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

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Abstract
Measurements were made of the extents of ATP splitting and the creatine kinase reaction and the heat + work (h + w) produced during 5 s isometric tetani of frog semitendinosus muscle at 0.degree. C. A comparison was made of tetani at 2 different muscle lengths. These lengths were l0 (sarcomere length 2.3 .mu.m before stimulation), which is near the optimum for interaction of actin and myosin, and lmax (sarcomere length 3.8 .mu.m) at which actin-myosin interaction is largely prevented. 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. The main new finding is that a significant amount of unexplained energy is produced at lmax where there is a negligible amount of actin-myosin interaction, suggesting that the unexplained energy cannot be due solely to actin-myosin interaction. On average less unexplained energy is produced at lmax than at l0. The process (or 1 of the processes) producing this energy may be dependent on muscle length. The observed h + w was divided on the basis of its time course into the 2 parts, labile and stable, 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 is influenced by muscle length in 2 ways: the total amount and the rate of its production are significantly smaller at lmax than at l0. At both lengths the labile H + w is equal, within experimental error, to the unexplained h + w.