Expression and localisation of IGF-binding protein mRNAs in regenerating rat skeletal muscle

The expression of the insulin-like growth factor-binding proteins (IGFBP) - 3, -4, -5 and -6 was investigated in neonatal, in normal adult and in regen erating rat skeletal muscle. Semi-quantification was done by densitometric scannings of Northern blots. The expression of all investigated IGFBPs, wit h the exception of IGFBP-5, was higher in neonatal than in adult muscle. Du ring postischaemic regeneration the expression of all IGFBPs increased, but with different time schedules. IGFBP-3 increased transiently during the ea rly phase of regeneration, while IGFBP-4, -5 and -6 increased during the la ter phase of regeneration. In situ hybridisation on regenerating muscle sho wed that the expression of the various IGFBPs was cell specific; thus, IGFB P 3 was mainly expressed in macrophages, IGFBP-4 in connective tissue, IGFB P-5 in regenerating muscle cells, and IGFBP-6 in muscle cells, connective t issue and endothelium. Ligand blotting, using 125I-IGF-I as the ligand, sho wed a number of bands ranging between 24 and 44 kDa. Samples from neonatal and regenerating muscle contained much higher levels of all IGFBPs than tho se from normal adult muscle. An ordered and cell-specific expression of IGF BPs, allowing a strict regulation of IGF actions, is probably necessary to ensure an optimal regeneration process.