Insulin-like growth factor I receptor mRNA and protein expression in pig corpora lutea

Insulin-like growth factor I (IGF-I) is believed to play a luteotrophic rol e in the pig corpus luteum during the oestrous cycle. Since the actions of IGF-I in target tissues are mediated by the type I IGF receptor, the concen trations of IGF-I receptor mRNA and protein were examined in pig corpora lu tea at different stages of the oestrous cycle. Corpora lutea were collected from normally cyclic gilts on days 4, 7, 10, 13, 15 and 16 of the oestrous cycle (n = 4 animals per day). Corpora lutea on days 7, 10 and 13 were dis sociated with collagenase, and large and small luteal cell sub-populations were separated by elutriation. Northern and slot blots were used to examine mRNA, and western blots were used to measure the concentrations of IGF-I r eceptor protein in the pig corpus luteum. On northern blots, luteal IGF-I r eceptor mRNA was present as a single 11 kb transcript. The slot blots showe d that the steady state expression of IGF-I receptor mRNA increased signifi cantly (P < 0.05) from its lowest value on day 4, to reach a maximum on day s 13-16. IGF-I receptor mRNA was also expressed to a greater extent in larg e compared with small luteal cells (P < 0.05). On western blots, IGF-I rece ptor appeared as a 95 kDa protein band (P-subunit) and IGF-I receptor prote in concentrations were significantly higher (P < 0.05) on days 4-10 than on days 13-16. Finally, large luteal cells appeared to contain more IGF-I rec eptor protein than the small luteal cells. In conclusion, since IGF-I recep tor was detected in the pig corpus luteum, it is a likely target tissue for IGF-I, especially during the early luteal phase. Furthermore, IGF-I recept or was localized primarily on large luteal cells, thus it is hypothesized t hat IGF-I may play a paracrine role in the pig corpus luteum.