Differential Ca2+ sensitivities of BK(Ca) isochannels in bovine mesenteric vascular smooth muscle.

The planar bilayer method was used to characterize the properties of large Ca(2+)-activated K+ [BK(Ca)] channels of smooth muscle from bovine mesenteric arteries. We found two isochannels of BK(Ca), differing in sensitivity to Ca2+ on the intracellular side of the channel. The first isochannel, Kc1, had a single-channel conductance of 287 +/- 8 pS and required a potential of -33 mV to activate to an open probability (Po) of 0.5 with 1 microM Ca2+. The single-channel conductance of the second isochannel, Kc2 (282 +/- 8 pS), was not statistically different from that of Kc1 but required a potential of 41 mV to activate to a Po of 0.5 with 1.0 microM Ca2+. At a channel voltage of 0 mV, the Ca2+ concentrations for activating Po to 0.5 were 0.2 and 10 microM for Kc1 and Kc2, respectively. The equivalent gating charges, estimated from the Boltzmann equation, were 2.4 and 2.2 for Kc1 and Kc2, respectively. The K/Cl selectivity of Kc1 was > 40 and not significantly different from Kc2. The Po of either isochannel did not change when protein kinase A or alkaline phosphatase was added to the intracellular side. We conclude that bovine mesenteric arteries contain two distinct isochannels of BK(Ca) that differ in Ca2+ sensitivity but are identical with respect to single-channel conductance, equivalent gating charge, and K+/Cl- selectivities.