Abstract

The contraction of both muscle and cell models is the result of the reaction of the contractile protein with ATP. The contraction may be studied by observation of the change in shape in the case of the cell models, of the superprecipitation of actomyosin gels, and of the unloaded shortening of actomyosin threads of glycerol-extracted fibres. It may also be studied by determining the tensionproduced by a model under isometric conditions, or by measuring the work done by a model. The extent of the contraction or the final tension, as well as the rate of shortening, may be investigated. It must be borne in mind that conditions affecting contraction have been shown to influence these reactions in a different way. Decrease of the Mg++ concentration, for instance, markedly affects the velocity of contraction of a muscle model, but has much less effect on the extent.

The contraction depends in a complex way on the active concentrations of the factors and cofactors of the system, such as actomyosin, ATP, Mg⁺⁺, ionic strength and pH. Any change influencing the concentration of one of them will indirectly affect the process (e.g., EDTA, hexametaphosphate, fluoride, etc.). Interpretation of the marked difference in the effects of various concentrations of ATP is complicated by the importance of the diameter of the models. A model containing a core which is free of ATP will develop a lower tension, or shorten more slowly, than will a thinner model. In a solution with a high concentration of ATP, the outer part of the model might react to a superoptimal concentration of ATP, while the center has not been reached by ATP.

The complexity of the reaction is also demonstrated in the behaviour of the enzymatic activity. With increasing ionic strength, the enzyme changes from a Mg⁺⁺-activated to a Mg⁺⁺-inhibited form. When, at low ionic strength, the actomyosin enzyme is present in its Mg⁺⁺-activated form, contraction is inhibited by Ca⁺⁺ if its concentration increases far above that of Mg⁺⁺.

The presence of the relaxing factors drastically changes the reaction of the system in that concentrations of ATP and Mg⁺⁺ which otherwise produce contraction bring about relaxation. While the addition of an ion, such as Ca⁺⁺ in low concentrations, has no influence on the contraction of pure actomyosin, calcium ion in the presence of a relaxing factor initiates contraction.

As previously stated, some substances act by altering the effective concentration of the components of the system, others attack the sulfhydryl groups, or the amino groups, thereby modifying the contraction. In addition, several other substances, e.g., urea, compounds containing sulfonic groups, have been found to influence contraction of the muscles reversibly by, as yet, unknown mechanisms.