McKeever Kh

Forosemide magnifies the exercise-induced elevation of plasma vasopressin concentration in horses

The purpose of this study was to test the hypothesis that furosemide administration before exercise would cause greater increases in plasma arginine vasopressin (AVP) concentration in exercising horses than exercise alone. Six adult, clinically normal, unfit mares underwent three randomly ordered 60 minute standard exercise tests on an equine treadmill to examine the effect of furosemide administration on plasma AVP concentration. In one trial, furosemide (1 mg kg-1) was infused four hours before exercise (FUR-4) and a placebo (10 ml saline) was infused two minutes before exercise; in another trial the placebo was infused four hours before exercise and drug was infused two minutes before exercise (FUR-2); in a third trial a placebo was infused four hours and two minutes before exercise (CON). During the treadmill test each mare ran up a fixed 4 degrees incline for one hour at a speed previously determined to produce a heart rate of 65 per cent of each horses maximum heart rate. Venous blood samples were collected at rest in the stall, immediately before exercise while standing on the treadmill, and at 15 minute intervals during the treadmill test. Plasma AVP concentration was measured by radioimmunoassay. In the CON trial, plasma AVP concentration increased 561 per cent (P < 0.05) from 6.3 +/- 1.0 pg ml-1 (mean +/- SE) at rest to 38.8 +/- 12.8 pg ml-1 at the end of the 60 minute run. During the FUR-2 trial, AVP increased 1185 per cent (P < 0.05) from 5.9 +/- 1.7 pg ml-1 to 75.8 +/- 17.7 pg ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)

EFFECT OF PHENYLBUTAZONE ON THE HAEMODYNAMIC, ACID-BASE AND EICOSANOID RESPONSES OF HORSES TO SUSTAINED SUBMAXIMAL EXERTION

The systemic haemodynamic and acid-base effects of the administration of phenylbutazone (4.4 mg kg-1 intravenously) to standing and running horses were investigated. Phenylbutazone, or a placebo, was administered to each of six mares either 15 minutes before, or after 30 minutes of a 60-minute submaximal exercise test which elicited heart rates approximately 55 per cent of maximal, and to the same horses at rest. The variables examined included the cardiac output, heart rate, systemic and pulmonary arterial pressures, right atrial and right ventricular pressures, and arterial and mixed venous blood gases and pH. Serum sodium, potassium and chloride concentrations, and plasma thromboxane B2, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), and prostaglandin E2 (PGE2) concentrations were measured in separate studies using similar protocols in the same horses. Running produced increases in heart rate, cardiac output, mean arterial and right ventricular pressure, and decreases in total peripheral resistance. The acid:base responses to exertion were characterised by respiratory alkalosis. Exertion did not significantly influence plasma 6-keto-PGF1 alpha or PGE2 concentrations but plasma thromboxane B2 concentrations were increased significantly by 60 minutes of exertion in the untreated horses. This exercise-induced increase in plasma thromboxane B2 concentration was inhibited by the previous administration of phenylbutazone, but phenylbutazone did not produce detectable changes in systemic haemodynamic or acid-base variables in either standing or running horses.

Effects of Sympathomimetic and Sympatholytic Drugs on Exercise Performance

This article has presented information on the importance of the sympathetic nervous system in the response to exercise. The authors have reviewed the very limited information on the effects of sympathomimetic and sympatholytic drugs on exercise performance in the horse. Most of these drugs are specifically prohibited under the rules of racing, and they have significant side effects that either decrease performance or make their use dangerous to both the horse and horse-man. Additionally, all of these drugs or their metabolites are readily detected by current drug testing protocols. Further information is needed to expand our knowledge of how each of these substances affects the health of the equine athlete.