Effects of Temperature and Thyroid Hormone on Oxidative and Lipid Metabolism in Juvenile Lake Whitefish (Coregonus Clupeaformis)

Abstract

Nearly every aspect of biological function is influenced by temperature, including metabolism. Factors such as seasonal change and climate change both contribute to environmental temperature variability. While many eurythermal fish species can acclimate to varying environmental conditions by adjusting metabolic processes, the cellular mechanisms and endocrine control of these shifts have not been fully elucidated. I examined the interactions between elevated temperature exposure and hyperthyroidism on various metabolic markers in the cool-water teleost, lake whitefish (Coregonus clupeaformis). Juveniles were exposed to 13 (control), 17 or 21 ºC for 4, 8 or 24 days. Widespread changes in the abundance of mRNA transcripts were observed in enzymes associated with oxidative metabolism (citrate synthase, cytochrome c oxidase subunit 1), lipogenesis (acetyl-coA carboxylase , acetyl-coA carboxylase ) and peroxisomal -oxidation (acyl-coA oxidase 3) following exposure to elevated temperatures. However, the absence of effects on mitochondrial -oxidation enzyme isoforms, carnitine palmitoyltransferase  and carnitine palmitoyltransferase , combined with a decline in citrate synthase and cytochrome c oxidase activities, suggests that exposure to elevated temperature used in this study did not significantly increase overall metabolic demands in juvenile lake whitefish. To assess the role of hyperthyroidism in mediating thermal responses, thyroid status of fish was altered using thyroxine implants prior to the initiation of temperature change. Exogenous thyroxine treatment resulted in notable effects of on citrate synthase, acetyl-coA carboxylase , acetyl-coA carboxylase  and carnitine palmitoyltransferase  mRNA abundance, but only in fish exposed to 17 ºC or 21 ºC. Furthermore, these effects were transient, occurring primarily within the first 8 days of temperature change. Pronounced short-term decreases in carnitine palmitoyltransferase  mRNA abundance in the presence of exogenous thyroxine at all three temperatures suggests thyroid hormones may drive towards a less oxidative phenotype in liver, perhaps in an effort to promote/protect lipid stores. Overall, these results suggest temperature-dependent thyroid hormone action on the transcription of metabolic enzymes in juvenile lake whitefish which may be particularly influential in the early stages of temperature response.