Exogenous lactate supply affects lactate kinetics of rainbow trout, not swimming performance

Abstract

Intense swimming causes circulatory lactate accumulation in rainbow trout because lactate disposal (Rd) is not stimulated as strongly as lactate appearance (Ra). This mismatch suggests that maximal Rd is limited by tissue capacity to metabolize lactate. This study uses exogenous lactate to investigate what constrains maximal Rd and minimal Ra. Our goals were to determine how exogenous lactate affects: 1) Ra and Rd of lactate under baseline conditions or during graded swimming, and 2) exercise performance (critical swimming speed, Ucrit) and energetics (cost of transport, COT). Results show that exogenous lactate allows swimming trout to boost maximal Rd lactate by 40% and reach impressive rates of 56 μmol·kg(-1)·min(-1). This shows that the metabolic capacity of tissues for lactate disposal is not responsible for setting the highest Rd normally observed after intense swimming. Baseline endogenous Ra (resting in normoxic water) is not significantly reduced by exogenous lactate supply. Therefore, trout have an obligatory need to produce lactate, either as a fuel for oxidative tissues and/or from organs relying on glycolysis. Exogenous lactate does not affect Ucrit or COT, probably because it acts as a substitute for glucose and lipids rather than extra fuel. We conclude that the observed 40% increase in Rd lactate is made possible by accelerating lactate entry into oxidative tissues via monocarboxylate transporters (MCTs). This observation together with the weak expression of MCTs and the phenomenon of white muscle lactate retention show that lactate metabolism of rainbow trout is significantly constrained by transmembrane transport.

Coping with an exogenous glucose overload: glucose kinetics of rainbow trout during graded swimming

“…In addition, they suppress hepatic glucose production, but not completely as they do at rest (8). Supplying extra carbohydrates in the form of glucose does not improve swimming performance because U crit remains constant, as previously observed for exogenous lactate (30). However, the metabolic rate of fish receiving exogenous glucose is consistently lower than in controls, and the total cost of transport is reduced accordingly.…”

“…It is not known whether enhancing glucose availability could increase critical swimming speed (U crit ) in trout, but a previous study testing exogenous lactate showed no effect (30). The supply of exogenous glucose could also affect fuel selection by causing a switch from lipids to carbohydrates.…”

Metabolic fuel kinetics in fish: swimming, hypoxia and muscle membranes

“…More recently, the lactate kinetics of rainbow trout have been characterized in more detail using continuous tracer infusion, a method that allows quantification of the rates of appearance (R a ) in the circulation and disappearance from it (R d ) separately. The effects of environmental hypoxia (Omlin and Weber, 2010), graded exercise (Teulier et al, 2013) and exogenous lactate supply (Omlin et al, 2014) on endogenous R a and R d lactate have been measured (Figs 1, 2). After 90 min of hypoxia (at 25% air saturation), lactate accumulates to 9 mmol l −1 in the circulation because of a mismatch in fluxes resulting from the more rapid rise in R a (+98%) than in R d (+52%) (Fig.…”

“…2). This extra increase in disposal is made possible by elevated circulating lactate levels that accelerate MCT-mediated transport into oxidative tissues such as red muscle and heart (Omlin et al, 2014). MCT protein levels and activity may also be upregulated by exercise or exogenous lactate, but such mechanisms have never been investigated in fish.…”

“…Another possible explanation might be continuous lactic acid production by the gas gland to release O 2 from hemoglobin into the swim bladder by Root effect and to control buoyancy (Umezawa et al, 2012). Finally, when exogenous lactate is supplied during graded swimming, exercise performance is neither improved nor decreased because U crit remains unchanged (Omlin et al, 2014).…”