1Institute of Biology, Odense University, Campusvej 55, DK-5230 Odense M, Denmark;, 2Department of Integrative Biology, Cancer Research Laboratory and Bodega Marine Laboratory, University of California, Berkeley, California 94720, U.S.A.
The effect of simultaneously-injected prolactin and growth hormone on short-term adaptation to seawater was investigated in two salmonid teleosts, Salmo trutta and Oncorhynchus mykiss. Freshwater-adapted fish were given 5-6 injections of prolactin, growth hormone or combinations of the two hormones on alternate days and subsequently challenged by transfer to seawater. Major plasma ions, osmolarity, muscle water content and gill Na+, K+-ATPase activity were measured. In both species, increased plasma ions and decreased muscle water were observed 2-3 days after transfer to seawater. Growth hormone significantly reduced post-transfer changes in plasma ions and muscle water, whereas prolactin injections alone had no effect on either freshwater-adapted or seawater-challenged fish. However, when prolactin was injected simultanously with growth hormone, prolactin significantly abolished the seawater-adaptive effect of growth hormone in a dose-related manner. Prolactin also significantly reduced the growth hormone-induced stimulation of gill Na+, K+-ATPase activity. We conclude that prolactin is antagonistic to growth hormone during seawater adaptation and development of hypoosmoregulatory mechanisms in salmonids.