β-Adrenoceptor stimulation activates large-conductance Ca²⁺-activated K⁺ channels in smooth muscle cells from basilar artery of guinea pig

Abstract

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 (V _H) of 0 mV, which was used to inactivate the outward rectifier. The outward macroscopic current available from aV _H 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 μM. With [Ca²⁺]_i between 0.1 and 1.0 μM, 0.4 μM isoproterenol increased this current by 58.6±17.1%, whereas with [Ca²⁺]_i at 0.01 μM a sixfold smaller increase was observed. With [Ca²⁺]_i≥0.1 μM, 100 μM dibutyryl-adenosine 3′:5: cyclic monophosphate (cAMP) and 1 μM forskolin increased this current by 58.5±24.1% and 59.7±10.3%, respectively. The increase with isoproterenol was blocked by 4.0 μM propranolol extracellularly, and by 10 U/ml protein kinase inhibitor intracellularly. Single-channel openings during depolarizing test pulses from aV _H of 0 mV recorded in the whole-cell configuration under the same conditions (outside-outwhole-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 μM. Outside-out-whole-cell recordings with [Ca²⁺]_i≥0.1 μM indicated that 100 μM 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 μM Ca²⁺ compared to 0.01 μM Ca²⁺. We conclude that the BK channel in cerebrovascular smooth muscle cells can be activated byβ-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 inβ-adrenoceptor-mediated relaxation of cerebral vessels, although the importance of this pathway in obtaining vasorelaxation remains to be determined.