Quantifying Ca 2+ release and inactivation of Ca 2+ 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.