Background It is well known that β-adrenoceptor agonists (β-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+]i). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the β-adrenergic relaxation.
Objective This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in β-adrenergic relaxation (Ca2+-independent relaxation by β-agonists).
Methods Isometric tension and [Ca2+]i were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+]i were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.
Results The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+]i. In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+]i. This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca2+ channels, was associated with that in [Ca2+]i. In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+]i. The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase.
Conclusion β-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, Gs-direct action is more potent in Ca2+-independent relaxation by β-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by β-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting β-agonists in acute asthma management.