Effect of acetylcholine on single Ca2(+)-activated K+ channels in bovine chromaffin cells

The single channel current amplitudes of "maxi" Ca2(+)-activated K+ channels from bovine chromaffin cell membranes are reduced when acetylcholine is applied to the internal surface of the membrane, which can be explained by a fast channel block. The block is concentration dependent with moderate affinity. It becomes progressively greater with depolarization although the voltage dependence is not pronounced. Acetylcholine reduces the probability of the open state in the same concentration range and in an essentially voltage independent manner. The changes in the channel kinetics are complex. Whilst the long component of the open intervals is shortened (by 48%; from 9.5 to 5.1 ms), the long component of the closed intervals is prolonged (by 96%; from 45 to 89 ms). The short components (open and closed) are essentially unaffected. Short open intervals are reduced by 4% (from 1.09 to 1.05 ms), whilst short closed intervals are reduced by 5% (from 2.3 to 2.2 ms). These changes in the channel kinetics can be explained at least partly if one assumes that acetylcholine, in addition to its fast channel blocking activity, acts also as a slow blocker. If so, both binding sites are expected to be located close to the mouth of the channel pore. Alternatively, acetylcholine may be affecting the gating mechanism, presumably by interfering with the Ca2+ binding.