EFFECT OF REPETITIVE CORTISOL AND THYROXINE INJECTIONS ON CHLORIDE CELL NUMBER AND NA+/K+-ATPASE ACTIVITY IN GILLS OF FRESH-WATER ACCLIMATED RAINBOW-TROUT, SALMO-GAIRDNERI

1. 1. Freshwater nonanadromous rainbow trout, Salmo gairdneri, were injected three times a week with either saline, 10μg cortisol/g, 1.0μg thyroxine/g or 10μg cortisol/g + 1.0μg thyroxine/g during a period of 28 days (12 injections). A separate group was derived as a subgroup from the thyroxine group on day 14 and received Cortisol + thyroxine from day 14 until day 28 (six injections). 2. 2. Gill chloride cell number and Na+/K+-ATPase activity increased by cortisol treatment, the changes being significant on days 7 and 14, respectively. 3. 3. Thyroxine treatment did not affect gill Na+/K+-ATPase activity or chloride cell number directly. Neither did it modify the stimulatory effect of cortisol on these parameters. 4. 4. Muscle water decreased in cortisol-treated fish and increased in thyroxine-treated fish, while no changes were observed in the combined hormone groups. 5. 5. No changes were observed in plasma chloride in any group during the experiment. 6. 6. The results demonstrate a putative role of cortisol in stimulating hypo-osmoregulatory mechanisms and suggest that thyroxine is without a direct or a supportive effect for cortisol action. © 1989.