Action of sympathomimetic drugs on the bronchial circulation of the horse.

This study examines the effects of certain drugs, known as sympathomimetic drugs, on blood flow in horse bronchial arteries. The research found that these drugs, which mimic the effects of the human nervous system, either increased, decreased, or had no effect on bronchial artery flow and arterial pressure, suggesting the presence of specific receptors in the bronchial artery bed.

Experiment and Findings

• The study examined the effects of intravenous administration of sympathomimetic drugs – adrenaline, dobutamine, phenylephrine, noradrenaline, and clenbuterol – on the blood flow in the bronchial artery of horses.
• Adrenaline decreased bronchial artery flow, whereas dobutamine increased it. The changes in blood flow with these two drugs were accompanied by corresponding changes in arterial pressure.
• Phenylephrine and noradrenaline, on the other hand, caused the bronchial artery flow to decrease while the arterial pressure increased. This contradicts the responses seen with adrenaline and dobutamine, suggesting an active narrowing (vasoconstriction) of the bronchial circulation.
• Clenbuterol, however, had an opposite effect – an increase in bronchial artery flow was accompanied by a decrease in systemic blood pressure. This suggests a widening (vasodilation) of the bronchial arteries.

Interpretation of Results

• The different responses to the sympathomimetic drugs indicate the presence of two types of receptors in the bronchial artery – alpha-constrictor receptors and beta 2-dilator receptors.
• The alpha-constrictor receptors likely respond to drugs like phenylephrine and noradrenaline, causing vasoconstriction.
• The beta 2-dilator receptors may respond to dobutamine and clenbuterol, resulting in vasodilation.
• The researchers postulate that stimulation of these receptors could explain changes in bronchial artery flow that are seen in horses during exercise and anaesthesia.