Relationship between ATPase activity and optical density during contraction and relaxation of myofibrils

Abstract

Contraction and relaxation of myofibrils, as induced by various ATP concentrations, has been studied by measuring simultaneously changes in optical density and ATPase activity. The optical density of myofibrils is increased by low ATP concentrations and decreased by high ATP concentrations. Changes in optical density and syneresis of myofibrils occur immediately after addition of ATP, and are accompanied by the high initial rate of ATPase activity. The ATPase activity of myofibrils is influenced by the ATP concentrations particularly during the initial period, and to a much lower extent after the occurrence of the physical changes in the myofibrils. Addition of Mg 2+ enhances all ATP effects, both on contraction and relaxation of myofibrils. Addition of Ca 2+ produces a small increase of the optical density but a powerful stimulation of the ATPase activity. Inhibition of SH groups produces relaxation of myofibrils even at low ATP concentrations. It is concluded that only the initial high rate of ATPase is expression of the mechano-chemical coupling process. The effects of Mg 2+ and Ca 2+ on this process are discussed.