Relation between adenosinetriphosphate activity and sarcomere length in stretched glycerol-extracted frog skeletal muscle.

Abstract

A now familiar feature of myofibrillar structure is the interdigitating array of thick and thin filaments first described by Hanson and Huxley.' Based on this structural feature, a model of muscular contraction has been proposed' which is briefly as follows. Contraction takes place by sliding of thin filaments (I filaments) toward the center of each sarcomere through interaction with thick filaments (A filaments) at projections on the latter termed bridges. Adenosinetriphosphate (ATP) is hydrolyzed at the bridges, and the interaction of thin filaments at these sites accelerates the rate of hydrolysis. The thick filaments are composed of myosin and the thin filaments, at least partially, of actin. Huxley3, 4 has summarized a number of lines of evidence which support this model of the contractile mechanism. Experiments on isolated contractile proteins of skeletal muscle indicate that actin greatly enhances the magnesium-activated adenosine triphosphatase (ATPase) activity of myosin which has a pH optimum at pH 7.3.5 We thought it would be of interest, in relation to the model of contraction proposed by Huxley, to examine the relationship between extent of overlap of thick and thin myofilaments and the rate of Mg++ activated ATP hydrolysis at pH 7.3. Glycerol-extracted skeletal muscle bundles were used, following the method of preparation introduced by SzentGy6rgyi.6 These bundles contract in the presence of ATP, and the amount of tension developed is approximately that developed by the intact muscle on excitation.6 ATPase activity was assayed in bundles of approximately rest length and in ones stretched to various lengths beyond rest length. Sarcomere lemngths in unfixed and uncontracted bundles were estimated by phase microscopy. The amount of myofilament overlap was gauged by electron microscopic examination of sections taken at random points along the length of representative bundles included in the