Glycerol and fatty acid kinetics in rainbow trout: effects of endurance swimming

Abstract

Continuous infusions of 2-[3H]glycerol and 1-[14C]palmitate were performed in vivo in rainbow trout to measure the effects of prolonged swimming on (1) the rate of appearance of glycerol (Ra glycerol or lipolytic rate), (2) the rate of appearance of non-esterified fatty acids (Ra NEFA) and (3) the rate of triacylglycerol:fatty acid cycling (TAG:FA cycling or re-esterification). Our goals were to test the hypothesis that sustained exercise for up to 4 days causes the progressive mobilization of triacylglycerol reserves to supply fuel to contracting muscles, and to assess whether TAG:FA cycling plays a role in the regulation of NEFA availability in teleosts. Contrary to expectation, the rates of lipolysis and fatty acid release in resting trout are not affected by endurance exercise. Unlike mammals, which increase the rate of lipolysis by two- to fourfold during submaximal exercise, these active teleosts do not mobilize triacylglycerol reserves beyond resting levels to supply more NEFAs to working muscles. Furthermore, they maintain Ra glycerol and Ra NEFA well in excess of oxidative fuel requirements even at rest. More than two-thirds of the NEFAs produced are re-esterified, but the results show that TAG:FA cycling is not involved in the regulation of NEFA availability during or after swimming. We propose that the observed high rates of re-esterification represent an important feature of ectothermic metabolism that allows the restructuring of membrane phospholipids to be synchronized with frequent changes in body temperature.

On the Utilization of Dietary Glycerol in Carnivorous Fish—Part II: Insights Into Lipid Metabolism of Rainbow Trout (Oncorhynchus mykiss) and European Seabass (Dicentrarchus labrax)

“…On the other hand, dorsal aorta cannulation in combination with a continuous infusion of a radiolabeled metabolite requires a surgical procedure, which is highly dependent on fish size for access through the mouth and on the skills of highly trained personnel (Haman and Weber, 1996). This is an invasive method that, if properly setup, can allow for accurate estimation of substrate fluxes (Weber et al, 2016), including glycerol (Bernard et al, 1999;Magnoni et al, 2008), in fully recovered fish under minimal stress. If using radioactive isotopes like [ 3 H] or [ 14 C] glycerol, additional laboratory security procedures and safe discard of leftover solutions and biomass must be in place, amounting to experimental costs.…”

Lactate signaling and fuel selection in rainbow trout: mobilization of energy reserves

“…These tracer methods have been thoroughly validated in fish (28) and specifically tested in rainbow trout to assess the effects of a variety of physiological stresses on R a glucose (14,19,20,29). The method to quantify R a glycerol is also well-established for rainbow trout (21)(22)(23). All infusates were freshly prepared immediately before each experiment by drying an aliquot of the tracer obtained from the supplier under N 2 and resuspending in Cortland's saline.…”

“…The rate of appearance of glycerol in the circulation is the best index of wholeorganism lipolytic rate (30) and it has been widely used to assess lipid mobilization in mammals (33,(44)(45)(46)(47) as well as fish (21)(22)(23). This study is the first to investigate the effects of exogenously increased circulating lactate on lipolytic rate in vivo for any species.…”

“…Rainbow trout always maintain very high rates of lipolysis and triacylglycerol/fatty acid substrate cycling under a wide range of physiological conditions that include rising plasma lactate concentration up to ~10 mM in the resting state (Fig. 4), endurance exercise lasting several days (22), and graded exercise up to critical swimming speed (23). In mammals, lactate inhibits lipolysis (5) and similar stresses of endurance and strenuous exercise cause major changes in fat mobilization (45,52).…”

“…The mobilization of lipid reserves (lipolysis or rate of appearance of glycerol in the circulation: R a glycerol) is also increased by epinephrine, but contrary to its effect in mammals, it is inhibited by norepinephrine in fish (21). Surprisingly, fish lipolytic rate remains unaffected by endurance swimming (22) or graded exercise up to critical swimming speed (U crit ) (23). Whether lactate signaling or Hcar1 are involved in modulating fuel selection in fish is unknown.…”