(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 human s. Although equine O-2 uptake ((V) over dotO(2)) kinetics are reported ly fast, they have not been precisely characterized, nor has their exe rcise intensity dependence been elucidated. To address these issues, a dult male horses underwent incremental treadmill testing to determine their lactate threshold (T-lac) and peak (V) over dotO(2) ((V) over do tO(2peak)), and kinetic features of their (V) over dotO(2) response to ''square-wave'' work forcings were resolved using exercise transition s from 3 m/s to a below-T-lac speed of 7 m/s or an above-T-lac speed o f 12.3 +/- 0.7 m/s (i.e., between T-lac and (V) over dotO(2peak)) sust ained for 6 min. (V) over dotO(2) and CO2 output were measured using a n open-flow system: pulmonary artery temperature was monitored, and mi xed venous blood was sampled for plasma lactate. (V) over dotO(2) kine tics at work levels below T-lac were well fit by a two-phase exponenti al 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 f ound in humans performing leg cycling exercise, but the time constant was substantially faster. For speeds above T-lac, TD1 was unchanged (2 0.3 +/- 1.2 s); however, the phase 2 time constant was significantly s lower (tau(1) = 20.7 +/- 3.4 s, P < 0.05) than for exercise below T-la c. 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 c onclude that, at exercise intensities below and above T-lac, qualitati ve features of (V) over dotO(2) kinetics in the horse are similar to t hose in humans. However, at speeds below T-lac the fast component of t he response is more rapid than that reported for humans, likely reflec ting different energetics of O-2 utilization within equine muscle fibe rs.