Insulin-like growth factors and insulin-like growth factor binding proteins in porcine serum and milk throughout lactation

IGF-I, IGF-II, and IGF binding proteins (IGFBP) were characterized in porcine serum, colostrum, and milk on d 1-28 postpartum. IGF-I and -II were measured by heterologous RIA. Serum IGFBP were characterized by Western ligand blotting and milk IGF binding activity by [¹²⁵I]-IGF binding assay. IGF-II accounted for 70-85% of serum IGF and rose 2-fold between d 1 and d 28. Serum IGF-I was unaffected by duration of lactation. Milk IGF-II concentrations were higher than IGF-I concentrations on d 1-7 postpartum. After d 10, milk IGF-I and IGF-II contents were not significantly different. Serum contained IGFBP with M_r of 43, 39, 34, 28, and 24 kD. Over the course of lactation, the 43- and 39-kD bands increased, the 24-kD band decreased, and the 34- and 28-kD bands were unchanged. Milk IGF binding activity increased between d 1 (28%) and d 3 (44%), then declined until d 28 (7%). Serum and milk were separated by isoelectric focusing into 20 fractions, across a gradient from pH 3 to 10, that were screened for IGFBP by Western ligand blotting. Milk contained six IGFBP of similar M_r as serum IGFBP; however, the relative amounts of the IGFBP and their apparent isoelectric points differed. In conclusion, porcine milk contains both IGF-I and -II, with IGF-II predominating. Several IGFBP with similar M_r as those found in serum are present in milk. IGF peptide concentrations were highest in prepartum secretions and colostrum, whereas IGF binding activity peaked on d 4 of lactation.