Sodium and potassium currents of larval zebrafish muscle fibres

Abstract

.p.f.) are described. In inner muscle, the outward currents were half-activated at -1.0·mV and half-inactivated at -30.4·mV, and completely inactivated within 100·ms of depolarization. The inward currents of inner fibres were half-activated at -7.3·mV and half-inactivated at -74.5·mV and completely inactivated within 5·ms of depolarization. Inner muscle fibres were found to support action potentials, while no action potentials could be evoked in outer muscles. In inner muscle fibres, all tested levels of depolarizing current above a threshold value evoked only one action potential. However, spiking at frequencies of up to 200·cycles·s -1 was evoked by the injection of depolarizing pulses separated by short hyperpolarizing currents. We suggest that the properties of the inward sodium and outward potassium currents permit high frequency firing in response to a pulsatile depolarizing input of the kind expected in fast swimming, whilst safeguarding against tetany during a strong depolarization.

A mechanism for graded motor control encoded in the channel properties of the muscle ACh receptor

“…In fish, the slow (red, tonic) muscle fibers are responsible for slow sustained swimming, whereas the fast fibers are for heavy bending during fast swimming or escapes. Graded control of the motion is achieved through regulation of the number and types of the fibers activated (1-3), and in fish, moreover, the slow fibers can make graded contractions by themselves (35,37). The slow muscle fibers in zebrafish larvae lack Na V -dependent APs and Table S2.…”

“…are tightly coupled across myotomes to distribute the synaptic signal to neighboring cells for averaging of the depolarization drive, as well as for priming the activation of the next segment (35,37). Such a signaling process may also work in the Ciona muscle band, whereas it remains unclear how these tonic fibers of vertebrates load Ca 2+ to make graded responses.…”

Computer simulations of high-pass filtering in zebrafish larval muscle fibres

“…where V 50,act , V 50,inact , s a and s i are the values for V 50 (voltage of half activation) and slope of activation/inactivation, respectively, taken from Buckingham and Ali (2004). The values for m τ and h τ were taken as the time constants of activation or inactivation, respectively.…”

“…Immature retinal ganglion neurons (RGN) exhibit single-spike firing during postnatal days P1 to P11, thought to be a consequence of insufficient calcium-activated potassium current [I K(Ca) ], while retinal amacrine cells fire single spikes because of insufficient Na + conductance and a lack of removal of Na + inactivation (Eliasof et al, 1987). In a recent paper in which we described the steady-state and kinetic properties of a sodium and potassium current present in the inner muscles of the zebrafish (Buckingham and Ali, 2004), we suggested that once-only firing may arise from the fact that the sodium current is rapidly inactivated and requires a rebound to more hyperpolarized voltages before sufficient channels are reactivated. Since at a resting potential of approximately -70·mV in the larvae (Buss and Drapeau, 2000) and approximately -80·mV in the adult (Westerfield et al, 1986) up to 70% of sodium current is inactivated (Buckingham and Ali, 2004), a second spike might only be possible after a hyperpolarization.…”

“…In a recent paper in which we described the steady-state and kinetic properties of a sodium and potassium current present in the inner muscles of the zebrafish (Buckingham and Ali, 2004), we suggested that once-only firing may arise from the fact that the sodium current is rapidly inactivated and requires a rebound to more hyperpolarized voltages before sufficient channels are reactivated. Since at a resting potential of approximately -70·mV in the larvae (Buss and Drapeau, 2000) and approximately -80·mV in the adult (Westerfield et al, 1986) up to 70% of sodium current is inactivated (Buckingham and Ali, 2004), a second spike might only be possible after a hyperpolarization. We proposed that this hyperpolarization is provided by the rapidly inactivating potassium current.…”