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

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.