Lipid mobilization and the role of lipoproteins during swimming in salmonids

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Title: Lipid mobilization and the role of lipoproteins during swimming in salmonids
Authors: Magnoni, Leonardo J
Date: 2008
Abstract: Lipids circulate as non-esterified fatty acids (NEFA) or as triacylglycerol (TAG) associated with phospholipids (PL), the two main components of lipoproteins. By analogy to mammals, previous studies have focused on NEFA, but lipoproteins have not been considered as an energy shuttle to working muscles. The main goal of this thesis was to study the effects of exercise on fish lipoprotein metabolism. Additional objectives of this thesis were to clarify the mechanisms of action of lipoprotein lipase (LPL) on circulating lipids and to study how lipolytic rate may be regulated in vivo by catecholamines in fish. Circulating NEFA, TAG and PL were measured in sockeye salmon (Oncorhynchus nerka) at different stages of their spawning migration (Chapter 2). NEFA represent less than 7% of total plasma fatty acids and only show a minor decrease during migration. In contrast, lipoproteins account for more than 93% of all the energy of circulating lipids, and their main constituents show a 27-fold decrease (TAG) and a 6-fold decrease (PL) in concentration. These results are consistent with the idea that lipoproteins may be used to cover the metabolic demands of exercise. In Chapter 3, I investigated the effects of endurance swimming on the lipoprotein metabolism of rainbow trout (Oncorhynchus mykiss) under controlled laboratory conditions. Red muscle LPL activity increased from 18 +/- 5 (rest) to 49 +/- 9 nmol FA min-1 g-1 (swimming for 4 days at 1.5 body lengths/s). In Chapter 4, I measured TAG turnover rate for the first time in a non-mammalian vertebrate. Results show that the baseline TAG turnover rate of trout ranges from 24 to 49 mumol TAG kg-1 min -1 and is not affected by prolonged exercise (6 h at 1.5 body length/s), exceeding all values measured to date in endotherms. Finally, I measured lipolytic rate in intact rainbow trout at rest, and show contrasting effects of norepinephrine (NE) and epinephrine (Epi) administration on lipolysis (Chapter 5). Results show that trout maintain particularly high baseline lipolytic rate, because only 13% of their lipolytic rate can fuel resting energy metabolism entirely (87% reesterification). Baseline glycerol turnover rate (4.6 +/- 0.4 mumol kg-1 min-1) is inhibited by NE (-56%), instead of being stimulated as in mammals, whereas Epi has the same activating effect in both groups of vertebrates (+167%). I show that the TAG turnover rate in trout at rest can cover several times the energy requirements of locomotion. Additionally, I demonstrate that trout maintain high lipolytic rates, indicating that fatty acid mobilization and reesterification are particularly active in fish. Both characteristics of lipid metabolism can allow for the rapid cycling of fatty acids, and may be crucial for restructuring membrane phospholipids and, therefore, necessary in all ectotherms for adequate homeoviscous adaptation.
URL: http://hdl.handle.net/10393/29616
http://dx.doi.org/10.20381/ruor-19827
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
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