# Zebrafish fast muscle contractions avoid the mammalian requirement for voltage-gated Na+ channels

## TL;DR

Zebrafish fast muscle fibers can contract without voltage-gated Na+ channels, unlike mammalian muscles, due to unique physiological and anatomical features.

## Contribution

This study reveals that zebrafish fast muscle fibers do not require voltage-gated Na+ channels for contraction, challenging the assumption of their universal necessity.

## Key findings

- Zebrafish fast muscle fibers have faster NaV kinetics and a more negative inactivation threshold compared to mouse fibers.
- Action potentials in zebrafish fibers disappear at resting potentials less negative than -80 mV, unlike in mice.
- Zebrafish fast fibers are multi-innervated, allowing efficient depolarization propagation without voltage-gated Na+ channels.

## Abstract

Fast skeletal muscle fibers from zebrafish share a number of functional properties with mammalian twitch muscle fibers, making this vertebrate a precious model to investigate the pathophysiology of neuromuscular disorders. We previously reported that action potentials (APs) from zebrafish fast fibers exhibit low amplitude and require unusually strong negative resting membrane voltage levels to be elicited. In this study, using voltage-clamp and current-clamp techniques, we explored the properties of voltage-gated Na+ channels (NaV) responsible for initiation and propagation of AP in isolated adult zebrafish fast skeletal muscle fibers and compared them to mouse fast-twitch muscle fibers using the same experimental conditions. We found that kinetics of activation and inactivation of NaV were faster in zebrafish fibers and, overall, that the voltage-dependence of inactivation was shifted by 25 mV toward negative voltages as compared to mouse fibers, yielding a mean half-inactivation potential of −90 mV. In agreement with these findings, recording of APs at various resting membrane potentials indicated that APs vanished for resting membrane potentials less negative than −80 mV in zebrafish, whereas APs could still be elicited from resting membrane potentials as low as −60 mV in mice. In addition, Ca2+ transients induced by field stimulation were insensitive to Na+ current blockade in zebrafish but not in mouse fibers. Fluorescence labeling of nicotinic acetylcholine receptors showed that zebrafish fast fibers were multi-innervated with a mean distance between extra-synaptic sarcolemma and motor endplates of 14 µm, expected to lead to negligible attenuation of depolarization propagated from endplates. Finally, knock out of the two genes encoding pore-forming NaV subunits in zebrafish muscles did not induce any change in locomotion and escape behavior of the animals. Taken together, these data question the role of NaV and the occurrence of APs in zebrafish fast muscle.

Mammalian muscle fiber contractions require voltage-gated Na+ channels, and this has been assumed to be universal. This study shows that zebrafish fast muscle fibers do not require these channels and suggests how anatomical and physiological differences from mouse muscles can ensure efficient contraction.