Effect of Atropine on Milk Protein Yield by Dairy Cows with Different β-Lactoglobulin Phenotypes

Abstract

The objective of this study was to determine the response of individual milk proteins to a reduction in amino acid (AA) availability induced by atropine and to determine whether the response was different in cows with different β-lactoglobulin (LG) phenotypes. Six cows that were homozygous for the A variant of β-LG and six cows that were homozygous for the B variant of β-LG were each given a single subcutaneous injection of saline or 20 mg of atropine. In both groups of cows, atropine decreased milk yield by 30% and reduced the concentration of α-lactalbumin (LA) by 25 to 30% at 8 h following injection. Eight hours after atropine injection, yield of β-LG was 41% lower than it was following saline injection, and yield of β-casein (CN) after atropine injection declined 16% relative to saline. Concentrations of BSA and the ratio of γ-CN to β-CN, which reflects plasmin activity in milk, were significantly increased after administration of atropine. Although the response to atropine tended to be more pronounced in cows that were homozygous for β-LG B, they were not significantly different from the response of cows that were homozygous for β-LG A. The differential response of individual proteins to a reduction in AA concentrations in whole blood suggested that susceptibility to restriction in substrate availability differed for individual proteins. The concentration of lactose in plasma did not change, which implied that the integrity of the mammary epithelial barrier was not compromised when AA derived from blood were diminished. The consistent concentration of lactose combined with the minimal increase in total yield of BSA in milk following atropine treatment indicated that the increased concentration in milk of proteins derived from serum was due to the concentrating effect of lower milk volume.