V-H+-ATPase, Na+/K+-ATPase and NHE2 immunoreactivity in the gill epithelium of the Pacific hagfish (Epatretus stoutii)

We report the presence of the ion transporting proteins V-H+-ATPase, Na+/K+-ATPase and NHE2 in the gill epithelium of the Pacific hagfish Epatretus stoutii. Heterologous antibodies recognized single bands of the appropriate sizes for the three transporters in western blots. Immunohistochemical staining revealed that the distribution of labeled cells in the gill epithelium was identical for the three proteins. Immunopositive cells were most abundant in the primary filament from the afferent side of the gill pouch, and their number diminished towards the lamella. Na+/K+-ATPase-like immunoreactivity (L-IR) occurred throughout the cell cytoplasm, probably associated to the basolateral tubular system. V-H+-ATPase L-IR was similar to Na+/K+-ATPase, although some cells had slightly heavier staining in either the supra- or infra-nuclear region. NHE2 L-IR was also generally cytoplasmic, but a minority of the cells had stronger immunoreactivity in the apical region. In general, all three ion transporting proteins were localized in the same cells, as estimated from 4-µm immunostained consecutive sections. We hypothesize that these putative ion-transporting cells are involved in systemic acid/base regulation and discuss other possible roles. This is the first report of V-H+-ATPase in myxinoids, and the first NHE2 report in the Pacific hagfish.