The electrogenic Na+-HCO3- cotransporter NBCe1-B can be regulated by intracellular Mg2+ (Mg-i(2+)). We previously reported that under whole-cell voltage-clamp conditions, bovine NBCe1-B (bNBCe1-B) currents heterologously expressed in mammalian cells are strongly inhibited by Mg-i(2+), and the inhibition is likely mediated by electrostatic interaction and relieved by truncation of the cytosolic NBCe1-B specific N-terminal region. Intriguingly, NBCe1-B-like currents natively expressed in bovine parotid acinar (BPA) cells are much less sensitive to Mg-i(2+) inhibition than bNBCe1-B currents. Here, we hypothesized that this apparent discrepancy may involve IRBIT, a previously identified NBCe1-B-interacting protein. RT-PCR, Western blot and immunofluorescence confocal microscopy revealed that IRBIT was not only expressed in the cytosol, but also colocalized with NBCe1-B in the region of plasma membranes of BPA cells. IRBIT was coimmunoprecipitated with NBCe1-B by an anti-NBCe1 antibody in bovine parotid cell lysate. Whole-cell patch-clamp experiments showed that coexpression of IRBIT lowered the Mg-i(2+) sensitivity of bNBCe1-B currents stably expressed in HEK293 cells. Collectively, these results suggest that IRBIT may reduce the apparent affinity for Mg-i(2+) in inhibition of NBCe1-B activity in mammalian cells. (C) 2012 Elsevier Inc. All rights reserved.