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 Ca²⁺-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, K(c1), had a single-channel conductance of 287 ± 8 pS and required a potential of -33 mV to activate to an open probability (P(o)) of 0.5 with 1 μM Ca²⁺. The single-channel conductance of the second isochannel, K(c2) (282 ± 8 pS), was not statistically different from that of K(c1) but required a potential of 41 mV to activate to a P(o) of 0.5 with 1.0 μM Ca²⁺. At a channel voltage of 0 mV, the Ca²⁺ concentrations for activating P(o) to 0.5 were 0.2 and 10 μM for K(c1) and K(c2), respectively. The equivalent gating charges, estimated from the Boltzmann equation, were 2.4 and 2.2 for K(c1) and K(c2), respectively. The K/Cl selectivity of K(c1) was >40 and not significantly different from K(c2). The P(o) 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.