INTEGRATED PHYSIOLOGICAL-RESPONSE OF TILAPIA, OREOCHROMIS-MOSSAMBICUS, TO SUBLETHAL COPPER EXPOSURE

Juvenile and mature tilapia (Oreochromis mossambicus) were exposed to a range of sublethal copper (Cu) concentrations for 6 days to examine the mechanisms underlying the acclimation to the toxic effects of the metal. The study focuses on the gills, the primary target for waterborne pollutants. To obtain a comprehensive picture of the branchial acclimation processes operating, multiple biochemical and morphological parameters were studied. At all concentrations tested, Cu exposure resulted in the accumulation of the metal in mature fish. At 100 and 200 mu g Cu 1(-1) only, chloride cell proliferation was observed, which was accompanied by an increase in average cell diameter in these groups. Whole body flux measurements in juvenile fish demonstrated a decrease in Na influx in fish exposed to 200 mu g 1(-1) Cu, in the absence of an effect on Ca influx. Gill Na+/K+-ATPase activity was also decreased in the crude branchial homogenates of the mature fish exposed to the highest Cu concentration only, but not in the purified branchial vesicle preparations of these fish, which may indicate reactivation of in vivo Cu-inhibited ATPase activity during the isolation process. Plasma pH, Na, Cl, K, glucose and ceruloplasmin concentrations were also affected in the 200 mu g Cu 1(-1) group exclusively. In accordance with the gill accumulation data, plasma Cu levels were clearly elevated in all groups exposed to the metal. The results underscore the integrated response of the gills to Cu, which, however, does not come into play until challenged by relatively high ambient concentrations. These results indicate that, in comparison to the Cu-sensitive rainbow trout, tilapia is more Cu-tolerant. The most sensitive parameters affected by Cu are gill and plasma metal levels, followed by chloride cell number and diameter.