Vasodilator efficacy of nitric oxide depends on mechanisms of intracellular calcium mobilization in mouse aortic smooth muscle cells

Background and purpose: Reduction of intracellular calcium ([Ca2+](i)) in smooth muscle cells (SMCs) is an important mechanism by which nitric oxide (NO) dilates blood vessels. We investigated whether modes of Ca2+ mobilization during SMC contraction influenced NO efficacy. Experimental approach: Isometric contractions by depolarization (high potassium, K+) or alpha-adrenoceptor stimulation (phenylephrine), and relaxations by acetylcholine chloride (ACh), diethylamine NONOate (DEANO) and glyceryl trinitrate (GTN) and SMC [Ca2+](i) (Fura-2) were measured in aortic segments from C57Bl6 mice. Key results: Phenylephrine-constricted segments were more sensitive to endothelium-derived (ACh) or exogenous (DEANO, GTN) NO than segments contracted by high K+ solutions. The greater sensitivity of phenylephrine-stimulated segments was independent of the amount of pre-contraction, the source of NO or the resting potential of SMCs. It coincided with a significant decrease of [Ca2+](i), which was suppressed by sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) inhibition, but not by soluble guanylyl cylase (sGC) inhibition. Relaxation of K+-stimulated segments did not parallel a decline of [Ca2+](i). However, stimulation (BAY K8644) of L-type Ca2+ influx diminished, while inhibition (nifedipine, 1-100 nM) augmented the relaxing capacity of NO. Conclusions and implications: In mouse aorta, NO induced relaxation via two pathways. One mechanism involved a non-cGMP-dependent stimulation of SERCA, causing Ca2+ re-uptake into the SR and was prominent when intracellular Ca2+ was mobilized. The other involved sGC-stimulated cGMP formation, causing relaxation without changing [Ca2+](i), presumably by desensitizing the contractile apparatus. This pathway seems related to L-type Ca2+ influx, and L-type Ca2+ channel blockers increase the vasodilator efficacy of NO.