Delineating Potential Control Mechanisms of Mammary Protein Synthesis Utilizing Atropine

¹ Department of Animal Science, McGill University, Ste. Anne-de-Bellevue, QC, H9X 3V9, Canada
² Department of Animal and Poultry Science, University of Guelph, ON, N1G 2W1, Canada
³ Agriculture and Agri-Food Canada, Lennoxville, QC, J1M 1Z3 Canada

Amino acids (AA) and/or somatotropin (ST) were infused into a jugular vein of lactating cows receiving atropine to determine the effect on milk protein secretion. Atropine decreased milk protein yield by 35%. Plasma -amino nitrogen, which was decreased by 31% due to atropine, was restored to control levels upon AA infusion. Plasma ST concentration was not affected by atropine and, upon ST infusion, increased by 236% in treated animals. Amino acids, ST, or the combination of the two were unable to return milk protein to control levels in atropine-treated animals. Plasma insulin concentration decreased by 37% in atropine-treated animals and was not significantly different for cows receiving atropine as well as ST, AA, or both ST and AA. Similarly, glucagon decreased by 36% in atropine-treated animals but was partially restored in cows receiving AA. Overall, the insulin-to-glucagon ratio was not significantly affected by treatment. Plasma glucose concentration was not affected by treatment. These data lend support to the importance of the insulin-to-glucagon ratio in terms of whole-body metabolism, with the exception of the mammary glands, which are glucagon-insensitive and, therefore, sensitive to the observed decrease in circulating insulin concentration.

Key Words: amino acids • somatotropin • insulin • glucagon

Submitted on November 5, 2001
Accepted on January 3, 2002