The purpose of this study is to investigate the mechanism underlying sarcoplasmic reticulum (SR) Ca2+ leakage at recovery phase after in vivo contractions. Rat gastrocnemius muscles were electrically stimulated in vivo, and then mechanically skinned fibers were prepared from the muscles excised 30 min after repeated high-intensity contractions. SR Ca2+ leakage was increased in the skinned fibers from stimulated muscles. Thereafter, SR Ca2+ leakage in skinned fibers was measured (1) under a continuously depolarized condition and (2) in the presence of nifedipine in the sealed transverse tubular system. In either of the two conditions, SR Ca2+ leakage in the rested fibers reached a level similar to that in the stimulated fibers. Furthermore, 1 mM tetracaine (Tet) treatment, but not 3 mM Mg2+ (3 Mg) treatment, lessened SR Ca2+ leakage in stimulated fibers. Depolarization-induced force in skinned fibers was more greatly decreased by Tet treatment than by 3 Mg treatment (92% reduction in Tet versus 31% reduction in 3 Mg), whereas caffeine-induced force in skinned fibers was similarly decreased by either treatment (73% reduction in Tet versus 75% reduction in 3 Mg). This difference indicates that Tet exerts a greater inhibitory effect on the dihydropyridine receptor (DHPR) signal to ryanodine receptor (RYR) than 3 Mg, although their inhibitory effects on RYR are almost similar. These results suggest that the increased Ca2+ leakage after muscle contractions is mainly caused by the orthograde signal of DHPRs to RYRs.

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