EFFECT OF ATROPINE ON MILK PROTEIN YIELD BY DAIRY-COWS WITH DIFFERENTBETA-LACTOGLOBULIN PHENOTYPES

The objective of this study was to determine the response of individua l milk proteins to a reduction in amino acid (AA) availability induced by atropine and to determine whether the response was different in co ws with different beta-lactoglobulin (LG) phenotypes. Six cows that we re homozygous for the A variant of beta-LG and six cows that were homo zygous for the B variant of beta-LG were each given a single subcutane ous injection of saline or 20 mg of atropine. In both groups of cows, atropine decreased milk yield by 30% and reduced the concentration of alpha-lactalbumin (LA) by 25 to 30% at 8 h following injection. Eight hours after atropine injection, yield of beta-LG was 41% lower than it was following saline injection, and yield of beta-casein (CN) after a tropine injection declined 16% relative to saline. Concentrations of B SA and the ratio of gamma-CN to beta-CN, which reflects plasmin activi ty in milk, were significantly increased after administration of atrop ine. Although the response to atropine tended to be more pronounced in cows that were homozygous for beta-LG B, they mere not significantly different from the response of cows that were homozygous for beta-LG A . The differential response of individual proteins to a reduction in A A concentrations in whole blood suggested that susceptibility to restr iction 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 w hen AA derived from blood were diminished. The consistent concentratio n of lactose combined with the minimal increase in total yield of BSA in milk following atropine treatment indicated that the increased conc entration in milk of proteins derived from serum was due to the concen trating effect of lower milk volume.