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

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.

Effects of fluoxetine on the reproductive axis of female goldfish (Carassius auratus)

“…After 12 h a significant increase in E 2 and a concurrent decrease in circulating T levels were observed, resulting in a significant fivefold increase in the circulating E 2 -to-T ratio in female goldfish injected with isotocin. This suggests that isotocin may act directly on the fish ovary to regulate estrogen production, as observed in porcine granulosa cells in vitro (44). While this is plausible, the hypothesis that isotocin acts on the goldfish ovary to regulate estrogen production requires further testing.…”

“…Isotocin could potentially act at several points of the reproductive axis. Oxytocin has been shown to stimulate GnRH-mediated LH release in rats (42) but also to increase ovarian E 2 production from porcine cells (44). The isotocin receptor is also expressed in both brain and ovaries in teleost fish (24), allowing for possible modulation of the reproductive axis at both levels.…”

The effect of oxytocin on progesterone secretion, phosphoinositide hydrolysis and intracellular mobilisation of Ca 2+ in porcine luteal cells

“…In the current study, OT stimulated P 4 secretion only during the mid-luteal phase of the porcine oestrous cycle, i.e. when the CL is highly responsive to endocrine regulators (Hunter, 1981;Pitzel et al, 1990;Yuan and Connor, 1993;Gregoraszczuk, 1997). This is in agreement with data presented by Pitzel et al (1990) who also examined the effect of OT on porcine luteal steroidogenesis.…”

“…when the CL is highly responsive to endocrine regulators (Hunter, 1981;Pitzel et al, 1990;Yuan and Connor, 1993;Gregoraszczuk, 1997). This is in agreement with data presented by Pitzel et al (1990) who also examined the effect of OT on porcine luteal steroidogenesis. In addition, similar stimulatory effects of OT on P 4 secretion were observed in bovine luteal cells (Sakumoto et al, 1996;Skarzynski and Okuda, 1999).…”

Effect of intra-ovarian infusion of oxytocin on plasma progesterone concentrations in pregnant ewes

“…These results are in agreement with our earlier study on oxytocin receptors in sheep (Sernia et ai, 1989). However, there are reports in pigs (Einspanier et al, 1986;Przala et ai, 1986a, b;Pitzel et al, 1988Pitzel et al, , 1990, cattle (Schams, 1987(Schams, , 1989 and women (Tan et al, 1982b;Bennegard et ai, 1987) which, while supporting an inhibitory role, shows oxytocin effects in the nonpregnant condition. These obser¬ vations need to be reconciled with the comparable studies by Richardson & Masson (1985), Duncan et ai (1961) and Gilbert et al (1989), which show no oxytocin effect on progesterone secretion.…”