The time course of ATP cleavage by contracting amphibian and mammalian skeletal muscles.

Abstract

The pattern of energy liberation by tetanically stimulated muscle is reasonably well defined. The energy liberation and ATPase rate are affected by a variety of mechanical factors which, aside from the case of shortening mammalian muscles, are well documented. The amount of ATP consumed by processes associated with the calcium release and sequestration is not trivial, amounting to 30-40% of the energy liberation in an isometric contraction. The energy liberated by isometric and shortening muscles is ultimately accounted for by known chemical reactions. However, there are two instances in which the time course of energy liberation does not correspond to the time course of high energy phosphate utilization. In an isometric tetanus, 30-40 mJ of energy per gram of muscle is produced by reactions probably associated with intracellular circulation of calcium, but not immediately involving ATP hydrolysis. Second, in rapidly shortening muscles, an unknown reaction can produce 6-7 mJ of energy per gram of muscle which is not immediately associated with high energy phosphate splitting. However, immediately after the cessation of shortening, this reaction is reversed by an ATP hydrolysis. Finally, a technique is now available which permits one to perform transient kinetic studies on skinned muscle fibers which are shortening and/or developing force. This development should enable the design of experiments in which the effect of mechanical conditions upon specific steps of the ATPase mechanism can be examined.