Insulin-like growth factors and insulin: comparative aspects.

IGF I and IGF II are two insulin-like growth factors resembling insulin in many respects. They stem from a common precursor, act through receptors similar to the insulin receptor with which they cross-react. When administered in large amounts they produce hypoglycemia. Their major effects, however, are on replication and differentiation of cells of mesodermal origin. IGF I is the major growth promoting factor in vivo. The synthesis and secretion of IGF I by the liver depend on the growth hormone status, insulin and nutrition. In contrast to insulin, the IGFs circulate in blood bound to the carrier proteins. Their half-life in man is in the order of 16 h. IGF I deficiency results in dwarfism (pygmy, Laron dwarf, toy poodle) despite normal or elevated growth hormone secretion. The anabolic actions of insulin and of the IGFs appear to complement each other as shown in Figure 7.