Local changes of IGF-I mRNA, GH receptor mRNA, and fiber size in rat plantaris muscle following compensatory overload.

Abstract

We investigated whether local insulin-like growth factor-I (IGF-I) mRNA and growth hormone (GH) receptor mRNA expressions in plantaris muscle were related to the region-specific hypertrophy following compensatory overload. Adult male normal or hypophysectomized rats were subjected to unilateral distal-half removals of the gastrocnemius and soleus muscles. The contralateral hindlimb was used as the control. Two weeks later, fiber areas in the distal and proximal parts of the plantaris muscle were measured. All the fiber areas measured in the distal part of the plantaris muscle in normal and hypophysectomized rats were significantly increased following the compensatory overload. In the proximal part, fiber areas of type I, IIA, and IIC were significantly increased, but fiber area of type IIB did not change. IGF-I mRNA expressions in the distal and proximal parts were increased 3 d after the compensatory overload in normal and hypophysectomized rats. The increase of IGF-I mRNA expression in the distal part 3 d after compensatory overload was greater than those in the proximal part. IGF-I mRNA expressions in the distal and proximal parts were increased 14 d after the compensatory overload in hypophysectomized rats, but not in normal rats. GH receptor mRNA expressions were decreased following compensatory overload, and almost disappeared 14 d after the compensatory overload in hypophysectomized rats. Thus muscle fiber hypertrophy following compensatory overload was different among the parts in a muscle and IGF-I mRNA was expressed in concert with the region-specific hypertrophy, but not GH receptor mRNA.