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.
Kurebayashi
N
N
Department of Pharmacology, Juntendo University School of Medicine, Tokyo 113-8421, Japan. nagomik@med.juntendo.ac.jp
Ogawa
Y
Y
D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006873:Hydroquinones / Q000494:pharmacology; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008345:Manganese / Q000493:pharmacokinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019284:Thapsigargin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D004791:Enzyme Inhibitors; D006873:Hydroquinones; D007211:Indoles; C019359:2,5-di-tert-butylhydroquinone; D008345:Manganese; D019284:Thapsigargin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D009543:Nifedipine; D011188:Potassium; D002118:Calcium; C000543:cyclopiazonic acid