High intracellular [Ca2+] alters sarcoplasmic reticulum function in skinned skeletal muscle fibres of the rat

The effect on sarcoplasmic reticulum (SR) function of exposure to high intracellular [Ca 2+ ] was studied in mechanically skinned fibres from the extensor digitorum longus muscle of the rat, using caffeine to assay the SR Ca 2+ content. A 15 s exposure to 50 μ m Ca 2+ irreversibly reduced the ability of the SR to load/retain Ca 2+ and completely abolished depolarization-induced Ca 2+ release, whereas a 90 s exposure to 10 μ m Ca 2+ had no detectable effect on either function. The reduction in net SR Ca 2+ uptake: (a) was near-maximal with treatment at 50 μ m Ca 2+ , (b) was unrelated to voltage-sensor function, and (c) persisted unchanged for > 20 min. The reduction was primarily due to a threefold increase in leakage of Ca 2+ out of the SR. This increased leakage was not substantially blocked by the presence of 10 mM Mg 2+ or 2 μ m Ruthenium Red. The adverse effect on SR function of exposure to high [Ca 2+ ] could also be observed by the reduction in the ability of the SR to maintain a low [Ca 2+ ] within the skinned fibre in the face of elevated [Ca 2+ ] in the bathing solution. When bathed in a solution with ≈1.5 μ m Ca 2+ (0.75 mM CaEGTA-EGTA), skinned fibres produced only low force responses for many minutes, but after high [Ca 2+ ] treatment (15 s exposure to 50 μ m Ca 2+ ) they showed large, steady or oscillatory force responses. These findings indicate that, in addition to uncoupling the Ca 2+ release channels from the voltage sensors, exposure of skinned fibres to high [Ca 2+ ] causes a persistent increase in resting Ca 2+ efflux from the SR. Such efflux in an intact fibre would alter the distribution of Ca 2+ between the SR, the cytoplasm and the extracellular solution. These results may be relevant to the basis of low-frequency fatigue and possibly other conditions in muscle.