MECHANISM OF ACTIVATION OF THE CA2+-ACTIVATED K+ CHANNEL BY CYCLIC-AMP IN CULTURED PORCINE CORONARY-ARTERY SMOOTH-MUSCLE CELLS

Activation of the Ca2+-activated K+ channel (K(Ca)-channel) by adenosine 3', 5'-cyclic monophosphate (cAMP) and cAMP-dependent protein kinase (A-kinase) in cultured smooth muscle cells from porcine coronary artery was investigated using the patch-clamp technique. In cell-attached patches, the K(Ca)-channel was activated when forskolin (10 muM) was applied to the bath. In excised inside-out patches, application of 50 muM cAMP to the bath activated the K(Ca)-channel in the presence of A-kinase (10 units/ml) and ATP (1 mM). In addition, the K(Ca)-channel was activated directly by application of cAMP to the cytoplasmic side of the membrane in the absence of A-kinase. The activation by cAMP or by A-kinase required intracellular Ca2+, and was enhanced by increase of intracellular Ca2+. At a low concentration (3X10(-7) M) of Ca2+, more than 2 mM cAMP was required for activation of the K(Ca)-channel, but with 10(-6) M Ca2+, 100 muM cAMP was sufficient for activation. These results suggest that there are two mechanisms of activation of the K(Ca)-channel by cAMP : direct activation, and indirect activation via phosphorylation of the channel by A-kinase.