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

Lactate is now recognized as a regulator of fuel selection in mammals because it inhibits lipolysis by binding to the hydroxycarboxylic acid receptor 1 (HCAR1). The goals of this study were to quantify the effects of exogenous lactate on: 1) lipolytic rate or rate of appearance of glycerol in the circulation (Ra glycerol) and hepatic glucose production (Ra glucose), and 2) key tissue proteins involved in lactate signaling, glucose transport, glycolysis, gluconeogenesis, lipolysis, and β-oxidation in rainbow trout. Measurements of fuel mobilization kinetics show that lactate does not affect lipolysis as it does in mammals (Ra glycerol remains at 7.3 ± 0.5 µmol·kg-1·min-1), but strongly reduces hepatic glucose production (16.4 ± 2.0 to 8.9 ± 1.2 µmol·kg-1·min-1). This reduction is likely induced by decreasing gluconeogenic flux through the inhibition of cytosolic phosphoenolpyruvate carboxykinase (Pck1, alternatively called Pepck1; 60% and 24% declines in gene expression and protein level, respectively). It is also caused by lactate substituting for glucose as a fuel in all tissues except white muscle that increases glut4a expression and has limited capacity for monocarboxylate transporter (Mct)-mediated lactate import. We conclude that lipolysis is not affected by hyperlactatemia because trout show no activation of autocrine Hcar1 signaling (gene expression of the receptor is unchanged or even repressed in red muscle). Lactate regulates fuel mobilization via Pck1-mediated suppression of gluconeogenesis and by replacing glucose as a fuel. This study highlights important functional differences in the Hcar1 signaling system between fish and mammals for the regulation of fuel selection.

The mammalian insulin antagonist S961 does not exhibit insulin receptor antagonism in rainbow trout in vivo

Due to their reported 'glucose-intolerant' phenotype, rainbow trout have been the focus of comparative studies probing underlying endocrine mechanisms at the organismal, tissue and molecular level. A particular focus has been placed on the investigation of the comparative role of insulin, an important glucoregulatory hormone, and its interaction with macronutrients. A limiting factor in the comparative investigation of insulin is the current lack of reliable assays to quantify circulating mature and thus bioactive insulin. To circumvent this limitation, tissue-specific responsiveness to postprandial or exogenous insulin has been quantified at the level of post-translational modifications of cell signalling proteins. These studies revealed that the insulin responsiveness of these proteins and their post-translational modifications are evolutionarily highly conserved and thus provide useful and quantifiable proxy indices to investigate insulin function in rainbow trout. While the involvement of specific branches of the intracellular insulin signalling pathway (e.g., mTor) in rainbow trout glucoregulation have been successfully probed through pharmacological approaches, it would be useful to have a functionally validated insulin receptor antagonist to characterize the glucoregulatory role of the insulin receptor pathway in its entirety for this species. Here, we report two separate in vivo experiments to test the ability of the mammalian insulin receptor antagonist, S961, to efficiently block insulin signalling in liver and muscle in response to endogenously released insulin and to exogenously infused bovine insulin. We found that, irrespective of the experimental treatment or dose, activation of the insulin pathway in liver and muscle was not inhibited by S961, showing that its antagonistic effect does not extend to rainbow trout.

Alanine alters the carbohydrate metabolism of rainbow trout: glucose flux and cell signaling

In rainbow trout, dietary carbohydrates are poorly metabolized compared with other macronutrients. One prevalent hypothesis suggests that high dietary amino acid levels could contribute to the poor utilization of carbohydrates in trout. In mammals, alanine is considered an important gluconeogenic precursor, but has recently been found to stimulate AMP-activated protein kinase (AMPK) to reduce glucose levels. In trout, the effect of alanine on glucose flux is unknown. The goal of this study was to determine the effects of 4 h exogenous alanine infusion on glucose metabolism in rainbow trout. Glucose flux, and the rate of glucose appearance (Ra) and disposal (Rd) were measured in vivo. Key glycolytic and gluconeogenic enzyme expression and activity, and cell signaling molecules relevant to glucose metabolism were assessed in the liver and muscle. The results show that alanine inhibits glucose Ra (from 13.2±2.5 to 7.3±1.6 μmol kg-1 min-1) and Rd (from 13.2±2.5 to 7.4±1.5 μmol kg-1 min-1) and the slight mismatch between Ra and Rd caused a reduction in glycemia, similar to the effects of insulin in trout. The reduction in glucose Rd can be partially explained by a reduction in glut4b expression in red muscle. In contrast to mammals, trout alanine-dependent glucose-lowering effects did not involve hepatic AMPK activation, suggesting a different mechanistic basis. Interestingly, protein kinase B (AKT) activation increased only in muscle, similar to effects observed in insulin-infused trout. We speculate that alanine-dependent effects were probably mediated through stimulation of insulin secretion, which could indirectly promote alanine oxidation to provide the needed energy.