Block of BK (maxi K) channels of rat pituitary melanotrophs by Na+ and other alkali metal ions.

The block of large-conductance calcium-activated potassium (BK) channels by internal and external alkali metal ions was studied in adult rat melanotrophs. Internal but not external 20 mM Na+ produced a strongly voltage-dependent, flickery block that was well-fitted to the Woodhull model by using a value of 140 mM for the dissociation rate constant at 0 mV [Kd(0)] and an equivalent valence (zdelta) of 0.9. At a concentration of 20 mM external K+, Cs+ and Rb+, but not Li+, caused a rightward shift of the voltage dependence of the intracellular Na+ (Na+i ) block. This effect of K+, Cs+ and Rb+ was modelled by an equilibrium knock-out mechanism in which the block-relieving ion binds to a site located within the voltage field and consequently increases the off-rate of Na+. Internal Li+ caused little or no block whereas internal Cs+ caused a voltage-dependent block [Kd(0) approximately 150 mM]. Flickery channel block observed in cell-attached patches was consistent with a cytoplasmic Na+ activity between 1 and 10 mM.