Insulinlike growth factor binding proteins.

Insulinlike growth factor binding proteins (IGFBPs), which are often present in extracellular fluids, have the potential to markedly alter IGF-receptor interaction. There are at least four specific forms of IGFBPs in blood. These forms are also secreted by a number of cell types. Their rates of secretion and clearance appear to be regulated, mostly by hormones. Their major role seems to be to alter the interaction of IGFs with their cell surface receptors. Other functions such as transport out of the vasculature and modulating target cell actions also appear to be important.