Effects of cortisol on chloride cells in the gill epithelium of Japanese eel, Anguilla japonica

The purpose of the present study was to determine the effects of cortisol o n the development of the freshwater chloride cell (CC), using flow cytometr y. Scanning electron microscopy was used to determine the corresponding mod ifications in CC apical structure. Simultaneously, biochemical analyses wer e conducted to determine the activities of transport ATPases, mitochondrial enzymes (succinate dehydrogenase (SDH) and Mg2+-ATPase) and lactate dehydr ogenase (Ldh). The effects of daily i.m. injection of 2 mug/g cortisol were compared with sham-injected freshwater-, control freshwater- and seawater- adapted fish. The hormone did not affect the activities of Ca2+-ATPases in CCs. However, it stimulated the proliferation and differentiation of the tw o freshwater CC subtypes (F1, 66 +/- 2.18% (S.E.M.) and F2, 34 +/- 2.18%), in which the relative proportion of F1 CCs was transiently reduced in the f irst 5 days of treatment (F1, 53 +/- 1.83%; F2, 47 +/- 1.83%) but was then restored to a higher relative percentage on day 10 (F1, 70 +/- 1.42%; F2, 3 0 +/- 1.42%). Biochemically, it induced the activities of Na+/K+-ATPase, Mg 2+-ATPase, SDH and Ldh, suggesting an increase in ion pumping and its assoc iated metabolic activities. CCs from cortisol-treated fish demonstrated rec essed apical morphology, accompanied by an increase in cell density (2012 t o 2413/mm(2)). Nevertheless, the extent of cell proliferation and different iation and the biochemical changes were significantly lower than those of s eawater fish. Our results indicate that cortisol alone cannot stimulate a c omplete differentiation of freshwater CCs to seawater CCs. However, the res pective roles of the two CC subtypes in freshwater and seawater environment s are indicated.