Expression of the locally produced IGF-I splice variants in damaged-regenerating and stretched and stimulated skeletal muscle

Muscle is a mechanical tissue in which new nuclei are obtained by the fusion of satellite cells with damaged muscle fibres following local damage. Up regulation of the expression of growth factors is essential to nurture the proliferation of these muscle precursor cells. Increased expression of IGF-I was found to be associated with regeneration of muscle fibres that is produced by muscle. Two alternatively spliced isoforms of IGF-I are produced locally in active muscle and appear to be positive regulators of muscle hypertrophy; IGF-IEa with a similar structure to the major endocrine form produced by liver and IGF-IEb or MGF that has an extra 52bp insert in exon 5 in rodents and was found to be up regulated in exercised muscle. Muscle cells are known to produce and secrete IGFBPs that are important in modifying the availability and biological effects of IGFs. In order to study the mechanism of local muscle repair, two systems of inducing local damage were applied; muscle stretch combined with an initial bout of electrical stimulation and injection of a myotoxic agent, bupivacaine. Using Real-Time PCR with primers that distinguish these spliced forms of IGF-I, their expression and the expression of MyoD, M-cadherin, IGFBP-4 and IGFBP-5 were measured in the damaged tibialis anterior muscle, its contralateral muscle as well as sham and normal muscles from age matched controls. The appearance and localisation of their proteins was investigated by immunohistochemistry. Both models resulted in extensive damage followed by activation of satellite cells, which decreased after muscle regeneration commenced. The two IGF-I isoforms were differentially regulated at the RNA level. A significant, acute surge of MGF in the damaged muscle was associated with the activation of satellite cells and an increased expression of IGFBP-5. IGF-IEa mRNA and protein expression appeared to be related to the long-term recovery of muscle