Different Steroidogenic Response of Young and Aged Porcine Small and Large Luteal Cells to Prostaglandin F_2α, Oxytocin and Estradiol

Summary

The role of oxytocin (OXT) and prostaglandin F_2α (PGF_2α) in the process of luteal regulation, particularly their function in the early luteal phase is poorly understood. Therefore the effects of both compounds on in vitro steroid release of porcine luteal cells harvested from young/milddleaged (day 4-6, day 0 = 1st estrous day) or old (day 12-14) corpora lutea were tested. As corpora lutea (CL) contain at least two different steroidogenic cell populations, fractions of the so called small (SLC) and large (LLC) luteal cells were prepared and tested in separate experiments. In SLC as well as LLC from young CL OXT and PGF_2α inhibited progesterone (P) production but induced a strong increase of estradiol (E₂) release. In old SLC and LLC OXT and PGF_2α were still inhibitory to P release but OXT was ineffective and PGF_2α had a moderate stimulatory effect on luteal E₂ secretion. In SLC cultures from young but not from old CL E₂ exerted a powerful stimulatory effect on progesterone (P) secretion, i.e. E₂ has strong luteotrophic effects in the early luteal phase. Indeed, the pronounced inhibitory effect of OXT and PGF_2α on P release from SLC could be counteracted by the addition of exogenous E₂ to the culture media. Therefore, we suggest that in the early luteal phase OXT as well as PGF_2α have an indirect, E₂-mediated luteotrophic effect on P release which is stronger than the direct inhibitory action on P secretion. Indeed, under in vivo conditions we have demonstrated these stimulatory effects of OXT and PGF_2α on luteal P secretion in the early luteal phase. Although in old CL the direct inhibitory effect of OXT and PGF_2α on P release was less pronounced than in young CL the reduced luteotrophic power of E₂ may sum up to an inhibitory effect on luteal P release. In conclusion, our experiments support the concept of a dual action of OXT and/or PGF_2α in the regulation of the CL, with a luteotrophic effect in the early luteal phase and a luteolytic effect in the late luteal phase.

Key words

Luteal cells - PGF_2α-Oxytocin