OXYTOCIN STIMULATES PROGESTERONE RELEASE FROM MICRODIALYZED BOVINE CORPUS-LUTEUM INVITRO

A microdialysis system (MDS) was implanted in corpora lutea (CL) from cows (Days 5-7, 8-12, and 15-18 of the estrous cycle); the CL were maintained in organ culture chambers. With this system, active substances can be applied, and a collection of steroids released from luteal cells surrounding the microcapillary (cut-off point = 100 kDa) is possible, while luteal cells maintain cell-to-cell contact. Spontaneous pulses of progesterone release were observed in 90% of control (perfused with Ringer's solution only) at 60-80-min intervals. The infusion of bovine LH (bLH) for 20 min (0.1-10-mu-g/ml) stimulated dose-dependent release of progesterone. Both results indicate that the CL maintains the activity of progesterone release and the ability to respond to LH stimulation in this system. Oxytocin (1-100-mu-M) also stimulated progesterone release in a dose-dependent manner. Preexposure with oxytocin antagonist blocked the stimulatory effect of oxytocin (p < 0.01) but not of LH (p < 0.05), confirming the specificity of the effect. When CL were prestimulated with a low dose of oxytocin (1-mu-M, 20 min) twice before bLH application, the release of progesterone by bLH (1-mu-g/ml, 20 min) was more pronounced (p < 0.05). A long-term infusion (3 h) with oxytocin and/or bLH stimulated the release of progesterone for the whole period of time. Oxytocin was most stimulative during the early luteal phase (Days 5-7) and decreased continuously from Days 8-12 to Days 15-18. The acute stimulatory effect on progesterone release by oxytocin described here has not been reported in studies using dispersed luteal cell incubations. Therefore, the overall results of this study suggest that cell-to-cell contact is important for the luteal response to oxytocin and support the hypothesis that ovarian oxytocin is involved in mechanisms of progesterone secretion as an autocrine/paracrine factor, especially during the early luteal phase.