In vitro responses of equine small airways and lung parenchyma.

The researchers investigated how small peripheral airways and lung parenchyma of horses respond in a lab setting, particularly focusing on their reactions to certain substances that could be involved in recurring airway obstruction, or ‘heaves’.

Overview of the Study

• The researchers primarily aimed to study how small peripheral airways (SA) and lung parenchyma (LP) of horses behave in vitro. They focused their attention on their contractile and relaxant responses.
• These body parts were chosen as they are implicated in the development or exacerbation of a common respiratory problem in horses known as recurrent airway obstruction (RAO) or ‘heaves’.
• The study also sought to determine the effects of certain bioactive compounds on these tissues, such as histamine, and endogenous prostanoids. These compounds are thought to be involved in RAO’s pathophysiology.

Key Findings

• When stimulated electrically, the SA and LP showed distinct reactions; SA contracted substantially, whereas LP’s contractions were only partially proportional to the stimulation frequency.
• Use of atropine (ATR) and tetrodotoxin (TTX) virtually eliminated contractions in SA, while only partial inhibition occurred for LP.
• Methacholine (MCh) had a higher potency and elicited a greater tension in SA than in LP.
• The SA response to electrical field stimulation (EFS) was significantly boosted at 1-4 Hz by both meclofenamate, a cyclooxygenase inhibitor, and 3 microM histamine. This amplification did not happen with LP.

Implications and Conclusions

• The study’s findings suggest that cholinergic nerves mostly determine SA tone, but they appear to have less effect on LP. This means that nerve supplies greatly influence the behavior of horse’s small peripheral airways, but not much in lung parenchyma.
• Prostanoids, naturally occurring hormone-like substances, may dampen the SA’s contraction response to nerve stimuli.
• Inhibition of cyclooxygenase, an enzyme linked with inflammation, and histamine greatly accentuate nerve stimulation responses in SA. Therefore, substances involved in inflammation and immune reaction processes may heighten airway contractions.
• These findings support the hypothesis that in ‘heaves’, histamine release and a dysregulated prostanoid balance may lead to obstruction of the small peripheral airways, potentially mediated by nerves related to the horse’s parasympathetic nervous system.