Na+/K+-ATpase immunoreactivity in branchial chloride cells of Oreochromis mossambicus exposed to copper

Chloride cells were identified by Na+/K+-ATPase immunocytochemistry at the light and electron microscope levels in gills of freshwater tilapia Oreochr omis mossambicus. Turnover of chloride cells was enhanced by exposing the f ish to waterborne copper (3.2 mu mol(-1)) for 14 days, as indicated by a 38 % increase in cells expressing proliferating cell nuclear antigen (PCNA) r elative to controls. The expression of PCNA was most marked in the central area of the filamental epithelium, from where the chloride cells are though t to originate and migrate. In control fish, chloride cells were associated exclusively with the filamental epithelium. In both controls and copper-ex posed fish, two chloride cell populations were seen after Na+/K+-ATPase imm unostaining, These probably represent subpopulations of newly emerged chlor ide cells: (1) strongly stained cells (mature chloride cells) in the filame ntal and lamellar epithelium and (2) weakly stained cells, identified by el ectron microscopy as apoptotic and necrotic chloride cells, mainly in the f ilamental epithelium. Absolute numbers of mature chloride cells fell, while necrotic and apoptotic chloride cell numbers increased, in copper-exposed fish. A strong correlation could be established for gill Na+/K+-APase speci fic activity and the number of strongly stained chloride cells in controls and copper-exposed fish and for Na+/K+-ATPase specific activity and total n umbers of immunoreactive cells in copper-exposed fish owing to an increased incidence of weakly staining cells.