1 Laboratory of Biochemistry, Department of Biomedical Sciences, Graduate School of Veterinary Medicine, Hokkaido University, Japan
2 Production Disease Team, National Institute of Animal Health, Japan
Received 26 May 2009; accepted for publication 4 August 2009
Retinol-binding protein 4 (RBP4) is a plasma protein involved in retinol transportation, and recent
evidence in rodents suggests that RBP4 is also a metabolic regulator that modifies insulin
sensitivity. To assess how RBP4 levels are regulated in ruminants, we determined the RBP4
concentrations in bovine plasma and milk using Western blot analysis. Plasma RBP4 levels in
non-pregnant non-lactating (control) cows were around 45 mg/ml, which were sustained during
60-h fasting, but decreased significantly 4 h after lipopolysaccharide (LPS) administration. Basal
plasma retinol concentration was around 30 mg/dl, but this decreased to approximately one-third
and one-half of these values during fasting and 8 h after LPS challenge, respectively. Plasma
RBP4 and retinol levels in cows 3–6 d before parturition were comparable to those of the controls.
However, on the day of parturition both were significantly decreased and had returned
to basal levels by two weeks after calving. Interestingly, RBP4 was clearly detected in colostrum
(16.4±5.6 mg/ml) but was only faintly detected in milk from cows at 7 d and 15 d after calving.
Retinol concentrations in colostrum were almost 10-fold higher than those in plasma, while those
in milk were comparable to those in plasma. These results suggest that RBP4 and retinol levels
are independently regulated under physiological and pathophysiological conditions and that
RBP4, like retinol, is transferred from maternal stores to calves through colostrum.
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