ATP synthesis and heat production during Ca2+ efflux by sarcoplasmic reticulum Ca2+-ATPase

Vesicles derived from the sarcoplasmic reticulum of rabbit white skeletal muscle were loaded with Ca2+ and used to measure the rates of Ca2+ efflux, heat production, and ATP synthesis from ADP and P-i. It was found that the Ca2+-ATPase can function in three different forms: (i) it absorbs heat from the medium (5 Kcal/mol Ca2+) when the efflux was coupled with ATP synthesis; (ii) it converts the energy derived from the gradient into heat (30 Kcal/mol Ca2+) when Mg2+ 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 Ca2+ efflux. The Ca2+ efflux, ATP synthesis, and heat production were inhibited by thapsigargin, a specific inhibitor of the Ca2+-ATPase. (C) 2000 Academic Press.