EFFECTS OF GROWTH-FACTORS AND HORMONES ON BASAL AND FSH-STIMULATED INHIBIN PRODUCTION BY PORCINE GRANULOSA-CELLS INVITRO

The effect of several growth factors, protein and steroid hormones on follicle stimulating hormone (FSH)-stimulated and basal inhibin secretion by mature porcine granulosa cells (g-cells) in culture was examined in order to elucidate the putative role of growth factors and hormones in the regulation of inhibin secretion by porcine g-cells in vitro. Cells were incubated with the respective hormones over a timespan of 0-144 h and immunoreactive inhibin was measured with a radioimmunoassay against porcine inhibin. Epidermal growth factor (EGF) and human transforming growth factor type beta (TGF-beta) decreased basal and gonadotrophin-stimulated inhibin and progesterone in a dose-dependent manner. In the absence of insulin, insulin-like growth factor type I (IGF-1) caused a 4-fold enhancement of basal inhibin secretion, but inhibin secretion was elevated only to 20% above control in the presence of 500 nM insulin. Porcine platelet-derived growth factor (PDGF) had no significant effect on basal or FSH-induced inhibin secretion by g-cells. In addition, neither gonadotrophin-releasing hormone (GnRH) nor prolactin (PRL), arginine vasopressin (AVP) and oxytocin affected basal or FSH-stimulated inhibin release by porcine g-cells. Oestradiol caused a slight but significant (P < 0.01) rise of basal inhibin production (158% of control) in the last 2 days of culture (96-144 h) and the effect of androstenedione on basal (158% of control) and FSH-stimulated (140% of control) inhibin release (P < 0.01) was also only visible on Days 4-6 of culture. In contrast to androstenedione and oestradiol, progesterone did not show any effect during 6 days of culture in a dose range of 10(-5) to 10(-9) M. Like steroids, prostaglandin E2 (PGE2) had a stimulatory effect on basal inhibin production (250% of control) by porcine g-cells, visible on Days 3-6 of culture, but an inhibitory effect on FSH-stimulated release (less than 40% of control). Over all the experiments with different hormones and growth factors, tested in varying doses and over a time span of 0-144 h, there was a strong correlation between progesterone and inhibin secretion by g-cells (0-48 h = 0.78; 48-96 h = 0.92; 96-144 h = 0.92). These results suggest that EGF, TGF-beta, IGF-I, oestradiol and androstendione as well as PGE2 have para- and/or autocrine modulatory effects on basal and FSH-stimulated inhibin secretion by mature porcine g-cells in vitro and further demonstrate that the secretion of the proteohormone inhibin and the steroid progesterone are closely related.