Functional expression of the human cardiac Na+/Ca2+ exchanger in Sf9 cells: rapid and specific Ni2+ transport

Although inhibition of the Na+/Ca2+ exchanger normally increases [Ca2+](i) in neonatal cardiac myocytes, application of the inhibitor Ni2+ appears to reduce [Ca2+](i) measured by fluo-3. To investigate how the apparent reduct ion in [Ca2+](i) occurs we examined Ca2+ transport by the human Na+/Ca2+ ex changer expressed in Sf9 cells. Transport of Ca2+ by the Na+/Ca2+ exchanger was examined using a laser-scanning confocal microscope and the fluorescen t Ca2+ indicator fluo-3, and the electrogenic function was determined by me asuring the Na+/Ca2+ exchange current (I-NaCa) using patch clamp methods. I -NaCA was elicited with voltage-clamp steps or flash photolysis of caged Ca 2+. We show significant expression of Na+/Ca2+ exchanger function in Sf9 ce lls infected with a recombinant Baculovirus carrying the Na+/Ca2+ exchanger . In addition to measurements of I-NaCa, characterization includes Ca2+ tra nsport via the Na+/Ca2+ exchanger and the voltage dependence of Ca2+ transp ort. Application of Ni2+ blocked I-NaCa but, contrary to expectation, decre ased fluo-3 fluorescence. Experiments with infected Sf9 cells suggested tha t Ni2+ was transported via the Na+/Ca2+ exchanger at a rate comparable to t he Ca2+ transport. Once inside the cells, Ni2+ reduced fluorescence, presum ably by quenching fluo-3. We conclude that Ni2+ does indeed block I-NaCa, b ut is also rapidly translocated across the cell membrane by the Na+/Ca2+ ex changer itself, most likely via an electroneutral partial reaction of the e xchange cycle.