Distinct role of insulin and IGF-I and its receptors in white skeletal muscle during the compensatory growth of gilthead sea bream (Sparus aurata)

In order to examine compensatory growth (CG) in gilthead sea bream (Sparus aurata), we used an experimental protocol involving 1-, 3- or 4-week fasting followed by a one- or two-month re-feeding period. After one month of re-feeding, final body weight gain and growth acceleration in groups fasted for 1 or 3 weeks (W1 and W3) were comparable to the control group values (C), indicating that partial CG was achieved. Fasting had a marked effect on depressing circulating glucose plasma levels, which were recovered to C group values after one month re-feeding. Glycogen content in liver and muscle decreased during fasting and was completely recovered after two month re-feeding. FFA plasma levels increased during fasting trial for all the different experimental groups, whereas lipid content in liver and muscle was depressed, indicating that lipid depot utilization contributes to the energy metabolism of gilthead sea bream. After re-feeding for 1 or 2 months, FFA and lipid content in liver and muscle were comparable to C group values. Plasma insulin and IGF-I values decreased during the fasting trial, while normal levels were restored after 1 month of re-feeding IGF-I and insulin binding assays in semi-purifications of soluble proteins from white skeletal muscle showed that IGF-I binding increased with fasting while insulin binding decreased. However, the specific binding for these two peptides was comparable to C group values after re-feeding for 1 or 2 months.In conclusion, after re-feeding insulin and IGF-I recovery was parallel with increases in weight gain and growth acceleration in those groups which showed diminutions in growth during the period of food restriction. Furthermore, the differential regulation of the specific binding for insulin and IGF-I in white skeletal muscle during fasting, indicates that these two receptors play distinct roles in response to the nutritional status of the animal.