EFFECTS OF DIETARY INSULIN-LIKE GROWTH-FACTOR-I ON GROWTH AND INSULIN-LIKE GROWTH-FACTOR RECEPTORS IN NEONATAL CALF INTESTINE

Colostrum is rich in IGF-I and IGF-II, and the dietary effects of recombinant human (rh)IGF-I on the newborn are of interest. The objective of this study was to examine the effects of dietary rhIGF-I on intestinal tissue growth and populations of IGF receptors. Twenty-three male dairy calves were fed one of three experimental diets: 1) milk replacer plus isolated colostrum-derived globulins (MR(-)), 2) same as 1 plus 750 ng of rhIGF-I/mL (MR(+)), or 3) pooled cow colostrum (COL). After the first four feedings, all calves received milk replacer without additional globulins; calves fed the MR(+) diet continued to receive the addition of 750 ng of rhIGF-I/ mt until the experiment ended at 7 d after birth. Calves were killed and intestinal tissue was collected for in vitro [3H]thymidine incorporation studies. Incorporation differed among intestinal regions (duodenum, jejunum, and ileum). The MR(+) calves had greater (P <.01) [H-3]thymidine incorporation per unit of DNA than either the COL or MR(-) calves (31.8 vs 18.6 and 11.5 x 10(3) dpm/mu g of DNA, respectively). Competitive binding analysis indicated the presence of specific type 1 and type 2 intestinal IGF receptors. The IGF-I was more potent than IGF-II and insulin at inhibiting [I-125]rhIGF-I binding (ED(50) was 1.84, 9.17, and 1.91 ng/mL, respectively). The IGF-II was the only ligand capable of inhibiting [I-125]rhIGF-II binding (ED(50) was .30 nmol/mL). Membranes from COL calves had less (P <.05) [I-125]IGF-II binding capacity than those from MR(-) and MR(+) calves (16.9 vs 35.5 and 29.9 fmol/mg of protein, respectively). However, membranes from calves fed MR(+) had greater(P <.05) [I-125]rhIGF-I binding capacity than those fed MR(-) and COL (6.9 vs 4.6 and 4.2 fmol/mg of protein, respectively). These data support the hypothesis that dietary IGF increases intestinal growth and alters IGF intestinal receptor populations.