Mechanochemical coupling in muscle: Attempts to measure simultaneously shortening and ATPase rates in myofibrils

We studied the ATPase of shortening myofibrils at 4 degrees C by the rapid flow quench method. The progress curve has three phases: a P-i burst, a fast linear phase k(F) of duration t(B), and a deceleration to a slow k(S). We propose that k(F) is the ATPase of myofibrils shortening under zero external load; at t(B) shortening and ATPase rates are reduced by passive resistance. The total ATP consumed during the rapid shortening is ATP(c). Our purpose was to obtain information on the myofibrillar shortening velocity from their ATPase progress curves. We tested t(B) as an indicator of shortening velocity by determining the effects of different probes upon it and the other ATPase parameters. The dependence of t(B) upon the initial sarcomere length was linear, giving a shortening velocity close to that of muscle fibres (V-o). The K-m of ATP was larger for t(B) than for k(F), as found with fibers for V-o and their ATPase. ADP and 2,3-butanedione monoxime, but not P-i, inhibited t(B) to the same extent as V-o. The Delta H-double dagger for t(B) and V-o were similar. ATP, was independent of the sarcomere length, implying that the more the myofibrils shorten, the less ATP expended per myosin head per micron shortened. We propose that t, can be used as an indicator for myofibrillar shortening velocities.