DEVELOPMENTAL EXPRESSION AND LOCATION OF IGF-I AND IGF-II MESSENGER-RNA AND PROTEIN IN SKELETAL-MUSCLE

To investigate the role of IGF in muscle development in vivo, developm ental expression and location of IGF-I and -II protein and mRNA were e xamined in fetal, postnatal, and adult skeletal muscle. Muscle tissue was collected from 30-, 44-, 59-, 68-, 75-, 89-, and 109-d porcine fet uses, 21-d neonatal pigs, and 6-mo-old (adult) pigs. Relative amounts of IGF-II mRNA peaked (P < .05) in 59-d fetal muscle and decreased the reafter. Inversely, muscle IGF-I expression increased (P < .05) to max imal levels around birth. For in situ hybridization, frozen muscle tis sue sections (10 mu m) mere hybridized with a hydrolyzed form of the s ame riboprobes or incubated with polyclonal or monoclonal antibodies t o IGF-I or -II, respectively. The majority of IG;F-I and IGF-II mRNA w as localized to developing muscle fibers, whereas little signal was fo und in the surrounding connective tissues. Immunofluorescent localizat ion of IGF-I and -II confirmed that muscle IGF are present in developi ng muscle fibers. Collectively, these data show that IGF-I and -II are expressed and produced primarily in muscle cells within developing mu scle tissue and support the hypothesis that IGF-I and -II modulate fet al muscle development.