Muscle enthalpy production and its relationship to actomyosin ATPase.

During a maintained tetanus most of the energy liberated is produced by ATP hydrolysis at the cross-bridge. The energy produced by ATP hydrolysis by the sarcoplasmic reticulum is, however, a significant fraction (0.35) of the total. The energy liberation rate depends on a variety of factors (including the shortening velocity, sarcomere length, temperature, fiber type, and duration of contraction) and is, for the most part, consistent with current cross-bridge models. At the beginning of tetanic stimulation, a significant amount of heat (approximately 30 mJ/g) is produced by calcium binding reactions in the sarcoplasm. In the transition from an isometric to a shortening contraction, the cross-bridge cycling rate increases, and as much as 6 mJ/g of unexplained heat is produced. This unexplained heat appears to involve enthalpy changes accompanying a redistribution of cross-bridge intermediates, and it is reversed by high-energy phosphate splitting after the cessation of shortening. The mechanistic significance of these enthalpy changes remains to be elucidated.