Uptake of 5-hydroxytryptamine by equine digital vein endothelial cells: inhibition by amines found in the equine caecum.
Authors: Bailey S R, Wheeler-Jones C, Elliott J
Journal: Equine veterinary journal
Summary
# Editorial Summary Serotonin (5-HT) is a potent vasoconstrictor in equine digital blood vessels and is thought to contribute to acute laminitis development, yet the mechanisms regulating its activity in this vascular bed remain poorly understood. Bailey and colleagues investigated whether endothelial cells from equine digital veins possess active uptake mechanisms for serotonin and whether other amines produced in the caecum might interfere with this clearance system. Using cultured digital vein endothelial and smooth muscle cells exposed to radioactive serotonin at varying concentrations, the researchers found that endothelial cells—but not smooth muscle cells—demonstrated saturable, facilitated uptake of serotonin; notably, the affinity was substantially lower (Km 41.6 micromol/l) than in pulmonary vein endothelial cells, suggesting digital vessels are less efficient at clearing serotonin. Critically, all six caecal-derived amines tested (tyramine, spermine, tryptamine, phenylethylamine and others) competitively inhibited serotonin uptake in a dose-dependent fashion, with tyramine showing the strongest inhibitory effect. These findings establish a mechanistic link between gastrointestinal disturbances and laminitis pathophysiology: disrupted caecal flora or gut dysfunction increasing circulating amines could amplify serotonin-mediated digital vasoconstriction by saturating the endothelium's limited clearance capacity, suggesting that nutritional and digestive management warrant closer scrutiny in laminitis prevention strategies.
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Practical Takeaways
- •Gastrointestinal health management may be critical in laminitis prevention, as caecal dysbiosis producing excess amines could amplify serotonin-induced digital vasoconstriction
- •Acute laminitis episodes may be triggered or worsened by concurrent GI disturbances; managing gut health could reduce vascular crisis in susceptible horses
- •Understanding this 5-HT uptake mechanism provides a biological explanation for why some laminitis cases are preceded by colic or digestive upset
Key Findings
- •Equine digital vein endothelial cells (EDVEC) demonstrated saturable facilitated uptake of 5-HT with Km of 41.6 ± 9.3 micromol/l, significantly higher than pulmonary vein controls (9.9 ± 2.1 micromol/l)
- •All six caecally-derived amines (tyramine, spermine, isoamylamine, tryptamine, phenylethylamine, isobutylamine) inhibited 5-HT uptake in concentration-dependent manner, with tyramine showing lowest IC50 of 3.7 × 10⁻⁶ mol/l
- •EDVEC exhibited active facilitated uptake mechanism for 5-HT while EDVSMC showed only nonfacilitated diffusion
- •Gut-derived amines can potentiate vasoconstrictor action of 5-HT, linking gastrointestinal disturbances to laminitis pathophysiology