Ni2+ transport by the human Na+/Ca2+ exchanger expressed in Sf 9cells

The mechanism ofNi²⁺ block of theNa⁺/Ca²⁺exchanger was examined in Sf 9 cells expressing the human heartNa⁺/Ca²⁺exchanger (NCX1-NACA1). As predicted from the reported actions ofNi²⁺, its application reducedextracellular Na⁺-dependentchanges in intracellular Ca²⁺concentration (measured by fluo 3 fluorescence changes). However, contrary to expectation, the reduced fluorescence was accompanied bymeasured⁶³Ni²⁺entry. The⁶³Ni²⁺entry was observed in Sf 9 cells expressing theNa⁺/Ca²⁺exchanger but not in control cells. The established sequential transport mechanism of theNa⁺/Ca²⁺exchanger could be compatible with these results if one of the two iontranslocation steps is blocked byNi²⁺ and the other permitsNi²⁺ translocation. We concludethat, because Ni²⁺ entry wasinhibited by extracellular Ca²⁺and enhanced by extracellular Na⁺,the Ca²⁺ translocation step movedNi²⁺, whereas theNa⁺ translocation step wasinhibited by Ni²⁺. A model ispresented to discuss these findings.