BETA-ADRENERGIC AGONISTS REGULATE K-CA CHANNELS IN AIRWAY SMOOTH-MUSCLE BY CAMP-DEPENDENT AND CAMP-INDEPENDENT MECHANISMS

Stimulation of calcium-activated potassium (K-Ca) channels in airway s mooth muscle cells by phosphorylation-dependent and membrane-delimited , G protein actions has been reported (Kume, H. A. Takai, H. Tokuno, a nd T. Tornita. 1989. Nature [Lend.]. 341:152-154; Kume, H., M. P. Graz iano, and M. I. Kotlikoff. 1992. Proc. Natl. Acad. Sci. USA. 89:11051- 11055). We show that beta-adrenergic receptor/channel coupling is not affected by inhibition of endogenous ATP, and that activation of K-Ca channels is stimulated by both or,and cAMP-dependent protein kinase (P KA). PKA stimulated channel activity in a dose-dependent fashion with an EC(50) of 0.12 U/ml and maximum stimulation of 7.38 +/- 2.04-fold. Application of alpha(S) to patches near maximally stimulated by PKA si gnificantly increased channel activity to 15.1 +/- 3.65-fold above bas eline, providing further evidence for dual regulatory mechanisms and s uggesting that the stimulatory actions are independent. Analysis of ch annel open-time kinetics indicated that isoproterenol and alpha(S) sti mulation of channel activity primarily increased the proportion of lon ger duration events, whereas PKA stimulation had little effect on the proportion of short and long duration events, but resulted in a signif icant increase in the duration of the long open-state. cAMP formation during equivalent relaxation of precontracted muscle strips by isoprot erenol and forskolin resulted in significantly less cAMP formation by isoproterenol than by forskolin, suggesting that the degree of activat ion of PKA is not the only determinant of tissue relaxation. We conclu de that beta-adrenergic stimulation of K-Ca channel activity and relax ation of tone in airway smooth muscle occurs, in part, by means indepe ndent of cyclic AMP formation.