One of the postulated main luteolytic actions of prostaglandin (PG) F(2 alpha) is to decrease ovarian blood flow. However, before Day 5 of the normal cycle, the corpus luteum (CL) is refractory to the luteolytic action of PGF(2 alpha). Therefore, we aimed to determine in detail the real-time changes in intraluteal blood flow after PGF(2 alpha) injection at the early and middle stages of the estrous cycle in the cow. Normally cycling cows at Day 4 (early CL, n = 5) or Days 10--12 (mid CL, n = 5) of the estrous cycle (estrus = Day 0) were examined by transrectal color and pulsed Doppler ultrasonography to determine the blood flow area, the time-averaged maximum velocity (TAMXV), and the volume of the CL after an i.m. injection of a PGF(2 alpha) analogue. Ultrasonographic examinations were carried out just before PG injection (0 h) and then at 0.5, 1, 2, 4, 8, 12, 24, and 48 h after the injection. Blood samples were collected at each of these times for progesterone (P) determination. The ratio of the colored area to a sectional plane at the maximum diameter of the CL was used as a quantitative index of the changes in blood flow within the luteal tissue. Blood flow within the midcycle CL initially increased (P < 0.05) at 0.5-2 h, decreased at 4 h to the same levels observed at 0 h, and then further decreased to a lower level from 8 h (P < 0.05) to 48 h (P < 0.001). Plasma P concentrations decreased (P < 0.05) from 4.7 +/- 0.5 ng/ml (0 h) to 0.6 +/- 0.2 ng/ml (24 h). The TAMXV and CL volume decreased at 8 h (P < 0.05) and further decreased (P < 0.001) from 12 to 24 h after PG injection, indicating structural luteolysis. These changes were not detected in the early CL, in which luteolysis did not occur. In the early CL, the blood flow gradually increased in parallel with the CL volume, plasma P concentration, and TAMXV from Day 4 to Day 6. The present results indicate that PGF(2 alpha) induces an acute blood flow increase followed by a decrease in the midcycle CL but not in the early CL. This transitory increase may trigger the luteolytic cascade. The lack of intraluteal vascular response to PG injection in the early CL appears to be directly correlated with the ability to be resistant to PG.

The effects were investigated of basic fibroblast growth factor (bFGF), transforming growth factor-beta (TGF-beta) and nerve growth factor (NGF) on the release of progesterone and oxytocin from the bovine corpus luteum (CL) at different stages of the oestrous cycle. A microdialysis system (MDS) of CL and a cell culture system with a reduced number of endothelial cells were used. In the MDS of CL from the mid-luteal stage (days 8-12 of the oestrous cycle), infusion with bFGF (0.1, 1, 10 and 100 ng/ml), TGF-beta (0.1, 1 and 10 ng/ml) and NGF (0.1, 1, 10 and 100 ng/ml) for 30 min induced significant acute effects on the release of progesterone. Both bFGF and NGF stimulated the release of progesterone during peptide infusion, TGF-beta and also bFGF in the period thereafter. This stimulation was dose-dependent during and after the infusion only for bFGF. This response pattern was observed at all luteal stages for the three growth factors, but bFGF was more stimulatory at the early (days 5-7) and mid-luteal stages during and after peptide infusion. The release of oxytocin was stimulated by bFGF in a dose-dependent manner. At the highest dose, bFGF, TGF-beta and NGF stimulated the release of oxytocin throughout all three luteal stages. When luteal cells were cultured with growth factors, only TGF-beta showed a dose-dependent inhibition of both basal and LH-stimulated progesterone as well as oxytocin release (measured between 48 and 52 h of culture). NGF had an inhibitory effect only on the basal release of oxytocin. bFGF had no effect on the release of either hormone under continuous stimulation in cell culture. The results indicate that bFGF, TGF-beta and NGF act directly and acutely on the secretory function of bovine CL in the MDS but also have long-term effects as shown in cell culture. bFGF appears to be an important autocrine/paracrine regulator of CL function, since local expression of its mRNA, peptide synthesis and its mitogenic and non-mitogenic actions have now been confirmed. Endothelial cells from the CL have been identified as target cells for bFGF. Differences observed between the two systems might thus be attributed to the presence or absence of cell-to-cell contact and a reduced number of endothelial cells, as well as to the duration of peptide stimulation and medium changes every 24 h compared with the flow-through conditions in the MDS.

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Prostaglandin F2 alpha (PGF2 alpha) is a primary luteolysin in the cow. Although the mechanisms involved in luteolysis are thought to be a complex of its direct action on luteal cells and indirect effect on luteal blood flow, the detailed mechanisms remain to be elucidated. This study focuses on the possible interaction of endothelial cells-derived endothelin-1 (ET-1) with PGF2 alpha in the rapid suppression of progesterone release from the bovine corpus luteum (CL). In in vitro microdialysis system (MDS) of CL, PGF2 alpha acutely stimulated the release of progesterone and oxytocin during infusion and ET-1 release after infusion. Moreover, PGF2 alpha induced slight decrease of progesterone release during the last period of the experiment (8-11 h after PGF2 alpha exposure). Two 1 h-perfusions of ET-1 at 3 h intervals induced only a slight decrease of progesterone release after the second perfusion. This treatment also affected the oxytocin release; the first ET-1 perfusion produced an acute stimulation, whereas the second ET-1 perfusion inhibited the release to below 50%. When the CL pieces were pre-perfused with PGF2 alpha for 2 h, the two consecutive perfusion of ET-1 at 3 h intervals induced drastic decrease in progesterone and oxytocin release only after the second ET-1 perfusion. Thus, a pre-exposure with PGF2 alpha clearly potentiated the inhibiting activity of ET-1 in the progesterone release. These results suggest a physiological impact of PGF2 alpha and ET-1 in the rapid cascade of functional luteolysis in vivo, and a possible interaction between endothelial cells and luteal cells.