Objective—To determine in vitro vasoactive potency
of monoamines formed in the cecum and found in the
systemic circulation of horses.
Sample Population—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.
Procedure—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 (α-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 and Clinical Relevance—Amines from
the equine cecum cause digital vasoconstriction. The
most potent (TRP and TYR) cause selective venoconstriction.
Tyrosine activates predominantly α-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.
(Am J Vet Res 2003;64:1124–1131)
Objective—To determine the effect of endotoxin (lipopolysaccharide [LPS]) on vasoactive mediator production by cultured equine digital vein endothelial cells (EDVECs).
Sample Population—EDVECs obtained from forelimb digital veins of 7 healthy adult horses.
Procedures—EDVECs were incubated with or without LPS (1 μg/mL) for 0, 2, 4, 6, 22, and 24 hours. The EDVECs were incubated for 18 hours with LPS (10 pg/mL to 1 μg/mL) with or without ibuprofen, cycloheximide, or L-nitroarginine methyl ester. Medium concentrations of prostacyclin, cyclic guanosine monophosphate, endothelin-1, and thromboxane A2 were determined. Changes in inducible nitric oxide synthase and cyclooxygenase-2 expression were determined.
Results—LPS stimulated mean 4.2- and 14.1-fold increases in EDVEC prostacyclin and cyclic guanosine monophosphate production, respectively, after 22 hours. These effects were LPS concentration–dependent (LPS concentrations that induced a response halfway between the maximum response and baseline of 1.50 and 1.22 ng/mL, respectively). The LPS-induced cyclic guanosine monophosphate production was significantly inhibited (to basal concentrations) by L-nitroarginine methyl ester, and prostacyclin production was inhibited by cycloheximide and ibuprofen. Production of thromboxane A2 by EDVECs was not detected. Endothelin-1 accumulated in the medium, but LPS did not enhance its production. Inducible nitric oxide synthase expression in EDVECs was not detected with the available antibodies, whereas LPS stimulated cyclooxygenase-2 expression in a time- and concentration-dependent manner.
Conclusions and Clinical Relevance—LPS stimulated vasoactive mediator production by equine endothelial cells, which may play a role in LPS-induced digital hypoperfusion.
Objective—To determine whether pasture, and specifically the addition of fructan carbohydrate to the diet, induces exaggerated changes in serum insulin concentration in laminitispredisposed (LP) ponies, compared with ponies with no history of the condition, and also to determine insulin responses to the dexamethasone suppression test.
Animals—10 LP and 11 control adult nonobese mixed-breed ponies.
Procedures—Insulin-modified IV glucose tolerance tests were performed (5 ponies/group). In diet studies, ponies were kept on pasture and then changed to a hay diet (10 ponies/group). Second, ponies were maintained on a basal hay diet (4 weeks) before being fed a hay diet supplemented with inulin (3 g/kg/d [1.4 g/lb/d]). Serum insulin and plasma glucose concentrations were analyzed before and after dietary changes. Serum cortisol and insulin concentrations were also measured in a standard dexamethasone suppression test.
Results—The LP ponies were insulin resistant (median insulin sensitivity of 0.27 × 104 L•min−1•mU−1 in LP ponies, compared with 0.64 × 104 L•min−1•mU−1 in control ponies). Median insulin concentration in LP ponies was significantly greater than that in control ponies at pasture, decreased in response to feeding hay, and was markedly increased (5.5fold) following the feeding of inulin with hay. The LP ponies had a greater increase in serum insulin concentration at 19 hours after dexamethasone administration (median, 222.9 mU/L), compared with control ponies (45.6 mU/L).
Conclusions and Clinical Relevance—Nonobese ponies predisposed to develop laminitis had compensated insulin resistance, and this phenotype was revealed by feeding plant fructan carbohydrate or by dexamethasone administration.
Objective—To measure concentrations of amines
formed in the cecum of clinically normal ponies,
determine amine concentrations in plasma samples
collected in spring and winter, and compare concentrations
of amines and serotonin in plasma samples
obtained from clinically normal ponies and ponies predisposed
Sample Population—Cecal contents obtained from
10 ponies euthanatized at an abattoir and blood samples
obtained from 42 adult ponies.
Procedure—Cecal contents were assayed for amines
by high-performance liquid chromatography (HPLC).
Blood samples were collected at various times of the
year from 20 ponies predisposed to acute laminitis
and 22 clinically normal ponies. Plasma serotonin concentration
was measured by HPLC, and tryptamine
(TRP), tyramine (TYR), phenylethylamine (PEA), and
isoamylamine (IAA) were measured by liquid chromatography-
Results—15 amines were identified in cecal contents.
Plasma TRP, TYR, PEA, and IAA concentrations
ranged from 10pM to 100nM in both groups of
ponies. Plasma concentrations of serotonin or other
amines did not differ between clinically normal ponies
and those predisposed to laminitis; however, significantly
higher concentrations of TRP, PEA, and IAA
were found in samples obtained in the spring, compared
with winter samples.
Conclusions and Clinical Relevance—Various
amines are found in the cecum of ponies, several of
which can be detected in the plasma. Concentrations
increase significantly in the spring and may reach
concentrations close to the threshold for causing
vasoconstriction. Release of amines from the cecum
into the systemic circulation may contribute to hemodynamic
disturbances in horses and ponies with
acute laminitis. (Am J Vet Res 2003;64:1132–1138)