Phenylbutazone increases right atrial pressure and heart rate of running horses.

The research investigates the influence of phenylbutazone, a cyclooxygenase inhibitor, on the cardiovascular responses in exercising horses. The findings suggest that phenylbutazone elevates heart rate and right atrial pressure during physical exertion, without noticeably affecting other hemodynamic parameters.

Study Design and Methodology

• The research was conducted on six horses which were subjected to a systematic exercise test on a treadmill.
• During the experiment, measurements were taken for rates of O2 consumption and CO2 production, and pressures in the carotid, pulmonary arterial, and right atrial areas.
• Testing was done after the horses were treated with either phenylbutazone, a known cyclooxygenase inhibitor, or a placebo for comparison.
• The dosage of phenylbutazone given was 8.8 mg/kg orally for two days and 4.4 mg/kg intravenously 60 minutes prior to exertion.

Main Findings

• The use of phenylbutazone resulted in an elimination of the usually observed increase in plasma 6-ketoprostaglandin F1 alpha and thromboxane B2 concentrations during exertion. This confirmed the successful inhibition of the cyclooxygenase activity.
• Phenylbutazone treatment led to noticeably (P < 0.05) higher heart rates and right atrial pressures during exertion when compared to the placebo-treated horses.
• This increase in heart rate and right atrial pressure due to phenylbutazone may have been triggered by an enhanced myocardial sensitivity to sympathetic stimulation or a reduction in venous compliance.
• No discernable impact of phenylbutazone was found on carotid or pulmonary arterial pressures, O2 consumption, CO2 production, or blood lactate concentration.
• Phenylbutazone also did not affect changes in plasma volume during exertion.

Conclusion and Implications

• The results indicate that cyclooxygenase byproducts likely play a role in mediating or altering some systemic hemodynamic responses to exertion in horses.
• This study, therefore, highlights the influence of cyclooxygenase activity on certain cardiovascular responses during physical exertion. The specific role of such influence warrants further investigation.
• The knowledge gained from this study could be important for understanding the physiological responses of horses during physical exertion, which could potentially influence strategies for managing cardiovascular health in these animals.