The IP 3 receptor‐binding protein IRBIT reduces phosphatidylinositol 4,5‐bisphosphate (PIP 2 ) stimulation of Na/bicarbonate cotransporter NBCe1 variants expressed in Xenopus laevis oocytes

We have recently shown that PIP 2 stimulates NBCe1 activity, and injecting PIP 2 directly into an oocyte expressing NBCe1‐B or NBCe1‐C stimulates the NBC‐mediated, HCO 3 − ‐induced outward current (I NBC ) by ~150%. Furthermore, PIP 2 stimulation of the C variant requires the N terminal 87 residues found in the B and C variants, but not the A variant— a finding similar to the reported stimulatory effect of IRBIT on the B but not A variant. To explore the hypothesis that the PIP 2 and IRBIT regulatory pathways overlap, we used the 2‐electrode voltage clamp technique to monitor the effect of injecting ~10 μM PIP 2 (final) on I NBC in oocytes (V h = −60 mV) injected with a rat NBCe1 variant ± IRBIT cRNA(s). I NBC was normalized to plasma membrane expression as determined by single‐oocyte chemiluminescence. For A variant‐expressing oocytes, coexpressing IRBIT augmented the small PIP 2 ‐induced % increase in I NBC from 8% ± 1 (n=2) to 66 ± 18% (n=8). Coexpressing IRBIT increased B variant I NBC as previously reported, and increased C variant I NBC by 448 ± 104% (n=8). For B variant‐expressing oocytes, coexpressing IRBIT reduced the PIP 2 ‐induced % increase in I NBC from 162 ± 23% (n=7) to 42 ± 7% (n=8). Similar results were obtained for the C variant, with a reduction in the PIP 2 ‐induced % increase in I NBC from 185 ± 30% (n=3) to 12% ± 3% (n=5). For oocytes expressing a C variant missing its 87 N terminal residues, coexpressing IRBIT did not affect I NBC appreciably, and the PIP 2 ‐induced % increase in I NBC was less than 45% in the absence or presence of IRBIT. According to these data, IRBIT and PIP 2 may compete with one another in stimulating NBCe1‐B and NBCe1‐C. NIH NS046653 , AHA‐SE 0755255B