Osmoregulatory action of PRL, GH, and cortisol in the gilthead seabream (Sparus aurata L.)

The osmoregulatory actions of ovine prolactin (oPRL), ovine growth hormone (oGH), and cortisol were tested in the eurylialine gilthead seabream Sparus aurata. Acclimated to sea water (SW, 40 ppt salinity, 1000 mOsm/kg H2O) or brackish water (BW, 5 ppt, salinity, 130 mOsm/kg H2O), injected every other day for one week (number of injections, 4) with saline (0.9% NaCl), oPRL (4 mug/g body weight), oGH (4 mug/g body weight) or cortisol (5 mug/g body weight), and transferred from SW to BW or from BW to SW 24 h after the last injection. Fish were sampled before and 24 h after transfer. Gill Na+, K+-ATPase activity, plasma osmolality, plasma ions (sodium and chloride), plasma glucose, and muscle water moisture were examined. SW-adapted fish showed higher gill Na+, K+-ATPase activity, plasma osmolality, and plasma ions levels than BW-adapted fish. Transfer from SW to BW decreased plasma osmolality and ions levels after 24 h, while transfer from BW to SW increased these parameters, whereas gill Na+, K+-ATPase activity was unaffected. oPRL treatment significantly decreased gill Na+, K+-ATPase activity and increased plasma osmolality and ions in SW- and BW-adapted fish. This treatment minimizes loss of osmolality and ions in plasma after transfer to BW and increased these values after transfer to SW. No significant changes were observed in gill Na+, K+-ATPase activity, plasma osmolality, and plasma ions in oGH-treated group with respect to saline group before or after transfer from SW to BW or from BW to SW. Treatment with cortisol induced, in SW-adapted fish, a significant increase of gill Na+, K+-ATPase activity and decrease of plasma osmolality and plasma ions. In BW-adapted fish this treatment induced a significant increases in gill Na+, K+-ATPase activity, plasma osmolality, and plasma ions. After transfer to SW cortisol-treated fish had higher plasma osmolality than the saline group. Our results support the osmoregulatory role of PRL in the adaptation to hypoosmotic environment in the gilthead seabream S. aurata. Further studies will be necessary to elucidate the osmoregulatory role of GH in this species. Cortisol results suggest a "dual osmoregulatory role" of this hormone in S. aurata. (C) 2002 Elsevier Science (USA). All rights reserved.