PHENYLBUTAZONE INCREASES RIGHT ATRIAL PRESSURE AND HEART-RATE OF RUNNING HORSES

The effect of inhibition of cyclooxygenase activity on the hemodynamic response to exertion was examined In 6 horses. Rates of O-2 consumpti on and CO2 production and carotid, pulmonary arterial, and right atria l pressures were measured while the horses performed a standardized ex ercise test on a treadmill after treatment with phenylbutazone or a pl acebo. Phenylbutazone (8.8 mg/kg po for 2 days and 4.4 mg/kg iv 60 min before exertion) abolished the exertion-induced Increases in plasma 6 -ketoprostaglandin F-1 alpha and thromboxane Ba concentrations, confir ming inhibition of cyclooxygenase activity Phenylbutazone treatment re sulted In significantly (P < 0.05) higher heart rates and right atrial pressures during exertion than did treatment with placebo, which may have been due to increased myocardial sensitivity to sympathetic stimu lation and/or decreased venous compliance. There was not a detectable effect of phenylbutazone on carotid or pulmonary arterial pressures, O -2 consumption, CO2 production, or blood lactate concentration. Change s in plasma volume during exertion were not influenced by phenylbutazo ne. These results demonstrate that cyclooxygenase products likely medi ate or modulate some of the systemic hemodynamic responses to exertion in horses.