INTRACELLULAR ION CONCENTRATIONS IN BRANCHIAL EPITHELIAL-CELLS OF BROWN TROUT (SALMO-TRUTTA L) DETERMINED BY X-RAY-MICROANALYSIS

The intracellular concentrations of sodium, chloride, phosphorus and p otassium under normal conditions in pavement epithelial (PE) cells of brown trout (Salmo trutta) gill were 66, 51, 87 and 88 mmoll(-1) respe ctively. The concentrations of these elements under identical conditio ns in mitochondria-rich (MR) cells were not significantly different, e xcept for that of chlorine, which was lower in MR cells (40 mmoll(-1)) . The concentration of sodium in the PE cells decreased slightly after exposure of the fish to low external [Na+] (25 mu moll(-1)) for 7 day s but increased greatly within 5 min of subsequent exposure to 1 mmoll (-1) external Na+. These changes in external [Na+] had no significant effect on MR cells. Exposure of fish to low [Cl-] (25 mu moll(-1)) had no effect on PE or MR cells, but on exposure to 1 mmoll(-1) Cl- the c oncentrations of chlorine, phosphorus and potassium in both types of c ells increased, whilst the intracellular sodium concentration decrease d only in MR cells. The PE cells were little affected by exposure of t he fish to the carbonic anhydrase inhibitor acetazolamide. in contrast , 0.5 mmoll(-1) external acetazolamide caused a significant decrease i n intracellular phosphorus, chlorine and potassium concentrations in M R cells. This suggests that the PE cells are the sites of sodium uptak e in the gills of the brown trout and that chloride uptake occurs via the MR cells. These results are discussed with respect to the sites an d possible mechanisms of ionic exchange in freshwater vertebrates.