Role of IGF-I and IGF-binding proteins within diaphragm muscle in modulating the effects of nandrolone

Abstract

Recent studies suggest that the anabolic effects of testosterone in muscle may be mediated, in part, by the insulin-like growth factor (IGF) system. The aim of this study was to examine the effects of nandrolone (NAN) on both IGF-I and IGF-binding proteins (IGFBPs) in the diaphragm muscle of 1-yr-old female rats. NAN (6.6 mg · kg−1 · day−1) was infused continuously for 17 days using a subcutaneous Silastic implant, whereas controls (CTL) received blank capsules. Muscle fibers were classified immunohistochemically, and fiber cross-sectional areas (CSA) were determined quantitatively. IGF-I levels in both serum and muscle were determined by RIA. Immunoreactivity to an IGF-I antibody was used to localize IGF-I expression within individual muscle fibers. Muscle IGFBPs were determined by SDS-PAGE and Western ligand blotting and measured by scanning densitometry. Body weight was higher in the NAN group compared with CTL (9.4 ± 4.5% vs. −0.6 ± 3.1%). There were no changes in the fiber composition of the diaphragm. NAN increased the CSA of type IIa (20%) and type IIx/b (30%) diaphragm fibers. Levels of IGF-I in the diaphragm muscle were significantly higher (50%) in NAN-treated animals. Immunohistochemistry revealed increased localization of IGF-I within type IIx/b diaphragm fibers. In addition, NAN increased IGFBP-3 within the diaphragm (69%), whereas IGFBP-4 decreased (40%). We conclude that NAN-induced diaphragm muscle fiber hypertrophy is mediated, in part, by influences of the IGF system within the muscle, such that coordinated changes in IGFBPs reflect a direction of change that has been associated with an anabolic response in other test systems.