Abstract
The objectives of this study were to (1) identify changes in plasma and mammary intracellular amino acid (AA) profiles in dairy cows treated with growth hormone (GH), and (2) evaluate the expression of mammary gland genes involved in the transport of AA identified in (1). Eight non-pregnant (n = 4 per group) lactating dairy cows were treated with a single subcutaneous injection of either a slow-release formulation of commercially available GH (Lactotropin 500 mg) or physiological saline solution. Six days after treatment, cows were milked and blood collected from the jugular vein for the analysis of free AA in the plasma. Cows were euthanized and mammary tissue harvested. Treatment with GH increased milk, protein, fat and lactose yields, with no effect on dry matter intake. Plasma concentrations of lysine and group I AA decreased significantly, and arginine, methionine, tyrosine and arginine-family AA tended to decrease in GH-treated cows. Concentrations of intracellular glycine, serine and glutamate increased significantly, with a trend for decreased arginine observed in the mammary gland of GH-treated cows. A trend for increased concentrations of intracellular total AA, NEAA and arginine-family AA were observed in the mammary gland of GH-treated cows. Variance in the concentration of plasma methionine, tyrosine, valine, alanine, ornithine, BCAA, EAA was significantly different between treatments. Variance in the concentration of intracellular lysine, valine, glutamine, EAA and group II was significantly different between treatments. AA changes were associated with increased mRNA abundance of the mammary gland AA transporter SLC3A2. We propose that these changes occur to support increased milk protein and fatty acid production in the mammary gland of GH-treated cows via potential mTOR pathway signaling.
Highlights
Amino acids (AA) are used by tissues both as building blocks for protein synthesis and as signaling molecules to regulate the protein synthetic machinery [1]
Significant differences in milk, protein, fat and lactose (Fig 1A to 1D) yields were observed between Growth hormone (GH) and control cows by d 4 of GH treatment
Milk proteins in the lactating ruminant mammary gland are primarily synthesized from circulating plasma AA, which can be classed into three groups based on their requirement for supporting milk protein synthesis, group I, group II and non-essential AA (NEAA) [10]
Summary
Amino acids (AA) are used by tissues both as building blocks for protein synthesis and as signaling molecules to regulate the protein synthetic machinery [1]. During lactation the ruminant mammary gland must import from the plasma and endogenously synthesize sufficient quantities of AA to support cell turnover, epithelial cell differentiation and elevated milk protein synthesis [2]. Growth hormone (GH) is a well-established treatment model used by researchers to study mechanisms involved in the regulation of increased milk protein synthesis by the lactating ruminant mammary gland [3]. Treatment of ewes with exogenous GH can yield milk protein increases of 7% (mid-lactation Rahmani ewes) [4], and 12% (late-lactation Comisana ewes) [5]. The mammary glands of cows treated with GH elevate arterio-venous extraction of a range of AA from the blood to support increased milk protein production [10]
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