PROLACTIN ANTAGONIZES THE SEAWATER-ADAPTIVE EFFECT OF CORTISOL AND GROWTH-HORMONE IN ANADROMOUS BROWN TROUT (SALMO-TRUTTA)

Two experiments are described in which the interaction of prolactin, c ortisol and growth hormone on hypoosmoregulation in the anadromous bro wn trout was studied. In experiment 1, fish at the postsmolt stage in freshwater (RN) were given four injections on alternate days of 5 mu g cortisol (F)/g in combination with 0, 0.2, 1 or 2 mu g ovine prolacti n (oPRL)/g. Additional groups received 2 mu g oPRL/g or saline as cont rol. In experiment 2, RN parr were given four injections on alternate days with 5 mu g F/g and 2 mu g ovine growth hormone (oGH)/g in combin ation with 0, 0.2, 1 or 2 mu g oPRL/g. Additional groups received 2 mu g oPRL/g, 2 mu g oGH/g, 5 mu g F/g or saline as control. In both expe riments, the fish were subjected to a 48 hr seawater (SW) challenge te st 24 hr after the last injection. Muscle water content, plasma osmola lity and ion levels, kidney and gill Na+,K+-ATPase activity were measu red. In experiment 1, F-treated fish had better hypoosmoregulatory cap acity than control fish as judged by a higher level of muscle water co ntent and lower plasma osmolality after 48 hr in SW. All three doses o f oPRL completely abolished this action of F. Gill Na+,K+-ATPase activ ity was stimulated by F and unaffected by oPRL at any dose. In experim ent 2, oPRL impaired, whereas F and oGH (injected individually or toge ther) improved performance in the 48 hr SW test relative to control fi sh, judged by plasma osmolality and muscle water content. Ovine PRL in hibited the combined action of F and oGH in a dose-related manner but could not completely counteract the combined effect of these hormones. F and oGH had additive stimulatory effects on gill Na+,K+-ATPase acti vity. This activation was counteracted by coinjection of oPRL in a dos e-related manner but not below the level found in the F-treated fish. We conclude that exogenous oPRL inhibits the hypoosmoregulatory action of F. However, oPRL does not antagonize the F-dependent stimulation o f gill Na+,K+-ATPase activity. The observed antagonism between PRL and F in SW-acclimation may therefore occur on a target different from gi ll Na+,K+-ATPase.