Effects of Exhaustive Exercise on Oxygen Consumption and Metabolism of Juvenile Rainbow Trout: An Evaluation of the O_2 Debt Hypothesis

Abstract

This thesis examined the effects of exhaustive exercise and post-exercise recovery on metabolism and respiratory gas exchange in juvenile rainbow trout, in relation to the classical oxygen (O₂) debt hypothesis (Hill & Lupton, 1923). The initial study was a detailed quantitative analysis of the metabolic cost of post-exercise recovery in terms of O₂ and ATP equivalents. A 5 min bout of exhaustive exercise resulted in about a 2.0-2.5 fold increase in O₂ consumption (MO₂), a 6-8 fold increase in whole-body lactate (LAC) levels and a near depletion in whole-body glycogen (GLY), adenosine triphosphate (ATP) and creatine phosphate (CP) stores. Recovery of MO₂, LAC and GLY was usually complete by 6 h, though GLY did not always return to resting levels. Recovery of resting whole-body ATP required 1.0-1.5 h, whereas restoration of CP required only 5 min. Quantitative budgets of the cost of recovery were prepared, based on two opposite assumptions. The first scenario (A) assumed that all the GLY resynthesized came from LAC, and that the remaining 25% of the total LAC cleared was oxidized. This scenario accounted for 48% of the excess post-exercise O₂ consumption (EPOC) and 94% of the ATP budget. The alternative scenario (B), attributed 100% of the EPOC to LAC oxidation (64% of the total LAC cleared), while the remaining LAC (36%) was resynthesized into GLY. As scenario B accounted for only 35% of the ATP budget, scenario (A) appeared more probable. Subsequent experiments attempted to experimentally dissociate LAC disappearance from EPOC, via repetitive exercise bouts and prior GLY depletion. A 2nd bout of exhaustive exercise, given 6 h after the 1st, significantly reduced EPOC by 40%, whereas metabolite status (LAC disappearance and GLY, ATP, and CP restoration) remained essentially unchanged. In contrast, prior GLY depletion by 5 days starvation significantly lowered the LAC burden while EPOC remained unchanged. Neither the two scenarios of the theoretical analysis could adequately explain the relationships between EPOC and the various metabolites. It is likely that some blend of the two approaches may be more realistic. Nevertheless, these approaches all point to the conclusion that the EPOC is not directly related to the metabolism of LAC after exercise in the rainbow trout. Thus, in contrast to the classical O₂ debt hypothesis, LAC disappearance does not determine the magnitude EPOC after exhaustive exercise in rainbow trout.