DIFFERENTIAL CA2-MUSCLE( SENSITIVITIES OF BK(CA) ISOCHANNELS IN BOVINE MESENTERIC VASCULAR SMOOTH)

The planar bilayer method was used to characterize the properties of l arge 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 firs t 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- 0) of 0.5 with 1 mu M Ca2+. The single-channel conductance of the seco nd 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-0 o f 0.5 with 1.0 mu M Ca2+. At a channel voltage of 0 mV, the Ca2+ conce ntrations for activating P-0 to 0.5 were 0.2 and 10 mu M 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 f rom K-c2. The P-0 of either isochannel did not change when protein kin ase A or alkaline phosphatase was added to the intracellular side. We conclude that bovine mesenteric arteries contain two distinct isochann els of BK(Ca) that differ in Ca2+ sensitivity but are identical with r espect to single-channel conductance, equivalent gating charge, and K/Cl- selectivities.