Influence of cortisol, growth hormone, insulin-like growth factor I and 3,3 ',5-triiodo-L-thyronine on hypoosmoregulatory ability in the euryhaline teleost Fundulus heteroclitus

The capacity of cortisol, ovine growth hormone (oGH), recombinant bovine in sulin-like growth factor I (rbIGF-I) and 3,3',5-triiodo-L-thyronine (T-3) t o increase hypoosmoregulatory capacity in the euryhaline teleost Fundulus h eteroclitus was examined. Fish acclimated to brackish water (BW, 10 ppt sal inity) were injected with a single dose of hormone suspended in oil and tra nsferred to seawater (SW, 35 ppt salinity) 10 days post-injection. Fish wer e sampled 24 h after transfer and plasma osmolality and gill Na+, K+-ATPase activity were examined. Transfer from BW to SW induced significantly incre ased plasma osmolality but not gill Na+, K+-ATPase activity. Cortisol (50 m u g g(-1) body weight) improved the ability to maintain plasma osmolality a nd to increase gill Na+, K+-ATPase activity. oGH (5 mu g g(-1) body weight) also increased hypoosmoregulatory ability and gill Na+, K+-ATPase activity . A cooperation between oGH and cortisol was observed in increasing hypoosm oregulatory ability but not in increasing gill Na+, K+-ATPase activity. rbI GF-I (0.5 mu g g(-1) body weight) alone was without effect in increasing sa linity tolerance or gill Na+, K+-ATPase activity. rbIGF-I and oGH showed a positive interaction in increasing salinity tolerance, but not gill Na+, K-ATPase activity. Treatment with T-3 (5 mu g g(-1) body weight) alone did n ot increase salinity tolerance or gill Na+, K+-ATPase activity, and there w as no consistent significant interaction between cortisol and T-3 or betwee n GH and T-3. The results confirm the classical role of cortisol as a seawa ter-adapting hormone and indicate an interaction between cortisol and the G H/IGF-I axis during seawater acclimation of Fundulus heteroclitus.