Insulin-like growth factor-1 (IGF-1) is a naturally occurring polypeptide produced in the liver, muscle and fat tissues (Hossner et al., 1997). IGF-1 has multiple effects on animal growth and metabolism. Original studies focussed on its endocrine role, where IGF-1 interacts with and influences the effects of growth hormone (GH). However, IGF-1 also has autocrine and paracrine functions in or close to tissues of synthesis (Hossner et al., 1997). The cellular level production and functions are probably more significant than endocrine functions prior to maturation of the IGF-1/GH axis (Harrell et al., 1999).
Circulating IGF-1 is usually bound with one of six specific binding proteins (BPs). Binding proteins control the distribution, function and activity of IGF-1 in vivo and are produced and expressed in at least one form in almost all tissues. It is thought that mechanisms controlling the expression of BPs during various physiological states (as yet largely undefined) will determine whether IGFs (both IGF-1 and IGF-2) have stimulatory or inhibitory effects on IGF-mediated growth (Hossner et al., 1997). The total concentration of circulating IGF-1 can be measured relatively easily in blood plasma with appropriate assays that first strip the BPs from IGF-1. In the absence of stress and due to the presence of BPs, the total pool of circulating IGF-1 is relatively stable (Hossner et al., 1997).