Fibrillation potentials of denervated rat skeletal muscle are associated with expression of cardiac-type voltage-gated sodium channel isoform Nav1.5

Abstract

Objective

The molecular mechanisms underlying fibrillation potentials are still unclear. We hypothesised that expression of the cardiac-type voltage-gated sodium channel isoform Nav1.5 in denervated rat skeletal muscle is associated with the generation of such potentials.

Methods

Muscle samples were extracted and analysed biologically from surgically denervated rat extensor digitorum longus muscle after concentric needle electromyographic recording at various time points after denervation (4 h to 6 days).

Results

Both nav1.5 messenger RNA (mRNA) signal on northern blotting and Nav1.5 protein expression on immunohistochemistry appeared on the second day after denervation, exactly when fibrillation potentials appeared. Administration of lidocaine, which has much stronger affinity for sodium channels in cardiac muscle than for those in skeletal muscle, dramatically decreased fibrillation potentials, but had no effect on contralateral compound muscle action potentials.

Conclusions

Expression of Nav1.5 participates in the generation of fibrillation potentials in denervated rat skeletal muscle.

Significance

We proposed an altered expression of voltage-gated sodium channel isoforms as a novel mechanism to explain the occurrence of fibrillation potentials following skeletal muscle denervation.

Introduction

Fibrillation potentials are hallmarks of pathological change in electromyographic (EMG) studies in the diagnosis of neuromuscular disorders. Fibrillation potentials have been hypothesised to arise from denervation acetylcholine hypersensitivity in response to small quantities of circulating acetylcholine (Denny-Brown and Pennybacker, 1938, Thesleff, 1982). However, denervation hypersensitivity appears along the entire length of denervated fibres, whereas fibrillation potentials originate only from the former endplate zone. In addition, infusion of large quantities of d-tubocurarine failed to abolish such discharges (Muchnik et al., 1973, Brumback et al., 1978, Kimura, 2001). Thus, these findings indicate that the underlying mechanisms of fibrillation cannot be accounted for by this hypothesis alone.

It has been found that sodium channels play an important role in generation of fibrillation potentials, as fibrillation potentials were abolished by sodium-free solution (Purves and Sakmann, 1974). Localised sodium conductance change in denervated muscle cell membrane participates in the generation of fibrillation potentials (Thesleff, 1982, Kimura, 2001). Following denervation, mammalian skeletal muscle undergoes dramatic changes in conductance, including the new appearance of tetrodotoxin (TTX)-resistant sodium current (Pappone, 1980). Expression of the ‘cardiac type’ of sodium channel in denervated skeletal muscle has been reported to contribute to TTX-resistant sodium current (Rogart et al., 1989, Lupa et al., 1995). Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha subunit and two auxiliary beta subunits. The alpha subunits form a gene family with 10 members including cardiac specific isoform Nav1.5 (Catterall et al., 2005).

There have, however, been no reports of the relationship between the generation of fibrillation potentials and expression of Nav1.5 in denervated skeletal muscle. The goal of this study was to clarify the association between fibrillation potentials and the cardiac sodium channel isoform by evaluating temporal changes in both the electromyogram and sodium channel expression in an animal model of denervation.