Submitted to: Aquaculture America Conference
Publication Type: Abstract only
Publication Acceptance Date: 10/6/2004
Publication Date: 1/18/2005
Citation: Shepherd, B.S., Drennon, K.D., Johnson, J.K., Nichols, J.W., Playle, R.C., Singer, T.D., Vijayan, M.M. 2005. The effects of gradual salinity acclimation on the somatotropic axis in rainbow trout, oncorhynchus mykiss. Aquaculture America Conference. On CD. Interpretive Summary:
Technical Abstract: In fish, the pituitary hormone, growth hormone (GH), and its intermediary Insulin-like growth factor-I (and plasma IGF-binding proteins), are important hormones that control growth and salt and water balance. For instance, the movement of a euryhaline fish from fresh water (FW) to seawater (SW) is accompanied by hormonal changes (e.g., increases in GH and IGF-I) that attenuate the physiological stresses (salt and water balance) which result from such movements. While in some fishes, such as salmon and tilapia, there is an increase in growth following SW acclimation, the hormonal mechanisms responsible for this increase in growth are not completely understood. In fact, the involvement of the IGF-binding proteins, which are important regulators of GH-dependent IGF-I, have not been studied. Consequently, the hormones that link salinity acclimation and growth in rainbow trout (a salmonid) are of great interest. We examined the relationships between the endocrine and physiological response of rainbow trout to gradual increases in salinity. Fresh water (FW)-adapted rainbow trout were exposed to gradual increases in salinity, up to 66% seawater, over a period of 5 days. During this acclimation process, elevations in plasma Ca2+ and Cl- were seen in the salinity-acclimated groups compared to FW controls. There were no changes in plasma Na+ levels. The salinity challenged animals responded with transient changes in plasma growth hormone (GH) and cortisol levels, while there were sustained elevations in plasma IGF-I levels and gill Na+,K+-ATPase activity. We identified IGF-binding proteins (IGFBPs) of 21, 32, 42 and 50 kDa in size in the plasma of these animals. Levels of the 21 kDa IGFBP roughly paralleled plasma cortisol levels, whereas levels of the 32, 42 and 50 kDa IGFBPs were found to be highest in the salinity-exposed groups and correlated with the elevated plasma IGF-I concentrations. This is the first demonstration of concurrent increases in blood levels of the growth-axis hormones (GH and IGF-I), and IGFBPs, following gradual salinity acclimation in a salmonid. These findings underscore the dual role of these hormones in growth and salinity acclimation in euryhaline teleosts. A better understanding of the multiple roles of the hormones of the growth/somatotropic-axis, and how these hormones can be stimulated via alterations in environmental conditions, will illuminate novel ways to enhance growth in economically-important fishes without adversely affecting other important physiological processes.