Cyclic AMP modulates Ca-activated K channel in cultured smooth muscle cells of rat aortas

Effects of adenosine 3',5'-cyclic monophosphate (cAMP) on single Ca-activated K current (IK(Ca)) in cultured smooth muscle cells of the rat aorta were investigated with the patch-clamp technique. In cell-attached patch configurations, extracellular application of isoproterenol (10(-5) M) increased the Ca-activated K currents. The increase in the currents was due to an increase in the probability of channel openings (Po). Neither unit conductance nor the maximum number of the channel in the patch was affected by the drug. The effects were inhibited by adding propranolol (10(-6) M). The extracellular application of forskolin (10(-5) M) or dibutyryl cAMP (10(-4) M) mimicked the effects of isoproterenol. In inside-out patch configurations, activated cAMP-dependent protein kinase (A kinase) in the bathing solution increased the sensitivity of the Ca-activated K channels to intracellular free calcium concentration ([Ca]i) and enhanced Po. Kinetic analyses of the IK(Ca) showed that cAMP-dependent phosphorylation of the Ca-activated K channels significantly reduced the mean closed time between bursting openings. We conclude from these observations that the Ca-activated K channels in aortic cells may increase Po through cAMP-dependent phosphorylation.