Effects of water-borne copper on branchial chloride cells and Na+/K+-ATPase activities in Mozambique tilapia (Oreochromis mossambicus)

Freshwater tilapia (Oreochromis mossambicus) were exposed for different per iods up to 28 days to 3.2 mu M of water-borne Cu. Electron microscopical an alysis of the gills demonstrated significant changes in the structure and n umber of chloride cells (CCs) from Cu-exposed fish when compared to control s. These cells, which are the main location of the Na+/K+-ATPase of the gil ls and which play a crucial role in transepithlial Na+ transport, showed a time-related increase of degeneration by apoptosis and necrosis in the Cu-e xposed fish. After 28 days of Cu exposure, apoptotic CCs had doubled in num ber while necrotic CCs had even increased by a factor of ten. The activity of the gill Na+/K+-ATPase and the plasma Na+ concentration decreased in tim e and in parallel. An inverse relationship between the Na+/K+-ATPase specif ic activity and the branchial Cu content further supports the notion that t his enzyme is very sensitive to Cu2+ inhibition. In contrast to controls, n o significant correlation was found in the Cu-exposed fish between the oper cular CC number and the gill Na+/K+-ATPase total activities, despite the la rge increase in number of these cells. This study provides further evidence that not only the number but also the quality of the CCs, may determine to a large extent the branchial capacity of a freshwater fish to absorb Na+ f rom the surrounding water. (C) 1998 Elsevier Science B.V. All rights reserv ed.