Components of the porcine anterior pituitary insulin-like growth factor system throughout the estrous cycle

Components of the circulating and anterior pituitary insulin-like growth factor (IGF) system vary in response to steroids in pigs. However, whether serum and anterior pituitary concentrations of the IGF system vary throughout the estrous cycle has not been determined. To further examine this relationship, estrus was synchronized in 40 gilts of similar age and weight (180 d; 120 kg) by feeding 15 mg altrenogest for 15 d to synchronize estrus. Gilts were checked twice daily for expression of estrus beginning 3 d after the end of altrenogest treatment and continuing for 7 d. The first day each gilt exhibited estrus was designated as day 1 of the estrous cycle. Blood samples were obtained by jugular venipuncture on days 1, 4, 7, 10, 13, 16, 19, and 22 of the estrous cycle. On days 7, 13, 19, and 22 of the estrous cycle 10 pigs were killed and anterior pituitary glands (AP) were collected. Serum concentrations of IGF-I and AP concentrations of IGF-I were determined by radioimmunoassay. Relative amounts of AP IGF binding protein (IGFBP) were determined by western ligand blot analysis. Relative expression of AP IGF-I, IGF-I receptor (IGF-I-R), gonadotropin-releasing hormone receptor (GnRHR), and luteinizing hormone (LH)-β subunit were determined by real-time reverse transcription polymerase chain reaction. Serum concentrations of IGF-I fluctuated throughout the estrous cycle. Mean serum concentrations of IGF-I decreased (P < 0.02) from day 1 through day 10, increased (P < 0.02) on days 13 through 16, and then decreased (P < 0.02) from days 19 through 22. Mean AP concentrations of IGF-I were greater (P < 0.03) on day 19 than on all other days, whereas no difference was detected (P > 0.05) in mean AP concentrations of IGF-I on days 7, 13, and 22. Mean relative amounts of AP IGFBP-2 and -5 were each greater (P < 0.02) in gilts on day 19 than on all other days, whereas no difference was detected (P > 0.05) in mean relative amounts of AP IGFBP-2 and -5 among pigs on days 7, 13, and 22 of the estrous cycle. Relative expression AP IGF-I was greater (P < 0.05) on days 13, 19, and 22 than on day 7 of the estrous cycle. Similarly, the relative expression of AP IGF-IR was increased (P < 0.05) in gilts on days 13, 19, and 22 compared with day 7. The relative expression of GnRHR was greater (P < 0.05) on days 13 and 22 of the estrous cycle than on day 7. The relative expression of LHβ subunit was greater (P < 0.05) on day 19 of the estrous cycle than on days 7, 13, and 22. Anterior pituitary release of LH throughout the porcine estrous cycle may be modulated by changes in the intrapituitary IGF system.