Description:
<jats:p>The mechanism of Ni<jats:sup>2+</jats:sup>block of the Na<jats:sup>+</jats:sup>/Ca<jats:sup>2+</jats:sup>exchanger was examined in Sf 9 cells expressing the human heart Na<jats:sup>+</jats:sup>/Ca<jats:sup>2+</jats:sup>exchanger (NCX1-NACA1). As predicted from the reported actions of Ni<jats:sup>2+</jats:sup>, its application reduced extracellular Na<jats:sup>+</jats:sup>-dependent changes in intracellular Ca<jats:sup>2+</jats:sup>concentration (measured by fluo 3 fluorescence changes). However, contrary to expectation, the reduced fluorescence was accompanied by measured<jats:sup>63</jats:sup>Ni<jats:sup>2+</jats:sup>entry. The<jats:sup>63</jats:sup>Ni<jats:sup>2+</jats:sup>entry was observed in Sf 9 cells expressing the Na<jats:sup>+</jats:sup>/Ca<jats:sup>2+</jats:sup>exchanger but not in control cells. The established sequential transport mechanism of the Na<jats:sup>+</jats:sup>/Ca<jats:sup>2+</jats:sup>exchanger could be compatible with these results if one of the two ion translocation steps is blocked by Ni<jats:sup>2+</jats:sup>and the other permits Ni<jats:sup>2+</jats:sup>translocation. We conclude that, because Ni<jats:sup>2+</jats:sup>entry was inhibited by extracellular Ca<jats:sup>2+</jats:sup>and enhanced by extracellular Na<jats:sup>+</jats:sup>, the Ca<jats:sup>2+</jats:sup>translocation step moved Ni<jats:sup>2+</jats:sup>, whereas the Na<jats:sup>+</jats:sup>translocation step was inhibited by Ni<jats:sup>2+</jats:sup>. A model is presented to discuss these findings.</jats:p>