Repriming and reversal of the isometric unexplained enthalpy in frog skeletal muscle.

1. heat production and high-energy phosphate hydrolysis by frog sartorius muscles in 6 s isometric tetani were measured to test the hypothesis that the isometric unexplained enthalpy (u.e.) and labile maintenance heat (l.m.h.) were of similar origin. Muscles were first given a conditioning tetanus to deplete the u.e. and l.m.h. A second (test) 6 s tetanus was given 6-300 s later to ascertain the extent to which l.m.h. and u.e. had recovered (reprimed). 2. The labile maintenance heat repriming was biphasic: 42% of the conditioning l.m.h. reprimed with a time constant of 10 s, the remainder with a time constant of 500 s. 3. The u.e. produced in the test tetanus 6 s after the conditioning tetanus was reduced to 18% of its conditioning value. By 30 s, u.e. had returned to conditioning values even though the amount of high-energy phosphate splitting was 17% less than that in the conditioning tetanus. 4. This observation is supported by measurements revealing that during the 30 s following a 6 s tetanus an amount of enthalpy was absorbed (less heat produced than expected from the measured metabolic changes) whose absolute value (after correction for oxygen consumption) was not different from the amount of unexplained enthalpy liberated during the tetanus. 5. The difference in repriming time course shows that l.m.h. and u.e. are not produced by the same reactions. The data are consistent with the hypothesis that calcium binding to troponin and parvalbumin produce u.e. while calcium binding to troponin and a non-linear time course of ATP hydrolysis produce l.m.h.