EXPRESSION OF THE MESSENGER RIBONUCLEIC-ACIDS FOR INSULIN-LIKE GROWTH-FACTOR-I AND INSULIN-LIKE GROWTH-FACTOR BINDING-PROTEINS IN PORCINE CORPORA LUTEAL

In the pig, the corpus luteum (CL) can develop and function autonomous of pituitary gonadotropins for similar to 12 days. We hypothesized th at the insulin-like growth factor (IGF) system may play an autocrine/p aracrine luteotrophic role(s) during this period. In this study, we mo nitored the expression (i.e., steady-state levels of mRNAs) of IGF-I a nd IGF binding proteins (IGFBP)-2, -3,-4,-5, and -6 mRNAs in whole CL and in small and large luteal cells on Days 4-16 of the estrous cycle. CL were dissociated with collagenase, and large and small luteal cell s were isolated by centrifugal elutriation. Whole CL and luteal cells were extracted to isolate total or poly(A)(+) RNA, which was subjected to Northern and/or dot-blot analyses using [P-32]-labeled cDNA probes for IGF-I and IGFBP-2,-3, -4, -5, and -6. Northern blots showed readi ly detectable transcripts for IGF-I (6.7 and 0.9 kb), IGFBP-2 (1.8 kb) , IGFBP-3 (2.8 kb), IGFBP-4 (2.6 kb), and IGFBP-5 (6.0 kb), but not fo r IGFBP-6. IGFBP-3 and -5 transcripts were observed mainly in small lu teal cells, while IGFBP-2 and -4 were seen in both cell types. Dot-blo t analyses for IGF-I and IGFBP-3 mRNAs were performed on total RNA fro m small and large luteal cells; blots were counter-probed with 3-phosp hoglyceraldehyde dehydrogenase (p-GAD) cDNA to assess RNA quantity and quality. IGF-I mRNA (ratio IGF-I:p-GAD mRNA) expression was similar t o 2-fold greater in small than in large luteal cells on Days 4-10. How ever, steady-state levels of IGF-I mRNA in small, but not large, lutea l cells decreased significantly on Days 12-16 (vs. Days 4-10). IGFBP-3 mRNA expression was significantly greater (similar to 3-fold) in smal l than in large luteal cells but did not vary significantly between Da ys 4-10 and 12-16 for either cell type. We conclude that porcine CL ex press mRNAs for IGF-I and IGFBP-2, -3, -4, and -5, and that while smal l luteal cells are the major sources of IGF-I and IGFBP-3 and -5, IGFB P-2 and -4 appear to be expressed to approximately the same extent: in small and large luteal cells. These results further suggest that the IGF-I/IGF system may have autocrine/paracrine regulatory actions in CL development/function in the pig.