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Departments of Veterinary Biosciences and Clinical Medicine, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
The present study was carried out to examine the
effects of nitric oxide synthase inhibition with
N
-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 heart
rate (HRmax). Experiments were
carried out on seven healthy, sound, exercise-trained Thoroughbred
horses. Using catheter-tip manometers, with signals referenced at the
point of the shoulder, we determined phasic and mean right atrial and
pulmonary 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 were
carried out in random order 7 days apart. Measurements were made at
rest and during treadmill exercise performed on a 5% uphill grade at
6, 8, and 14.2 m/s. Exercise on a 5% uphill grade at 14.2 m/s elicited
HRmax and could not be sustained
for >90 s. In quietly standing horses,
L-NAME administration caused a
significant 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. In
both treatments, exercise caused progressive significant increments in
right atrial and pulmonary vascular pressures, but the values recorded
in the L-NAME study were not
different from those in the control study. The extent of
exercise-induced tachycardia was significantly decreased in the
L-NAME study at 6 and 8 m/s but
not at 14.2 m/s. Thus, L-NAME
administration may not modify the equine pulmonary vascular tone during
exercise at HRmax. However, as
indicated by a significant reduction in heart rate,
L-NAME seems to modify the
sympathoneurohumoral response to submaximal exercise.
pulmonary hemodynamics; right heart pressures; N-nitro-L-arginine methyl ester; nitric oxide synthase inhibition; cardiac catheterization; exercise-induced pulmonary hemorrhage ; exercise-induced pulmonary
hypertension
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