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 compare the responses of equine digital
arteries (EDAs) and equine digital veins (EDVs) to
endothelin-1 (ET-1) and determine the role of the
endothelium and type of receptors involved in the
modulation and mediation of those responses,
Sample Population—5 to 9 palmar digital
vessels/experiment from 28 healthy horses.
Procedure—Rings of dissected vessels were mounted
under tension between force transducer wires in
organ baths containing Krebs-Henseleit solution at
30oC. Responses of EDAs and EDVs (with intact [+e]
or denuded [–e] endothelium) to cumulative concentrations
of ET-1 (10–10 to 3 × 10–7 M) were compared.
For (+e)EDAs and (+e)EDVs precontracted with a
thromboxane-mimetic (U44069; 10–8 M) and (–e)EDAs
and (–e)EDVs, responses to an ETB receptor agonist
(S6c; 10–10 to 3 × 10–7 M) were evaluated. Responses to
ET-1 (10–7 M) in (–e)EDAs and (–e)EDVs were evaluated
after incubation with an ETA receptor antagonist (BQ-
123; 3 × 10–7 M), an ETB receptor antagonist (BQ-788;
3 × 10–7 M), or vehicle solution.
Results—Endothelin-1 induced a concentrationdependent
contraction of endothelium-intact and
-denuded EDAs and EDVs; EDVs were more sensitive.
Neither vessel type relaxed in response to S6c,
although 2 of the (–e)EDAs contracted mildly.
Whereas BQ-123 inhibited the (–e)EDA and (–e)EDV
responses to ET-1, BQ-788 had no effect.
Conclusions and Clinical Relevance—Endothelin-1
induced digital vasoconstriction (marked constriction
in veins). This action was unaffected by endothelium
and mediated predominantly by ETA receptors. These
findings suggest ET-1 can induce selective digital
venoconstriction. (Am J Vet Res 2003;64:1438–1443
Objective—To compare responses of equine digital arteries (EDAs) and veins (EDVs) to human-acalcitonin gene-related peptide (hαCGRP), evaluate effect of the endothelium, and characterize receptors and sources of endogenous CGRP.
Sample—Palmar digital vessels (5 to 9/experiment) from healthy adult horses killed at an abattoir.
Procedures—Vessel rings were mounted under tension in organ baths containing Krebs-Henseleit solution at 30°C, with relaxation responses examined in vessels preconstricted with a thromboxane-mimetic (3 × 10−8M). Responses of endothelium-intact (+e) and -denuded (−e) EDAs and EDVs to hαCGRP C10−10 to 3 × 10−7M) were compared. Following incubation with an hαCGRP receptor antagonist (hαCGRP8–37; 1μM), responses of EDA(−e) and EDV(−e) to hαCGRP (10−7M) were obtained. Responses of endothelium-intact and -denuded arteries and veins to hαCGRP (3 × 10−7M) or capsaicin (10−5M) were evaluated as well as responses of endothelium-intact and -denuded EDA and EDV to hαCGRP (10−10 to 10−6M) after incubation with endothelin-1 (ET-1; 10−12M).
Results—hαCGRP resulted in nonendothelium, concentration-dependent relaxation in EDAs and EDVs, with greater responses in EDAs. Treatment with hαCGRP8–37 had minimal effect on responses to hαCGRP in either vessel type. Capsaicin induced relaxation in both vessel types. There were no differences between responses to hαCGRP for vessels pretreated with ET-1 or vehicle.
Conclusions and Clinical Relevance—Both hαCGRP and capsaicin induced digital vasodilation unaffected by a functional endothelium. This suggested that endogenous CGRP likely emanates from sensory-motor nerves and may contribute to digital vasodilation.
Objective—To determine whether cats in the nonazotemic stages of chronic kidney disease have increased plasma parathyroid hormone (PTH) concentrations as a compensatory physiologic mechanism to maintain plasma phosphate concentration within the reference interval.
Design—Prospective longitudinal study.
Animals—118 client-owned geriatric cats with various degrees of renal function.
Procedures—For each cat, a blood sample was obtained for plasma biochemical analysis and determination of plasma PTH concentration, and a urine sample was obtained for determination of urine specific gravity at study entry (baseline) and after 12 months. For a subset of 30 cats, plasma calcitriol concentration was determined at baseline. Cats were categorized into 1 of 3 groups on the basis of kidney function at the end of 12 months. At baseline and after 12 months, plasma concentrations of variables associated with calcium homeostasis were compared between the 3 groups and also within groups over time. Multivariable linear regression was used to identify variables associated with plasma PTH concentration.
Results—Plasma PTH concentration was significantly increased in cats that developed azotemia, compared with PTH concentration in cats that remained nonazotemic, and PTH concentration increased before changes in plasma calcium and phosphate concentrations were detected. A moderate positive association between plasma calcitriol and PTH concentrations was identified. Plasma PTH concentration was associated with age and plasma urea, creatinine, and total calcium concentrations in the final multivariable model.
Conclusions and Clinical Relevance—Results suggested that renal secondary hyperparathyroidism can develop prior to azotemia in cats, even in the absence of hyperphosphatemia and hypocalcemia.
Objective—To determine prevalence of systolic
hypertension and associated risk factors in cats with
chronic renal failure evaluated in first-opinion practice.
Animals—103 cats with chronic renal failure.
Procedure—Systolic arterial blood pressure (SABP)
was measured with a noninvasive Doppler technique,
and cats that had SABP > 175 mm Hg on 2
occasions or that had SABP > 175 mm Hg and compatible
ocular lesions were classified as hypertensive.
Information from the history (previous treatment
for hyperthyroidism, age), physical examination
(sex, body weight), routine plasma biochemical
analyses (creatinine, cholesterol, potassium, sodium,
chloride, and calcium concentrations), and thyroid
status were evaluated as potential risk factors for
systolic hypertension. Variables associated with systolic
hypertension were evaluated by use of logistic
Results—20 (19.4%; 95% confidence interval, 13 to
28%) cats had systolic hypertension. Plasma potassium
concentration was significantly and inversely
associated with systolic hypertension.
Conclusions and Clinical Relevance—Prevalence of
systolic hypertension, although clinically important,
was lower than that reported previously. The cause of
the inverse association between systolic hypertension
and plasma potassium concentration is not yet
known. (J Am Vet Med Assoc 2002;220:1799–1804)
Objective—To compare flow-mediated vasodilation (FMD) measurements in brachial and femoral arteries of healthy dogs habituated to the assessment method, evaluate repeatability of these measurements, and investigate effects of blood pressure cuff inflation time on femoral artery FMD measurements.
Animals—11 healthy adult Miniature Schnauzers.
Procedures—Arterial luminal diameter and blood flow velocity integral (FVI) were measured before and after cuff inflation of 5 minutes' (brachial and femoral arteries) or 3 minutes' duration (femoral artery) in separate experiments. A blood pressure cuff was inflated to > 200 mm Hg distal to each imaging site to increase local blood flow to induce reactive hyperemia. Changes in FVI after cuff deflation, FMD, and between-dog and within-dog coefficients of variation (CVs) were determined.
Results—After cuff inflation of 5 minutes' duration, greater changes were detected in median change in FVI and FMD of brachial arteries (174.0% and 8.0%, respectively), compared with values determined for femoral arteries (32.0% and 2.1%, respectively). Between-dog CV for brachial artery FMD was 34.0%, compared with 89.6% for femoral arteries, and within-dog CV was 32.5% for brachial arteries versus 51.6% for femoral arteries after cuff inflation of 5 minutes' duration.
Conclusions and Clinical Relevance—In healthy Miniature Schnauzers, FMD was greater and more repeatable in brachial arteries than in femoral arteries. Reactive hyperemia was inconsistently induced in femoral arteries following 3- or 5-minute cuff inflation times. Brachial, but not femoral, artery FMD measurement is a potentially useful research technique for measurement of endothelial function in dogs.
Objective—To compare concentrations of urinary iodide (UI) in euthyroid and untreated hyperthyroid cats.
Animals—118 euthyroid and 88 hyperthyroid client-owned cats from 2 nonreferral veterinary practices.
Procedures—Iodide concentration was measured in 5 urine samples collected every 3 to 12 months from selected cats, and variability of results between euthyroid cats and hyperthyroid cats prior to the diagnosis of hyperthyroidism was evaluated via 1-way ANOVA, after logarithmic transformation of UI concentrations (logUIs). The UI concentration in hyperthyroid cats was measured at diagnosis and 2 to 6 weeks and 3 to 6 months after treatment for hyperthyroidism. The pretreatment logUI in hyperthyroid cats was compared with that in euthyroid cats, taking into account the effects of renal function on UI concentration. Iodine intake was estimated in euthyroid cats following calculation of the volume of daily urine output, with a fixed value for iodine concentration in feces.
Results—The variability of UI concentrations did not differ significantly between hyperthyroid (n = 10) and euthyroid (8) cats. The logUI increased 2 to 6 weeks after initiation of treatment in hyperthyroid cats (n = 80) and was lower in azotemic versus nonazotemic cats. Hyperthyroid cats had a lower logUI than euthyroid cats, and there was no evidence of deficient iodine intake in euthyroid cats.
Conclusions and Clinical Relevance—The logUI was lower in cats with azotemia and with untreated hyperthyroidism, compared with that in euthyroid cats from the same population. Additional studies are needed to determine whether iodine intake plays a role in the development of hyperthyroidism in cats.
Objective—To validate a nonautomated technique for the measurement of urinary N-acetyl-β-D-glucosaminidase (NAG) activity in cats and assess the correlation between NAG index, plasma creatinine concentration, and proteinuria.
Animals—197 client-owned cats (≥ 9 years old; 119 neutered males and 78 neutered females) of which 103 had previously been determined to have chronic kidney disease (CKD).
Procedures—Preliminary assay validation was performed for a nonautomated colorimetric technique for quantification of NAG activity. The effect of storage of samples was examined. A cross-sectional study was performed to assess urinary NAG index in cats with variable plasma creatinine concentrations and with proteinuria, as quantified by use of the urine protein-to-creatinine ratio (UP:C).
Results—Interassay coefficients of variance (CVs) in cats with low (mean, 0.64 U/L), medium (mean, 4.38.U/L), and high (mean, 8.48 U/L) urine NAG activity were 25.9%, 14.4%, and 25.1%, respectively, but intra-assay CVs were < 20%. Urine NAG activity was stable for 4 freeze-thaw cycles and for storage at −20°C. There was no significant difference in log NAG index when cats (n = 197) were grouped according to plasma creatinine concentration, but a moderate positive correlation was found between log NAG index and log UP:C (r2 = 0.259).
Conclusions and Clinical Relevance—N-acetyl-β-D-glucosaminidase activity can be quantified in feline urine by use of a nonautomated colorimetric technique. However, data should be interpreted cautiously because of high interassay CVs. The NAG index in cats with CKD may be indicative of ongoing lysosomal activity rather than active proximal tubular cell damage.
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 evaluate the roles of 5-hydroxytryptamine (5-HT), thromboxane A2 (T×A2), and platelet-activating factor (PAF) in endotoxin-induced digital hypoperfusion in horses.
Animals—6 healthy adult Thoroughbreds.
Procedures—Horses were treated with IV administration of saline (0.9% NaCl) solution (control treatment) or the 5-HT1B/D selective antagonist, GR55562 (0.3 mg/kg), prior to tryptamine infusion (1.6 μg/kg/min for 30 minutes) to establish an effective GR55562 dose. In a crossover study, horses were treated with IV administration of saline solution (control treatment), aspirin (4 mg/kg, 2 hours or 4 days before lipopolysaccharide [LPS] infusion), GR55562 (0.3 mg/kg), the PAF antagonist WEB2086 (3 mg/kg), or aspirin plus GR55562 prior to LPS infusion (30 ng/kg for 30 minutes). Digital blood flow was measured by use of Doppler ultrasonography. Concomitant measurements of hoof wall and coronary band surface temperatures were made. Serial blood samples were collected and plasma 5-HT and T×A2 concentrations determined.
Results—GR55562 abolished tryptamine-induced digital hypoperfusion. Neither WEB2086 nor GR55562 affected LPS-induced alterations in digital perfusion or plasma mediator concentrations. Aspirin given 2 hours before LPS administration abolished the increase in plasma T×A2 concentration and significantly attenuated LPS-induced digital hypoperfusion. Aspirin given 4 days before LPS significantly attenuated the increase in plasma T×A2 concentration and digital hypothermia. Aspirin plus GR55562 had a greater effect on LPS-induced digital hypothermia than aspirin alone.
Conclusions and Clinical Relevance—Thromboxane A2 and 5-HT played a role in mediating LPS-induced digital hypoperfusion in horses. Platelet-activating factor appeared unimportant in mediating LPS-induced 5-HT or T×A2 release or digital hypoperfusion.