Quantifying Ca²⁺release and inactivation of Ca²⁺release in fast‐ and slow‐twitch muscles

Key points Calcium ions initiate muscle contraction and are an important determinant of the force and energy cost of contraction. Existing estimates of the amount of calcium released into mammalian muscle cells are uncertain and existing descriptions of the effects of prior stimulation on calcium release are inconsistent. Calcium release was quantified by measuring the amount of ATP used to pump Ca 2+ back to its pre‐release storage site. Calcium release was greater in fast‐ than slow‐twitch muscle, was reduced immediately following a stimulus and recovered with a time course that depended on muscle type and temperature. Ca 2+ release and inactivation in fast‐twitch muscle resembles that of amphibian muscle but that of slow‐twitch muscle is distinctive in exhibiting a slower and less temperature‐sensitive reversal. Abstract The aims of this study were to quantify the Ca 2+ release underlying twitch contractions of mammalian fast‐ and slow‐twitch muscle and to comprehensively describe the transient inactivation of Ca 2+ release following a stimulus. Experiments were performed using bundles of fibres from mouse extensor digitorum longus (EDL) and soleus muscles. Ca 2+ release was quantified from the amount of ATP used to remove Ca 2+ from the myoplasm following stimulation. ATP turnover by crossbridges was blocked pharmacologically ( N ‐benzyl‐ p ‐toluenesulphonamide for EDL, blebbistatin for soleus) and muscle heat production was used as an index of Ca 2+ pump ATP turnover. At 20°C, Ca 2+ release in response to a single stimulus was 34 and 84 μmol (kg muscle) −1 for soleus and EDL, respectively, and increased with temperature (30°C: soleus, 61 μmol kg −1 ; EDL, 168 μmol kg −1 ). Delivery of another stimulus within 100 ms of the first produced a smaller Ca 2+ release. The maximum magnitude of the decrease in Ca 2+ release was greater in EDL than soleus. Ca 2+ release recovered with an exponential time course which was faster in EDL (mean time constant at 20°C, 32.1 ms) than soleus (65.6 ms) and faster at 30°C than at 20°C. The amounts of Ca 2+ released and crossbridge cycles performed are consistent with a scheme in which Ca 2+ binding to troponin‐C allowed an average of ∼1.7 crossbridge cycles in the two muscles.