Large-conductance calcium-activated potassium channels of cultured rat melanotrophs.

A large conductance, Ca(2+)-activated K+ channel of the BK type was examined in cultured pituitary melanotrophs obtained from adult male rats. In cell-attached recordings the slope conductance for the BK channel was approximately 190 pS and the probability (Po) of finding the channel in the open state at the resting membrane potential was low (< < 0.1). Channels in inside-out patches and in symmetrical 150 mM K+ had a conductance of approximately 260 pS. The lower conductance in the cell-attached recordings is provisionally attributed to an intracellular K+ concentration of approximately 113 mM. The permeability sequence, relative to K+, was K+ > Rb+ (0.87) > NH4+ (0.17) > Cs+ > or = Na+ (< or = 0.02). The slope conductance for Rb+ was much less than for K+. Neither Na+ nor Cs+ carried measurable currents and 150 mM internal Cs+ caused a flickery block of the channel. Internal tetraethylammonium ions (TEA+) produced a fast block for which the dissociation constant at 0 mV (KD(0 mV)) was 50 mM. The KD(0 mV) for external TEA+ was much lower, 0.25 mM, and the blocking reaction was slower as evidenced by flickery open channel currents. With both internal and external TEA+ the blocking reaction was bimolecular and weakly voltage dependent. External charybdotoxin (40 nM) caused a large and reversible decrease of Po. The Po was increased by depolarization and/or by increasing the concentration of internal Ca2+. In 0.1 microM Ca2+ the half-maximal Po occurred at approximately 100 mV; increasing Ca2+ to 1 microM shifted the voltage for the half-maximal Po to -75 mV. The Ca2+ dependence of the gating was approximated by a fourth power relationship suggesting the presence of four Ca2+ binding sites on the BK channel.