IGF-I/IGFBP system: metabolism outline and physical exercise

The GH/IGF-I system plays a well-known hormonal role and its effects, mainly anabolic and insulin-sensitizing, are mediated through endocrine as well as paracrine/ autocrine mechanisms. This system includes the binding proteins, namely GH binding proteins and IGF-I binding proteins (IGFBP). As expected, this axis plays a key role in organism modification in consequence of a physical exercise. Physical activity, training, and exercise capacity chiefly involve anabolism process modifications of various tissues, in particular muscular adjustments. Numerous investigators found a correlation among the level of exercise tolerance, muscle strength or walking speed and IGF-I/IGFBP-3 concentrations. However, also inverse and absent correlations between circulating IGF-I concentrations and acute or chronic exercise responses have been reported. IGF-I is generally accepted as an important GH mediator with metabolic effects, through both endocrine and paracrine or autocrine mechanisms. GH is the main regulator of the hepatic synthesis of IGF-I and IGFBP-3, which is the most abundant IGF carrier in human plasma. Recently, it has been shown that the physical exercise stimulatory impact on skeletal muscles is mediated through an increased local IGF-I synthesis with an IGFPB involvement. An absent association of exercise performance and circulating IGF-I may indicate that exercise will exert muscle strength by predominately locally derived paracrine or autocrine mediators rather than endocrine circulating IGF-I. The present review considers the general aspects of the IGF/IGFPB system and the role of the IGF/IGFPB system in relation to physical exercise (type, duration, etc.) taking into account the training aspects.