In post-natal animals, plasma concentrations of IGF-I are tightly regulated by nutritional status. The current study reports that plasma levels of IGF-II in sheep are also regulated by nutrition, but whether plasma IGF-II is increased, decreased or remains the same, depends on the age of the animal. Ewe lambs, ranging in age from 2 days to 2 years, were fed or fasted for lengths of time between 24 and 72 h. Blood samples were taken at intervals of 24 h throughout the treatment period and immediately before slaughter. Plasma concentrations of IGF-I increased with advancing age in fed animals (P<0.001) and were reduced by fasting in all age groups (P<0.001). Plasma concentrations of IGF-II also increased as animals matured (P<0.001), but did not show an overall effect of the fasting treatment. An interaction between age and nutrition (P<0.001) resulted from a decrease in plasma IGF-II in response to fasting in neonatal animals (P<0.01) and, conversely, increased levels of plasma IGF-II in fasted mature animals (P<0.01 or P<0.001). Fasted sheep of peripubertal age showed no change in plasma levels of IGF-II. The nutritional sensitivity of serum IGF-binding proteins (BPs) also changed with age. The 29 kDa BP, which we presume to be BP1, was elevated by fasting in young animals and reduced slightly in older animals. BP2 was increased to a similar magnitude by fasting at all ages. BP3 was depressed by fasting in young animals and showed little change in adults. In contrast, a 24 kDa BP, which is probably BP4, showed little change in young animals and was reduced substantially in older sheep. In conclusion, the response of plasma IGF-II to fasting suggests that this peptide has functions in mediating nutritional stress which depend on the age of the animal, and also that the role of IGF-II may differ from that of IGF-I in adults.

The IGF axis is nutritionally sensitive in vivo and IGFs stimulate myoblast proliferation and differentiation in vitro, while myostatin inhibits these processes in vitro. We hypothesised that underfeeding would reversibly inhibit the myogenic activity of satellite cells in vivo together with decreased IGF-I and increased myostatin in muscle. Satellite cell activity was measured indirectly from the expression of proliferating cell nuclear antigen (PCNA) and the myogenic regulatory factors (MRFs), MyoD, Myf-5 and myogenin. Young sheep were underfed (30% of maintenance) and some killed after 1, 4, 12, 17, 21 and 22 weeks. Remaining underfed animals were then re-fed a control ration of pellets and killed after 2 days, and 1, 6 and 30 weeks. Expression of PCNA and MRFs decreased during the first week of underfeeding. This coincided with reduced IGF-I and myostatin mRNA, and processed myostatin. Subsequently, Myf-5, MyoD, myostatin mRNA and processed myostatin increased, suggesting that satellite cells may have become progressively quiescent. Long-term underfeeding caused muscle necrosis in some animals and IGF-I and MRF expression was increased in these, indicating the activation of satellite cells for muscle repair. Re-feeding initiated rapid muscle growth and increased expression of PCNA, IGF-I and the MRFs concurrently with decreased myostatin proteins. In conclusion, these data indicate that IGF-I and myostatin may work in a coordinated manner to regulate the proliferation, differentiation and quiescence of satellite cells in vivo.