Divergent effects of prolactin upon steroidogenesis by porcine granulosa cells in vitro: Influence of cytodifferentiation

Ovine PRL (20-1, 000 ng/ml) produced dose-dependent inhibition of progesterone production by cultured granulosa cells isolated from small (1-2 mm) immature follicles of porcine ovaries. The inhibition was most prominent during the initial 4 days of culture, when PRL decreased progesterone accumulation by 30-67%. The suppressive effects of PRL persisted when steroid accumulation was corrected per cell [control (0.51 ± 0.03) vs. PRL-treated (0.24 ± 0.03 ng progesterone/10⁵ cells-48 h)]. In contradistinction, with granulosa cells isolated from large (>6 mm) mature follicles, PRL (4-1, 000 ng/ml) exerted an opposite effect upon steroidogenesis, stimulating progesterone secretion by 30-100%. Previous studies had suggested that the two granulosa cell populations employed differed with regard to PRL receptor content. To clarify the relationship of PRL binding to its divergent action, specific ovine [¹²⁵I]PRL ([¹²⁵I]oPRL) binding was compared serially in immature and mature granulosa cells before and during 6-8 days in culture. At the outset of culture, immature cells exhibited higher levels of specific [¹²⁵I]oPRL binding than mature granulosa cells (respectively, 9.4 ± 1.2 vs. 4.1 ± 1.1% [¹²⁵I]oPRL binding/mg DNA). However, binding in mature cells increased from 5.2 ± 1.3% on day 2 to 14.2 ± 1.8% on days 4 and 6 in culture, equaling or surpassing the binding maintained by immature cells. These studies suggest that the porcine ovary is a target organ for PRL action. The effects of PRL on porcine granulosa cells depend critically on the level of follicular maturation. Although PRL binding changes during the cytodifferentiation of these cells in vivo and in vitro, such changes cannot readily account for the divergent PRL effects observed.