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

The effect of nitric oxide synthase (NOS) inhibition during exercise on lac tate production was investigated in five Thoroughbred horses. A standard ex ercise test (SET), consisting of three canters (similar to 55 per cent (V) over dot O-2max), with walking and trotting between each canter, was perfor med twice (control and test, in random order) by each horse. N-phi-nitro-L- arginine methyl eater (L-NAME; 20 mg kg(-1)), a competitive inhibitor of No s, induced a significant increase (P < 0.05) in plasma lactate [5.7 (2.9) v s 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 ((V) over dot o) between ea ch SET. The results show that non-specific inhibition of Nos isoforms durin g exercise in the horse increases plasma lactate concentration, although th e mechanism's remain uncertain.