The mean rate of high-energy phosphate splitting during shortening near half-maximum velocity was about fivefold higher (1.60 mumol/g/s) than during the isometric tetanus (0.32 mumol/g/s).
In frog skeletal muscle shortening at half-maximal velocity, heat and work produced can be accounted for by simultaneous chemical changes, contrasting with shortening at Vmax.
Absolute Event Rate: 1.6% vs 0.32%
High-energy phosphate metabolism and energy liberated as heat and work were measured in 3-s tetani of frog sartorius muscle at 0 degree C. Two contraction periods were studied: (a) a 0.35-s period of shortening near half-maximum velocity beginning after 2 s of isometric stimulation, and (b) a 0.65-s isometric period immediately following the shortening. There were no significant changes in levels of ATP, ADP, or AMP in the two contraction periods. The observed changes in inorganic phosphate and creatine levels indicated that the only significant reaction occurring was phosphocreatine splitting. The mean rate of high-energy phosphate splitting during the shortening, 1.60 +/- 0.23 mumol X g-1 X s-1 (n = 24), was about fivefold higher than that in the 1-s period in the isometric tetanus, 0.32 +/- 0.11 mumol X g-1 X s-1 (n = 17), observed in our previous study. The mean rate in the post-shortening period, 0.46 +/- 0.13 mumol X g-1 X s-1 (n = 17), was not significantly different from that in the 1-s period in the isometric tetanus. A large amount of heat plus work was produced during the shortening period, and this could be accounted for by simultaneous chemical changes. In the post-shortening period, the observed enthalpy was also accounted for by simultaneous chemical reactions. Thus, the present result is in sharp contrast to that obtained from a similar study performed at a shortening at Vmax, where an enthalpy excess was produced during shortening and an enthalpy deficit was produced during the period following the shortening.
Homsher et al. (Sat,) conducted a other in Frog skeletal muscles. Shortening near half-maximum velocity vs. Isometric period was evaluated on Rate of high-energy phosphate splitting. The mean rate of high-energy phosphate splitting during shortening near half-maximum velocity was about fivefold higher (1.60 mumol/g/s) than during the isometric tetanus (0.32 mumol/g/s).