ATP synthesis and heat production during Ca(2+) efflux by sarcoplasmic reticulum Ca(2+)-ATPase

Vesicles derived from the sarcoplasmic reticulum of rabbit white skeletal muscle were loaded with Ca(2+) and used to measure the rates of Ca(2+) efflux, heat production, and ATP synthesis from ADP and P(i). It was found that the Ca(2+)-ATPase can function in three different forms: (i) it absorbs heat from the medium (5 Kcal/mol Ca(2+)) when the efflux was coupled with ATP synthesis; (ii) it converts the energy derived from the gradient into heat (30 Kcal/mol Ca(2+)) when Mg(2+) is removed from the medium and the synthesis of ATP is impaired; and (iii) the ATPase becomes uncoupled when the different ligands of the enzyme are removed from the medium. As a result, there is no ATP synthesis and no heat production or absorption during Ca(2+) efflux. The Ca(2+) efflux, ATP synthesis, and heat production were inhibited by thapsigargin, a specific inhibitor of the Ca(2+)-ATPase.