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 FACTOR

Active immunization against GRF at 6 months of age delays puberty in b eef heifers. The objectives of the present study were to determine whe ther active immunization against GRF at an earlier age would affect no rmal onset of puberty and follicular growth and to determine whether t hese changes were related to alterations in ovarian insulin-like growt h factor I (IGF-I) or IGF binding protein (IGFBP) 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 ag e. A third group of heifers was not immunized (CON; n = 16). Immunizat ion against GRF delayed puberty beyond 13 months of age in 75% of trea ted heifers. Unilateral ovariectomy at 191 days of age revealed that t he delay in puberty was associated with a reduction in the number of l arge (greater than or equal to 7 mm in diameter) follicles. Large foll icles were present in only 22% of GRFi heifers compared to 77% of HSAi heifers. The number of small (less than or equal to 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 dif ferent between HSAi (65%) and GRFi (62%) heifers. Unilateral ovariecto my 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 8 2 +/- 3 ng/ml in HSAi heifers to 48 +/- 6 ng/ml (P < 0.01). Levels of IGFBP-5 (determined by Ligand blot analysis) in serum and FFL of small follicles were decreased by GRFi (P < 0.01). In contrast, IGFBP-2 ser um 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 FF L 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 I GFBP levels were lower in FFL from large vs. small follicles. The band intensites of IGFBP-4 and -5 were drastically reduced (>80%) while th e decreases in IGFBP-2 and -3 were less marked (<50%). The decreased l evels of IGFBP-5 in FFL from large follicles was not associated with a n increase in proteolytic fragments detectable by immunoblot analysis. While mRNA transcripts for IGF-I, GH receptor, and IGFBP-2, -3, -4, a nd -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 re lated to changes in circulating IGF-I and/or IGFBPs.