Nitric oxide synthase inhibition does not affect the exercise-induced arterial
hypoxemia in Thoroughbred horses.
Manohar, Murli, Thomas E. Goetz, and Aslam S. Hassan.
Departments of 1Veterinary Biosciences and 2Clinical Medicine, College of
Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois
61801
APStracts 8:0270A, 2001.
Because sensitivity of equine pulmonary vasculature to endogenous as well as exogenous
nitric oxide (NO) has been demonstrated, we examined whether endogenous NO
production plays a role in exercise-induced arterial hypoxemia. We hypothesized that
inhibition of NO synthase may alter the distribution of ventilation-perfusion
mismatching, which may affect the exercise-induced arterial hypoxemia. Arterial blood-
gas variables were examined in seven healthy, sound Thoroughbred horses at rest during
incremental exercise protocol leading to galloping at maximal heart rate without (control;
placebo = saline) and with N?-nitro-l-arginine methyl ester (l-NAME) administration (20
mg/kg iv). The experiments were carried out in random order, 7 days apart. At rest, l-
NAME administration caused systemic hypertension, pulmonary hypertension, and
bradycardia. During 120 s of galloping at maximal heart rate, significant arterial
hypoxemia, desaturation of hemoglobin, hypercapnia, hyperthermia, and acidosis
occurred in the control as well as in NO synthase inhibition experiments. However,
statistically significant differences between the treatments were not found. In both
treatments, exercise caused a significant rise in hemoglobin concentration, but the
increment was significantly attenuated in the NO synthase experiments, and therefore,
arterial O2 content («cao2») increased to significantly lower values. These data suggest
that whereas l-NAME administration does not affect pulmonary gas exchange in
exercising horses, it may affect splenic contraction, which via an attenuation of the rise in
hemoglobin concentration and «cao2» may limit performance at higher workloads.
Received 8 March 2001; accepted in final form 24 April 2001
APS Manuscript Number A214-1.
Article publication pending J Appl Physiol
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 18 June 2001