GH regulation of the Type 2 IGF receptor in regenerating skeletal muscle of rats

Abstract

GH enhances skeletal muscle growth, and IGF-II peptide is highly expressed during regeneration. We have therefore investigated the effect of GH administration on IGF-II binding and expression in regenerating rat skeletal muscle using the techniques of receptor autoradiography and in situ hybridisation. Notexin, a myotoxin, was injected into the right M. biceps femoris (day 0), causing affected fibres to undergo necrosis followed by rapid regeneration. Animals were administered either GH (200 micrograms/100 g body weight) or saline vehicle daily. Contralateral muscles were used as regeneration controls. GH administration during regeneration resulted in significant increases in body weight, and damaged and undamaged muscle weights (P < 0.001). IGF-II expression, which was examined in regenerating fibres, survivor fibres and undamaged fibres, varied according to tissue type (P < 0.001). Specifically, IGF-II expression in regenerating fibres was elevated relative to control and survivor fibres after day 3 (P < 0.05), with a peak on day 9 (P < 0.001). GH did not affect IGF-II message levels. 125I-IGF-II binding in regenerating muscle was examined in the same fibre types as well as in connective tissue. 125I-IGF-II binding in regenerating fibres was higher (P < 0.001) than in other tissue types on day 5. GH administration increased 125I-IGF-II binding in all damaged muscle tissues on day 5 (P < 0.001, regenerating fibres; P < 0.01, others). We believe that this shows for the first time an effect of GH on the Type 2 IGF receptor in regenerating skeletal muscle.

Myostatin regulation during skeletal muscle regeneration

“…Upregulation of myostatin expression concommitant with down-regulation of IGF-II expression has also been noted during muscle wasting (Shivji et al, 1999). Taken together, these studies and the present findings support a role for myostatin as a regulator of myoblast and satellite cell proliferation, and a role for IGF-II in the regulation of myoblast and satellite cell differentiation (Tollefsen et al, 1989;Kirk et al, 1996).…”

“…In the current study, dw/dw rats undergoing muscle fibre necrosis and regeneration following notexin (a potent myotoxin) injection in the M. biceps femoris were administered exogenous GH in order to enhance the muscle regeneration process. Previously we have shown that GH-treatment of the dw/dw rats from this study increased muscle weight but did not alter regeneration time (Kirk et al, 1996). Further, we identified an increase in IGF-II message associated with regenerating muscle which was not affected by GH-administration.…”

“…In an earlier report on this trial (Kirk et al, 1996), we showed an elevation of insulin-like growth factor-II (IGF-II) message concommitant with myotube formation during skeletal muscle regeneration in dw/dw rats, which we now know is the time that myostatin is absent. Similarly, the related factors, TGF-␤1 and TGF-␤3, are negatively regulated by IGF-II in L6A1 myoblasts just prior to the upregulation of myogenin mRNA (Bosche et al, 1995).…”

“…Although GH had a significant effect on the muscle mass of regenerated muscle (Kirk et al, 1996) no effect of GH-administration on the intensity or distribution of myostatin immunostaining was observed in necrotic, regenerating, or survivor fibres.…”

“…A full description of muscle regeneration has been presented elsewhere (Kirk et al, 1996). In brief, both normal and dw/dw rats showed a similar pattern of regeneration, with necrotic muscle fibres present from day 1 after notexin injection onwards.…”