Ion channels in transit: voltage-gated Na and K channels in axoplasmic organelles of the squid Loligo pealei

Abstract

Ion channels that give rise to the excitable properties of the neuronal plasma membrane are synthesized, transported, and degraded in cytoplasmic organelles. To determine whether plasma membrane ion channels from these organelles could be physiologically activated, we extruded axoplasm from squid giant axons, dissociated organelles from the cytoskeletal matrix, and fused the free organelles with planar lipid bilayers. Three classes of ion channels normally associated with the plasma membrane were identified based on conductance, selectivity, and gating properties determined from steady-state single-channel recordings: (i) voltage-dependent Na channels, (ii) voltage-dependent delayed rectifier K channels, and (iii) large, voltage-independent K channels. The identity of the delayed rectifier channels was confirmed by reconstructing the time course of activation from single-channel responses to depolarizing voltage steps applied across the bilayer. These observations suggest that several classes of plasma membrane ion channels are transported in cytoplasmic organelles in physiologically active forms.