β-Adrenergic agonists regulate K_Ca channels in airway smooth muscle by cAMP-dependent and -independent mechanisms

Stimulation of calcium-activated potassium (K_Ca) channels in airway smooth muscle cells by phosphorylation-dependent and membrane-delimited, G protein actions has been reported (Kume, H. A. Takai, H. Tokuno, and T. Tomita. 1989. Nature [Lond.]. 341:152-154; Kume, H., M. P. Graziano, and M. I. Kotlikoff. 1992. Proc. Natl. Acad. Sci. USA. 89:11051-11055). We show that β-adrenergic receptor/channel coupling is not affected by inhibition of endogenous ATP, and that activation of K_Ca channels is stimulated by both as and cAMP-dependent protein kinase (PKA). PKA stimulated channel activity in a dose-dependent fashion with an EC₅₀ of 0.12 U/ml and maximum stimulation of 7.38±2.04-fold. Application of α_S to patches near maximally stimulated by PKA significantly increased channel activity to 15.1±3.65-fold above baseline, providing further evidence for dual regulatory mechanisms and suggesting that the stimulatory actions are independent. Analysis of channel open-time kinetics indicated that isoproterenol and as stimulation of channel activity primarily increased the proportion of longer duration events, whereas PKA stimulation had little effect on the proportion of short and long duration events, but resulted in a significant increase in the duration of the long open-state. cAMP formation during equivalent relaxation of precontracted muscle strips by isoproterenol and forskolin resulted in significantly less cAMP formation by isoproterenol than by forskolin, suggesting that the degree of activation of PKA is not the only determinant of tissue relaxation. We conclude that β-adrenergic stimulation of K_Ca channel activity and relaxation of tone in airway smooth muscle occurs, in part, by means independent of cyclic AMP formation.