Na⁺はスキンドフェレット心筋におけるCa²⁺誘発Ca²⁺放出を促進する

抄録

It is well known that Mg²⁺ inhibits Ca²⁺-induced Ca²⁺ release (CICR) (physiological Ca²⁺ release mechanism) in sarcoplasmic reticulum (SR) of cardiac muscles. Previously, we suggested that intracellular Na⁺ enhances Ca²⁺ release from SR in aequorin-injected ferret ventricular muscles. In the present study, we directly demonstrate the enhancement of CICR by Na⁺ using skinned myocardium. We measured the relative amount of Ca²⁺ released by Ca²⁺ (2 min application of solutions at pCa7-4) from SR using a fluorescent Ca²⁺ indicator (fluo-3) in saponin-treated ferret thin ventricular muscle bundles. Ca²⁺ was released in the solutions containing 1.5 mM free-Mg²⁺ with or without Na⁺. Na⁺ was replaced with three monovalent cations (K⁺, Li⁺, N-methyl-D-glucamine⁺). We also measured Ca²⁺ release in Mg²⁺-free solutions with or without Na⁺. The amount of released Ca²⁺ was normalized to that by 50 mM caffeine which releases all Ca²⁺ in the SR. The amount of Ca²⁺ released by Ca²⁺ in the presence of 100% Na⁺ was higher than that in Na⁺-free solutions containing other monovalent cations. There were no significant differences in the relative amount of Ca²⁺ released by Ca²⁺ (pCa6) among these three different cations. CICR was inhibited by Mg²⁺. However, the inhibitory effect of Mg²⁺ on CICR disappeared in the solutions with 100% Na⁺, i.e., in the absence of Mg²⁺, the enhancement of CICR by Na⁺ was not observed. We conclude that Na⁺ might act on the same site where Mg²⁺ binds to inhibit CICR. [Jpn J Physiol 55 Suppl:S91 (2005)]