The role of intra-luteal factors in the control of the porcine corpus luteum

In this paper we review three intra-luteal factors and their roles in the corpus luteum (CL).Insulin-like growth factor (IGF)-I, together with its receptor and IGF-binding proteins (IGFBPs),represent an important control system in the CL.IGF-I is a product of small luteal cells and has steroidogenic (i.e.luteotrophic) actions on large luteal cells via the type I receptor, while IGFBPs (e.g.BP-2 and 3; small cells) generally inhibit IGF-Is actions.IGF-I is particularly important in early CL development (up to day 7 of the oestrous cycle) in the pig.Tumour necrosis factor (TNF)-a is a product of luteal macrophages that infiltrate CLs in increasing numbers as the cycle progresses.TNF-a has been shown to play an important role in luteolysis, but we hypothesise that in the pig, this factor plays an additional role during the mid-luteal phase (days 7-13) in promoting the acquisition of luteal sensitivity to the luteolytic actions of prostaglandin (PG)F2a( = luteolytic sensitivity; LS).Endothelin (ET)-1 is a product of (luteal) endothelial cells, and along with its receptors (ETaand ETB)and endothelin-converting enzyme (ECE)-1, represent an intra-luteal system that also plays a role in luteolysis, in association with PGF2a.Since TNF-a induces endothelial cells to secrete ET-1, we hypothesise that ET-1 mediates the sensitising effects of TNF-a on the porcine CL during the mid-luteal phase (days 7-13).Finally, we hypothesise that TNF-a and/or ET-1 act to up-regulate luteal protein kinase C (e.g.isoforms gll and e) activity and thereby sensitises luteal cells to PGF2a.