Inotropic mechanisms of dopexamine hydrochloride in horses.

This research article investigates the mechanisms behind the positive heart-strengthening effects of the drug dopexamine in horses. The study found that baroreflex activity and stimulation of beta 1 and beta 2-adrenergic receptors contribute significantly to these effects.

Introduction and Methodology

The researchers conducted the study on eight halothane-anesthetized horses. They specifically examined the cardiostimulatory effects of incrementally increased infusions of dopexamine, a drug known to strengthen the heart’s contractions.

• The effects of dopexamine were studied both before and after the administration of specific antagonists or drugs that counteract certain body actions.
• The antagonists were glycopyrrolate and chlorisondamine (to block muscarinic and nicotinic ganglionic activity respectively) and atenolol and ICI 118,551 (to block beta 1 and beta 2-adrenergic receptor activity respectively).

Results

The dopexamine infusions led to:

• A rise in heart rate, cardiac output (the quantity of blood the heart pumps per minute), systolic and mean arterial blood pressure, and maximum rate of left ventricular pressure development (a measure of heart contractility).
• A decrease in right atrial pressure and systemic vascular resistance (resistance to blood flow in blood vessels).

Impact of Antagonist Administration

The administration of glycopyrrolate, chlorisondamine, atenolol, and ICI 118,551 led to:

• Attenuation of the effects of dopexamine on cardiac output, blood pressures, and heart contractility.
• Potentiation (increase in potency) of dopexamine’s effect on heart rate.
• Decrease in heart rate, cardiac output, arterial blood pressure, and heart contractility from baseline values when beta 1-Adrenergic receptor was blocked.
• Further decrease in hemodynamic variables from baseline values and complete abolition of the cardiostimulatory effects of dopexamine when beta 2-Adrenergic receptor was blocked.

Conclusion

The study’s results indicate that baroreflex activity, and stimulation of beta 1- and beta 2-adrenergic receptors contribute to the positive heart-strengthening effects of dopexamine in horses. This could help better understand and optimize the use of dopexamine in veterinary medicine.