Ionophoretic model for Na–Ca counter transport

Abstract

IN most mammalian cells the cytosolic concentration of free ionised Ca²⁺ is thought to approximate 10⁻⁶–10⁻⁷ M, whereas the extracellular Ca²⁺ concentration is close to 10⁻³ M (ref. 1). The efflux of Ca²⁺ across the cell membrane, which takes place against an electrochemical gradient, is apparently mediated either by a Ca²⁺-stimulated ATPase² or by a Na–Ca counter transport process. In the latter process, the entry of Na⁺ into the cell, which is a passive event driven by the electrochemical gradient of Na⁺ across the plasma membrane, is thought to be coupled with the uphill outwards transport of Ca²⁺ (ref. 3). Although a possible modulatory effect of ATP on such a process has been suggested, it is generally assumed that the Na-Ca counter transport mechanism does not consume ATP (discussed in ref. 4). Because several passive ionic movements (such as the influx of Na⁺ or Ca²⁺ into the cell) could be mediated by native ionophoretic molecules, we have explored whether it was possible, using an artificial system for the study of ionophoresis, to reproduce the major characteristics of the Na–Ca counter transport process. We report here our success.