Control of energy fluxes by the sarcoplasmic reticulum Ca2+-ATPase: ATP hydrolysis, ATP synthesis and heat production.

The experiments described indicate that heat is released when Ca2+ leaks through the Ca2+-ATPase of sarcoplasmic reticulum vesicles. In the presence of a transmembrane Ca2+ concentration gradient, agents that modify the amount of ATP synthesized from ADP and Pi also modify the amount of heat produced by the hydrolysis of each ATP molecule. Thus, in the presence of heparin, less ATP is synthesized and more heat is produced. Conversely, with dimethyl sulfoxide more ATP is synthesized and less heat is produced. The data indicate that between limits (-10 to -30 kcal/mol) the Ca2+-ATPase can regulate the interconversion of energy in such a way as to vary the fraction of energy derived from ATP hydrolysis which is converted into heat and that which is converted into other forms of energy.