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Simultaneous recording of intramembrane charge movement components and calcium release in wild‐type and S100A1−/− muscle fibres

In the preceding paper, we reported that flexor digitorum brevis (FDB) muscle fibres from S100A1 knock‐out (KO) mice exhibit a selective suppression of the delayed, steeply voltage‐dependent component of intra‐membrane charge movement current termed Qγ. Here, we use 50 μm of the Ca2+ indicator fluo‐4 in the whole cell patch clamp pipette, in addition to 20 mm EGTA and other constituents included for the charge movement studies, and calculate the SR Ca2+ release flux from the fluo‐4 signals during voltage clamp depolarizations. Ca2+ release flux is decreased in amplitude by the same fraction at all voltages in fibres from S100A1 KO mice compared to fibres from wild‐type (WT) littermates, but unchanged in time course at each pulse membrane potential. There is a strong correlation between the time course and magnitude of release flux and the development of Qγ. The decreased Ca2+ release in KO fibres is likely to account for the suppression of Qγ in these fibres. Consistent with this interpretation, 4‐chloro‐m‐cresol (4–CMC; 100 μm) increases the rate of Ca2+ release and restores Qγ at intermediate depolarizations in fibres from KO mice, but does not increase Ca2+ release or restore Qγ at large depolarizations. Our findings are consistent with similar activation kinetics for SR Ca2+ channels in both WT and KO fibres, but decreased Ca2+ release in the KO fibres possibly due to shorter SR channel open times. The decreased Ca2+ release at each voltage is insufficient to activate Qγ in fibres lacking S100A1.