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.