Cortisol increases Na+/K+-ATPase density in plasma membranes of gill chloride cells in the freshwater tilapia Oreochromis mossambicus

The effect of cortisol on Na+/K+-ATPase expression in the gill chloride cel ls of tilapia Oreochromis mossambicus was studied by immunocytochemistry at the light and electron microscope levels, One of three doses of cortisol ( low, 125 mg kg(-1) food; middle, 375 mg kg(-1) food; high, 750 mg kg(-1) fo od) was administered via the food (at a ration of 1.5 % of body mass) and t he fish were sampled after 5 days, Plasma osmolality and Na+ levels were el evated in the middle- and high-dose groups, and plasma cortisol levels in t he high-dose groups. Hematocrit values were not affected by the treatments. Opercular membrane chloride cell density increased by 94 % and 286 % in th e middle- and high-dose fish, respectively, whereas the gill chloride cell frequency increased by up to 28 % maximally in the high-dose fish. Lamellar gill chloride cells were absent in the control and low-dose groups, but we re observed in the middle- and high-dose groups, Cortisol increased the vol ume of the tubular membrane system in mature gill chloride cells, Quantific ation of immunogold-labelled Na+/K+-ATPase antigen (a 104 kDa protein speci es, as demonstrated by western blot) revealed that the high dose of cortiso l increases the Na+/K+-ATPase density in the tubular system of chloride cel ls. This is the first direct evidence that cortisol not only increases chlo ride cell numbers but also Na+/K+-ATPase density in these cells.