HINKLE, L.; GARCIA, M.; VEILLETTE, P.A.; SPECKER, J.L.; University of Rhode Island; University of Rhode Island; University of Rhode Island; University of Rhode Island: Cortisol concentrations, the glucocorticoid-like receptor, and Na⁺/K⁺-ATPase in the juvenile summer flounder (Paralichythys dentatus) subjected to osmotic stress

Cortisol is the glucocorticoid and mineralocorticoid in teleost fishes and is regarded as the �seawater-adapting hormone.� In marine fishes such as the summer flounder salt and water balance is maintained by drinking seawater and desalinating blood plasma by excreting salt through the chloride cells in the gills. The chloride cells are mitochondria-rich cells known to contain dense amounts of Na⁺/K⁺-ATPase, an enzyme important to salt excretion. In this study juvenile summer flounder maintained in 30 ppt (bay water), were transferred for 5 d to 5, 30, or 50 ppt salinity. Whole-body levels of cortisol were measured by RIA following extraction. Immunocytochemistry was used to localize the glucocorticoid-like receptor (GLR) and the enzyme Na⁺/K⁺-ATPase. Five days in 50 ppt, but not 5 ppt, elevated cortisol levels (P < 0.05). The GLR was found in the gills, stomach, intestine, and kidney tubules. In the gills it was most strongly present in the mitochondria-rich cells throughout the cytosol. In the stomach GLR had a strongly nuclear localization in the epithelium of the gastric glands and the surface, but not the mucus neck cells of the gastric glands nor the mesenchyme. GLR was also present in the nucleus of many, but not all, epithelial cells of the intestine. Na⁺/K⁺-ATPase, as expected, was localized in the mitochondria-rich cells of the gills and the basolateral surface of the epithelial cells in the kidney tubules and intestine; however, the stomach did not seem to contain appreciable Na⁺/K⁺-ATPase. The co-localization of the GLR and Na⁺/K⁺-ATPase in many tissues supports the direct regulatory role of cortisol in salt balance in this marine teleost fish. [Supported by an REU Supplement to NSF IBN-0220196]