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  • Title: Environmental temperature increases plasma GH levels independently of nutritional status in rainbow trout (Oncorhynchus mykiss).
    Author: Gabillard JC, Weil C, Rescan PY, Navarro I, Gutiérrez J, Le Bail PY.
    Journal: Gen Comp Endocrinol; 2003 Aug; 133(1):17-26. PubMed ID: 12899843.
    Abstract:
    Like many poecilotherms, salmonids exhibit seasonal variations of growth rate in relation with seasonal temperatures and plasma GH level. However, temperature alters other parameters like food intake, which may directly modify the level of plasma GH. In order to determine whether temperature regulates plasma GH levels independently of nutritional status, fish were reared at 8, 12, or 16 degrees C and either fed ad libitum (fish with different food intake) to determine the global effect of temperature, or with the same ration (1.2%/body weight) to observe the temperature effect in fish with the same growth rate. Plasma insulin level was inversely proportional to the temperature (8, 12, and 16 degrees C) in fish fed ad libitum (12.1+/-0.3 ng/ml, 10.9+/-0.3 ng/ml, 9.5+/-0.4 ng/ml; P<0.001) and in restricted fish (14.0+/-0.3 ng/ml, 11.3+/-0.3 ng/ml, 10.0+/-0.2 ng/ml; P<0.0001), probably due to a prolonged nutrient absorption, and delayed recovery of basal insulin level at low temperature. Conversely, temperature did not affect plasma T3 level of fish fed ad libitum (2.5+/-0.2 ng/ml, 2.4+/-0.1 ng/ml, 2.5+/-0.1 ng/ml at 8, 12, and 16 degrees C) while fish fed with the same ration present less T3 at 16 degrees C than at 8 degrees C (1.83+/-0.1 ng/ml versus 1.2+/-0.1 ng/ml; P<0.001) throughout the experiment; these observations indicate that different plasma T3 levels reflect the different nutritional status of the fish. The levels of GH1 and GH2 mRNA, and GH1/GH2 ratio were not different for whatever the temperature or the nutritional status. Pituitary GH content, of fish fed ad libitum did not exhibit obvious differences at 8, 12, or 16 degrees C (254+/-9 ng/g bw, 237+/-18 ng/g bw, 236+/-18 ng/g bw), while fish fed with the same ration have higher pituitary GH contents at 16 degrees C than at 8 degrees C (401+/-30 ng/g bw versus 285+/-25 ng/g bw; P<0.0001). Interestingly, high temperature strongly increases plasma GH levels (2.5+/-0.3 ng/ml at 8 degrees C versus 4.8+/-0.6 ng/ml at 16 degrees C; P<0.0001) to the same extent in both experiments, since at a given temperature average plasma GH was similar between fish fed ad libitum or a restricted diet. Our results, demonstrate that temperature regulates plasma GH levels specifically but not pituitary GH content, nor the levels of GH1 and GH2 mRNA. In addition no differential regulation of both GH genes was evidenced whatever the temperature.
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