Effects of local growth factors on the secretory function of bovine corpus luteum during the oestrous cycle and pregnancy in vitro

The impact of insulin-like growth factor-I (IGF-I), basic fibroblast growth factor (bFGF), endothelin-1 (ET-1), tumour necrosis factor-α (TNF-α), transforming growth factor-α (TGF-α) and platelet-derived growth factor (PDGF) on the release of progesterone (P₄) and oxytocin (OT) from individual bovine corpora lutea at different stages of the oestrous cycle and pregnancy was evaluated with a microdialysis system (MDS) in vitro. IGF-I (1 μg mL^-1) induced significantly the acute effects on P₄ release at the late luteal stage (Days 15-18) and early pregnancy (Days 60-120), whereas bFGF (100 ng mL^-1) was extremely effective in stimulating P₄ release particularly during the mid-luteal stage (Days 8-12). Both peptides stimulated (P < 0.05) the release of OT throughout the three luteal stages and during early and late pregnancy (Days 30-60 and Days 150-210). ET-1 (100 ng mL^-1) clearly inhibited P₄ release during the early (Days 5-7) and mid-luteal phase and stimulated OT release only during the mid-luteal stage (P < 0.001). TNF-α (100 ng mL^-1) stimulated the release of P₄ exclusively at the early luteal phase (P < 0.05), whereas OT secretion was increased by TNF-α during all stages of the oestrous cycle (P < 0.001). TGF-α and PDGF (100 ng mL^-1) were effective in stimulating P₄ release particularly during late pregnancy (P < 0.05). In contrast, stimulation of OT secretion by TGF-α was maximal during the late-luteal stage (P < 0.001), whereas PDGF significantly increased OT secretion during the oestrous cycle (except the early luteal stage) and pregnancy (P < 0.001). The data demonstrate distinct and stage-specific effects of growth factors on P₄ and OT secretion in vitro. IGF-I, bFGF and TGF-α may play an important role in corpus luteum (CL) function during the oestrous cycle and pregnancy since they are locally expressed and synthesized, there are receptors for these growth factors, and they have been demonstrated to exert biological effects on the CL.