Effects of activity on growth factor expression.For some time, it has been appreciated that muscle mass is regulated locally as well as systemically. We have cloned the cDNA of two isoforms of IGF-1, which are derived from the IGF-1 gene by alternate splicing. The expression of one of these was only detectable after mechanical stimulation. For this reason, this has been called mechano growth factor ( MGF). The MGF is not glycosylated, is smaller, and has a shorter half-life in the unbound state than the systemic liver type IGF-1. As the result of a reading frame shift the MGF peptide also has a different C terminal sequence and thus has different binding protein/receptor affinities. Another splice variant (muscle L.IGF-I) is expressed in muscle during rest but is also upregulated by exercise. The latter is similar to the systemic liver type IGF-1. The evidence suggests that MGF has a high potency for inducing local protein synthesis and preventing apoptosis and therefore has an important role in local tissue repair and remodeling. Our physiological experiments show that stretch and particularly stretch combined with electrical stimulation, rather than stimulation alone are important in inducing MGF expression. The mechanotransduction mechanism is believed to involve the muscle cytoskeleton. During aging, the production of growth hormone and IGF-1 by the liver declines markedly. The discovery of MGF and muscle IGF-1 provides a link between physical activity and gene expression. This underlines the need for the elderly to remain active as the locally produced growth factors supplement the circulating IGF-1 levels.