Acid-sensing ion channels are involved in epithelial Na⁺ uptake in the rainbow trout Oncorhynchus mykiss

A role for acid-sensing ion channels (ASICs) to serve as epithelial channels for Na+uptake by the gill of freshwater rainbow trout was investigated. We found that the ASIC inhibitors 4′,6-diamidino-2-phenylindole and diminazene decreased Na+uptake in adult rainbow trout in a dose-dependent manner, with IC50values of 0.12 and 0.96 μM, respectively. Furthermore, we cloned the trout ASIC1 and ASIC4 homologs and demonstrated that they are expressed differentially in the tissues of the rainbow trout, including gills and isolated mitochondrion-rich cells. Immunohistochemical analysis using custom-made anti-zASIC4.2 antibody and the Na+-K+-ATPase (α5-subunit) antibody demonstrated that the trout ASIC localizes to Na+/K+-ATPase-rich cells in the gill. Moreover, three-dimensional rendering of confocal micrographs demonstrated that ASIC is found in the apical region of mitochondrion-rich cells. We present a revised model whereby ASIC4 is proposed as one mechanism for Na+uptake from dilute freshwater in the gill of rainbow trout.