Nitric oxide synthase inhibition in thoroughbred horses augments O2 extraction at rest and submaximal exercise, but not during short-term maximal exercise.
BACKGROUND
Work is required to establish the role of endogenous nitric oxide (NO) in metabolism of resting and exercising horses.


OBJECTIVE
To examine the effects of NO synthase inhibition on O2 extraction and anaerobic metabolism at rest, and during submaximal and maximal exertion.


METHODS
Placebo and NO synthase inhibition (with Nomega-nitro-L-arginine methyl ester [L-NAME] administered at 20 mg/kg bwt i.v.) studies were performed in random order, 7 days apart on 7 healthy, exercise-trained Thoroughbred horses at rest and during incremental exercise leading to 120 sec of maximal exertion at 14 m/sec on a 3.5% uphill grade.


RESULTS
At rest, NO synthase inhibition significantly augmented the arterial to mixed-venous blood O2 content gradient and O2 extraction as mixed-venous blood O2 tension and saturation decreased significantly. While NO synthase inhibition did not affect arterial blood-gas tensions in exercising horses, the exercise-induced increment in haemoglobin concentration and arterial O2 content was attenuated. In the L-NAME study, during submaximal exercise, mixed-venous blood O2 tension and haemoglobin-O2 saturation decreased to a greater extent causing O2 extraction to increase significantly. During maximal exertion, arterial hypoxaemia, desaturation of haemoglobin and hypercapnia of a similar magnitude developed in both treatments. Also, the changes in mixed-venous blood O2 tension and haemoglobin-O2 saturation, arterial to mixed-venous blood O2 content gradient, O2 extraction and markers of anaerobic metabolism (lactate and ammonia production, and metabolic acidosis) were not different from those in the placebo study.


CONCLUSIONS
Endogenous NO production augments O2 extraction at rest and during submaximal exertion, but not the during short-term maximal exercise. Also, NO synthase inhibition does not affect anaerobic metabolism at rest or during exertion.


CONCLUSIONS
It is unlikely that endogenous NO release modifies aerobic or anaerobic metabolism in horses performing short-term maximal exertion.
Manohar
M
M
Department of Veterinary Biosciences and Clinical Medicine, College of Veterinary Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
Goetz
T E
TE
Hassan
A S
AS
D000818:Animals; D018592:Cross-Over Studies; D004734:Energy Metabolism / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005260:Female; D006736:Horses / Q000097:blood / Q000378:metabolism / Q000502:physiology*; D008297:Male; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology*; D009569:Nitric Oxide / Q000378:metabolism / Q000502:physiology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors* / Q000378:metabolism; D010100:Oxygen / Q000097:blood*; D010805:Physical Conditioning, Animal / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology
D004791:Enzyme Inhibitors; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D010100:Oxygen; D019331:NG-Nitroarginine Methyl Ester