which could be abolished significantly by methylene blue. These results suggest that NO selectively inhibits intracellular Ca⁺⁺ release stimulated by inositol 1,4,5-triphosphate, but not caffeine in vascular smooth muscle.