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<p>Abstract</p> <p>Background</p> <p>It has been shown that the contractile state of airway smooth muscle cells (SMCs) in response to agonists is determined by the frequency of Ca²⁺oscillations occurring within the SMCs. Therefore, we hypothesized that the relaxation of airway SMCs induced by agents that increase cAMP results from the down-regulation or slowing of the frequency of the Ca²⁺oscillations.</p> <p>Methods</p> <p>The effects of isoproterenol (ISO), forskolin (FSK) and 8-bromo-cAMP on the relaxation and Ca²⁺signaling of airway SMCs contracted with methacholine (MCh) was investigated in murine lung slices with phase-contrast and laser scanning microscopy.</p> <p>Results</p> <p>All three cAMP-elevating agents simultaneously induced a reduction in the frequency of Ca²⁺oscillations within the SMCs and the relaxation of contracted airways. The decrease in the Ca²⁺oscillation frequency correlated with the extent of airway relaxation and was concentration-dependent. The mechanism by which cAMP reduced the frequency of the Ca²⁺oscillations was investigated. Elevated cAMP did not affect the re-filling rate of the internal Ca²⁺stores after emptying by repetitive exposure to 20 mM caffeine. Neither did elevated cAMP limit the Ca²⁺available to stimulate contraction because an elevation of intracellular Ca²⁺concentration induced by exposure to a Ca²⁺ionophore (ionomycin) or by photolysis of caged-Ca²⁺did not reverse the effect of cAMP. Similar results were obtained with iberiotoxin, a blocker of Ca²⁺-activated K⁺channels, which would be expected to increase Ca²⁺influx and contraction. By contrast, the photolysis of caged-IP₃in the presence of agonist, to further elevate the intracellular IP₃concentration, reversed the slowing of the frequency of the Ca²⁺oscillations and relaxation of the airway induced by FSK. This result implied that the sensitivity of the IP₃R to IP₃was reduced by FSK and this was supported by the reduced ability of IP₃to release Ca²⁺in SMCs in the presence of FSK.</p> <p>Conclusion</p> <p>These results indicate that the relaxant effect of cAMP-elevating agents on airway SMCs is achieved by decreasing the Ca²⁺oscillation frequency by reducing internal Ca²⁺release through IP₃receptors.</p>