The efficiency of contraction in rabbit skeletal muscle fibres, determined from the rate of release of inorganic phosphate

1 The relationship between mechanical power output and the rate of ATP hydrolysis was investigated in segments of permeabilized fibres isolated from rabbit psoas muscle. 2 Contractions were elicited at 12 °C by photolytic release of ATP from the P 3 ‐1‐(2‐nitrophenyl)ethyl ester of ATP (NPE‐caged ATP). Inorganic phosphate (P i ) release was measured by a fluorescence method using a coumarin‐labelled phosphate binding protein. Force and sarcomere length were also monitored. 3 ATPase activity was determined from the rate of appearance of P i during each phase of contraction. The ATPase rate was 10.3 s −1 immediately following release of ATP and 5.1 s −1 during the isometric phase prior to the applied shortening. It rose hyperbolically with shortening velocity, reaching 18.5 s −1 at a maximal shortening velocity > 1 ML s −1 (muscle lengths s −1 ). 4 Sarcomeres shortened at 0.09 ML s −1 immediately following the photolytic release of ATP and at 0.04 ML s −1 prior to the period of applied shortening. The high initial ATPase rate may be largely attributed to initial sarcomere shortening. 5 During shortening, maximal power output was 28 W l −1 . Assuming the free energy of hydrolysis is 50 kJ mol −1 , the efficiency of contraction was calculated from the power output at each shortening velocity. The maximum efficiency was 0.36 at a shortening velocity of 0.27 ML s −1 , corresponding to a force level 51 % of that in the isometric state. 6 At the maximal shortening velocity, only 10 % of the myosin heads are attached to the thin filaments at any one time.