Effects of monoamines formed in the cecum of horses on equine digital blood vessels and platelets.
OBJECTIVE
To determine in vitro vasoactive potency of monoamines formed in the cecum and found in the systemic circulation of horses.


METHODS
Segments of digital blood vessels obtained from 6 healthy mixed-breed horses and ponies euthanatized at an abattoir and platelets isolated from 4 healthy ponies.


METHODS
Paired rings of digital artery and vein from the same horse were examined, and isometric tension was recorded. Concentration-response curves for tryptamine (TRP), tyramine (TYR), phenylethylamine (PEA), isoamylamine (IAA), and isobutylamine (IBA) were obtained. Vasoconstrictor mechanisms were investigated for TRP and TYR by the use of antagonists. Washed platelets loaded with [3H]-5-hydroxytryptamine (5-HT) were incubated with monoamines; the amount of radioactivity displaced after 30 minutes was estimated.


RESULTS
TRP, TYR, and PEA were potent constrictors of arteries and veins, with TRP and TYR being more potent in veins than arteries. Constrictions induced by TYR were inhibited by benextramine (alpha-antagonist) and nisoxetine (neuronal-uptake blocker), whereas TRP responses were inhibited by ketanserin (5-HT receptor antagonist). All 5 amines displaced 5-HT from platelets with the order of potency being TYR > TRP > PEA > IAA > IBA.


CONCLUSIONS
Amines from the equine cecum cause digital vasoconstriction. The most potent (TRP and TYR) cause selective venoconstriction. Tyrosine activates predominantly alpha-adrenoceptors through the release of neuronal norepinephrine, whereas TRP activates 5-HT receptors. All amines tested released 5-HT from platelets. Amines formed in the cecum and released into the systemic circulation warrant additional investigation as trigger factors for digital ischemia and subsequent laminitis.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D001679:Biogenic Amines / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002432:Cecum / Q000187:drug effects / Q000378:metabolism*; D003538:Cystamine / Q000031:analogs & derivatives* / Q000494:pharmacology; D005263:Femoral Artery / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D005268:Femoral Vein / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D005473:Fluoxetine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006736:Horses / Q000097:blood*; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects / Q000502:physiology; D007650:Ketanserin / Q000494:pharmacology; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D015539:Platelet Activation / Q000187:drug effects / Q000502:physiology; D012702:Serotonin Antagonists / Q000494:pharmacology; D014661:Vasoconstriction / Q000187:drug effects / Q000502:physiology
D000316:Adrenergic alpha-Agonists; D001679:Biogenic Amines; D012702:Serotonin Antagonists; D005473:Fluoxetine; C011386:nisoxetine; C029998:benextramine; D007650:Ketanserin; D003538:Cystamine