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

Abstract

Background and purpose: Reduction of intracellular calcium ([Ca 2+] i) in smooth muscle cells (SMCs) is an important mechanism by which nitric oxide (NO) dilates blood vessels. We investigated whether modes of Ca 2+ mobilization during SMC contraction influenced NO efficacy. Experimental approach: Isometric contractions by depolarization (high potassium, K +) or α-adrenoceptor stimulation (phenylephrine), and relaxations by acetylcholine chloride (ACh), diethylamine NONOate (DEANO) and glyceryl trinitrate (GTN) and SMC [Ca 2+] 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 [Ca 2+] i, which was suppressed by sarcoplasmic reticulum (SR) Ca 2+ ATPase (SERCA) inhibition, but not by soluble guanylyl cylase (sGC) inhibition. Relaxation of K +-stimulated segments did not parallel a decline of [Ca 2+] i. However, stimulation (BAY K8644) of L-type Ca 2+ 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 Ca 2+ re-uptake into the SR and was prominent when intracellular Ca 2+ was mobilized. The other involved sGC-stimulated cGMP formation, causing relaxation without changing [Ca 2+] i, presumably by desensitizing the contractile apparatus. This pathway seems related to L-type Ca 2+ influx, and L-type Ca 2+ channel blockers increase the vasodilator efficacy of NO. © 2009 The British Pharmacological Society.