Osmoregrulatory action of 17β-estradiol in the gilthead sea bream Sparus auratus

The osmoregulatory action of 17beta-estradiol (E-2) was examined in the euryhaline teleost Sparus auratas. In a first set of experiments, fish were injected once with vegetable oil containing E-2 (1, 2 and 5 mug/g body weight), transferred 12h after injection from sea water (SW, 38 ppt salinity) to hypersaline water (HSW, 55 ppt) or to brackish water (BW, 5 ppt salinity) and sampled 12h later (i.e. 24 h post-injection). In a second experiment, fish were injected intraperitoneally with coconut oil alone or containing E-2 (10 mug/g body weight) and sampled after 5 days. In the same experiment, after 5 days of treatment, fish of each group were transferred to HSW, BW and SW and sampled 4 days later (9 days post-implant). Gill Na+,K+-ATPase activity, plasma E-2 levels, plasma osmolality, and plasma levels of ions (sodium and calcium), glucose, lactate, protein, triglyceride, and hepatosomatic index were examined. Transfer from SW to HSW produced no significant effects on any parameters assessed. E-2 treatment did not affect any parameter. Transfer from SW to BW resulted in a significant decrease in plasma osmolality and plasma sodium but did not affect gill Na+,K+-ATPase activity. A single dose of E-2 attenuated the decrease in these parameters after transfer from SW to BW, but was without effect on gill Na+,K+-ATPase activity. An implant of E-2 (10 mug/g body weight) for 5 days significantly increased plasma calcium, hepatosomatic index, plasma metabolic parameters, and gill Na+,K+-ATPase activity. In coconut oil-implanted (sham) fish, transfer from SW to HSW or BW during 4 days significantly elevated gill Na+,K+-ATPase. Gill Na+,K+-ATPase activity remained unaltered after transfer of E-2-treated fish to HSW or BW. However, in E-2-treated fish transferred from SW to SW (9 days in SW after E-2-implant), gill Na+,K+-ATPase activity decreased with respect to HSW- or BW-transferred fish. Shams transferred to HSW showed increased levels of lactate, protein, and trygliceride in plasma, while those transferred to BW only displayed increased trygliceride levels. E-2-treated fish transferred to HSW showed higher protein levels without any change in other plasmatic parameters, while those transferred to BW displayed elevated plasma glucose levels but decreased osmolality and protein levels. These results substantiate a chronic stimulatory action of E-2 on gill Na+,K+-ATPase activity in the euryhaline teleost Sparus auratas. (C) 2004 Wiley-Liss, Inc.