Correlations of length and volume measurements in myofibril suspensions.

Instrumentation has been developed for the rapid electronic sizing of large numbers of myofibrils. The response of myofibrils in the presence of ATP to changes in Ca(++) concentration was examined. Shortening of myofibrils upon addition of Ca(++) was accompanied by an increased protein effective volume of approximately 10-40%. Whereas ATPase activation and increased turbidity of myofibrils upon addition of Ca(++) were reversible upon subsequent addition of EGTA, the shortening and swelling were irreversible. It is proposed that the swelling may result from the breaking of hydrophobic bonds within myosin. The ATPase activity and turbidity are measures of the input, while the shortening and swelling are measures of the output of a coupled nonequilibrium process; failure of reversal of the output indicates an uncoupling under the experimental conditions.