Objective—To measure and compare the concentration
and composition of very low-density lipoprotein
(VLDL) in plasma and selected lipids in serum of horses
fed mixed grass hay ad libitum or denied feed for
Animals—4 healthy adult mares.
Procedure—Mares were either fed mixed grass hay
ad libitum or denied feed for 36 hours beginning at
8:00 AM. Blood samples were collected every 2 hours
during the study period and analyzed for nonesterified
fatty acid (NEFA), triglyceride (TG), VLDL, and glucose
concentrations and composition of VLDL.
Results—Withholding feed significantly increased
mean serum concentrations of NEFA. By 36 hours, a
16-fold increase in mean serum NEFA concentration
and 2-fold increase in mean plasma VLDL concentration,
compared with baseline values, were detected.
Mean plasma TG concentrations significantly
increased with time in feed-deprived horses.
Significantly lower overall mean plasma glucose concentrations
were detected in feed-deprived horses.
Mean percentage of protein in VLDL was significantly
lower in feed-deprived horses. Plasma VLDL concentrations
varied widely among horses in response
to withholding feed. Plasma TG and VLDL concentrations
remained unaltered in 2 horses.
Conclusions and Clinical Relevance—Withholding
feed significantly increases blood lipid concentrations
in horses, but individual horses respond differently.
Serum NEFA concentrations were increased in all 4
horses denied feed, indicating mobilization of tissue
triglyceride stores. Variation in plasma VLDL concentration
in response to withholding feed suggests that
its metabolism is strongly influenced by other, as yet
undetermined, factors in horses. Differences in the
plasma VLDL concentrations among horses in
response to withholding feed may be used as an indication
of susceptibility to the hyperlipemic syndrome
of Equidae. (Am J Vet Res 2002;63:1018–1021)
Objective—To evaluate selected concentrations of
blood lipids and lipase activities in euthyroid and
hypothyroid horses deprived of feed for 96 hours.
Animals—4 healthy adult mares and 4 thyroidectomized
Procedure—Horses were deprived of feed for 96
hours. Blood samples were collected at 24-hour intervals
and analyzed to determine concentrations of nonesterified
fatty acid (NEFA), triglyceride (TG), total cholesterol
(TC), and very-low-density lipoprotein (VLDL)
as well as composition of VLDL. Plasma lipase activities
were measured after feed was withheld for 96
hours and 12 days after resumption of feeding.
Results—Time significantly affected plasma NEFA,
VLDL, TG, and TC concentrations in both groups of
horses. During the 96-hour period, mean plasma concentrations
of NEFA and VLDL increased 10-fold in
euthyroid horses and increased 5-fold and 9-fold,
respectively, in hypothyroid horses. Mean plasma TG
concentrations increased 8-fold in both groups, and
plasma TC concentrations significantly increased by
33 and 30%, respectively. Composition of VLDL was
significantly affected by feed deprivation in euthyroid
horses. Activities of lipoprotein lipase and hepatic
lipase were significantly higher in feed-deprived horses.
Activity of hepatic lipase was significantly lower in
hypothyroid horses than in euthyroid horses.
Conclusions and Clinical Relevance—Hypothyroidism
did not significantly alter the magnitude of the response
of blood lipids to feed deprivation. Thyroid hormones
may reduce variability in blood lipid concentrations but
do not determine susceptibility to hyperlipemia.
Hypothyroidism does not appear to be a factor in the
pathogenesis of hyperlipemia in horses. (Am J Vet Res 2003;64:823–828)
Objective—To determine whether urine protein-to-creatinine (UP:C) ratio assessment provides an estimate of urine protein excretion (UPE) over a 24-hour period in horses and ponies, establish a preliminary UP:C ratio reference range, and determine UP:C ratio variation over time in healthy equids.
Animals—11 female horses and 6 female ponies.
Procedures—Urine was collected from all equids at 4-hour intervals for 24 hours. Total 24-hour UPE (mg of protein/kg of body weight) and UP:C ratio were determined; these variables were also assessed in aliquots of urine collected at 4-hour intervals. On 2 additional days, urine samples were also obtained from 6 horses (1 sample/horse/d) to determine day-to-day variation in UP:C ratio. Correlation between 4-hour or 24-hour UPE and UP:C ratio values was assessed. Reference ranges for 24-hour UPE, 24-hour UP:C ratio, and 4-hour UP:C ratios were calculated as central 95th percentiles of observed values.
Results—Mean 24-hour UPE (4.28 ± 2.99 mg/kg) and 24-hour UP:C ratio (0.0 to 0.37) had excellent correlation (R = 0.826; P < 0.001) in both horses and ponies; analysis of 4-hour data also revealed good correlation (R = 0.782; P < 0.001) with these variables. Calculated UPE and UP:C ratio reference ranges were similar to established ranges in other species. Day-to-day variability in UP:C ratio was minimal, and all results were within the reference range calculated by use of the 24-hour urine samples.
Conclusions and Clinical Relevance—Assessment of the UP:C ratio appears to be a reliable method for estimating 24-hour UPE in horses and ponies.
Objective—To evaluate the effect of ovariectomy on insulin sensitivity in horses and determine whether the effects of suppression of the hypothalamo-pituitary-adrenal axis differ before and after ovariectomy.
Animals—6 healthy mares.
Procedures—The horses underwent an IV glucose tolerance test (IVGTT), an insulin sensitivity test, and a dexamethasone suppression test before and 5 weeks after ovariectomy. Body weight, serum cortisol and plasma ACTH concentrations, serum insulin-to-blood glucose concentration ratios, and changes in blood glucose concentration with time after injection of glucose or insulin were compared before and after ovariectomy.
Results—The dexamethasone injection resulted in a decrease in serum cortisol concentration before and after ovariectomy. In all horses, baseline plasma ACTH concentrations were within the reference range before and after ovariectomy. For each mare, results of an IVGTT before and after ovariectomy were considered normal. No significant differences in basal blood glucose concentration or time to reach baseline glucose concentration after an IVGTT were observed. Basal serum insulin concentration and serum insulin-to-blood glucose concentration ratios were not significantly different before or after ovariectomy, nor was the mean time to attain a 50% decrease in blood glucose concentration after insulin injection.
Conclusions and Clinical Relevance—Results indicated that ovariectomy does not appear to modify dexamethasone response in horses and that it does not modify short-term measures of insulin sensitivity. Findings suggested that horses undergoing ovariectomy are not at higher risk of developing equine metabolic syndrome or hypothalamo-pituitary-adrenal axis dysfunction and associated morbidity.
Objective—To compare endogenous ACTH and α-melanocyte-stimulating hormone (α-MSH) concentrations after administration of thyrotropin-releasing hormone (TRH) and to compare ACTH concentrations after TRH administration with those following domperidone administration in healthy horses and horses with pituitary pars intermedia dysfunction (PPID).
Design—Prospective case series.
Animals—69 clinically normal horses and 47 horses with or suspected to have PPID.
Procedures—ACTH concentrations were measured during 108 TRH stimulation tests in 88 horses, and α-MSH concentrations were measured during 56 TRH stimulation tests in 50 horses. In 28 of these horses, ACTH concentrations after domperidone administration were measured and test results were compared. The pituitary gland was histologically examined in all horses that were euthanatized.
Results—ACTH and α-MSH concentrations increased in all horses afterTRH administration, with a greater and more prolonged increase in horses with PPID. Percentage increase was significantly greater for α-MSH concentration than for ACTH concentration. The change in ACTH concentration after domperidone administration was less consistent in differentiating clinically normal horses from those with PPID than was the response to TRH.
Conclusions and Clinical Relevance—Results suggested that ACTH concentration in response to TRH administration was useful for the diagnosis of PPID in horses and appeared more accurate than response to domperidone administration. Use of an α-MSH concentration ≥ 30 or 50 pmol/L did not appear superior to use of an ACTH concentration ≥ 36 pg/mL for the diagnosis of PPID, either before or 30 minutes after TRH administration.
Objective—To compare kinetics of the metabolism of
very-low-density lipoprotein (VLDL) apolipoprotein B
(apoB) before and after thyroidectomy in mares.
Animals—5 healthy adult mares.
Procedure—Thyroidectomy was performed in euthyroid
mares. Kinetics of VLDL apoB metabolism were
measured before and after thyroidectomy by use of a
bolus IV injection of 5,5,5-2H3 (98%) leucine (5 mg/kg)
and subsequent isolation of labeled amino acid from
plasma and VLDL. Labeled leucine was quantified by
use of gas chromatography-mass spectrometry.
Production rate (PR), delay time, and fractional catabolic
rate (FCR) were calculated for the 2 forms of
equine VLDL, apoB-48 VLDL, and apoB-100 VLDL.
Plasma lipid concentrations were measured, and
VLDL composition was determined.
Results—Physical appearance of horses was not
altered by thyroidectomy. Significantly lower mean
blood concentrations of thyroid hormones and nonesterified
fatty acids were detected following thyroidectomy.
Mean percentage of free cholesterol in
VLDL was significantly higher after thyroidectomy.
Mean plasma VLDL concentration or kinetics of
apoB-48 or apoB-100 were not significantly altered
by thyroidectomy. Mean ± SEM PR was significantly
lower (8.70 ± 1.61 mg/kg/d) and mean delay time
significantly longer (1.58 ± 0.12 hours) for apoB-48
VLDL in euthyroid mares, compared with values for
thyroidectomized mares (16.15 ± 2.24 mg/kg/d and
0.93 ± 0.10 hours, respectively).
Conclusions and Clinical Relevance—Hypothyroidism
did not significantly alter plasma VLDL concentrations
or kinetics of VLDL apoB metabolism.
Metabolism of apoB-48 VLDL differed significantly
from that of apoB-100 VLDL in euthyroid mares.
(Am J Vet Res 2003;64:1052–1058)
Objective—To investigate the effects of heparin administration on urine protein excretion during the developmental stages of experimentally induced laminitis in horses.
Procedures—Horses received unfractionated heparin (80 U/kg, SC, q 8 h; n = 7) or no treatment (control group; 6) beginning 3 days prior to induction of laminitis. All horses were given 3 oligofructose loading doses (1 g/kg each) at 24-hour intervals and a laminitis induction dose (10 g of oligofructose/kg) 24 hours following the final loading dose (designated as 0 hours) via nasogastric tube. Serum glucose and insulin concentrations were measured before administration of the first loading dose (baseline) and at 0 and 24 hours; urine protein-to-creatinine (UP:C) ratio was determined at 0 hours and every 4 hours thereafter. Lameness was evaluated every 6 hours, and horses were euthanized when Obel grade 2 ameness was observed.
Results—Mean ± SD time until euthanasia did not differ significantly between the heparin-treated (28.9 ± 6.5 hours) and control (29.0 ± 6.9 hours) horses. The UP:C ratio was significantly increased from baseline at 20 to 28 hours after induction of laminitis (ie, 4 ± 4 hours before lameness was evident) in control horses but did not change significantly from baseline in heparin-treated horses. Serum glucose or insulin concentration did not change significantly from baseline in either group.
Conclusions and Clinical Relevance—Urine protein excretion increased during the developmental stages of carbohydrate-induced laminitis in horses; administration of heparin prevented that increase, but did not delay onset or decrease severity of lameness.