Effects of hypophysectomy and growth hormone administration on the mRNA levels of collagen I, III and insulin-like growth factor-I in rat skeletal muscle

The effect of short-term treatment of normal or hypophysectomized rats with biosynthetic growth hormone (GH) was studied in extensor digitorum longus and soleus muscles. in situ hybridization revealed that in normal rats, mRN A for collagen I, collagen III and insulin-like growth factor-I (IGF-I) are expressed by fibroblasts between the muscle fibre areas and that the speci ficity of this location was not altered by GH administration. Hypophysectom y appeared to cause a decrease in IGF-I and decreased collagen I and III ge ne expression (P < 0.001, P < 0.001, respectively). GH administration seeme d to increase IGF-I mRNA levels in all the animals studied. Quantitative im age analysis that GH administration to hypophysectomized rats caused an inc rease in collagen I gene expression after 2 days (P < 0.05) and an increase in collagen III gene expression after 4 days (P < 0.05). The results indic ate that the fibroblast cells are an important target for the action of GH on skeletal muscle and that the fibroblasts respond to GH by increases in t he expression of mRNA for collagen I and collagen III.