Fluorescently labelled Na⁺ channels are localized and immobilized to synapses of innervated muscle fibres

Abstract

Segregation of voltage-dependent sodium channels to the hillock of motoneurones and nodes of Ranvier in myelinated axons is crucial for conduction of the nerve impulse^1,2. Much less is known, however, about the distribution of voltage-dependent Na⁺ channels on muscle fibres. Recently, Beam et al.³ have shown that Na⁺ channels are concentrated near the neuromuscular junction. To determine the topography and mechanisms governing the distribution of voltage-dependent Na⁺ channels on muscle, microfluorimetry and fluorescence photobleach recovery (FPR) have now been used to measure the density and lateral mobility of fluorescently labelled Na⁺ channels on uninnervated and innervated muscle fibres. On uninnervated myotubes, Na⁺ channels are diffusely distributed and freely mobile, whereas after innervation the channels concentrate at neuronal contact sites. These channels are immobile and co-localize with acetylcholine receptors (AChRs). At extrajunctional regions the Na⁺ channel density is lower and the channels more mobile. The results suggest that the nerve induces Na⁺ channels to redistribute, immobilize and co-localize with AChRs at sites of neuronal contact.