Preexercise hypervolemia does not affect arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise

Nitric oxide synthase inhibition in thoroughbred horses augments O2 extraction at rest and submaximal exercise, but not during short-term maximal exercise

REASON FOR PERFORMING STUDY

Work is required to establish the role of endogenous nitric oxide (NO) in metabolism of resting and exercising horses.

OBJECTIVES

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.

CONCLUSION

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.

POTENTIAL RELEVANCE

It is unlikely that endogenous NO release modifies aerobic or anaerobic metabolism in horses performing short-term maximal exertion.

Pre-exercise hypervolaemia is not detrimental to arterial oxygenation of horses performing a prolonged exercise protocol simulating the second day of a 3-day equestrian event

REASONS FOR PERFORMING STUDY

Hyperhydration, prior to prolonged moderate-intensity exercise simulating the 2nd day of a 3-day equestrian event (E3DEC), may induce arterial hypoxaemia detrimental to performance.

OBJECTIVES

Because moderate-intensity exercise does not induce arterial hypoxaemia in healthy horses, the effects of pre-exercise hypervolaemia on arterial oxygenation were examined during a prolonged exercise protocol.

METHODS

Blood-gas studies were carried out on 7 healthy, exercise-trained Thoroughbred horses in control and hyperhydration experiments. The study conformed to a randomised crossover design. The sequence of treatments was randomised for each horse and 7 days were allowed between studies. Hyperhydration was induced by administering 0.425 g/kg bwt NaCl via nasogastric tube followed by free access to water. The exercise protocol was carried out on a treadmill set at a 3% uphill grade and consisted of walking at 2 m/sec for 2 min, trotting for 10 min at 3.7 m/sec, galloping for 2 min at 14 m/sec (which elicited maximal heart rate), trotting for 20 min at 3.7 m/sec, walking for 10 min at 1.8 m/sec, cantering for 8 min at 9.2 m/sec, trotting for 1 min at 5 m/sec and walking for 5 min at 2 m/sec.

RESULTS

NaCl administration induced a significant mean +/- s.e. 15.5 +/- 1.1% increase in plasma volume as indicated by a significant reduction in plasma protein concentration. In either treatment, whereas arterial hypoxaemia was not observed during periods of submaximal exercise, short-term maximal exertion caused significant arterial hypoxaemia, desaturation of haemoglobin, hypercapnoea, and acidosis in both treatments. However, the magnitude of exercise-induced arterial hypoxaemia, desaturation of haemoglobin, hypercapnoea, and acidosis in both treatments remained similar, and statistically significant differences between treatments could not be demonstrated.

CONCLUSIONS

It was concluded that significant pre-exercise expansion of plasma volume by this method does not adversely affect the arterial oxygenation of horses performing a prolonged exercise protocol simulating the 2nd day of an E3DEC.

Hyperhydration prior to a simulated second day of the 3-day moderate intensity equestrian competition does not cause arterial hypoxemia in Thoroughbred horses

Dehydration and the associated impairment of cardiovascular and thermoregulatory function comprise major veterinary problems in horses performing prolonged exercise, particularly under hot and humid conditions. For these reasons, there is considerable interest in using pre-exercise hyperhydration to help maintain blood volume in the face of the excessive fluid loss associated with sweat production during prolonged exertion. However, recently it was reported that pre-exercise hyperhydration causes arterial hypoxemia in horses performing moderate intensity exercise simulating the second day of an equestrian 3-day event competition (E3DEC) which may adversely affect performance (Sosa Leon et al. in Equine Vet J Suppl 34:425-429, 2002). These findings are contrary to data from horses performing short-term maximal exertion, wherein hyperhydration did not affect arterial O2 tension/saturation. Thus, our objective in the present study was to examine the impact of pre-exercise hyperhydration on arterial oxygenation of Thoroughbred horses performing an exercise test simulating the second day of an E3DEC. Control and hyperhydration studies were carried out on seven healthy Thoroughbred horses in random order, 7 days apart. In the control study, horses received no medications. In the hyperhydration experiments, nasogastric administration of NaCl (0.425 g/kg) 5 h pre-exercise induced a plasma volume expansion of 10.9% at the initiation of exercise. This methodology for inducing hypervolemia was different from that of Sosa Leon et al. (2002). Blood-gas tensions/pH as well as plasma protein, hemoglobin and blood lactate concentrations were measured pre-exercise and during the exercise test. Our data revealed that pre-exercise hyperhydration neither adversely affected arterial O2 tension nor hemoglobin-O2 saturation at any time during the exercise test simulating the second day of an E3DEC. Further, it was observed that arterial blood CO2 tension, pH, and blood lactate concentrations also were not affected by pre-exercise hyperhydration. However, hemodilution in hyperhydrated horses caused an attenuation of the expansion in the arterial to mixed-venous blood O2 content gradient during phases B and D of the exercise protocol, which was likely offset by an increase in cardiac output. It is concluded that pre-exercise hyperhydration of horses induced in the manner described above is not detrimental to arterial oxygenation of horses performing an exercise test simulating the second day of an E3DEC.

The Opioid Haemorphin-7 in Horses During Low-speed and High-speed Treadmill Exercise to Fatigue

The opioid neuropeptide haemorphin-7 was measured, by immunoreactivity, in Standardbred horses during low-speed (7 m/s) and high-speed (10 m/s) endurance exercises, lasting 49-58 and 12-16 min respectively. In parallel, heart rate, muscle temperature and plasma lactate concentrations were measured. The profile of the low-speed exercise showed significantly increased heart rate after 10 min [154 beats per minute (bpm)]. After the exercise, muscle temperature (42.1 degrees C) and plasma lactate (4.8 mmol/l) were significantly increased. The profile of the high-speed exercise was comparatively characterized by a higher increase of heart rate after 5 min (194 bpm) and higher increases of muscle temperature (43.2 degrees C) and lactate levels (15.8 mmol/l) after the exercise. The horses were probably exhausted by glycogen depletion in the low-speed exercise and by muscle pH decrease in the high-speed exercise. Haemorphin-7 increased significantly during the high-speed exercise (274.8 fmol/ml) but not during low speed (108.3 fmol/ml), coincident with the results of lactate. These results suggest that plasma haemorphin-7 is measurable in the horse by immunoreactivity, and that intense exercise stimulates release of this opioid. Such endogenous opioids are most likely involved in regulatory functions associated with pain, physical effort, inflammation, and blood pressure variation in horses, as have been established in other species.

Intrapulmonary arteriovenous shunts of >15 microm in diameter probably do not contribute to arterial hypoxemia in maximally exercising Thoroughbred horses

The present study examined whether Thoroughbred horses performing strenuous exercise exhibit intrapulmonary arteriovenous shunting that may contribute to the observed arterial hypoxemia. Experiments were carried out on seven healthy, exercise-trained Thoroughbreds at rest, maximal exercise (galloping at 14 m/s on a 3.5% uphill grade for 120 s), and submaximal exertion (8 m/s on a 3.5% uphill grade for 150 s). Along with blood gas/hemodynamic parameters, intrapulmonary arteriovenous shunting was studied by injecting 15-microm-diameter microspheres, labeled with different stable isotopes, into the right atrium while simultaneous blood samples were being withdrawn at a constant rate from the pulmonary artery and the aorta. Arterial hypoxemia was observed only during maximal exercise. Also, despite significant pulmonary arterial hypertension during submaximal and maximal exertion, 15-microm microspheres did not traverse the pulmonary microcirculation to appear in the aortic blood. Thus our findings did not support a role for intrapulmonary arteriovenous shunts of >15 microm in diameter in the exercise-induced arterial hypoxemia in racehorses. Interestingly, our observation that, in going from 30 to 120 s of maximal exertion, arterial O2 tension had remained unchanged despite significant reductions in mixed venous blood O2 tension, hemoglobin-O2 saturation, and O2 content also discounts the importance of intrapulmonary arteriovenous shunts in causing arterial hypoxemia. This is because, assuming that a constant fraction of total pulmonary blood flow bypasses the gas-exchange areas of the equine lungs via intrapulmonary arteriovenous shunts during 30-120 s of maximal exertion, the observed significant reductions in mixed venous blood oxygenation should cause a significant reduction in arterial O2 tension, which was not the case in our horses. Thus it is suggested that intrapulmonary arteriovenous shunting probably does not contribute to the exercise-induced arterial hypoxemia in racehorses.

Acute hypervolemia does not improve arterial oxygenation in maximally exercising thoroughbred horses

Recently, it was reported that acute hypervolemia improves arterial oxygen tension in human athletes known to experience exercise-induced arterial hypoxemia. Since exercise-induced arterial hypoxemia is routinely observed in racehorses and is known to limit performance, we examined whether pre-exercise induction of acute hypervolemia would similarly benefit arterial oxygenation in maximally exercising thoroughbred horses. Two sets of experiments, namely, placebo [intravenous (IV) physiological saline] and acute hypervolemia (IV 7.2% NaCl, causing an 18.2% expansion of plasma volume) studies were carried out in random order on 13 healthy, exercise-trained thoroughbred horses, 7 days apart. An incremental exercise protocol leading to 120 s of galloping at 14 m s(-1) on a 3.5% uphill incline was used. Galloping at this workload elicited maximal heart rate and induced pulmonary hemorrhage in all horses in both treatments. In the placebo study, arterial oxygen tension decreased to 76.1 (2) mmHg (P<0.0001) at 30 s of maximal exertion, but further significant changes did not occur as exercise duration increased to 120 s [arterial oxygen tension 72.4 (2) mmHg]. A significant arterial hypoxemia also developed in galloping horses in the acute hypervolemia study [arterial oxygen tension at 30 and 120 s was 76.7 (1.7) and 71.9 (1.6) mmHg, respectively], but significant differences between treatments could not be demonstrated. In both treatments, a similar desaturation of arterial hemoglobin was also observed at 30 s of maximal exercise, which intensified with increasing exercise duration as hyperthermia, acidosis and hypercapnia intensified. Thus, acute expansion of plasma volume did not benefit arterial oxygenation in maximally exercising thoroughbred horses.

NaHCO3 does not affect arterial O2 tension but attenuates desaturation of hemoglobin in maximally exercising Thoroughbreds

The objective of the present study was to examine the effects of preexercise NaHCO3 administration to induce metabolic alkalosis on the arterial oxygenation in racehorses performing maximal exercise. Two sets of experiments, intravenous physiological saline and NaHCO3 (250 mg/kg iv), were carried out on 13 healthy, sound Thoroughbred horses in random order, 7 days apart. Blood-gas variables were examined at rest and during incremental exercise, leading to 120 s of galloping at 14 m/s on a 3.5% uphill grade, which elicited maximal heart rate and induced pulmonary hemorrhage in all horses in both treatments. NaHCO3 administration caused alkalosis and hemodilution in standing horses, but arterial O2 tension and hemoglobin-O2 saturation were unaffected. Thus NaHCO3 administration caused a reduction in arterial O2 content at rest, although the arterial-to-mixed venous blood O2 content gradient was unaffected. During maximal exercise in both treatments, arterial hypoxemia, desaturation, hypercapnia, acidosis, hyperthermia, and hemoconcentration developed. Although the extent of exercise-induced arterial hypoxemia was similar, there was an attenuation of the desaturation of arterial hemoglobin in the NaHCO3-treated horses, which had higher arterial pH. Despite these observations, the arterial blood O2 content of exercising horses was less in the NaHCO3 experiments because of the hemodilution, and an attenuation of the exercise-induced expansion of the arterial-to-mixed venous blood O2 content gradient was observed. It was concluded that preexercise NaHCO3 administration does not affect the development and/or severity of arterial hypoxemia in Thoroughbreds performing short-term, high-intensity exercise.