(V)over-dot-O-2 kinetics in the horse during moderate and heavy exercise

The horse is a superb athlete, achieving a maximal O-2 uptake (similar to 160 ml.min(-1).kg(-1)) approaching twice that of the fittest humans. Although equine O-2 uptake ((V) over dotO(2)) kinetics are reportedly fast, they have not been precisely characterized, nor has their exercise intensity dependence been elucidated. To address these issues, adult male horses underwent incremental treadmill testing to determine their lactate threshold (T-lac) and peak (V) over dotO(2) ((V) over dotO(2peak)), and kinetic features of their (V) over dotO(2) response to ''square-wave'' work forcings were resolved using exercise transitions from 3 m/s to a below-T-lac speed of 7 m/s or an above-T-lac speed of 12.3 +/- 0.7 m/s (i.e., between T-lac and (V) over dotO(2peak)) sustained for 6 min. (V) over dotO(2) and CO2 output were measured using an open-flow system: pulmonary artery temperature was monitored, and mixed venous blood was sampled for plasma lactate. (V) over dotO(2) kinetics at work levels below T-lac were well fit by a two-phase exponential model, with a phase 2 time constant (tau(1) = 10.0 +/- 0.9 s) that followed a time delay(TD1 = 18.9 +/- 1.9 s). TD1 was similar to that found in humans performing leg cycling exercise, but the time constant was substantially faster. For speeds above T-lac, TD1 was unchanged (20.3 +/- 1.2 s); however, the phase 2 time constant was significantly slower (tau(1) = 20.7 +/- 3.4 s, P < 0.05) than for exercise below T-lac. Furthermore, in four of five horses, a secondary, delayed increase in (V) over dotO(2) became evident 135.7 +/- 28.5 s after the exercise transition. This ''slow component'' accounted for similar to 12% (5.8 +/- 2.7 l/min) of the net increase in exercise (V) over dotO(2). We conclude that, at exercise intensities below and above T-lac, qualitative features of (V) over dotO(2) kinetics in the horse are similar to those in humans. However, at speeds below T-lac the fast component of the response is more rapid than that reported for humans, likely reflecting different energetics of O-2 utilization within equine muscle fibers.