High‐energy phosphate metabolism and energy liberation associated with rapid shortening in frog skeletal muscle

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Abstract
High-energy phosphate metabolism and energy liberated as heat and work were measured in 3 s tetani of frog sartorius muscles at 0.degree. C. Three contraction periods were studied: shortening at near-maximum velocity for 0.3 s from sarcomere length 2.6-1.8 .mu.m, beginning after 2 s of isometric stimulation, the 0.7 s isometric period immediately following such rapid shortening, the period from 2-3 s in an isometric tetanus at sarcomere length 1.8 .mu.m. There were no significant changes in levels of ATP, ADP or AMP in any contraction period. The observed changes in Pi and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting. The mean rate of high-energy phosphate splitting during rapid shortening, 0.48 .+-. 0.24 .mu.mol/g.cntdot.s (mean .+-. standard error of mean, n = 29; g refers to blotted muscle weight), did not significantly differ from that in the 1 s period in the isometric tetanus, 0.32 .+-. 0.11 .mu.mol/g.cntdot.s (n = 17). The mean rate in the post-shortening period, 0.71 .+-. 0.10 .mu.mole/g.cntdot.s (n = 22), was greater than that in the 1 s period in the isometric tetanus; this difference is significant (P < 0.02, t test). A large quantity of heat plus work was produced during the rapid shortening period, but < 1/2 of this could be accounted for by simultaneous chemical reactions. The unexplained enthalpy production was 6.5 .+-. 2.6 mJ/g (mean .+-. standard error of mean). No significant unexplained enthalpy was produced in the 1 s period in the isometric tetanus. In the post-shortening period the observed enthalpy was less, by 6.2 .+-. 2.6 mJ/g, than that expected from the simultaneous chemical reactions. The results are interpreted in terms of an exothermic shift in the population of cross-bridge states during rapid shortening. A relatively slow subsequent step apparently prevents many of these cross-bridges from completing the cycle and splitting ATP until after the end of shortening.