Effect of atropine on plasma amino acid levels and milk secretion of cows

Subcutaneous injections of 30 mg atropine into lactating cows induced a 20--40% decrease of free amino acid (AA) levels in arterial plasma. Minimum levels were observed after 30--50 min. The decline persisted for more than 6 h. The greatest fall in concentration was noted for tyrosine, methionine, lysine, arginine, phenylalanine and threonine. Arterial glucose levels remained unaffected. The effect of atropine on milk secretion was studied in 2 cows which were milked every hour with the aid of oxytocin. Maximal effects were observed after 3--4 h. They included reduction in concentration of casein and alpha-lactalbumin (alpha-la) and a decline in production of milk (20%), casein (35%), alpha-la (45%) and lactose (18%). Uptake by the lactating udder over a period of about 1 h after injection of atropine was studied in 2 cows. Mammary blood flow and glucose uptake remained unaffected. There was a positive correlation between arteriovenous differences of essential AA and arterial plasma concentrations. The uptake of essential AA decreased by approximately 50%. There was no evidence that atropine has a direct inhibiting effect on the udder. It is suggested that the decrease of alpha-la synthesis might induce an inhibition of lactose synthesis and milk production.