Rat Heart NCX1.1 Stoichiometry Measured in a Transfected Cell System

Expression of cDNA encoding the rat heart NCX1.1 protein in HEK-293 cells provides a simple system in which properties of the expressed protein can be determined in the absence of contaminating ionic conductances or parallel transport pathways. Using this system, we have determined that NCX1.1 transports 4 Na(+) in exchange for 1 Ca(2+) and accompanied by the movement of 2 positive charges. The measured stoichiometry is constant over a range of different intracellular Na(+) and Ca(2+) concentrations, independent of the nature of the Ca(2+) buffer used and not affected by the direction of the applied voltage ramp. We conclude that our measurements are unlikely to be confounded by poor control over ionic conditions close to the intracellular surface of the membrane. A stoichiometry of 4:1 for NCX1 has important implications for models of the role of NCX1 in cellular Ca(2+) handling and suggests that in the heart the exchanger will be cardioprotective over a wider range of conditions and have a greater contribution to action potential shape than previously appreciated.