Plasma 5-hydroxytryptamine constricts equine digital blood vessels in vitro: implications for pathogenesis of acute laminitis.

This research investigates the ways in which the neurotransmitter 5-Hydroxytryptamine (5-HT), also known as serotonin, induces contraction in blood vessels within the digit of horses. This biological action could have potential implications for explaining the pathogenesis of acute laminitis, a common, painful and debilitating hoof disease in the equine population.

Research Procedures

• At the heart of the study were comparative tests conducted on isolated arterial rings obtained from several locations—digital, facial, tail, and coronary—in horses.
• In these tests, the response of these vessels to different concentrations of 5-HT was observed and measured. This response was typically a sort of vasoconstriction or contraction of the blood vessels.
• The researchers also compared the response of these vessels when endothelium, the layer of cells that lines the inner surface of blood vessels, was present and absent.
• The concentration of 5-HT within the plasma of equine blood, both platelet-rich and platelet-poor, was also determined via high-performance liquid chromatography (HPLC).

Key Findings

• The study discovered that 5-HT was particularly potent in constricting the digital arteries—in fact, significantly more so than arteries located in the face and tail. The absence of the endothelium didn’t alter the sensitivity of the digital arteries to 5-HT.
• Interestingly, coronary arteries did not contract at all in response to 5-HT. Instead, if pre-contracted, they almost showed a relaxation response to 5-HT—a response that was partially dependent on the presence of endothelium.
• The tests revealed a distinct concentration of 5-HT in platelet-rich equine plasma—significantly higher than that in platelet-poor plasma.
• Digital arteries were observed to contract when exposed to diluted platelet-poor equine plasma. This contraction was largely inhibited by 5-HT receptor antagonists, indicating the role of 5-HT in this contraction process.
• A strong correlation was established between the results obtained through both bioassay for plasma 5-HT and the HPLC method of estimating 5-HT concentration.
• The researchers also concluded that the free concentrations of 5-HT circulating in healthy horses might be enough to partially constrict digital blood vessels, although they were below the contraction threshold for other examined peripheral blood vessels.

Implications

• The study’s findings suggest a role of 5-HT in the development of acute laminitis in horses. The potent contraction effect it has on digital arteries, which are responsible for delivering blood to the horse’s hoof, might implicate it in the abnormal blood flow often observed in this condition.
• This new knowledge could be used to facilitate further research and aid in the development of therapeutic strategies to manage laminitis.
• It also opens up the broader investigation into the role of 5-HT and similar neurotransmitters on vascular behaviour, which might have implications for understanding and treating vascular diseases in humans and animals.