Pre and postprandial regulation of ghrelin, amino acids and IGF1 in Atlantic salmon (Salmo salar L.) at optimal and elevated seawater temperatures

Atlantic salmon farmed in seacage facilities are exposed to changing environmental conditions. While growth decline under lower water temperatures are reasonably well understood, the mechanisms behind production decline under high water temperatures are yet to be well elucidated and have been hampered by a lack of data describing pre- and postprandial patterns of endocrine fluctuation. The present research therefore aims to determine whether peak nutrient flux into the blood plasma is the most appropriate time point to investigate the endocrine regulation of growth and appetite under conditions of normal and high temperature, and to investigate the interrelationship between appetite and growth on a pre- and post-prandial time scale. Two experiments are presented which examine ghrelin (GHRL) as an indicator of appetite stimulation, the GH-IGF (growth hormone-insulin-like growth factor) system to describe growth-regulating processes and free amino acids (FAA) to indicate postprandial nutrient influx and link appetite and anabolic processes. Postprandial sampling of plasma and white muscle tissue from short-term adapted postsmolt was conducted at 13 degrees C and 19 degrees C at 4, 8, 12, 16, 20 and 24 hours (h). The same samples were taken from long-term adapted big salmon at 12 degrees C, -4, -2, -1, 0 h pre-prandially and 2, 3, 4 and 6 h post-prandially. While limited relationship between plasma ghrelin concentration and meal times was found for short term adapted postsmolt, clear ghrelin peaks were described for long term adapted salmon prior to the timing of anticipated meals. Possible explanations and consequences to experimental design in this area will be discussed. Postprandial FAA in plasma and white muscle from postsmolt was reduced at 19 degrees C compared to 13 degrees C and plasma levels peaked 8 h post-prandially. Muscle Igf1 mRNA expression levels were consistently higher at 13 degrees C than 19 degrees C, with no clear postprandial patters. In contrast, plasma IGF-1 concentration was relatively constant over time at 12 degrees C and 13 degrees C, but significantly declined from 20 h postprandially at 19 degrees C. GH receptor (ghr1) mRNA expression in muscle was unaffected by temperature, peaking 4 h post-prandially at both temperatures. This paper describes growth and appetite-regulating processes under conditions of normal and elevated temperature for Atlantic salmon, which is fundamental to our understanding of growth limitations inherent to high water temperature situations. (C) 2014 Elsevier B.V. All rights reserved.