NITRIC-OXIDE AND THERMOREGULATION DURING EXERCISE IN THE HORSE

The effect of inhibition of nitric oxide production on sweating rate ( SR) and on core, rectal, and tail skin temperatures was measured in fi ve Thoroughbred horses during exercise of variable intensity on a high -speed treadmill. A standard exercise test consisting of three canters [similar to 55% maximum O-2 uptake ((V) over dot O-2max)], with walki ng (similar to 9% (V) over dot O-2max) and trotting (similar to 22% (V ) over dot O-2max) between each canter, was performed twice (control o r test), in random order, by each horse. N-G-nitro-L-arginine methyl e ster (L-NAME; 20 mg/kg), a competitive inhibitor of nitric oxide synth ase, was infused into the central circulation and induced a significan t reduction in the SR measured on the neck (31.6 +/- 6.4 vs. 9.7 +/- 4 .2 g . min(-1). m(-2); 69%) and rump (14.7 +/- 5.2 vs. 4.8 +/- 1.6 g . min(-1). m(-2); 67%) of the horses during canter (P < 0.05). Signific ant increases in core, rectal, and tail skin temperatures were also me asured (P < 0.05). L-Arginine (200 mg/kg iv) partially reversed the in hibitory effects of L-NAME on SR, but core, rectal, and tail skin temp eratures continued to increase (P < 0.05), suggesting a cumulation of body heat. The results support the contention that nitric oxide syntha se inhibition diminishes SR, resulting in elevated core and peripheral temperatures leading to deranged thermoregulation during exercise. Th e inhibition of sweating by L-NAME may be related to peripheral vasoco nstriction but may also involve the neurogenic control of sweating.