Abstract
The glucose homeostasis in dairy cattle is very well controlled, in line with the metabolic adaptation during the periparturient period. Former studies showed that nicotinic acid (NA) lowered plasma non-esterified fatty acids (NEFA) concentrations and increased insulin sensitivity in dairy cows. Thus, the purpose of this study was to investigate whether the expression of proteins involved in hepatic and adipose insulin signaling and protein expression of hepatic glucose transporter 2 (GLUT2) were affected by dietary NA and dietary concentrate intake in periparturient dairy cows. Twenty pluriparous German Holstein cows were fed with the same diet from about 21 days before the expected calving date (d-21) to calving. After calving, cows were randomly assigned in 4 groups and fed with diets different in concentrate proportion (“HC” with 60:40% or “LC” with 30:70% concentrate-to-roughage ratio) and supplemented with NA (24 g/day) (NA) or without (CON) until d21. Biopsy samples were taken from the liver, subcutaneous (SCAT) and retroperitoneal (RPAT) adipose tissues at d-21 and d21. Protein expression of insulin signaling molecules (insulin receptor (INSR), phosphatidylinositol-3-kinase (PI3K), protein kinase Cζ (PKCζ)) and hepatic GLUT2 was measured by Western Blotting. The ratio of protein expression at d21/at d-21 was calculated and statistically evaluated for the effects of time and diet. Cows in HC had significantly higher dietary energy intake than cows in LC. In RPAT a decrease in PI3K and PKCζ expression was found in all groups, irrespectively of diet. In the liver, the GLUT2 expression was significantly lower in cows in NA compared with cows in CON. In conclusion, insulin signaling might be decreased in RPAT over time without any effect of diet. NA was able to modulate hepatic GLUT2 expression, but its physiological role is unclear.
Highlights
The periparturient period is critical for the occurrence of metabolic disorders associated with severe energy deficit and excessive lipolysis in dairy cattle [1]
The main findings in this study were 1) the marked reduction of PI3K and protein kinase Cz (PKCz) protein expression over time in retroperitoneal adipose tissue (RPAT), and 2) the lower content of hepatic glucose transporter 2 (GLUT2) protein in cows supplemented with nicotinic acid (NA)
Modulation of insulin signaling at the onset of lactation occurred at the level of protein expression in a tissue-specific manner irrespectivley of diet
Summary
The periparturient period is critical for the occurrence of metabolic disorders associated with severe energy deficit and excessive lipolysis in dairy cattle [1]. Cows physiologically undergo a drastic shift in metabolism from anabolic to catabolic status around parturition. Insulin blood concentration and responsiveness of peripheral tissues to insulin are lowest in this period, so that shifting of glucose into the mammary gland for the milk synthesis is maximized [2]. In comparison to humans and monogastric animals, dairy cows have physiologically much lower glucose concentrations in the blood and are known to be less insulin sensitive [2]. The lower expression of this key component of insulin-dependent glucose uptake may contribute to the constitutively lesser insulin sensitivity of ruminants
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