Changes in progesterone secretion following treatment with transforming growth factor α (TGF-α) during the follicular phase of the sheep oestrous cycle

Abstract

The effects of transforming growth factor α (TGF-α) on ovarian steroid secretion were investigated. Three crossbred ewes synchronized for oestrus with ovarian autotransplants were infused with TGF-α (30 μg in 12 h) via the ovarian artery for 12 h before withdrawal of progestagen pessary. Three ewes were used as controls. Jugular and ovarian venous blood samples were taken at intervals of 10 min at two stages during the follicular phase (21-27 h and 38-42 h after pessary withdrawal) and every 2 h from 44 to 86 h. Plasma LH and FSH concentrations, and ovarian secretion rates of inhibin, androstenedione, oestradiol and progesterone were determined using radioimmunoassays. LH pulse amplitude increased in ewes treated with TGF-α in the early follicular phase (0.92 ± 0.25 μg l-1 in controls versus 3.10 ± 0.35 μg l-1 in TGF-α treated ewes; P < 0.05) and remained high in the late follicular phase. Plasma FSH concentrations were high during the follicular phase in ewes treated with TGF-α (P < 0.05). The infusion of TGF-α had no significant effect on the ovarian rate of secretion of androstenedione and, although the secretion rates of oestradiol and inhibin were consistently lower in TGF-α-infused ewes, the differences were not significant. The ratio of secretion of androstenedione to oestradiol was greater during the follicular phase in TGF-α-treated ewes (P < 0.05), suggesting that the efficiency of aromatization had been impaired. Progesterone secretion was enhanced by treatment with TGF-α; progesterone secretion was detectable 52 h and 24 h after pessary withdrawal and had reached a secretion rate of 16.4 ± 17.4 ng min-1 and 119.6 ± 21.9 ng min-1 by 86 h after pessary withdrawal in control and TGF-α-treated ewes, respectively. An increase in progesterone secretion would suggest that TGF-α had induced either premature luteinization or atresia in the large antral follicles.