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Insulin-Like Growth Factor (IGF)-I, IGF-I Receptor, and IGF Binding Protein-3 Messenger Ribonucleic Acids and Protein in Corpora Lutea from ProstaglandinF₂-Treated Gilts¹

^a Department of Anatomy, Physiological Sciences and Radiology, College of Veterinary Medicine and ^b Department of Animal Science, North Carolina State University, Raleigh, North Carolina 27695

Insulin-like growth factor-I (IGF-I) is produced within the porcine corpus luteum (CL) and is thought to play an autocrine/paracrine role in CL development/function during the early luteal phase. This study examines the hypotheses that the luteolytic actions of prostaglandin F₂ (PGF₂) during the early luteal phase may involve either a decrease in IGF-I or IGF receptor (IGF-IR), or an increase in IGF binding protein (IGFBP)-3, expression, any of which could interfere with the luteotropic actions of IGF-I in this tissue. Cycling gilts were treated twice daily with PGF₂ (or saline) on Days 5–9 of the cycle to induce premature luteolysis. CL were collected on Days 6–9, and RNA, protein, or progesterone was extracted. By slot blot analysis, steady-state levels of IGF-I and IGFBP-3 mRNA were not different in PGF₂-treated vs. control animals; however, IGF-IR mRNA was increased in treated animals on Day 9. No changes in IGF-I content (ng/CL measured by RIA) were observed with respect to treatment. According to ligand blot analysis, the levels of IGFBP-3 increased on Day 6 and decreased on Days 8–9, while IGFBP-2 was higher on Days 6–7 and decreased on Day 9 in treated animals. IGF-IR levels, determined from Western blots, were higher on Day 7 (P < 0.05) and lower on Day 9 in PGF₂-treated animals vs. control animals (P < 0.05). In conclusion, PGF₂-induced premature luteolysis was associated with an increase in steady-state levels of IGF-IR mRNA, but it did not appear to be linked to changes in mRNA levels for IGF-I or IGFBP-3. However, since IGFBP-2 and -3 protein levels increased early in the treatment period (Days 6–7), it is possible that they may mediate the luteolytic actions of PGF₂ by sequestering IGF-I and preventing its interaction with the IGF-IR.

¹ This work was supported by funds from the North Carolina Pork Producer's Association and the State of North Carolina.

² Correspondence: John Gadsby, College of Veterinary Medicine, 4700 Hillsborough St., Raleigh, NC 27606. FAX: 919 515 4237; john_gadsby{at}ncsu.edu

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