Effect of L-NAME on oxygen uptake kinetics during heavy-intensity exercise in the horse

There is evidence that oxidative enzyme inertia plays a major role in limiting/setting the O(2) uptake (VO(2)) response at the transition to higher metabolic rates and also that nitric oxide (NO) competitively inhibits VO(2) within the electron transport chain. To investigate whether NO is important in setting the dynamic response of VO(2) at the onset of high-intensity (heavy-domain) running in horses, five geldings were run on a treadmill across speed transitions from 3 m/s to speeds corresponding to 80% of peak VO(2) with and without nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor (20 mg/kg; order randomized). L-NAME did not alter (both P > 0.05) baseline (3 m/s, 15.4 +/- 0.3 and 16.2 +/- 0.5 l/min for control and L-NAME, respectively) or end-exercise VO(2) (56.9 +/- 5.1 and 55.2 +/- 5.8 l/min for control and L-NAME, respectively). However, in the L-NAME trial, the primary on-kinetic response was significantly (P < 0.05) faster (i.e., reduced time constant, 27.0 +/- 2.7 and 18.7 +/- 3.0 s for control and L-NAME, respectively), despite no change in the gain of VO(2) (P > 0.05). The faster on-kinetic response was confirmed independent of modeling by reduced time to 50, 63, and 75% of overall VO(2) response (all P < 0.05). In addition, onset of the VO(2) slow component occurred earlier (124.6 +/- 11.2 and 65.0 +/- 6.6 s for control and L-NAME, respectively), and the magnitude of the O(2) deficit was attenuated (both P < 0.05) in the L-NAME compared with the control trial. Acceleration of the VO(2) kinetics by L-NAME suggests that NO inhibition of mitochondrial VO(2) may contribute, in part, to the intrinsic metabolic inertia evidenced at the transition to higher metabolic rates in the horse.