[Findings and hormonal coordination of protein metabolism in ruminants]

Protein metabolism roughly exhibits the same characteristics in ruminants and non-ruminant species. Changes in whole-body protein mass are the result of the balance between the simultaneous protein synthesis and breakdown. Essential amino acids are either incorporated into protein or degraded. Insulin, growth hormone and glucagon have been shown to regulate those metabolic pathways in ruminants. Alteration of food intake both decreases protein synthesis and protein breakdown. Protein synthesis is affected to a greater extent than protein breakdown. So protein synthesis is the main factor controlling N balance in response to alteration of food intake. The decrease in protein synthesis may be related to an impairment of plasma insulin. Protein synthesis and breakdown both decrease throughout development. Protein deposition decreases throughout development because protein synthesis declines more rapidly than protein breakdown. The hormonal coordination of those changes depends on the age of the animals. For example the decrease in protein synthesis in the postnatal growth period may be due to either the decrease in plasma growth hormone or to the impairment in cellular insulin receptors. Due to milk protein synthesis in the mammary gland, lactating animals exhibit a large increase in whole-body protein synthesis. Surprisingly, protein synthesis decreases in some non-mammary tissues. This represents a mechanism for a greater partitioning of amino acids towards milk at the expense of body proteins. We recently demonstrated that insulin could be involved in that adaptative process.