Presence of Na-K-ATPase in mitochondria-rich cells in the yolk-sac epithelium of larvae of the teleost Oreochromis mossambicus

The purpose of this study is to provide biochemical evidence for the functions of the mitochondria-rich cell (MR cell) in the yolk-sac epithelium of the developing larvae of tilapia Oreochromis mossambicus. Western blotting with the antibody (6F) raised against avian Na-K-ATPase alpha 1 subunit demonstrated the presence of Na-K-ATPase in yolk-sac epithelium of tilapia larvae and about 1.46-fold more of the enzyme in seawater larvae than in freshwater ones. The yolk-sac MR cells were immunoreacted to the antibody (alpha 5) against the alpha subunit of avian Na-K-ATPase and were double-labeled with anthroylouabain and dimethylaminostyrylethyl-pyridiniumiodine, suggesting the existence and activity of Na-K-ATPase in these cells. Binding of H-3-ouabain in the yolk sac of seawater larvae was much higher than in that of freshwater larvae (4.183 +/- 0.143 pmol/mg protein versus 1.610 +/- 0.060 pmol/mg protein or 0.0508 +/- 0.0053 pmol/yolk sac versus 0.0188 +/- 0.0073 pmol/yolk sac). These biochemical results are further evidence that yolk-sac MR cells are responsible for a major role in the osmoregulatory mechanism of early developmental stages before the function of gills is fully developed.