Determinants of cardiac Na(+)/Ca(2+) exchanger temperature dependence: NH(2)-terminal transmembrane segments.

The cardiac Na(+)/Ca(2+) exchanger (NCX) in trout exhibits profoundly lower temperature sensitivity in comparison to the mammalian NCX. In this study, we attempt to characterize the regions of the NCX molecule that are responsible for its temperature sensitivity. Chimeric NCX molecules were constructed using wild-type trout and canine NCX cDNA and expressed in Xenopus oocytes. NCX-mediated currents were measured at 7, 14, and 30 degrees C using the giant excised-patch technique. By using this approach, the differential temperature dependence of NCX was found to reside within the NH(2)-terminal region of the molecule. Specifically, we found that approximately 75% of the Na(+)/Ca(2+) exchange differential energy of activation is attributable to sequence differences in the region that include the first four transmembrane segments, and the remainder is attributable to transmembrane segment five and the exchanger inhibitory peptide site.