Sodium channel distribution on uninnervated and innervated embryonic skeletal myotubes

Abstract

Acetylcholine receptor (AChR) and sodium (Na(+)) channel distributions within the membrane of mature vertebrate skeletal muscle fibers maximize the probability of successful neuromuscular transmission and subsequent action potential propagation. AChRs have been studied intensively as a model for understanding the development and regulation of ion channel distribution within the postsynaptic membrane. Na(+) channel distributions have received less attention, although there is evidence that the temporal accumulation of Na(+) channels at developing neuromuscular junctions (NMJs) may differ between species. Even less is known about the development of extrajunctional Na(+) channel distributions. To further our understanding of Na(+) channel distributions within junctional and extrajunctional membranes, we used a novel voltage-clamp method and fluorescent probes to map Na(+) channels on embryonic chick muscle fibers as they developed in vitro and in vivo. Na(+) current densities on uninnervated myotubes were approximately one-tenth the density found within extrajunctional regions of mature fibers, and showed several-fold variations that could not be explained by a random scattering of single channels. Regions of high current density were not correlated with cellular landmarks such as AChR clusters or myonuclei. Under coculture conditions, AChRs rapidly concentrated at developing synapses, while Na(+) channels did not show a significant increase over the 7 day coculture period. In vivo investigations supported a significant temporal separation between Na(+) channel and AChR aggregation at the developing NMJ. These data suggest that extrajunctional Na(+) channels cluster together in a neuronally independent manner and concentrate at the developing avian NMJ much later than AChRs.

Critical illness myopathy serum fractions affect membrane excitability and intracellular calcium release in mammalian skeletal muscle

“…As the I Na currents in the present study (see Figs. 4, 5, see also [35]) are very similar to I Na currents recorded with other techniques, e. g. the vaseline-gap technique [36], the 'loose-patch' clamp technique [37][38][39][40] or the Three-Micro-Electrode technique [41] the 2-MVC technique also allows the faithful recording of I Na currents in short fibres.…”

Clustering of Cyclic-Nucleotide-Gated Channels in Olfactory Cilia

“…Channel clustering is a phenomenon observed in many cell types and neuronal compartments, including neuronal axons, hair cells of the inner ear (27), skeletal myotubes (28), and pancreatic b cells (29). The functions of channel clustering include regulation of oscillation frequency in hair cells and neurosecretory cells, as well as compartmentalization of signaling components in neuronal synapses.…”