Regulation of protein metabolism by insulin: value of different approaches and animal models.

Insulin induces protein accretion by stimulating protein synthesis and inhibiting proteolysis. However, the mechanisms of regulation of protein metabolism by insulin are complex and still not completely understood. The use of approaches combining hyperinsulinemic clamp and isotopic methods, or measurement of the activation of intracellular kinases involved in insulin signaling, in addition to the use of different animal models in a comparative physiology process, provide better understanding of the potential regulation of protein metabolism by insulin. Studies using the clamp technique in lactating goats have shown a clear inhibitory effect of insulin on proteolysis, with an interaction between the effects of insulin and amino acids. Such studies revealed that the insulin-inhibited proteolysis is improved in lactating goats, this adaptative process limiting the mobilization of body protein under the conditions of amino acid deficit which occurs during early lactation. Insulin signaling studies in growing chickens have also provided some interesting features of insulin regulation compared to mammals. Refeeding or insulin injection leads to the activation of the early steps of insulin receptor signaling in the liver but not in the muscle. Muscle p70 S6 kinase, a kinase involved in the insulin activation of protein synthesis, was found to be markedly activated in response to insulin and to refeeding, suggesting that other signaling pathways than those classically described in mammalian muscles may be involved in signal transduction. Finally, although the role of insulin has been doubtful and has long been considered to be minor in ruminants and in avian species, this hormone clearly regulates protein metabolism in both species.