The cardiac sodium channel shows a regular substate pattern indicating synchronized activity of several ion pathways instead of one

Abstract

Cardiac sodium channel substates were induced by using different gating modifiers, namely S-DPI 201-106 (s), toxin II from Anemonia sulcata (a), veratridine (v) and mixtures of these agents (s + v, a + v). Current ratios (normalized substate currents), slope conductances, reversal potentials and saturation characteristics were evaluated for the individual channel substates. The results can be summarized as follows: (i) Current ratios fell into a pattern of six equidistant values (I to VI) irrespective of the modification applied (0.20, 0.34, 0.51, 0.69, 0.85, 1.00). Slope conductances, determinable for substates II, V and VI (4.8, 11.7 and 14.0, respectively), are also consistent with six conductance substates which are integer multiples of a smallest conductance (state I). (ii) The permeability ratio PNa+/PK+ (i.e., reversal potential of substate currents) of the sodium channel was conserved both for different modifications, i.e., by s, a, s + v and a + v, and for the different substates (at least for II, IV and VI) observed for each modification. (iii) Sodium binding to the channel is substate independent. Analysis of slope conductances of states II and VI for three sodium chloride concentrations (71.5, 140 and 303 mM) revealed different maximal conductances (geVImax = 2.9.geIImax) but similar apparent affinities for sodium (KNa + VI = 286 mM; KNa + II = 303 mM). These findings are shown to seriously challenge the commonly unquestioned conception that 'single-current events' reflect ion passage through only one single pathway. The alternative view, that not one pore, but either six or three pores with synchronized gating ('oligochannel') underlie 'single-channel events', is shown to readily account for the observed substate properties and appears not to contradict known properties of 'the sodium channel'. This fundamentally new view of the sodium channel aims to invoke further efforts to distinguish between conceptually distinct models of structure-function relationships for a variety of channels which show multiple substates and conserved ion selectivity.