Nitric oxide induces airway smooth muscle cell relaxation by decreasing the frequency of agonist-induced Ca(2+) oscillations

Nitric oxide (NO) induces airway smooth muscle cell (SMC) relaxation, but the underlying mechanism is not well understood. Consequently, we investigated the effects of NO on airway SMC contraction, Ca(2+) signaling, and Ca(2+) sensitivity in mouse lung slices with phase-contrast and confocal microscopy. Airways that were contracted in response to the agonist 5-hydroxytryptamine (5-HT) transiently relaxed in response to the NO donor, NOC-5. This NO-induced relaxation was enhanced by zaprinast or vardenafil, two selective inhibitors of cGMP-specific phosphodiesterase-5, but blocked by ODQ, an inhibitor of soluble guanylyl cyclase, and by Rp-8-pCPT-cGMPS, an inhibitor of protein kinase G (PKG). Simultaneous measurements of airway caliber and SMC [Ca(2+)](i) revealed that airway contraction induced by 5-HT correlated with the occurrence of Ca(2+) oscillations in the airway SMCs. Airway relaxation induced by NOC-5 was accompanied by a decrease in the frequency of these Ca(2+) oscillations. The cGMP analogues and selective PKG activators 8Br-cGMP and 8pCPT-cGMP also induced airway relaxation and decreased the frequency of the Ca(2+) oscillations. NOC-5 inhibited the increase of [Ca(2+)](i) and contraction induced by the photolytic release of inositol 1,4,5-trisphosphate (IP(3)) in airway SMCs. The effect of NO on the Ca(2+) sensitivity of the airway SMCs was examined in lung slices permeabilized to Ca(2+) by treatment with caffeine and ryanodine. Neither NOC-5 nor 8pCPT-cGMP induced relaxation in agonist-contracted Ca(2+)-permeabilized airways. Consequently, we conclude that NO, acting via the cGMP–PKG pathway, induced airway SMC relaxation by predominately inhibiting the release of Ca(2+) via the IP(3) receptor to decrease the frequency of agonist-induced Ca(2+) oscillations.