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The changes in body weight and serum levels of growth hormone (GH), so-matomedin insulin-like growth factor (IGF), and its carrier protein (CP) have been followed in rats from fetal to adult life. The weight gain was maximal (6.0 g/day) between Days 28 and 55 and minimal (1.4 g/day) between Days 79 and 120. Two distinct peaks of GH concentration have been found in normally growing animals. The first (mean ± SEM) was observed on the first day of life (371 ± 37 ng/ml) while the second peak (365 ± 97 ng/ml) coincided with an acceleration in growth rate beginning Day 40. An unexpectedly high fetal IGF level (812 ± 164 μU/ ml) rapidly declined to a nadir (189 ± 26 μU/ ml) at Day 21, and then rose gradually to a level of 572 ± 12 μU/ml on Day 120. The pattern of IGF CP concentration resembled that of weight gain rather than those of serum GH and IGF. The sex of animal appeared to have no influence on any component of growth-promoting system.
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Draznin et al. (1979) studied this question.
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