| Insulin-like growth factor I
|
| IGF-I is the most GH-dependent of a series of growth factors
|
| IGF-I is a 70-amino-acid single-chain basic peptide; it has considerable homology with proinsulin. In response to tropic hormones like GH, IGF-I is produced in many tissues, whose growth is then stimulated; that is, IGF-I acts as a parahormone. The liver is the major source of circulating IGF-I, whose function is primarily feedback inhibition of GH secretion. In plasma and other extracellular fluids, IGF-I is complexed to a series of IGF-binding proteins (IGFBPs) of which IGFBP-3 is the most abundant. IGF-I works through the type 1 IGF receptor, which is structurally similar to the insulin receptor and linked to intracellular tyrosine kinase activity. The relative affinities of insulin and IGF-I for their respective receptors mean that, in normal physiology, there is little cross-binding, although in pathologic and pharmacologic situations it is possible for insulin to have some action through the IGF-I receptor, and vice versa. This has some implications for the possible use of IGF-I as a therapeutic agent in type 2 diabetes mellitus.
|
| The plasma reference interval for IGF-I in adults aged 20-60 years is fairly constant. It is much lower in young children but rises dramatically during the period of growth and progression through puberty. IGF-I concentrations fall after the sixth decade of life. Thus, IGF-I appears to be a good marker of integrated GH activity.
|
|
