Maximum O2 uptake, O2 debt and deficit, and muscle metabolites in Thoroughbred horses

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Abstract
This study determined maximal O2 uptake (.ovrhdot.VO2max), maximal O2 deficit, and O2 debt in the Thoroughbred racehorse exercising on an inclined treadmill. In eight horses the O2 uptake (.ovrhdot.VO2) vs. speed relationship was linear until 10 m/s and .ovrhdot.VO2max values ranged from 131 to 153 ml .cntdot. kg-1 .cntdot. min-1. Six of these horses then exercised at 120% of their .ovrhdot.VO2max until exhaustion. .ovrhdot.VO2, CO2 production (.ovrhdot.VCO2), and plasma lactate (La) were measured before and during exercise and through 60 min of recovery. Muscle biopsies were collected before and at 0.25, 0.5, 1, 1.5, 2, 5, 10, 15, 20, 40, and 60 min after exercise. Muscle concentrations of adenosine 5''-triphosphate (ATP), phosphocreatine (PC), La, glucose 6-phosphate (G-6-P), and creatine were determined, and pH was measured. The O2 deficit was 128 .+-. 32 (SD) ml/kg (64 .+-. 13 liters). The O2 debt was 324 .+-. 62 ml/kg (159 .+-. 37 liters), approximately two to three times comparative values for human beings. Muscle [ATP] was unchanged, but [PC] was lower (P < 0.01) than preexercise values at .ltoreq. 10 min of recovery. [PC] and .ovrhdot.VO2 were negatively correlated during both the fast and slow phases of .ovrhdot.VO2 during recovery. Muscle [La] and [G-6-P] were elevated for 10 min postexercise. Mean muscle pH decreased from 7.05 (preexercise) to 6.75 at 1.5 min recovery, and the mean peak plasma La value was 34.5 mmol/l. Relating the metabolic mechanisms for the O2 debt to the changes in muscle metabolites indicated that terminal oxidation of the estimated changes in [La] would require a greater .ovrhdot.VO2 than was measured. Rephosphorylation of PC accounted for < 1.5% of the O2 debt. The role of elevated postexercise temperature as a contributing factor to the O2 debt could not be assessed.