The Regulation of Metabolism and Energy Release in Contracting Muscle

Abstract

A brief review has been given of our present knowledge of the nature of the chemical events coupled with muscular contraction. Although there are experiments that fail to demonstrate a breakdown of ATP in the contraction of living muscle, these may yet yield to special explanations, leaving the conclusion that a breakdown of ATP is the reaction most closely related to the mechanical activity of the contractile structure. Owing to a rapid rephosphorylation reaction, this event appears in the form of a breakdown of phosphocreatine. The amount of energy liberated in contraction may be written as the sum of 3 quantities (a) activation energy, (b) shortening energy, and (c) work. It appears that, chemically, these correspond to 3 distinct quantities of phosphocreatine breakdown. The matter is complicated by the fact that the activation energy is not constant but may diminish as the muscle shortens. This may be little noticeable in frog sartorius muscle (dependent on the treatment) but appears to be pronounced in the heart, where it leads to the classical observation that work against a high pressure is performed less efficiently than work raised by ejections of an increased volume.