Rapid Ca 2+ flux through the transverse tubular membrane, activated by individual action potentials in mammalian skeletal muscle

Periods of low frequency stimulation are known to increase the net Ca 2+ uptake in skeletal muscle but the mechanism responsible for this Ca 2+ entry is not known. In this study a novel high‐resolution fluorescence microscopy approach allowed the detection of an action potential‐induced Ca 2+ flux across the tubular (t‐) system of rat extensor digitorum longus muscle fibres that appears to be responsible for the net uptake of Ca 2+ in working muscle. Action potentials were triggered in the t‐system of mechanically skinned fibres from rat by brief field stimulation and t‐system [Ca 2+ ] ([Ca 2+ ] t‐sys ) and cytoplasmic [Ca 2+ ] ([Ca 2+ ] cyto ) were simultaneously resolved on a confocal microscope. When initial [Ca 2+ ] t‐sys was ≥ 0.2 m m a Ca 2+ flux from t‐system to the cytoplasm was observed following a single action potential. The action potential‐induced Ca 2+ flux and associated t‐system Ca 2+ permeability decayed exponentially and displayed inactivation characteristics such that further Ca 2+ entry across the t‐system could not be observed after 2–3 action potentials at 10 Hz stimulation rate. When [Ca 2+ ] t‐sys was closer to 0.1 m m , a transient rise in [Ca 2+ ] t‐sys was observed almost concurrently with the increase in [Ca 2+ ] cyto following the action potential. The change in direction of Ca 2+ flux was consistent with changes in the direction of the driving force for Ca 2+ . This is the first demonstration of a rapid t‐system Ca 2+ flux associated with a single action potential in mammalian skeletal muscle. The properties of this channel are inconsistent with a flux through the L‐type Ca 2+ channel suggesting that an as yet unidentified t‐system protein is conducting this current. This action potential‐activated Ca 2+ flux provides an explanation for the previously described Ca 2+ entry and accumulation observed with prolonged, intermittent muscle activity.