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Equine Centre, The Animal Health Trust, Newmarket, Suffolk CB8 7DW, United Kingdom
Received 8 July 1996; accepted in final form 22 November 1996.
Mills, Paul C., David J. Marlin, Caroline M. Scott, and
Nicola C. Smith. Nitric oxide and thermoregulation during exercise in the horse. J. Appl. Physiol. 82(4):
1035-1039, 1997.
The effect of inhibition of nitric oxide
production on sweating rate (SR) and on core, rectal, and tail skin
temperatures was measured in five Thoroughbred horses during exercise
of variable intensity on a high-speed treadmill. A standard exercise
test consisting of three canters [~55% maximum
O2 uptake
(
O2 max)], with
walking (~9%
O2 max) and trotting
(~22%
O2 max)
between each canter, was performed twice (control or test), in random
order, by each horse.
NG-nitro-L-arginine methyl ester
(L-NAME; 20 mg/kg), a
competitive inhibitor of nitric oxide synthase, was infused into the
central circulation and induced a significant 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). Significant increases in core, rectal, and tail
skin temperatures were also measured (P < 0.05).
L-Arginine (200 mg/kg iv)
partially reversed the inhibitory effects of
L-NAME on SR, but core, rectal,
and tail skin temperatures continued to increase
(P < 0.05), suggesting a cumulation
of body heat. The results support the contention that nitric oxide
synthase inhibition diminishes SR, resulting in elevated core and
peripheral temperatures leading to deranged thermoregulation during
exercise. The inhibition of sweating by
L-NAME may be related to
peripheral vasoconstriction but may also involve the neurogenic control
of sweating.
sweating rate; NG-nitro-L-arginine methyl ester; nitric oxide
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