Insulin action in man

Insulin action is crucial for the regulation of glucose metabolism. Insulin plays a key role in suppressing endogenous glucose production by the liver, both in fasting and postprandial states. Insulin is also necessary for the maintenance of normal rates of glucose oxidation and storage in insulin-sensitive tissues and for the prevention of excessive gluconeogenic substrate production. Various methods have been developed to assess insulin action in vivo, essentially at liver and muscle sites. Such methods evaluate the effect of exogenous or endogenous insulin, using respectively the open-loop approach (interruption of the feedback loop by inhibiting endogenous insulin secretion) or the closed-loop approach (mathematical modelling of the insulin-glucose feedback loop). Knowledge of the successive steps of cellular insulin action has markedly improved during the last ten years. Preceptor, receptor and postreceptor levels need to be considered since they may be affected in insulin-resistant states. This general progress in the understanding of insulin action in man improves our approach to the complex pathophysiology of non-insulin-dependent diabetes mellitus and opens up new prospects for treatment of the insulin-resistant syndrome which is associated with several atherosclerotic risk factors.