Effect of somatotropin and/or equine chorionic gonadotropin on serum and follicular insulin-like growth factor I and insulin-like growth factor binding proteins in cattle

Abstract

We investigated the effect of administration of somatotropin (ST) and/or eCG on insulin-like growth factor I (IGF-I) and IGF-binding proteins (IGFBP) in serum and follicular fluid (FFL) of cattle actively immunized against growth hormone-releasing factor (GRF). Cyclic beef cattle, previously immunized against GRF-(1-29)-Gly-Gly-Cys-NH2 conjugated to human serum albumin (synthesized and provided by Hoffmann-LaRoche, Inc., Nutley, NJ; GRFi, n = 31) or to human serum albumin alone (HSAi, n = 26), received (i.m.): 1) 25 mg recombinantly derived methionyl somatotropin (rbST, n = 14; sometribove provided by Monsanto Co., St. Louis, MO); 2) 1100 IU eCG (n = 10); 3) rbST and eCG (rbST-eCG, n = 15); or 4) vehicle (VEH, n = 17) at 0 and 24 h after receiving prostaglandin F(2α) (PGF(2α)). Serum samples were collected at 0 and 40 h after PGF(2α), and the ovary bearing the largest follicle (DOM) was removed 44.0 ± 0.5 h after PGF(2α); FFL was harvested from DOM and the subordinate follicle (SUB). Before treatment (0 h), GRFi cows had lower serum ST (0.6 ± 0.2 vs. 2.2 ± 0.2 ng/ml; p < 0.01) and IGF- I (26 ± 4 vs. 72 ± 4 ng/ml; p < 0.01), but greater IGFBP-2 (594 ± 48 vs. 384 ± 52 ng/ml; p < 0.01) than HSAi cows. Serum and FFL concentrations of IGF-I or IGFBP-2 were not different between rbST- and rbST-eCG-treated cows or between VEH- and eCG-treated cows at Hour 40 after the initial treatment injection; therefore, data were combined and designated as rbST and VEH, respectively. Serum IGF-I was increased to a greater extent (percentage increase above 0 h) by rbST treatment in GRFi (362 ± 24) than in HSAi (176 ± 16) cows (immunization by treatment, p < 0.01). Across GRFi and HSAi, rbST lowered serum IGFBP-2 (342 ± 31 vs. 541 ± 27 ng/ml, rbST vs. VEH; p < 0.01). Diameters of DOM or SUB were not affected by immunization or treatment. Concentrations of IGF-I and IGFBP-3 (determined by ligand blot analysis) in FFL from both DOM and SUB were lower (p < 0.05) in GRFi than in HSAi cows. In contrast, IGFBP-2 in FFL was elevated in SUB, but not DOM, in GRFi compared to HSAi cows. Ligand blot analyses indicated that IGFBP-4 and IGFBP-5 were markedly higher in FFL from SUB in GRFi than in HSAi cows, but not different for DOM. Administration of rbST increased IGF-I and decreased IGFBP-2 in DOM and SUB. In conclusion, GRFi decreased serum and FFL concentrations of IGF-I, while it increased concentrations of IGFBP-2 in serum and in FFL from SUB, but not DOM. Treatment with rbST increased serum and FFL IGF-I, but decreased both serum and FFL IGFBP-2 (in both DOM and SUB). The specific roles that IGF-I and IGFBP play in folliculogenesis are yet to be determined; of particular interest is the divergent effect of GRFi on IGFBP in dominant vs. subordinate follicles.