L-NAME does not affect exercise-induced pulmonary hypertension in thoroughbred horses.

The research examines the impact of nitric oxide synthase inhibition using L-NAME on the heart and blood pressures of exercise-trained thoroughbred horses, both at rest and during physical activity. Findings indicate that L-NAME doesn’t affect equine pulmonary vascular tone during high heart-rate exercise but significantly changes the response to submaximal exercise.

Study Design and Participants

• The experiment involved seven healthy, sound, exercise-trained Thoroughbred horses.
• Tests were performed to analyze the effect of Nitric Oxide synthase inhibition with L-NAME on different body pressures (right atrial, pulmonary arterial, capillary, and venous blood pressures).
• The horses were subjected to two sets of experiments – once without any medications (control set) and once with L-NAME (administered intravenously 10 minutes before the exercise studies).

Methodology and Measurements

• The phasic and mean right atrial and pulmonary vascular pressures were determined using catheter-tip manometers, with signals referenced at the point of the shoulder.
• Measurements were made at rest and also during treadmill exercise performed on a 5% uphill grade at different speeds (6, 8, and 14.2 m/s).
• The fastest speed of 14.2 m/s was established as the threshold speed, as it accelerated the horses’ heart rate to its maximum, and couldn’t be sustained for more than 90 seconds.

Effects of L-NAME

• Administration of L-NAME to resting horses induced a significant rise in right atrial, pulmonary arterial, capillary, and venous pressures, indicating a modification of the basal pulmonary vasomotor tone.
• During exercise, right atrial and pulmonary vascular pressures increased, irrespective of whether L-NAME had been administered. However, the pressure values recorded in the L-NAME trials were not different from the control trials. This suggests that L-NAME may not alter the equine pulmonary vascular tone during high heart-rate exercise.
• Interestingly, the extent of exercise-induced tachycardia (abnormally rapid heart rate) was significantly reduced in the L-NAME trials at speeds of 6 and 8 m/s but not at the maximum speed of 14.2 m/s. This indicates that L-NAME could modify the sympathoneurohumoral response (the body’s physiological response to physical exercise) during submaximal exercise.