The conductance and selectivity of the Ca-activated K channel in cultured rat muscle was studied. Shifts in the reversal potential of single channel currents when various cations were substituted for K(i)+ were used with the Goldman-Hodgkin-Katz equation to calculate relative permeabilities. The selectivity was Tl+>K+>Rb+>NH4+, with permeability ratios of 1.2, 1.0, 0.67, and 0.11. Na+, Li+, and Cs+ were not measurably permeant, with permeabilities < 0.05 that of K+. Currents with the various ions were typically less than expected on the basis of the permeability ratios, which suggests that the movement of an ion through the channel was not independent of the other ions present. For a fixed activity of K0+ (77 mM), plots of single channel conductance vs. activity of K(i)+ were described by a two-barrier model with a single saturable site. This observation, plus the finding that the permeability ratios of Rb+ and NH4+ to K+ did not change with ion concentration, is consistent with a channel that can contain a maximum of one ion at any time. The empirically determined dissociation constant for the single saturable site was 100 mM, and the maximum calculated conductance for symmetrical solutions of K+ was 640 pS. TEA(i)+ (tetraethylammonium ion) reduced single channel current amplitude in a voltage-dependent manner. This effect was accounted for by assuming voltage-dependent block by TEA+ (apparent dissociation constant of 60 mM at 0 mV) at a site located 26% of the distance across the membrane potential, starting at the inner side. TEA0+ was much more effective in reducing single channel currents, with an apparent dissociation constant of ~0.3 mM.
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