[Overrelaxation of muscle models in media with high concentrations of calcium ions and exposed to MgATP in "contraction-inducing" concentrations].
Cross-striation pattern and sarcomere length in isolated myofibrils (both glycerinated and fresh) as well as isometric tension of glycerinated fibers of rabbit m. psoas are unaffected by an evaluation in ionic strength of CaCl2 up to 0.2 in the absence of ATP. An addition of MgATP (1 to 3mM) to the Ca2+ media induces the changes which have been shown to be characteristic of overrelaxation [1, 2]: A band shortening occurs followed by a complete plastification of the fibres. A tentative mechanism of the process is discussed in terms of spontaneous rearrangement of calcium myosinate packing in thick filaments that follows disrupting of rigor crossbridges with thin filaments under the action of ATP. Released calcium myosinate heads fail to form "active" bridges with actin; thick filaments undergo a conformational change resulted in their shattening due to increase in the equilibrium region of LMM tail overlap. The effects do not depend on ionic strength only: on replacing CaCl2 by KCl at equal ionic strength 0.2, an addition of ATP induces normal contraction instead of overrelaxation. A possibility is discussed that in a living muscle overrelaxation could provide a siding to prevent damage in case of emergency.
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D066298:In Vitro Techniques; D008274:Magnesium; D009119:Muscle Contraction; D009210:Myofibrils / Q000187:drug effects* / Q000648:ultrastructure; D011817:Rabbits
D000255:Adenosine Triphosphate; D008274:Magnesium
Sverkhrasslablenie myshechnykh modeleĭ v srede s vysokoĭ kontsentratsieĭ ionov kal'tsiia pri deĭstvii MgATF v "sokrashchaiushchikh" kontsentratsiiakh