Induction of mRNA for IGF-I and -II during growth hormone-stimulated muscle hypertrophy

The expression of insulin-like growth factor (IGF) genes during skeletal and cardiac muscle hypertrophy was examined using adult 5-mo-old female Wistar-Furth rats implanted with growth hormone-secreting GH₃ cells. Control and treated animals were killed at 40, 60, and 80 days after initiation of the experiment. From the time of injection to day 80, body, heart, skeletal muscle, and liver weights increased 112, 93, 55, and 314%, respectively. RNA was extracted and steady-state levels of IGF-I and IGF-II mRNAs were quantitated using a solution-hybridization nuclease-protection assay. Low levels of mRNA for both growth factors were detected in control tissues. By day 80 IGF-I mRNA had increased eightfold and IGF-II mRNA sixfold in skeletal muscle from treated rats. In cardiac muscle the levels of mRNA for both growth factors rose three- to fourfold. Although growth hormone induced an increase in hepatic IGF-I mRNA, IGF-II mRNA remained nearly undetectable. This study shows that during growth hormone-stimulated muscle growth mRNAs for both IGF-I and IGF-II accumulate, supporting other observations implicating the IGFs as paracrine or autocrine factors involved in skeletal muscle growth.