Insulin-like growth factor-I (IGF-I) analogue, LR(3)IGF-I, ameliorates the loss of body weight but not of skeletal muscle during food restriction
F M Tomas. Growth Horm IGF Res. 2001 Apr.
Abstract
Insulin-like growth factor-I (IGF-I) is known to have anabolic effects in freely fed rats. We have investigated the ability of infused LR(3)IGF-I, an analogue of IGF-I, to attenuate the loss of lean tissue due to food restriction in young (5 weeks) and adult (12 weeks) rats. Groups of rats received food at 100%, 78%, 56% or 33% of ad libitum levels. Within each nutrition group the rats were continuously infused with LR(3)IGF-I at (98 nmol/day)/kg body weight or vehicle for 7 days. At each level of food intake, rats infused with LR(3)IGF-I maintained higher body weight (around 3-8%;P< 0.001) and nitrogen retention (P< 0.001) than those infused with vehicle alone but muscle protein was not conserved. LR(3)IGF-I infusion increased fat loss only in young rats (P< 0.05) despite a reduction in plasma insulin levels in both age groups (P< 0.01). Muscle protein turnover rates were unaffected by LR(3)IGF-I in young rats. In adult rats LR(3)IGF-I exacerbated the effects of food restriction through increased rates of protein breakdown, reduced RNA content and reduced rates of protein synthesis (P< 0.05) despite their larger fat reserves. Although young and adult rats show differing metabolic responses, we conclude that infusion of LR(3)IGF-I to either group during short-term food restriction does not ameliorate the loss of lean tissue by allowing more efficient utilization and/or partitioning of nutrients.
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