Pharmacokinetics and pharmacodynamics of three intravenous doses of yohimbine in the horse.

This study looks at the pharmacokinetics (how the body processes a drug) and pharmacodynamics (the effects the drug has on the body) of varying doses of yohimbine, which can counteract the depressive effects on the central nervous system from adrenergic receptor antagonists, in horses.

Introduction

• The research investigates yohimbine—an alpha 2 adrenergic receptor antagonist known to counteract the central nervous system’s (CNS) depressant effects from alpha 2 receptor agonists—in horses.
• This study was initiated after the detection of yohimbine in a regulatory sample from a racing horse, along with anecdotal evidence of CNS stimulation and documented cardiovascular changes when administered with an agonist.

Methodology

• Nine healthy adult horses received a single intravenous dose of yohimbine at three different concentrations: 0.1, 0.2, and 0.4 mg/kg.
• Blood samples were collected before drug administration and at various times up to 24 hours post-administration. These were subsequently analyzed using liquid chromatography-mass spectrometry (LC-MS).
• The data obtained from the plasma samples was analyzed using both noncompartmental and compartmental analysis to estimate pharmacokinetics parameters.

Findings

• Peak plasma concentrations after administration were 106.0 ± 28.9, 156.7 ± 34.3, and 223.0 ± 44.5 ng/mL for doses of 0.1, 0.2, and 0.4 mg/kg, respectively.
• Behavioral changes were observed immediately after administration: two horses showed signs of sedation, one appeared excited, while the other six appeared unaffected.
• All horses experienced episodes of tachycardia (rapid heart rate) within minutes of drug administration, regardless of the dose given. These episodes were not linked to the observed behavioral responses.
• A significant percentage of the horses (63%) showed signs of second-degree atrial-ventricular conduction blocks and bradycardia (slow heart rate) prior to yohimbine administration. However, these symptoms temporarily improved or disappeared upon yohimbine administration.
• Gastrointestinal sounds in all horses increased transiently after all doses.

Conclusion

• The result indicates that yohimbine induces different reactions in horses, such as sedation, excitement, and tachycardia.
• The cardiovascular changes observed support anecdotal suggestions of CNS stimulation and cardiovascular changes when yohimbine is administered in conjunction with an agonist.
• The findings also offer insight into the presence of yohimbine in racehorses, suggesting possible therapeutic use to treat bradycardia or atrial-ventricular conduction blockages in these animals.