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

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.