Relaxing Factors in Muscle

Journal/Book: Reprinted from Nature Vol. 172 p. 84 July 11 1953. 1953;

Abstract: Institute for Muscle Research Marine Biological Laboratory Woods Hole Massachusetts. Jan. 5. ACCORDING to Szent-Györgyi1 muscle in the relaxed state is characterized by the dissociation of actomyosin into actin and myosin. One of us (M. C. G.)2 using the glycerated fibre preparation1 of rabbit psoas muscle has confirmed this by showing that ionic media which produce dissociation of actomyosin3 cause a reduction in elastic modulus of the fibres under isometric conditions although in all these cases the conditions are unphysiological. Elastic modulus here is a well-defined quantity well-washed and preserved fibres having Hookean elasticity which corresponds to the value found by Hill4 for intact muscle in tetanus and is some five times greater than for the same muscle in the reating state. An alternative proof is given by the temperature dependence of tension (Varga5). Fibres under isometric tension in the presence of adenosine triphosphate are relaxed by reducing the temperature actomyosin formation being an endothermie reaction and there is also a corresponding reduction in elastic modulus as measured by quick stretch or release. The question was then : What makes a reversible dissociation under physiological conditions ? Attempting to answer this it was found that a watery extract (1 : 1 by weight) of fresh muscle mince contained a factor which in presence of adenosine triphosphate (4 mM) has a powerful dissociating action judged by complete relaxation of the fibre and fall of elastic modulus although the total ionic strength was only 0·05 M (adenosine triphosphate only dissociates at ionic strength greater than 0·25). Further this action was reversible with pH the range 6·0-7·5 sufficing for complete relaxation and contraction respectively. We then began a systematic investigation of this factor from muscle extract using the pH dependence of isometric tension of glycerated fibres as a quantitative method of assay the variation of pH being made by a current of carbon dioxide or nitrogen alternately and the percentage relaxation from maximum tension being taken as measure of activity. We finally obtained a crystalline substance (as calcium salt from ethanol and recrystallized four times) which was analysed as creatine phosphate and had the following properties : (1) 10 mM gives maximal activity ; (2) 2 m.equiv. magnesium ions is necessary ; (3) 0·8 m.equiv. calcium ions inhibits completely ; (4) pH range required 6·2-6·6. ... ___MH

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