To determine the involvement of Ca2+ sensitization in b-adrenergic action, we \ud
examined the relationship between isometric tension and intracellular Ca2+ \ud
concentration (F340/F380) in the inhibitory action of isoproterenol (ISO) against \ud
methacholine (MCh)- induced contraction, using fura-2 loaded tracheal smooth muscle.\ud
\ud
ISO reduces contraction more than Ca2+ concentration. This phenomenon was \ud
mimicked by forskolin and db-cAMP. In contrast, an inhibition in tension by SKF- \ud
96365, a non-selective Ca2+ channel inhibitor, was associated with that in F340/F380, \ud
different from ISO. In the presence of Rp-cAMP, a membrane-permeable inhibitor of \ud
protein kinase A (PKA), ISO caused an equivalent relaxation with a less reduction in \ud
F340/F380. The effects of ISO were not affected in the presence of Y-27632, an inhibitor \ud
of Rho-kinase, and bisindolylmaleimide, an inhibitor of protein kinase C. Even removal \ud
of Ca2+ from the extracellular surface, the effects of ISO was not affected. ISO also \ud
inhibits high K+-induced contraction without lowering F340/F380. However, response to \ud
ISO was completely diminished in the presence of calyculin A, an inhibitor of myosin \ud
phosphatase. In conclusion, b-adrenergic action antagonizes Ca2+ sensitization mediated \ud
by impairment of myosin phosphatase activity. Moreover, cAMP independent process, \ud
G protein direct action, is more potent in inhibiting this Ca2+ sensitization than PKA
