Plasma IGF-I binding proteins in sheep: effect of recombinant growth hormone treatment and nutritional status

In humans the IGF binding proteins (BP) are closely related to metabolic status. In this paper we have examined the influence of controlled feed intake and GH treatment on IGF binding proteins in growing lambs. Analyses were performed on plasma samples from animals maintained on two levels of feed intake (1.75% body weight as lucerne pellets or 3% body weight which is approximately equivalent to an ad libitum intake) either with or without recombinant bovine growth hormone (BST; 0.25 mg/kg body weight/day) administration. Samples used for the analyses reported in this paper were collected at 9.00 hr following 41 d of treatment. Total plasma IGF-I was increased on the higher plane of nutrition (P less than .01) and by BST (P less than .001) but only on high feed intake. IGF is associated with BP of 150 kDa and 40-50 kDa in sheep plasma. 150 kDa bound IGF-I was increased on the higher plane of nutrition (P less than .05) and by BST treatment (P less than .001) but only on the higher feed intake. By contrast no change in 40-50 kDa bound IGF-I was observed with treatment. Unbound IGF-I was also found in sheep plasma (2-5% of total) but demonstrated only minor changes in relation to treatment. Saturation analysis gave estimates of total binding capacity and saturation of the IGF-BP. In ovine plasma the binding capacity of the 150 kDa species is in excess of bound IGF (P less than .001). Saturation did not change with treatment despite the observed differences in 150 kDa bound IGF-I. Thus BP(s) contained in the 150 kDa fraction were responsive to treatment. By contrast large differences in saturation of the 40-50 kDa species were observed (P less than .001) despite little treatment dependent change in bound IGF-I. IGF-BP(s) in the 40-50 kDa fraction were elevated in the low nutrition group and suppressed on the higher feed intake resulting in near saturation. These data strongly suggest that the IGF BP are modulated according to metabolic status in the sheep.