BIOCHEMICAL-CHARACTERIZATION OF ISOLATED BRANCHIAL MITOCHONDRIA-RICH CELLS OF OREOCHROMIS-MOSSAMBICUS ACCLIMATED TO FRESH-WATER OR HYPERHALINE SEA-WATER

Mitochondria-rich cells have been separated from other epithelial cell s of tilapia (Oreochromis mossambicus) gills by density gradient centr ifugation on Percoll. During centrifugation two main bands of cells fo rmed. The viability of the cells in both bands was high (>90%). In one band, 45-74% of the total cell number was mitochondria-rich cells. Th e other band contained at least 80% pavement cells, representing the m ajority of other gill epithelial cell types. A comparison of the activ ities of four enzymes involved in major metabolic and ion regulatory f unctions was made between these two different fractions of cells. Furt hermore, the separation of gill epithelial cells and determination of enzymatic activity was carried out in tilapia after the fish were accl imated to fresh water or hyperhaline sea water (60 mg . ml-1 S) to gai n an indication of the relative contribution of mitochondria-rich cell s and pavement cells to both NaCl excretion and absorption. Regardless of acclimation salinity, the activities of Na+/K+-ATPase, glutamate d ehydrogenase and glucose-6-phosphate dehydrogenase were significantly higher in mitochondria-rich cells than in pavement cells. However, til apia acclimated to hyperhaline sea water possessed significantly lower carbonic anhydrase activity in mitochondria-rich cells than in paveme nt cells. In contrast, no significant difference of carbonic anhydrase activity was observed between the two cell fractions in tilapia accli mated to fresh water.