Regulation of bovine intra-luteal function by peptide hormones

There is increasing evidence for the existence of substances in ovarian tissues and fluids which are able to act locally, either alone or by modulating the actions of the gonadotropins, thus modifying the functions of ovarian cells. There are now clear data for oxytocin (OT), insulin-like growth factor-1 (IGF-1) and basic fibroblast growth factor (bFGF) in luteal tissue, with regard to specific expression of mRNA, secretion of peptid and receptors. Biological effects of these growth factors and others on luteal tissue were determined during the estrous cycle using two different in vitro systems; a novel microdialysis system (MDS) of intact luteal tissue and culture of luteal cells. The concentrations of secreted progesterone and OT were used as parameters. MDS; OT was most stimulative during the early luteal phase (days 5-7) and decreased continuously from days 8-12 to days 15-18. During early and mid stages bFGF was the most potent stimulator and at the late stage IGF-1 and IGF-2 were most stimulatory. Transforming growth factor beta (TGF-beta) stimulated the release of OT most effective at the early luteal stage. Results from the cell culture system (where no cell to cell contact exists) showed a different pattern. IGF-1 and IGF-2 had a stimulatory effect during long and short term stimulation, bFGF only during short term and OT showed no effect. Receptors were found for all peptides examined of luteal cells. A model of an intraovarian cascade-like system for the amplification of luteal function is presented.