H1-receptor antagonist, tripelennamine, does not affect arterial hypoxemia in exercising Thoroughbreds.

The research investigates whether the use of an H1-receptor antagonist can reduce exercise-induced arterial hypoxemia (EIAH), a condition that affects performance in racehorses. However, they found no significant difference in the extent of EIAH following treatment, suggesting that histamine release is not a major factor in this common equine condition.

Study Objective and Background

• The study was conducted to determine whether an H1-receptor antagonist, tripelennamine, could reduce exercise-induced arterial hypoxemia (EIAH) in Thoroughbred horses. EIAH is a condition marked by lower than normal levels of oxygen in the arterial blood during exercise. Some believe that this condition may result from the stress-induced rupture (i.e., stress failure) of tiny blood vessels (capillaries) in the horses’ lungs.
• Previous research suggested that inflammation and injuries to the lungs, which can cause cells called mast cells to release the chemical histamine, might play a role in EIAH. As blocking H1-receptors can mitigate the impact of histamine, the researchers wanted to see if an H1-receptor antagonist could help prevent the condition.

Methodology

• Researchers conducted two experiments with seven healthy, trained Thoroughbred horses. The horses underwent the same strenuous exercise regimen (galloping at 14 m/s up a 3.5% incline for 120 seconds) one week apart.
• In one trial, the horses were given a saline placebo before exercising. In the other, they were given tripelennamine 15 minutes before the workout.
• Scientist made measurements of arterial and venous blood gas and pH levels, both at rest and during peak physical exertion.

Findings

• Maximum heart rate during the exercise, and the resulting exercise-induced pulmonary hemorrhage (i.e., lung bleeding caused by capillary rupture), were observed in all horses in both trials, indicating that intense exercise did indeed result in lung injury.
• However, levels of EIAH, as well as other measures of respiratory stress such as hemoglobin desaturation, high levels of carbon dioxide in the blood (hypercapnia), and a pH imbalance suggesting acidosis were found to be similar in both the placebo and the drug trials.
• This lack of noticeable difference led the researchers to conclude that tripelennamine did not significantly impact EIAH in the horses, implying that histamine release may not be a major factor in EIAH.