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 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 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)