Ovarian expression of insulin-like growth factor-I (IGF-I), IGF binding proteins, and growth hormone (GH) receptor in heifers actively immunized against GH-releasing factors

Abstract

Active immunization against GRF at 6 months of age delays puberty in beef heifers. The objectives of the present study were to determine whether active immunization against GRF at an earlier age would affect normal onset of puberty and follicular growth and to determine whether these changes were related to alterations in ovarian insulin-like growth factor I (IGF-I) or IGF binding protein (IG-FBP) messenger RNA (mRNA) levels. Heifers were immunized against human serum albumin (HSAi; n = 15) or against GRF conjugated to HSA (GRFi; n = 18) at 3 months of age. A third group of heifers was not immunized (CON; n = 16). Immunization against GRF delayed puberty beyond 13 months of age in 75% of treated heifers. Unilateral ovariectomy at 191 days of age revealed that the delay in puberty was associated with a reduction in the number of large ( > or = 7 mm in diameter) follicles. Large follicles were present in only 22% of GRFi heifers compared to 77% of HSAi heifers. The number of small ( < or = 3 mm in diameter) and medium (4 to 6 mm in diameter) follicles was not affected by GRFi. The percentage of 1- to 3-mm follicles that were atretic was not different between HSAi (65%) and GRFi (62%) heifers. Unilateral ovariectomy had no effect on age at puberty. Immunization against GRF decreased (P < 0.01) concentrations of IGF-I in serum (23 +/- 2 ng/ml) compared to HSAi heifers (109 +/- 11 ng/ml). IGF-I levels in follicular fluid (FFL) of medium and small follicles were also decreased by GRFi from 82 +/- 3 ng/ml in HSAi heifers to 48 +/- 6 ng/ml (P < 0.01). Levels of IGFBP-3 (determined by ligand blot analysis) in serum and FFL of small follicles were decreased by GRFi (P < 0.01). In contrast, IGFBP-2 serum levels were increased from 422 +/- 32 ng/ml in HSAi heifers to 657 +/- 6 ng/ml in GRFi heifers (P < 0.05). Likewise, IGFBP-2 levels in FFL from small and medium follicles were increased from 785 +/- 44 ng/ml to 926 +/- 44 ng/ml (P < 0.05). Ligand blot analysis indicated that IGFBP levels were lower in FFL from large vs. small follicles. The band intensities of IGFBP-4 and -5 were drastically reduced ( > 80%) while the decreases in IGFBP-2 and -3 were less marked ( < 50%). The decreased levels of IGFBP-5 in FFL from large follicles was not associated with an increase in proteolytic fragments detectable by immunoblot analysis. While mRNA transcripts for IGF-I, GH receptor, and IGFBP-2, -3, -4, and -5 were readily detectable in ovarian tissue, GRFi had no effect on ovarian levels of mRNA for each of these proteins. This suggests that the decrease in follicular development associated with GRFi may be related to changes in circulating IGF-I and/or IGFBPs.

Growth Hormone Receptor, Insulin-Like Growth Factor (IGF)-1, and IGF-Binding Protein-2 Expression in the Reproductive Tissues of Early Postpartum Dairy Cows

“…We did not see this relationship in follicle where there was no correlation between GHR and IGF-1. The absence of a significant correlation agrees with a study that showed little effect of GH on ovarian IGF-1 expression (Cohick et al, 1996).…”

Energy Balance and Ovarian Follicle Development Prior to the First Ovulation Postpartum in Dairy Cows Receiving Three Levels of Dietary Fat

“…The relative importance of peripheral or locally produced levels of IGF-I to follicular development remains unclear, but data from the present study suggest that low concentrations of circulating IGF-I are related to low steroidogenic output of dominant ovarian follicles early PP. Furthermore, the results of a recent study strongly suggest that changes in systemic levels of IGF-I and IGFBPs affect their concentrations in follicular fluid and follicular development in heifers [42].…”

Effects of Milk Replacer Composition on Selected Blood Metabolites and Hormones in Preweaned Holstein Heifers

“…The IGFBP detected in serum included bands at approximately 44, 40, 34, 31, 29, 28, and 24 kDa. Although specific antibodies and deglycosylation techniques were not used to identify individual IGFBP bands, it can be deduced from previous studies that the 44-and 40-kDa bands were 2 different glycosylated forms of IGFBP-3, the 34 kDa band was IGFBP-2, IGFBP-5 migrated as a doublet at approximately 29 to 31 kDa, and the 24-and 28-kDa bands were nonglycosylated and glycosylated forms of IGFBP-4, respectively (Cohick et al, 1996;Funston et al, 1996;Roberts et al, 1997). The 44-and 40-kDa bands of IGFBP-3 and the 31-to 29-kDa bands of IGFBP-5 were not always distinguishable as individual bands on ligand blots.…”