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)