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

Abstract

To examine whether a capacitative Ca 2+ 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 Ca 2+ by repetitive treatments with high‐K+ solutions, initially in the absence and then in the presence of a sarcoplasmic/endoplasmic reticulum Ca 2+‐ATPase (SERCA) inhibitor. Depletion of the SR of Ca 2+ enabled us for the first time to show convincingly that the vast majority of the voltage‐sensitive Ca 2+ store overlaps the caffeine‐sensitive Ca 2+ 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 Ca 2+ influx pathway active enough to refill the depleted SR within several minutes was shown in skeletal muscle fibres. Ca 2+ entry was sensitive to Ni 2+, but resistant to nifedipine and was suppressed by plasma membrane depolarisation. Evidence for store‐operated Ca 2+ entry was provided by measurements of Mn 2+ entry. Significant acceleration of Mn 2+ entry was observed only when the SR was severely depleted of Ca 2+. The Mn 2+ influx, which was blocked by Ni 2+ but not by nifedipine, was inwardly rectifying, as is the case with the Ca 2+ entry. These results indicate that the store‐operated Ca 2+ entry is similar to the Ca 2+ release‐activated Ca 2+ channel (CRAC) current described in other preparations.