Insulin and glucagon
Insulin exerts its anabolic actions in the liver, the adipose tissue and muscle tissue. At the cellular level, such target cells possess specific insulin receptors. In the liver, insulin promotes glycogen synthesis by stimulating the enzyme glycogen synthetase and inhibiting glycogen phosphorylase although it has no direct effect on the GLUT 2 transporters and, hence, the uptake of glucose into hepatocytes. In contrast, insulin induces a rapid uptake of glucose in muscle and fat tissue by recruiting intracellular GLUT 4 transporters and, thus, increasing their cell-surface expression. As a consequence, muscle converts glucose to glycogen. In adipose tissue, glucose is converted to fatty acids for storage as triglycerides. Insulin also stimulates the uptake of amino acids into muscle. At the same time, insulin suppresses the glucose supply from the liver by inhibiting the breakdown of glycogen in the liver, as well as the release of amino acids from muscle and the release of free fatty acids from adipose tissue.