Acute IGF‐I infusion stimulates whole body protein synthesis but does not reduce proteolysis in neonates

Abstract

Skeletal muscle protein synthesis increases in response to a physiological rise in total insulin-like growth factor I (IGF-I) in neonatal pigs. To determine the response of whole body protein synthesis and degradation to IGF-I, fasted 7-day-old pigs (n=4/dose) were infused with IGF-I (0, 20, or 50 μg/(kg/hr)) to achieve levels within the physiological range. Because IGF-I infusion lowers plasma insulin, an additional group of IGF-I pigs was provided replacement insulin (10 ng/(kg0.66/min)). Plasma glucose was clamped with dextrose and AA were clamped with a balanced AA mixture at fasting levels. Whole body protein turnover was determined with 13C-leucine. Total IGF-I levels in controls, low-dose IGF-I, high-dose IGF-I, and high-dose IGF-I with replacement insulin were 15±2, 23±2, 37±3, and 41±3 ng/ml, respectively. High dose IGF-I increased flux (+26%, P<0.01), protein synthesis (+25%, P<0.009), AA oxidation (+28%, P<0.008), and protein balance (+29%, P<0.05) but had no effect on proteolysis. Provision of replacement insulin during the high dose IGF-I infusion did not further enhance endogenous flux, protein synthesis, AA oxidation, or protein balance, or alter proteolysis. Low dose IGF-I had no effect on any parameter. The results demonstrate that a physiological rise in total IGF-I stimulates whole body protein synthesis but does not alter protein degradation in neonates. NIH AR44474, USDA 58-6250-6-001