An examination of endocrine and nutrient controls of milk protein production /

The control of milk protein production was investigated utilising two different approaches. The first model is one of intravenous infusion of atropine. Atropine, which decreases milk protein yield, has been theorised to act either by decreasing blood somatotropin (ST) concentration or by decreasing blood amino acid (AA) concentration. Thus, the first experiment was designed to test which mechanism, or both, is responsible for the effects on milk protein yield. Five lactating dairy cows were assigned to the following treatments which were administered intravenously: Saline (CONT), atropine (ATR), ATR + ST, ATR + AAs, and ATR + ST + AAs. Atropine treatment failed to decrease plasma ST concentration but did decrease plasma alpha-amino nitrogen concentration. Atropine treatment decreased milk protein yield but neither ST, AAs, nor ST + AAs were able to maintain milk protein yield at the CONT level when infused with ATR. It is clear that the treatments tested are not directly responsible for the decrease in milk protein yield due to ATR. Therefore, neither ST, AAs, nor ST + AAs appear to have direct control of milk protein production. Plasma insulin (INS) concentration was decreased and plasma IGF-I concentration was not decreased by ATR treatment. Insulin, therefore, presents itself as a candidate for direct control over milk protein synthesis. The second model is one of monitoring endocrine response to abomasal infusion of AAs mimicking the profile of milk protein with selective deletion of certain AAs. Six lactating dairy cows were subjected to the following treatments: Saline (negative control, NC), AAs (positive control, PC), PC minus methionine (PC-Met), PC minus lysine (PC-Lys), PC minus histidine (PC-His), and PC minus the branched-chain AAs (PC-BCAAs). All endocrine factors studied (ST, INS, glucagon & IGF-I) were affected by treatment. Plasma IGF-I concentration responded similarly, except for the PC-Met treatment, to milk protein yield (Weekes and Cant, 20