Kinetic characterization of a stably expressed novel Na+/H+ exchanger (NHE-2)

We have recently reported the molecular cloning, sequencing and tissue distribution of a novel Na+/H+ exchanger (NHE-2). The cDNA for NHE-2 was cloned by screening a rat intestinal cDNA library. This clone was unique due to the fact that it lacks the first two transmembrane domains which are present in the other Na+/H+ exchanger isoforms (NHE-1, NHE-3, NHE-4). This structural change in the cDNA offered a unique opportunity to study in detail the properties of this stably expressed cDNA in chinese lung fibroblasts that lack the Na+/H+ exchanger (PS120) cells. Amiloride-sensitive Na+ uptake was linear up to 2 min in PS120 cells transfected with the cDNA. Kinetics of the amiloride-sensitive Na+ uptake showed a Vmax of 24.7 +/- 5 nmol/microliters ICW per min and a Km of 33.1 +/- 2.0 mM. The inhibitory constant (KI) for amiloride and its analogue 5-N-ethyl-N-isopropylamiloride (EIPA) was 0.15 microM and 0.66 microM, respectively. Epidermal growth factor, a known stimulator of NHE-1, also upregulated the expressed NHE-2. These results characterize the kinetic properties of this unique exchanger and suggests that the first two transmembrane domains of the Na+/H+ exchanger isoforms are not essential for the expression of amiloride-sensitive Na+ uptake.