Contractile response of horse deep dorsal penile vein to histamine.

This study examines how histamine, a compound that your body makes when you’re having an allergic reaction, affects the contraction of veins in a horse’s penis. The study found that histamine triggers these veins to contract without endothelium involvement. This effect is mainly mediated by H1 receptors.

Research Objective

• The objective of this research was to determine the effect of histamine on isolated rings of the horse deep dorsal penile vein. More particularly, the research aimed at understanding the type of histamine receptor responsible for the contractile response.

Research Methodology

• The horse deep dorsal penile vein rings were isolated and their contractions were induced by histamine.
• The response was measured under precontracted or basal conditions.
• To identify the response specificity, the vein rings were pretreated with mepyramine, cimetidine, thioperamide – these are respective antagonists (inhibitors) of H1, H2, and H3 receptors for histamine.
• Similar testing was performed with (R)-alpha-methylhistamine, which is a specific H3 receptor agonist, and with propranolol that impacts heart and blood vessels.

Research Findings

• Histamine evokes a contraction in the isolated sections of the horse deep dorsal penile vein that is independent of the endothelium, the inner lining cells of the blood vessels.
• The contraction was found to be mainly mediated by histamine’s H1 receptors. This was proven by using the H1 receptor antagonist, mepyramine, which shifted the concentration-response curve to the right in a competitive manner, verifying the involvement of H1 receptors.
• The use of cimetidine, an H2 receptor antagonist, did not change the pEC50 (the negative log of the EC50, which is a measure of efficacy of a substance in producing a response) and the maximal contraction of the histamine response. This suggested that the H2 receptors do not play a significant role in histamine-induced contraction.
• The tested H3 receptor antagonist, thioperamide, did not affect the contractions induced by histamine either, implicating that H3 receptors also don’t have a major role in the observed contraction.
• The contraction response remained even with the use of (R)-alpha-methylhistamine, an H3 receptor agonist, in the presence of either thioperamide or tetrodotoxin, which is a potent neurotoxin.

Conclusion

• The research concludes that histamine induces a contraction response in the horse deep dorsal penile vein which is independent of the endothelium and it is mainly mediated by H1 receptors.