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

The present study was carried out to examine theeffects of nitric oxide synthase inhibition withN-nitro-L-arginine methyl ester(L-NAME) on the right atrial as well as on the pulmonary arterial, capillary, and venous blood pressures of horses during rest and exercise performed at maximal heartrate (HR_max). Experiments werecarried out on seven healthy, sound, exercise-trained Thoroughbredhorses. Using catheter-tip manometers, with signals referenced at thepoint of the shoulder, we determined phasic and mean right atrial andpulmonary vascular pressures in two sets of experiments [control(no medications) and L-NAME (20 mg/kg iv given 10 min before exercise studies)]. The studies werecarried out in random order 7 days apart. Measurements were made atrest and during treadmill exercise performed on a 5% uphill grade at6, 8, and 14.2 m/s. Exercise on a 5% uphill grade at 14.2 m/s elicitedHR_max and could not be sustainedfor >90 s. In quietly standing horses,L-NAME administration caused asignificant rise in right atrial, as well as pulmonary arterial, capillary, and venous pressures. This indicates that nitric oxide synthase inhibition modifies the basal pulmonary vasomotor tone. Inboth treatments, exercise caused progressive significant increments inright atrial and pulmonary vascular pressures, but the values recordedin the L-NAME study were notdifferent from those in the control study. The extent ofexercise-induced tachycardia was significantly decreased in theL-NAME study at 6 and 8 m/s butnot at 14.2 m/s. Thus, L-NAMEadministration may not modify the equine pulmonary vascular tone duringexercise at HR_max. However, asindicated by a significant reduction in heart rate,L-NAME seems to modify thesympathoneurohumoral response to submaximal exercise.