The potential-dependent K+ channel in molluscan neurones is organized in a cluster of elementary channels

Abstract

Single potential-dependent K+ channels were studied using the patch-voltage-clamp method. Two types of channel with identical, but oppositely directed, potential dependences were found. The channels of the first type (slow channels) are assumed to be responsible for the outward rectification. The properties of the channels of the second type (fast channels) are similar to those of the K+ channels in neurone soma which create the fast transient currents. The kinetic characteristics of both types of channel are presented. The conductances of slow and fast K+ channels are approx. 30 and 40 pS, respectively, at zero membrane potential and a K+ concentration of 50 mmol/l at the inner side of the membrane. The following sequence of channel selectivity with respect to monovalent cations was found: T1+ greater than K+ greater than Rb+ much greater than Cs+ approximately equal to Li+ approximately equal to Na+. The probability of the channel open state monotonically decreases with free Ca2+ concentration at the inside membrane surface for both types of channel. It was found that the channels have discrete and multiple conductance substates . It is supposed that a unitary K+ channel consists of approx. 16 elementary ones with conductances of approx. 2 pS (slow channels) and approx. 2.5 pS (fast channels) at zero potential. At +100 mV the elementary conductances are equal to approx. 4 and 5.5. pS, respectively. Thus, according to this assumption, the unitary channel is a cluster of elementary channels.