NaCl uptake by the branchial epithelium in freshwater teleost fish: An immunological approach to ion-transport protein localization

Teleost fishes, living in fresh water, engage in active ion uptake to maint ain ion homeostasis, Current models for NaCl uptake involve Na+ uptake via an apical amiloride-sensitive epithelial Na+ channel (ENaC), energized by a n apical vacuolar-type proton pump (V-ATPase) or alternatively by an amilor ide-sensitive Na+/H+ exchange (NHE) protein, and apical Cl- uptake mediated by an electroneutral, SITS-sensitive Cl-/HCO3- anion-exchange protein. Usi ng non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models, Na+/K+-ATPas e was used as a cellular marker for differentiating branchial epithelium mi tochondria-rich (MR) cells from pavement cells. In both the freshwater tila pia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-AT Pase and ENaC-like immunoreactivity co-localized to pavement cells, althoug h apical labelling was also found in MR cells in the trout. In the freshwat er tilapia, apical anion-exchanger-like immunoreactivity is found in the MR cells, Thus, a freshwater-type MR chloride cell exists in teleost fishes, The NHE-like immunoreactivity is associated with the accessory cell type an d with a small population of pavement cells in tilapia.