The Pharmacological Profile of Brain Liver Intestine Na⁺ Channel: Inhibition by Diarylamidines and Activation by Fenamates

Abstract

The brain liver intestine Na ϩ channel (BLINaC) is a member of the degenerin/epithelial Na ϩ channel gene family of unknown function. Elucidation of the physiological function of BLINaC would benefit greatly from pharmacological tools that specifically affect BLINaC activity. Guided by the close molecular relation of BLINaC to acid-sensing ion channels, we discovered in this study that rat BLINaC (rBLINaC) and mouse BLINaC are inhibited by micromolar concentrations of diarylamidines and nafamostat, similar to acid-sensing ion channels. Inhibition was voltage-dependent, suggesting pore block as the mechanism of inhibition. Furthermore, we identified the fenamate flufenamic acid and related compounds as agonists of rBLINaC. Application of millimolar concentrations of flufenamic acid to rBLINaC induced a robust, Na ϩ -selective current, which was blocked partially by amiloride. The identification of an artificial agonist of rBLINaC supports the hypothesis that rBLINaC is opened by an unknown physiological ligand. Inhibition by diarylamidines and activation by fenamates define a unique pharmacological profile for BLINaC, which will be useful to unravel the physiological function of this ion channel.

Inhibition of Neuronal Degenerin/Epithelial Na+ Channels by the Multiple Sclerosis Drug 4-Aminopyridine

“…This explains why blockade of Deg/ENaC by amiloride is sensitive to voltage (18,46). A recent study reported that amiloride senses ϳ35% of the electric field across BLINaC (39). Whereas interpretation of such a finding is complicated and often dependent on variables that have not been fully quantified, a simple explanation is that the amiloride binding site or the inhibitory effector site for the cationic guanidinium moiety of amiloride lies about a third of the way through the BLINaC pore referenced from its extracellular mouth.…”

“…Whole cell macroscopic current recordings of BLINaC expressed in CHO cells also were performed under voltage clamp conditions with standard methods (24,39). Mutant BLINaC harboring the A443T substitution is constitutively active (24).…”

The bile acid-sensitive ion channel (BASIC), the ignored cousin of ASICs and ENaC

“…They are potently inhibited by diarylamidines, a class of anti-protozoal molecules and the related substance nafamostat. 29 Apparent affinity of BASIC for diarylamidines and nafamostat is in the low micromolar range and similar for mouse, rat, and human BASIC. 29 Diarylamidines and nafamostat likely function as pore blockers as their block is voltage-dependent and they compete with amiloride for its binding site, 29 which is in the pore region.…”

A Cytosolic Amphiphilic α-Helix Controls the Activity of the Bile Acid-sensitive Ion Channel (BASIC)

“…Furthermore, the UDCA-induced current could be inhibited by 10 M diminazene (Fig. 6D), a potent inhibitor for rBASIC (32). WT rBASIC expressed in Xenopus oocytes is characterized by unselectivity for monovalent cations, a rather uncommon feature of Deg/ENaC channels (17).…”