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Am J Physiol Regul Integr Comp Physiol 253: R896-R903, 1987;
0363-6119/87 $5.00
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AJP - Regulatory, Integrative and Comparative Physiology, Vol 253, Issue 6 896-R903, Copyright © 1987 by American Physiological Society

ARTICLES

Lactate kinetics in exercising Thoroughbred horses: regulation of turnover rate in plasma

J. M. Weber, W. S. Parkhouse, G. P. Dobson, J. C. Harman, D. H. Snow and P. W. Hochachka
Physiology Unit, Animal Health Trust, Newmarket, Suffolk, United Kingdom.

Plasma lactate turnover rate of Thoroughbred racehorses was measured by bolus injection of [U-14C]lactate at rest and two levels of submaximal treadmill exercise (3-4 m/s trot, 6% incline, and 6.5 m/s horizontal canter). Our goals were 1) to determine the relative effects of changes in cardiac output and in plasma lactate concentration on turnover rate [using cardiac output data from Weber et al. (28)] and 2) to assess the importance of lactate as a metabolic fuel in a trained animal athlete. Lactate turnover rates were 9.3 mumol.min-1.kg-1 (rest), 75.9 mumol.min-1.kg-1 at the beginning of the trot protocol [45% maximum O2 uptake (VO2max)], 50.3 mumol.min-1.kg-1 later in the same protocol (50% VO2max), and 66.1 mumol.min-1.kg-1 during the canter protocol (55% VO2max). Both changes in cardiac output and in plasma lactate concentration had a significant effect on turnover rate. Variation in plasma lactate fluxes of Thoroughbreds during exercise follows the standard mammalian pattern, but this substrate only plays a minor role as an oxidizable fuel in horses. The oxidation of plasma lactate accounts for less than 5% of metabolic rate (VO2) during submaximal work. Adjustments in cardiac output and in metabolite concentration represent, respectively, the coarse and fine controls for the regulation of plasma metabolite turnover rate.


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