In vitro effects of alpha 2-adrenergic receptor stimulation on cholinergic contractions of equine distal airways.

The research investigates the effects of alpha 2-adrenergic receptor stimulation on cholinergic contractions in horse airways, particularly in horses with reversible obstruction. The study found that the receptor stimulation inhibits contraction of bronchi induced by the electric field, offering insights into how certain sedatives improve lung function in affected horses.

Study Overview

• The study aims to understand the underlying mechanism through which certain sedative drugs (specifically the alpha 2-adrenergic agonist xylazine) improve the pulmonary function in horses suffering from reversible airway obstruction, a condition widely known as heaves.
• The major focus of the researchers was to assess how alpha 2-adrenergic receptor stimulation impacts the reaction of isolated distal airways to cholinergic stimulation.

Methodology

• The bronchial segments of 22 healthy horses were isolated and suspended in specialized organ baths. This set-up allowed the researchers to carefully study the mechanical responses of these tissues to various stimuli.
• The tissues were incubated with different concentrations of clonidine, a drug often used in hypertension treatment, either alone or in combination with tolazoline, another medication.
• After incubation, the contractile response of the tissues to either cumulative addition of acetylcholine (ACh) or cumulative electrical field stimulation (EFS) was recorded.

Results

• Any contractile responses invoked by EFS were found to have been mediated through the stimulation of cholinergic airway nerves.
• The researchers noted that clonidine did not influence the contractile response of distal airway segments to ACh. However, at concentrations higher than 10(-5) M, clonidine significantly diminished the contractile response of these airway segments to EFS.
• This inhibitory effect of clonidine was absent when tolazoline was present.
• Similar outcomes were observed when the less-selective alpha 2-adrenergic agonist xylazine was used instead of clonidine.

Conclusion

• The study concluded that alpha 2-adrenergic receptor stimulation seems to cause a presynaptic inhibition of cholinergic nerves that innervate distal parts of the bronchi in horses. This effect only seems to inhibit contractions that are induced by EFS but not those triggered by ACh.
• The study provides useful insights into potentially more effective methods of managing heaves in horses.