Interaction of Actomyosin and Adenosinetriphosphate as Studied With Glycerol-Extracted Cardiac Muscle Bundles

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Abstract
Glycerol-extracted muscle bundles were obtained from the right and left ventricles of dogs. Isometric tension developed on addition of adenosinetriphosphate (ATP) and resistance to stretch of the extracted bundles and the rate of ATP hydrolysis of the myofibrillar components of the bundle were studied. Effects of temperature, ionic strength, pH and concentrations of ATP, sodium, magnesium and calcium were evaluated. The results indicated that the contractile phenomenon producing tension is a complex of at least two underlying reactions one of which promotes contraction (R1) and the other (R2) relaxation. Which of these reactions limits tension development is determined by the concentration of ATP. The concentration at which the transition takes place between R1 as the limiting reaction to R2 is a function of temperature, ionic strength, pH and the concentration of calcium ions. R1 appears to be associated with the hydrolysis of ATP and this may be the energy-supplying reaction. Certain findings in regard to the effect of concentration of divalent cations on the contractile response and the rate of hydrolysis are difficult to reconcile, however, with the contention that R1 is a reaction in which ATP is hydrolyzed. At very low concentration of Mg and Ca, hydrolysis is strikingly inhibited though contraction takes place. Furthermore, concentrations of Mg of the order of 2 x 10–2 m inhibit enzymatic hydrolysis and promote contraction. Concentrations of Ca of 10–2 m inhibit contraction but accelerate ATP hydrolysis.