Increased secretion of insulin-like growth factor-binding proteins and decreased secretion of insulin-like growth factor-II by muscle from growth-retarded neonatal rats.
Insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs) may be important factors in the control of neonatal growth. We have examined the production, in vitro, of IGFBPs and IGFs by hind-limb skeletal muscle from normal and small-for-gestational age (SGA) neonatal rats. Conditioned medium was collected from muscle strips after incubation at 37 degrees C for 2 h in Ham's F-12 medium. The conditioned medium was subjected to acid-gel permeation chromatography to separate IGFBPs from IGFs. The binding of 125I-labelled IGF-I to IGFBPs from both control and SGA muscle was displaced equipotently by IGF-I and IGF-II and not at all by insulin. IGFBPs from control and SGA muscles bound IGF-I with comparable affinities (Kd = 0.071 and 0.069 nmol/l respectively). When IGF-II was used as tracer, neither IGF-I nor insulin competed for binding. Western ligand blots of IGFBPs in conditioned media from both control and SGA muscles showed three bands of radioactivity at molecular masses equivalent to 24, 30 and 40 kDa. When the release of IGFBPs by muscle tissue in vitro was quantified by measuring the number of IGF-I binding sites in acid-fractionated medium it was apparent that the muscles from SGA pups secreted significantly more IGFBPs (39.3 +/- 7.5 fmol/mg muscle protein per 2 h) than the muscles from control pups (17.8 +/- 2.7 fmol/mg protein per 2 h; P less than 0.05). In contrast to the IGFBPs, more IGF activity was secreted by the muscles from the control pups (61.1 +/- 15.6 fmol/mg muscle protein per 2 h) than the muscles from the SGA pups (12.6 +/- 5.8 fmol/mg muscle protein per 2 h; P less than 0.05). Analysis of the IGF activity with assays specific for IGF-I and IGF-II showed that both SGA and control muscles secreted predominantly IGF-II with approximately 10% of the total IGF activity measurable as IGF-I. This differential secretion of IGFBPs and IGFs may be associated with the reduced growth potential of the SGA neonate.
Frampton
R J
RJ
Department of Paediatrics, Monash University, Clayton, Victoria, Australia.
Jonas
H A
HA
Larkins
R G
RG
D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D046508:Culture Techniques; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D005317:Fetal Growth Retardation / Q000378:metabolism*; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I / Q000096:biosynthesis; D007335:Insulin-Like Growth Factor II / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism*; D011247:Pregnancy; D051381:Rats; D011919:Rats, Inbred Strains
D002352:Carrier Proteins; D018969:Insulin-Like Growth Factor Binding Proteins; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II