Mn quenching in activated zebrafish muscle fibers does not result from store-operated Ca entry

In mammalian skeletal muscle fibers, transmembrane Ca2+ influx is known to occur at rest and to increase in response to depolarization. In parallel to the well-identified dihydropyridine receptor (DHPR) pathway underlying this depolarization-induced Ca2+ influx, a tubular Ca2+ entry pathway activated by sarcoplasmic reticulum (SR) Ca2+ depletion, named store-operated Ca2+ entry (SOCE), has been identified. The use of the Mn2+ quenching technique has been instrumental for the characterization of these Ca2+ influxes. But, because both should be activated by depolarization, it is difficult to discriminate between these two Ca2+ entry pathways. In that context, the zebrafish muscle fiber is an ideal model to determine whether or not SOCE develops in response to depolarization, because the zebrafish DHPR is not conductive to any divalent cation. Using the technique of Mn2+ quenching of fura-2 fluorescence in voltage-clamped zebrafish fast muscle fibers, we show that depolarization pulses evoke slow transient Mn2+ quenching signals that persist after washout of external Mn2+. The Mn2+ quenching signal displays rate of recovery and voltage dependence correlated to the rate of recovery and voltage dependence of SR Ca2+ release, respectively. Our data suggest that the voltage-evoked Mn2+ quenching signal of zebrafish muscle fibers does not result from a Mn2+ influx provoked by depletion of SR Ca2+ content but from a displacement of Mn2+ accumulated on intracellular Ca2+ buffers by Ca2+ released from the SR. These findings should encourage to consider that increase in Mn2+ quenching can result from changes in intracellular Ca2+ and not from SOCE.