Developmental Histology, Segmental Expression, and Nutritional Regulation of Somatotropic Axis Genes in Small Intestine of Preweaned Dairy Heifers

Components of the somatotropic axis and nutrition regulate intestinal development and maturation of enterocytes. We measured gene expression in the mucosal layer of small intestine of preweaned dairy heifers to test the hypothesis that feeding increased amounts of protein and fat alters expression of somatotropic axis genes. Twenty-four newborn Holstein heifers were randomly assigned to 1 of 4 milk replacer (MR) diets: (1) 20% CP, 20% fat MR (DM basis) fed at 450 g/d (CON); (2) 28% CP, 20% fat MR fed at 970 g/d (HPLF); (3) 28% CP, 28% fat MR fed at 970 g/d (HPHF); and (4) 28% CP, 28% fat MR fed at 1,460 g/d (HPHF+). Dry calf starter (20% CP, 1.43% fat) was offered free choice. At 64 ± 3 d of age heifers were killed and intestinal tissues were harvested for RNA isolation and histological examination. We measured the mRNA expression of growth hormone receptor (GHR), insulin-like growth factor-I (IGF-I), IGF-I receptor (IGF-IR), and IGF binding proteins (IGFBP)-1 to -6 in duodenum, jejunum, and ileum by quantitative real-time reverse transcription PCR. Expression of IGFBP-3 mRNA was lowest in the duodenum of HPHF+ and greatest in the ileum of the CON group, whereas expression of IGFBP-4 mRNA was greatest in the jejunum of the HPHF+ group. Expression of IGFBP-5 mRNA was greatest in the CON and lowest in the HPHF+. However, overall diet did not affect expression of GHR, IGF-I, IGF-IR, or IGFBP-1, -2, and -6. Expression of somatotropic axis genes differed among small intestinal locations. The GHR, IGF-IR, IGFBP-1, and IGFBP-5 mRNA were greatest in the ileum. Duodenum produced less IGF-IR, IGF-I, and IGFBP-5 mRNA. Villi were shortest in the ileum, but there was no difference in villus height between the duodenum and jejunum. There was no difference in crypt depth or villus circumference between locations. In conclusion, some components of the somatotropic axis in preweaned dairy heifers are differentially expressed in regions of the small intestine, and the gene expression tended to be affected by dietary protein and fat.