Voltage-gated Ca2+ influx through L-type channels contributes to sarcoplasmic reticulum Ca2+ loading in skeletal muscle

Abstract

Muscle contraction is triggered by Ca2+ ions released from the sarcoplasmic reticulum (SR) in response to depolarization of skeletal muscle fibres. Muscle activation is also associated with a voltage-activated trans-sarcolemmal Ca2+ influx early identified as a current flowing through L-type Ca2+ channels. Because removal of external Ca2+ does not impede fibres from contracting, a negligible role was given to this voltage-activated Ca2+ entry, although the decline of Ca2+ release is more pronounced in the absence of Ca2+ during long-lasting activation. Furthermore, it is not clearly established whether Ca2+ exclusively flows through L-type channels or in addition through a parallel voltage-activated pathway distinct from L-type channels. Here, by monitoring the quenching of fura-2 fluorescence resulting from Mn2+ influx in voltage-controlled mouse and zebrafish isolated muscle fibres, we show that the L-type current is the only contributor to Ca2+ influx during long-lasting depolarizations in skeletal muscle. Calibration of the Mn2+ quenching signal allowed us to estimate a mean Mn2+ current of 0.31 ± 0.06 A F-1 flowing through L-type channels during a train of action potentials. Measurements of SR Ca2+ changes with fluo-5N in response to depolarization revealed that an elevated voltage-activated Ca2+ current potentiated SR Ca2+ loading and addition of external Mn2+ produced quenching of fluo-5N in the SR, indicating that voltage-activated Ca2+/Mn2+ influx contributes to SR Ca2+/Mn2+ loading.