Molecular characterization of the human airway smooth muscle Na+/Ca2+ exchanger.

The Na+/Ca2+ exchanger is an important plasma-membrane Ca2+ transport protein. Functional studies of the effects of removal of external Na+ on airway smooth muscle tone, and studies of Na+-dependent Ca2+ fluxes in membrane vesicles have provided indirect evidence of an important role for Na+/Ca2+ exchange in regulating airway smooth muscle tone. Recent molecular studies have identified seven isoforms of the Na+/Ca2+ exchanger (NCX1) gene in other tissues, but direct molecular characterization of the airway smooth muscle Na+/Ca2+ exchanger has not previously been performed. We have therefore used a reverse transcriptase-polymerase chain (RT-PCR) reaction method and DNA sequencing to study the Na+/Ca2+ exchanger isoform present in human airway smooth muscle. The dominant isoform was found to be virtually identical to the NACA3 isoform originally described in rabbit kidney cortex. The DNA sequence of the Na+/Ca2+ exchanger exon B that we obtained is previously unpublished for any human tissue. This study provides the first definitive molecular evidence of the existence of an Na+/Ca2+ exchanger in airway smooth muscle. The molecular characterization of the human airway smooth muscle Na+/Ca2+ exchanger will facilitate future electrophysiologic studies of its function.