Regulation of IGF‐I, IGFBP‐4 and IGFBP‐5 gene expression by loading in mouse skeletal muscle

Abstract
Gene expression of IGF‐I, IGFBP‐4 and IGFBP‐5 was studied in hindhimb skeletal muscle of mice, which were either overloaded or unloaded for 8 days. Overloading induced a 15% hypertrophy in soleus muscle associated with a 60% increase of IGF‐I transcript levels and a doubling of IGFBP‐4 mRNA levels. IGFBP‐5 mRNA levels were decreased to one third of the control value. Changes in IGFBPs mRNA always preceded changes in IGF‐I gene expression. Unloading by hindlimb suspension resulted in atrophy of soleus muscle (20%) and phenotype change towards the fast type associated with a transient decrease of IGF‐I mRNA (30%) and a sustained increase (×2) of IGFBP‐5 transcript. These alterations in IGFBPs expression, in unloaded or overloaded soleus, suggest that they may play a role in skeletal muscle adaptation to changes in loading.