An ATP-dependent Ca2+-pumping system in dog heart sarcolemma.

Abstract

Since the pioneering studies of Ringer of almost a century ago1,2, it has been known that the contraction of heart cells requires the presence of calcium in the extracellular medium. Ca2+ penetrates into the sarcoplasm where it either directly activates troponin C (ref. 3), or indirectly conveys the contraction signal to the myofibrils, possibly by inducing Ca2+ release from intracellular stores (sarcoplasmic reticulum, SR)4–6. After contraction Ca2+ must obviously leave the heart cell again to prevent its progressive accumulation. As the electrochemical potential of Ca2+ across the sarcolemma would prevent its passive outflow, the efflux of Ca2+ is an energy requiring process. Most of the Ca2+ is apparently ejected from heart cells in exchange for Na+ (refs 7, 8), on an electrophoretic antiporter which exchanges three Na+ ions for one Ca2+ (ref. 9). However, there is some evidence10–14 of a specific ATPase in cardiac sarcolemma, which could also have a role in the energy-dependent ejection of Ca2+. One important difficulty in trying to establish the existence of a specific Ca2+-ATPase in cardiac sarcolemma lies in the fact that most of the preparations used in such studies have been heavily contaminated by other sub-cellular fractions (mitochondria, SR) which also possess ATP-dependent Ca2+ transporting systems. In the present work we describe an ATP-dependent Ca2+ transport system which can be conclusively ascribed to heart sarcolemma. We show that a heart vesicular preparation particularly enriched in sarcolemma markers accumulates Ca2+ in the presence of ATP. That the process takes place in sarcolemmal vesicles is demonstrated by the fact that the accumulated Ca2+ is promptly and completely released in exchange for Na+.