Effects of Oxytocin on in Vitro Steroid Release of Midstage Small and Large Porcine Luteal Cells*

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Abstract
Previously, we have demonstrated an inhibitory effect of oxytocin (OXT) on progesterone (P) and androstenedione (A) release of porcine luteal cell cultures. The present study examines whether OXT modulates P, A, or estradiol (E2) release of so-called small luteal cells (SLC) or of granulosa-derived large luteal cells (LLC). To ensure clean Percoll-gradient separation of the 2 cell types, corpora lutea not older than 6 days were used. SLC, but not LLC, responded to human (h)CG (6 ng/ml) with increased P and A, but not E2, release. When OXT was added to the culture system, both basal as well as hCG-stimulated P release of SLC, but not of LLC, were dose dependently reduced. In contrast, E2 production of SLC and LLC was significantly stimulated by OXT whereas A release of SLC cultures, but not of LLC, was inhibited in response to OXT. In the presence of a specific OXT-antagonist, this inhibitory effect of OXT on P release was abolished, indicating a specific receptor-mediated effect of OXT on porcine luteal cells. When E2 was added to the culture medium, a dose-dependent stimulatory effect on P release of SLC was demonstrated. The presence of the E2 receptor antagonist monohydroxy-tamoxifen in the culture system prevented the E2-induced increase of P release of SLC. E2 was able to counteract dose dependently the OXT-induced inhibition of P release in SLC cultures. These results suggest that OXT may have a dual function in young corpora lutea. The reduction of P and A production can be interpreted as a luteolytic effect of OXT. The simultaneous increase of E2 production, however, may also point to an indirect luteotropic effect since E2 was shown to stimulate luteal P release and to counteract OXT-induced inhibition of P release excessively.