Circulating levels of insulin-like growth factor-1 and associated binding proteins in plasma and mRNA expression in tissues of growing pigs on a low threonine diet

The aim was to determine whether dietary threonine levels affected hepatic insulin-like growth factor-1 (IGF-1) mRNA expression as well as plasma IGF-1 concentration and IGF binding protein ( IGFBP) profile in growing pigs. Two male 6-week-old pigs from each of seven litters were used. Each littermate was assigned to one of two diets, control or low threonine (LT), providing per kg 14.3 MJ digestible energy in both diets, 170 g protein in the control diet and 167 g protein in the LT diet. The control diet contained all essential amino acids in the recommended amounts, including 8.2 g threonine per kg. The LT diet was similar but contained only 5.1 g threonine per kg. Pigs were pair-fed these diets for 3 weeks. Growth rate and food efficiency of pigs given the LT diet were significantly lower than those of pigs given the control diet (P < 0.001). Plasma IGF-1 concentration of pigs given the LT diet was proportionately 0.44 lower than that of pigs given the control diet (P < 0.01). Plasma free threonine concentration of pigs given the LT diet was lower than that of the pigs given the control diet (P < 0.001). Plasma IGFBP2 level of pigs given the LT diet was significantly higher than that of pigs given the control diet (P < 0.05). Pigs given the LT diet had a significantly lower plasma IGFBP3 level compared with their littermates given the control diet (P < 0.05) suggesting that clearance rate of circulating IGF-1 was higher in the LT group. Dietary threonine level did not affect IGF-1 mRNA abundance in the liver. It is concluded that lower plasma IGF-1 level caused by reduced dietary threonine level may have been partly due to increased clearance rate of circulating IGF-1 but not due to IGF-1 gene expression in the liver.