Evidence of histamine receptor function in isolated horse penile dorsal arteries.

This research investigates how histamine, acting on a specific type of receptor known as H1, influences the behavior of horse penile dorsal arteries. The study demonstrates that these arteries show a biphasic (two-phase) response to histamine – an initial relaxation followed by contraction. Importantly, it appears that this effect is mediated largely by H1 receptors, with nitric oxide also playing a partial role in promoting relaxation of precontracted vessels.

Study Design and Methods

• The researchers treated isolated horse penile dorsal arteries with various concentrations of histamine and observed the resulting responses, including both relaxation and contraction.
• They examined the role of H1 receptors in these responses, using PEA, an H1 receptor agonist (enhances the receptor’s action), and mepyramine, an H1 receptor antagonist (inhibits the receptor’s action).
• To understand the involvement of other components in the response, experiments with nitric oxide inhibitors, and different receptor agonists (dimaprit, (R)-alpha-methylhistamine) and antagonists (cimetidine, thioperamide) were also conducted.
• Endothelium (the inner lining of the blood vessels) was also removed in some experiments to see its effect on the histamine-induced responses.

Key Findings

• Histamine induced a two-part response: initial relaxation followed by contraction in precontracted vessels, but only a contractile effect in vessels at their baseline tone.
• The H1 receptor agonist PEA reproduced this response pattern, suggesting a key role for H1 receptors.
• The H1 receptor antagonist mepyramine blocked the relaxing effect of histamine, supporting the importance of H1 receptors in mediating this response.
• Researchers found no evidence for a role of H2 and H3 receptors as there was no response to their respective agonists and antagonists.
• The removal of the endothelium or nitric oxide synthesis inhibitors altered the relaxant response but did not affect the maximal contraction induced by histamine.

Conclusion and Implications

• The study concluded that the two-part response to histamine in horse penile dorsal arteries is most likely mediated by H1 receptors, with some involvement of nitric oxide in promoting relaxation.
• The findings could provide valuable insights into the mechanisms by which histamine influences blood vessels’ behavior and possibly contribute to understanding of diseases involving abnormal vasoreactivity.