Batrachotoxin-modified sodium channels from squid optic nerve in planar bilayers. Ion conduction and gating properties.

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Abstract
Squid optic nerve sodium channels were characterized in planar bilayers in the presence of batrachotoxin (BTX). The channel exhibits a conductance of 20 pS in symmetrical 200 mM NaCl and behaves as a sodium electrode. The single-channel conductance saturates with increasing the concentration of sodium and the channel conductance vs. sodium concentration relation is well described by a simple rectangular hyperbola. The apparent dissociation constant of the channel for sodium is 11 mM and the maximal conductance is 23 pS. The selectivity determined from reversal potentials obtained in mixed ionic conditions is Na+ .apprx. Li+ > K+ > Rb+ > Cs+. Calcium blocks the channel in a voltage-dependent manner. Analysis of single-channel membranes showed that the probability of being open (Po) vs. voltage relation is sigmoidal with a value of 0.5 between -90 and -100 mV. The fitting of Po requires at least two closed and one open state. The apparent gating charge required to move through the whole transmembrane voltage during the closed-open transition is four to five electronic charges per channel. Distribution of open and closed times are well described by single exponentials in most of the voltage range tested and mean open and mean closed times are voltage dependent. The number of charges associated with channel closing is 1.6 electronic charges per channel. Tetrodotoxin blocked the BTX-modified channel being the blockade favored by negative voltages. The apparent dissociation constant at zero potential is 16 nM. We concluded that sodium channels from the squid optic nerve are similar to other BTX-modified channels reconstituted in bilayers and to the BTX-modified sodium channel detected in the squid giant axon.