Choline blockage of currents through Ca(2+)-activated K+ channels in bovine chromaffin cells.

The action of choline on "maxi" Ca(2+)-activated K+ channels was studied in excised patches of bovine chromaffin cell membranes. Choline (20-70 mM) applied to the internal surface of the membrane reduced the single channel current amplitudes, which can be explained by a fast channel block. The block is concentration- and voltage-dependent and is rapidly and completely reversed upon washout. The block becomes progressively greater with depolarization. The estimates of blocking parameters vary from channel to channel but appear to fall in two groups. A larger group (two-thirds of cases) with moderate affinity [KD(0) = 88.5 mM] and low voltage dependence (delta = 0.26) and a smaller group (one-third of cases) with very low affinity (KD = 306 mM) and moderate voltage dependence (delta = 0.59). The open state probability appears not to be affected at any choline concentration (up to 70 mM) or membrane potential (from -20 to +60 mV) studied, suggesting that choline does not affect the channel gating kinetics. Since the affinity of the choline block is low to moderate, the intracellular choline is not expected to alter the current flow through "maxi" Ca(2+)-activated K+ channels unless the choline concentration close to the protoplasmic membrane is much higher than the mean cellular concentration.