Submitted to: American Journal of Physiology - Regulatory Integrative & Comparative Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/9/2004
Publication Date: 12/16/2004
Citation: Shepherd, B.S., Drennon, K., Johnson, J., Nichols, J.W., Playle, R.C., Singer, T.D., Vijayan, M.M. 2004. Salinity acclimation affects the somatotropic axis in rainbow trout. American Journal of Physiology - Regulatory Integrative & Comparative Physiology, Vol. 288, P. R1385-1395 Interpretive Summary: In fish, the pituitary hormone, growth hormone (GH), and its intermediary Insulin-like growth factor-I (and IGF-binding proteins), are important hormones that control growth and salt and water balance. For instance, the movement of a 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) that result from such movements. While in some fishes 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 IGF-I, have not been studied at all. Consequently, the hormones that link salinity acclimation and growth in rainbow trout (a salmonid) are of great interest. We examined the hormonal response of rainbow trout (Oncorhynchus mykiss) to the effects of gradual salinity acclimation by subjecting FW-adapted trout to gradual salinity increases, up to 66% SW, over a period of 5 days. Remarkably, we found that levels of the growth-axis hormones, GH, IGF-I and the IGF-binding proteins (of which there are 4, including a newly discovered one) were elevated in those animals undergoing changes in environmental salinity. This is the first demonstration of concurrent increases in blood levels of the growth-axis hormones and strengthens the idea for a dual role of these hormones in growth and salinity acclimation. A better understanding of the multiple roles of the hormones of the growth-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 such as the regulation of salt and water balance.
Technical Abstract: We examined the relationships between the endocrine and physiological response of rainbow trout to gradual salinity acclimation by subjecting fresh water (FW)-adapted rainbow trout (Onchorhyncus mykiss) to gradual salinity increases, 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 and they were modulated by salinity. Specifically, 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. Considered together, our findings suggest a key role for the somatotropic axis (GH, IGF-I and IGFBPs) in the salinity acclimation process in rainbow trout.