Salinity acclimation affects the somatotropic axis in rainbow trout

Abstract

In this study, we set out to examine the role of the somatotropic axis in the ion-regulation process in rainbow trout. Specifically, our objective was to examine whether plasma insulin-like growth factor-binding proteins (IGFBPs) are modulated by gradual salinity exposure. To this end, freshwater (FW)-adapted rainbow trout were subjected to gradual salinity increases, up to 66% seawater, over a period of 5 days. During this acclimation process, minimal elevations in plasma Ca 2ϩ and Cl Ϫ were seen in the salinity-acclimated groups compared with FW controls. There were no changes in plasma Na ϩ levels, and only a minor transient change in plasma cortisol levels was seen with salinity exposure. The salinity challenged animals responded with elevations in plasma growth hormone (GH) and IGF-I levels and gill Na ϩ -K ϩ -ATPase activity. We identified IGFBPs of 21, 32, 42, and 50 kDa in size in the plasma of these animals, and they were consistently higher with salinity. Despite the overall increase in IGFBPs with salinity, transient changes in individual BPs over the 5-day period were noted in the FW and salinity-exposed fish. Specifically, the transient changes in plasma levels of the 21-, 42-, and 50-kDa IGFBPs were different between the FW and salinity groups, while the 32-kDa IGFBP showed a similar trend (increases with sampling time) in both groups. Considered together, the elevated plasma IGFBPs suggest a key role for these binding proteins in the regulation of IGF-I during salinity acclimation in salmonids.insulin-like growth factor binding-proteins; insulin-like growth factor-I; growth hormone; rainbow trout; salinity EURYHALINE TELEOSTS, such as killifish, tilapia, and eels, survive abrupt full-strength seawater (SW) transfer without prior physiological preparation (14,42,92), whereas salmonids undergo a preacclimation process that is essential to SW survival (46). The preacclimation process involves physiological and morphological changes that improve salinity tolerance of developing/migrating salmonids (16,31,48). These physiological changes, such as salinity tolerance, are driven by changes in circulating hormones such as growth hormone (GH), prolactin (PRL), somatolactin (SL), insulin-like growth factor-I (IGF-I), and cortisol (8,24,45).Efforts to separate the endocrine and physiological response to salinity exposure, from the acquisition of salinity tolerance by migrating salmonids, have been made difficult by species differences (48) and aspects of the preacclimation process such as developmental and sex-specific differences (8,10,30,48,50,55), changes in temperature and photoperiod (5, 8, 9, 27), diet and rearing conditions (64, 97), growth, genetic lineage and domestication (24,27,57), and the method and duration used for salinity challenge (9,72,73,75,93).Aside from the aforementioned factors that influence salinity tolerance and hormone levels in salmonids, there is good evidence to show that the preacclimation process, SW tolerance, and enhanced growth of SW-acclimated salmonids are controlled by ...

Effects of short-term growth hormone treatment on liver and muscle transcriptomes in rainbow trout (Oncorhynchus mykiss)

“…Similar to the transgenic fish, we found modulation of the complement factor genes, CCAAT/enhancer-binding protein, and barrier-to-autointegration factor genes via microarray. These are in agreement with previous studies that observed increased immune stimulation following GH administration (4) or seawater acclimation, in which endogenous GH is typically elevated (56,66). However, in the present study, we have also identified immune-related genes that that were not differentially expressed in the GH transgenic fish, indicating a difference in the response between the short-term and long-term effects of GH on the immune system.…”

“…Neither the transcript identity nor the mechanisms of GHBP formation have been reported in the rainbow trout. However, during saltwater acclimation in the rainbow trout, which is characterized by a significant increase in GH secretion (56), serum GHBP was found to increase significantly (62). Here we provide evidence for regulation of GHR or GHBP in fish similar to that indicated in mammalian species.…”

IGF-I and branchial IGF receptor expression and localization during salinity acclimation in striped bass

“…This process is associated with increased hepatic expression of IGF-I mRNA (29) and circulating IGF-I (1), suggesting a hypoosmoregulatory role of the hormone. On the other hand, direct transfer of rainbow trout to 80% SW induced no change in liver IGF-I mRNA levels (28), whereas acclimation to 66% SW induced a gradual increase in circulating IGF-I levels over 5 days (33). More conclusive evidence for the role of IGF-I as a SW-adaptive hormone in salmonids (rainbow trout, 18; Atlantic salmon, 19; brown trout, 30) is offered by injection experiments in which IGF-I improves hypoosmoregulatory ability.…”

Metabolic hormones regulate insulin-like growth factor binding protein-1 mRNA levels in primary cultured salmon hepatocytes; lack of inhibition by insulin

“…Further, the zebrafish IGFBP-1 promoter contains a functional hypoxia-response element, indicating that cellular pathways for direct hypoxia induction of IGFBP-1 exist in fishes (Kajimura et al 2006). A 21 kDA IGFBP increased during gradual seawater adaptation in rainbow trout; however, cortisol increased only slightly and transiently (Shepherd et al 2005). Further studies are required to evaluate the role of cortisol in the regulation of fish IGFBP-1 by various physiological stressors.…”