Force decline during muscle relaxation promotes calcium release to the cytosol.
During relaxation of skeletal muscle, an initial rapid decline of intracellular Ca2+ concentration ([Ca2+]i) (phase 1), is followed by a brief phase in which the decline of [Ca2+]i is markedly slowed or even reversed (phase 2). Phase 2 appears as a prominent "bump" on records of the time course of declining [Ca2+]i during relaxation. The goal of this study was to test the hypothesis that phase 2 represents a release of Ca2+ to the cytosol that occurs with net cross-bridge detachment during relaxation. The experimental approach was to measure [Ca2+]i with indo 1 in stimulated bullfrog semitendinosus muscles and to determine if phase 2 was diminished during relaxation of contractions in which cross-bridge interactions had been reduced by two different methods: 1) stretching muscles to reduce the overlap between actin and myosin filaments or 2) decreasing stimulus duration. The results showed that, when either method was used to reduce cross-bridge interactions during contraction, then the size of phase 2 during relaxation was also decreased. Phase 2 was eliminated during relaxation of contractions in which cross-bridge interactions had been reduced to a lower contraction force approximately 30% of maximum. These findings are consistent with the hypothesis that the phase 2 of [Ca2+]i decline represents a release of Ca2+ to the cytosol that occurs with net cross-bridge detachment during relaxation. This conclusion is consistent with previous studies that suggest that cross-bridge detachment lowers the affinity of troponin for Ca2+.
Baker
A J
AJ
Magnetic Resonance Unit, Department of Veterans Affairs Medical Center, San Francisco, California, USA.
Weiner
M W
MW
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009218:Myosins / Q000378:metabolism*; D011892:Rana catesbeiana; D012518:Sarcomeres / Q000502:physiology*; D013314:Stress, Mechanical; D013997:Time Factors
D000199:Actins; D009218:Myosins; D002118:Calcium