Role of adenosine triphosphate in the relaxation of skeletal muscle myofibrils

Abstract

Recent studies of relaxation have shown that, in a calcium-depleted system, the ATPase of actomyosin is inhibited and syneresis is delayed. The hydrolysis of ATP during such a delay must somehow cause the onset of syneresis, and it probably does so by lowering the ATP concentration, although the liberated ADP and inorganic phosphate have also been reported to play a role. We have re-examined the roles of ATP and ADP by directly measuring the ATP concentration remaining at the moment when myofibrils flocculate. Our results indicate that the delay in flocculation represents primarily the time required for the concentration of ATP to fall below a critical “threshold” level which is required for relaxation, although added ADP appears to increase this threshold level. We have also found that, when “relaxation” is promoted by an increase in the concentration of EGTA, relaxing-factor granules, KCl, or MgCl 2, the threshold ATP level is correspondingly reduced. We offer an interpretation of these results based on the postulate that the actin-myosin interaction is inhibited by the binding of ATP to a “relaxing” site on the myosin, distinct from the ATPase site.