Ni2+ transport by the human Na+/Ca2+ exchanger expressed in Sf9 cells

The mechanism of Ni2+ block of the Na+/Ca2+ exchanger was examined in Sf9 c ells expressing the human heart Na+/Ca2+ exchanger (NCX1-NACA1). As predict ed from the reported actions of Ni2+, its application reduced extracellular Na+-dependent changes in intracellular Ca2+ concentration (measured by flu e 3 fluorescence changes). However, contrary to expectation, the reduced fl uorescence was accompanied by measured Ni-63(2+) entry. The Ni-63(2+) entry was observed in Sf9 cells expressing the Na+/Ca2+ exchanger but not in con trol cells. The established sequential transport mechanism of the Na+/Ca2exchanger could be compatible with these results if one of the two ion tran slocation steps is blocked by Ni2+ and the other permits Ni2+ translocation . We conclude that, because Ni2+ entry was inhibited by extracellular Ca2and enhanced by extracellular Na+, the Ca2+ translocation step moved Ni2+, whereas the Na+ translocation step was inhibited by Ni2+. A model is presen ted to discuss these findings.