Molecular operations of the sodium–calcium exchanger revealed by conformation currents

Abstract

THE sodium-calcium exchanger is critical in the normal functioning of many cells^1,2. In heart muscle, it is the principal way by which the cells keep the concentration of intracellular calcium low, pumping out the Ca²⁺ that enters the cytosol through L-type Ca²⁺ channels^3,4. The exchanger may also contribute to the triggering of Ca²⁺ release during voltage-activated excitation–contraction coupling in heart^5,6. Time resolved examination of the conforma-tional changes of macromolecules in living cells has so far been largely restricted to ion-channel proteins whose gating is voltage-dependent^7,8. We have now directly measured electrical currents arising from the molecular rearrangements of the sarcolemmal Na–Ca exchanger. Changes in the conformation of the exchanger protein were activated by a rapid increase in the intracellular calcium concentration produced by flash photolysis of caged calcium⁹ in voltage-clamped heart cells. Two components of membrane current were produced, reflecting a calcium-dependent con-formational change of the transporter proteins and net transport of ions by the exchanger. The properties of these components provide evidence that the Na–Ca exchanger protein undergoes two consecutive membrane-crossing molecular transitions that each move charge, and that there are at least 250 exchangers per µm² turning over up to 2,500 times per second.