Depletion of Ca2+ in the sarcoplasmic reticulum stimulates Ca2+ entry into mouse skeletal muscle fibres

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Abstract
To examine whether a capacitative Ca2+ entry pathway is present in skeletal muscle, thin muscle fibre bundles were isolated from extensor digitorum longus (EDL) muscle of adult mice, and isometric tension and fura‐2 signals were simultaneously measured. The sarcoplasmic reticulum (SR) in the muscle fibres was successfully depleted of Ca2+ by repetitive treatments with high‐K+ solutions, initially in the absence and then in the presence of a sarcoplasmic/endoplasmic reticulum Ca2+‐ATPase (SERCA) inhibitor. Depletion of the SR of Ca2+ enabled us for the first time to show convincingly that the vast majority of the voltage‐sensitive Ca2+ store overlaps the caffeine‐sensitive Ca2+ store in intact fibres from mouse EDL muscle. This conclusion was based on the observation that both high‐K+ solution and caffeine failed to cause a contracture in the depleted muscle fibres. The existence of a Ca2+ influx pathway active enough to refill the depleted SR within several minutes was shown in skeletal muscle fibres. Ca2+ entry was sensitive to Ni2+, but resistant to nifedipine and was suppressed by plasma membrane depolarisation. Evidence for store‐operated Ca2+ entry was provided by measurements of Mn2+ entry. Significant acceleration of Mn2+ entry was observed only when the SR was severely depleted of Ca2+. The Mn2+ influx, which was blocked by Ni2+ but not by nifedipine, was inwardly rectifying, as is the case with the Ca2+ entry. These results indicate that the store‐operated Ca2+ entry is similar to the Ca2+ release‐activated Ca2+ channel (CRAC) current described in other preparations.