Is the Contracting Muscle in a new elastic equilibrium?

Abstract

The contractile system can be isolated in two forms which are able to act like the living muscle : (1) as the water-glycerine-extracted muscle fibre, and (2) as the oriented actomyosin thread. The extensibility of the extracted fibre is similar in all conditions to that of the intact fibre. Both pass into a state of rigor when they do not contain ATP and extensibility is then very small; during a slow small stretch the modulus of elasticity has a value between 5000 and 10000 g/cm 2 . On the other hand, with a normal content of ATP (5 x 10 -3 M), the extensibility both of the intact and of the extracted fibre is 10 to 50 times larger (elastic modulus between 100 and 1000 g/cm 2 ), provided that contraction does not occur. If, however, the living or the extracted fibre containing ATP is in a contracting condition the extensibility is again smaller (elastic modulus nearly half as large as in rigor). The extracted fibre and the oriented actomyosin thread contract on treatment with ATP, or ITP, with the same amount of shortening and the same dependence on temperature as are characteristic of the living muscle. The extracted fibre thereby develops the tension of a maximal tetanic contraction (4 Kg/cm 2 ) of a skeletal muscle. Both models show the same gradual recovery of tension after release, as does the living muscle. During contraction the birefringence of the extracted fibre is diminished by the same amount as is that of the living fibre. Both systems when contracted by treatment with ATP relax again if ATP splitting is stopped and if rigor is avoided. That is to say, they relax in spite of the presence of ATP when its breakdown is inhibited by adding appropriate ATP-ase-poisons (e. g. salyrgan). They relax also if ATP is washed out provided that rigor is prevented by the presence of pyrophosphate, triphosphate or ADP. If the poison is removed, or if ATP is added again, they contract anew. Thus ATP has two effects: (1) it makes actomyosin more plastic and more extensible ; in this it can be replaced by other poly orthophosphates; (2) as an operative substance ATP causes contraction and provides the energy needed for the working cycle; in this it can be replaced only by the chemically closely related ITP which can also be split by actomyosin. The experimental separation of the working cycle from the other cycles of the living muscle fibre (excitation, recovery) shows that relaxation is the thermodynamically spontaneous phase of the working cycle, while contraction occurs only when free energy is supplied to the contractile mechanism by ATP splitting. If so, muscle contraction cannot be a thermokinetic process, for then energy would be required to stretch again the statistically coiled molecules. The dependence on temperature of the tension during contraction is apparently the dependence of a steady state of ATP splitting and is not a sign of a change of elastic equilibrium.