Proceedings of The Physiological Society
University College London (2011) Proc Physiol Soc 24, C06 and PC06
BLINaC: An epithelial Na+ channel in the liver
D. Wiemuth1, S. Gründer1
1. Physiology, RWTH Aachen, Aachen, Germany.
The Brain Liver Intestine Na+ Channel (BLINaC) is an ion channel of the DEG/ENaC gene family. Its physiological function is completely unknown. Expression of rat BLINaC (rBLINaC) in Xenopus oocytes leads to small unselective currents that are only weakly sensitive to amiloride1. rBLINaC is potently inhibited by micromolar concentrations of extracellular Ca2+ and removal of Ca2+ leads to robust currents and increases Na+ selectivity of the ion pore3. Strikingly, the species ortholog from mouse (mBLINaC) has an almost 250-fold lower Ca2+ affinity than rBLINaC, rendering mBLINaC constitutively active at physiological Ca2+ concentrations3. In addition, mBLINaC is more selective for Na+ and has a 700-fold higher amiloride affinity than rBLINaC3. A single amino acid in the extracellular domain determines these profound species differences3. These results suggest that rBLINaC is opened by an unknown ligand whereas mBLINaC is a constitutively open epithelial Na+ channel. In rodents, the blinac mRNA is expressed mainly in brain, liver, and intestine and to a lesser extent in kidney and lung1; in humans, it is mainly expressed in the small intestine2. To explore the expression of the BLINaC protein in these tissues, we generated a polyclonal antibody against BLINaC. This antibody revealed prominent expression of BLINaC in the apical membrane of cholangiocytes lining the bile duct of mice, potentially implicating BLINaC in the generation of secondary bile. Guided by the localisation of BLINaC in cholangiocytes we tested whether the channel is affected by bile. Interestingly the application of diluted bile to Xenopus oocytes expressing rBLINaC induces a strong, amiloride-sensitive Na+-selective current. This finding suggests the presence of an endogenous activator of rBLINaC in bile and supports the longstanding hypothesis that BLINaC is a ligand-gated ion channel. We are currently trying to unravel the nature of the ligand from bile. In addition we identified the fenamate flufenamic acid (FFA) and related compounds as agonists of rBLINaC. Application of millimolar concentrations of FFA to rBLINaC expressing oocytes induces a robust, Na+-selective current, which is partially blocked by amiloride. We also discovered that rBLINaC and mBLINaC, similar to the related acid sensing ion channels (ASICs), are inhibited by micromolar concentrations of diarylamidines and nafamostat. Thus, we identified pharmacological tools that will help to characterize the function of BLINaC in native cells, such as cholangiocytes.
Where applicable, experiments conform with Society ethical requirements