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

1.To examine whether a capacitative Ca2+ entry pathway is present in skelet al muscle, thin muscle fibre bundles were isolated fr om extensor digitorum longus (EDL) muscle of adult mice, and isometric tension and fura-2 signal s were simultaneously measured. 2. The sarcoplasmic reticulum (SR) in the muscle fibres was successfully de pleted of Ca2+ by repetitive treatments with high-K+ solutions, initially i n the absence and then in the presence of a sarcoplasmic/endoplasmic reticu lum Ca2+-ATPase (SERCA) inhibitor. 3. Depletion of the SR of Ca2+ enabled us for the first time to show convin cingly that the vast majority of the voltage-sensitive Ca2+ store overlaps the caffeine-sensitive Ca2+ store in intact fibres from mouse EDL muscle. T his conclusion was based on the observation that both high-K+ solution and caffeine failed to cause a contracture in the depleted muscle fibres. 4. The existence of a Ca2+ influx pathway active enough to refill the deple ted SR within several minutes was shown in skeletal muscle fibres. Ca2+ ent ry was sensitive to Ni2+, but resistant to nifedipine and was suppressed by plasma membrane depolarisation. 5. Evidence for store-operated Ca2+ entry was provided by measurements of M n2+ entry. Significant acceleration of Mn2+ entry was observed only when th e SR was severely depleted of Ca2+. The Mn2+ influx, which was blocked by N i2+ 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 si milar to the Ca2+ release-activated Ca2+ channel (CRAC) current described i n other preparations.