Metabolic effects of nitric oxide synthase inhibition during exercise in the horse

The effect of nitric oxide synthase (NOS) inhibition during exercise on lactate production was investigated in five Thoroughbred horses. A standard exercise test (SET), consisting of three canters (approximately 55 per cent VO2max), with walking and trotting between each canter, was performed twice (control and test, in random order) by each horse. Nphi-nitro-L-arginine methyl ester (L-NAME; 20 mg kg-1), a competitive inhibitor of NOS, induced a significant increase (P < 0.05) in plasma lactate [5.7 (2.9) vs 11.8 (3.8) mmol L-1], which continued to increase despite administration of L-arginine, the substrate for NOS. There were no differences in cardiac output (Q) or the total body oxygen consumption (VO) between each SET. The results show that non-specific inhibition of NOS isoforms during exercise in the horse increases plasma lactate concentration, although the mechanism/s remain uncertain.