Beta-Adrenoceptor Stimulation Activates Large-conductance Ca2+-activated K+ Channels in Smooth Muscle Cells from Basilar Artery of Guinea Pig

We studied the effect of isoproterenol on the Ca2+-activated K+ (BK) channel in smooth muscle cells isolated from the basilar artery of the guinea pig. Cells were studied in a whole-cell configuration to allow the clamping of intracellular Ca2+ concentration, [Ca2+]i. Macroscopic BK channel currents were recorded during depolarizing test pulses from a holding potential (VH) of 0 mV, which was used to inactivate the outward rectifier. The outward macroscopic current available from a VH of 0 mV was highly sensitive to block by external tetraethylammonium Cl (TEA) and charybdotoxin, and was greatly augmented by increasing [Ca2+]i from 0.01 to 1.0 microM. With [Ca2+]i between 0.1 and 1.0 microM, 0.4 microM isoproterenol increased this current by 58.6 +/- 17.1%, whereas with [Ca2+]i at 0.01 microM a sixfold smaller increase was observed. With [Ca2+]i > or = 0.1 microM, 100 microM dibutyryl -adenosine 3':5: cyclic monophosphate (cAMP) and 1 microM forskolin increased this current by 58.5 +/- 24.1% and 59.7 +/- 10.3%, respectively. The increase with isoproterenol was blocked by 4.0 microM propranolol extracellularly, and by 10 U/ml protein kinase inhibitor intracellularly. Single-channel openings during depolarizing test pulses from a VH of 0 mV recorded in the whole-cell configuration under the same conditions (outside-out-whole-cell recording) indicated a slope conductance of 260 pS. In conventional outside-out patches, this 260-pS channel was highly sensitive to block by external TEA, and in inside-out patches, its probability of opening was greatly augmented by increasing [Ca2+]i from 0.01 to 1.0 microM. Outside-out-whole-cell recordings with [Ca2+]i > or = 0.1 microM indicated that 100 microM dibutyryl-cAMP increased the probability of opening of the 260-pS channel by 152 +/- 115%. In inside-out patches, the catalytic subunit of protein kinase A increased the probability of opening, and this effect also depended on [Ca2+]i , with a 35-fold larger effect observed with 0.1-0.5 microM Ca2+ compared to 0.01 microM Ca2+. We conclude that the BK channel in cerebrovascular smooth muscle cells can be activated by beta-adrenoceptor stimulation, that the effect depends strongly on [Ca2+]i, and that the effect is mediated by cAMP-dependent protein kinase A with no important contribution from a direct G-protein or phosphorylation-independent mechanism. Our data indicate that the BK channel may participate in beta-adrenoceptor-mediated relaxation of cerebral vessels, although the importance of this pathway in obtaining vasorelaxation remains to be determined.