Effect of muscle length on energy balance in frog skeletal muscle.

1. Measurements were made of the extents of ATP splitting and the creatine kinase reaction and the heat + work (h+w) produced during 5s isometric tetani of frog semitendinosus muscle at 0 degrees C. A comparison was made of tetani at two different muscle lengths. These lengths were l0 (sarcomere length 2.3 micrometers before stimulation), which is near the optimum for interaction of actin and myosin, and lmax (sarcomere length 3.8 micrometers) at which actin-myosin interaction is largely prevented. 2. As in earlier studies of muscle at l0, the observed h+w was significantly greater than the amount explained by the energy from ATP splitting and the creatine kinase reaction. Our main new finding is that a significant amount of unexplained energy is also produced at lmax where there is a negligible amount of actin-myosin interaction. This suggests that the unexplained energy cannot be due solely to actin-myosin interaction. 3. On average less unexplained energy is produced at lmax than at l0. Thus it seems likely that the process (or one of the processes) producing this energy is dependent on muscle length. 4. The observed h+w was divided on the basis of its time course into the two parts, labile and stable, which were defined by Aubert (1956). The labile part of the h+w has an exponentially declining rate, and the stable part has a constant rate. The production of labile h+w influenced by muscle length in two ways: the total amount and the rate of its production are significantly smaller at lmax than at l0. 5. At both lengths the labile h+w is equal, within experimental error, to the unexplained h+w.