Insulin-like growth factor-I in man enhances lipid mobilization and oxidation induced by a growth hormone pulse

Summary Growth hormone (GH) secretion is suppressed during insulin-like growth factor-I (IGF-I) administration. The aim of the study was to examine whether IGF-I alters the metabolic response to a GH pulse. Seven healthy male subjects (age 27±4 years, BMI 21.8±1.7 kg/m²) were treated with NaCl 0.9% (saline) or IGF-I (8 g · kg^–1 · h^–1) for 5 days by continuous subcutaneous infusion in a randomized, crossover fashion while receiving an isocaloric diet (30 kcal · kg^–1 · day^–1). On the third treatment day an intravenous bolus of 0.5 U GH was administered. Forearm muscle metabolism was examined by measuring arterialized and deep venous blood samples, forearm blood flow by occlusion plethysmography and substrate oxidation by indirect calorimetry. IGF-I treatment significantly reduced insulin concentrations by 80% (p<0.02) and C-peptide levels by 78% (p<0.02), as assessed by area under the curve. Non-esterified fatty acid (NEFA), glycerol and 3-OH-butyrate levels were elevated and alanine concentration decreased. Forearm blood flow rose from 2.10±0.43 (saline) to 2.79±0.37 ml · 100ml^–1 · min^–1 (IGF-I) (p<0.02). GH-pulse: 10 h after i.v. GH injection serum GH peaked at 40.9±7.4 ng/ml. GH did not influence circulating levels of total IGFI, C-peptide, insulin or glucose, but caused a further increase in NEFA, glycerol and 3-OH-butyrate levels, indicating enhanced lipolysis and ketogenesis. This effect of GH was much more pronounced during IGF-I: NEFA rose from 702±267 (saline) and 885±236 (IGF-I) to 963±215 (saline) (p<0.05) and 1815±586 mol/l (IGF-I) (p<0.02), respectively; after 5 h, 3-OH-butyrate rose from 242±234 (saline) and 340±280 (IGF-I) to 678±638 (saline) (p<0.02) and 1115±578 mol/l (IGF-I) (p<0.02) respectively. After injection of GH, forearm uptake of 3-OH-butyrate was markedly elevated only in the subjects treated with IGF-I: from 44±195 to 300±370 after 20 min (p<0.03) and to 287±91 nmol · 100 ml^–1 · min^–1after 120 min (p<0.02). In conclusion, the lipolytic and ketogenic response to GH was grossly enhanced during IGF-I treatment, and utilization of ketone bodies by skeletal muscle was increased.Abbreviations AUC Area under the curve - C-peptide connecting peptide - EE energy expenditure - FFM fat-free mass - GH growth hormone - IGF-I insulin-like growth factor-I - NEFA non-esterified fatty acid - Ra rate of glucose appearance - Rd rate of glucose disposal - FGU forearm glucose uptake - CV coefficient of variation