Serum Concentrations of Insulin-Like Growth Factor II Are Not Changed by Short Term Fasting And Refeeding*

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Abstract
To determine the factors that regulate insulin-like growth factor II (IGF-II), we raised polyclonal antibodies to this peptide and developed a RIA that measures IGF-II in serum or plasma samples after extraction of IGF-binding proteins by C18 cartridge chromatography. The IGF-II antiserum was highly specific, exhibiting no cross-reactivity with IGF-I or insulin at the highest concentrations tested (10-6 mol/L). As little as 0.43 .mu.g/L IGF-II was detectable, and 50% displacement of tracer occurred at 1.7 .mu.g/L. The serum IGF-II concentrations of normal adults [mean, 634 .+-. 170 (.+-. SD) .mu.g/L], patients with acromegaly (570 .+-. 146 .mu.g/L), and patients with hypopituitarism (156 .+-. 58 .mu.g/L) were similar to those reported by others. In eight obese subjects injected with GH (0.1 mg/kg ideal BW, im, every 48 h for 16 days), serum IGF-II concentrations did not rise significantly, whereas IGF-I concentrations increased 67%. Sixteen normal subjects, within 15% of ideal body weight, were fasted for 5 days on two to four occasions and refed diets of differing protein and calorie contents. Their mean serum IGF-II concentration before fasting (691 .+-. 26 .mu.g/L) was not significantly different from that after fasting (674 .+-. 21 .mu.g/L) or after refeeding (641 .+-. 20 .mu.g/L). In contrast, their mean IGF-I concentration decreased 42% with fasting and rose with refeeding. Unlike IGF-I, serum IGF-II concentrations do not appear to be regulated by short term changes in nutritional status. It is clear for this study and others that IGF-II and IGF-I are regulated differently despite their structural homology and the similarity of their actions in vitro.

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