Effects of nitric oxide inhibition on thermoregulation during exercise in the horse.

We investigated the role of NO in the control of thermoregulation. We measured sweating rate and body temperatures (core, rectal and skin) in five thoroughbred horses during exercise of variable intensity on a high-speed treadmill. A standard exercise test (SET) consisting of three canters (8 m s-1), with walking and trotting between each canter, was performed twice, in random order, by each horse and N omega-nitro-L-arginine methyl ester (L-NAME; 20 mg ml-1), a competitive inhibitor of nitric oxide synthase (NOS), was infused into the central circulation after the first canter in the test SET only. L-Arginine (200 mg ml-1), a substrate of NOS, was injected after the second canter in both control and test SETs. L-NAME significantly (p < 0.05) reduced the sweating rate measured on the neck (31.6 +/- 6.4 versus 9.7 +/- 4.2 g/min/m2) and rump (14.7 +/- 5.2 versus 4.8 +/- 1.6 g/min/m2) while raising the core temperature (39.7 +/- 0.2 versus 40.6 +/- 0.7 degrees C, p < 0.05) during the second canter. In the third canter, sweating rate had increased after giving L-arginine during the test SET, but had not returned to levels measured at similar times during the control SET. Core, rectal and skin temperatures continued to rise and were significantly higher than control levels, despite giving L-arginine. The results show that inhibition of NO production reduces sweating rate in the horse during exercise thereby inducing a rise in body temperatures.