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

Abstract

Since the pioneering studies of Ringer of almost a century ago^1,2, 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 evidence^10–14 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⁺.