Unexplained enthalpy production in contracting skeletal muscles.

Energy balance experiments, testing the hypothesis that the observed enthalpy production can be accounted for by high-energy phosphate splitting, have shown that two additional sources of heat production not directly associated with high-energy phosphate splitting per se can be observed during contraction. The first is an energy production occurring during the first 5 s of a maintained isometric tetanus (0 C), which amounts to about 25 mJ/g of muscle. Because the amount of this unexplained enthalpy is not altered by varying the amount of thick and thin filament overlap, it is suggested that it is associated with Ca2+ redistribution (and related binding) within the muscle cell during contraction. The second type of unexplained enthalpy production, amounting to 5-8 mJ/g, occurs when thick and thin filaments slide rapidly past one another. It is suggested that its production is associated with a redistribution of cross-bridge states upon transition from isometric to rapidly shortening contraction. Both types of unexplained enthalpy production are thought to be reversed by an ATP splitting subsequent to their production.