Lactate kinetics in exercising Thoroughbred horses: regulation of turnover rate in plasma.
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.
Weber
J M
JM
Physiology Unit, Animal Health Trust, Newmarket, Suffolk, United Kingdom.
Parkhouse
W S
WS
Dobson
G P
GP
Harman
J C
JC
Snow
D H
DH
Hochachka
P W
PW
D000818:Animals; D050260:Carbohydrate Metabolism; D006736:Horses / Q000097:blood / Q000502:physiology*; D007700:Kinetics; D007773:Lactates / Q000097:blood*; D019344:Lactic Acid; D008657:Metabolic Clearance Rate; D005082:Physical Exertion; D012016:Reference Values
D007773:Lactates; D019344:Lactic Acid