The ovarian secretion of androstenedione and oestradiol during late pregnancy and the early postpartum period in sheep with an autotransplanted ovary.

During late pregnancy in the ewe, ovarian function is suppressed by placental steroids and following parturition ovarian function is restored. This experiment determined the ovarian secretion of oestradiol and androstenedione during late pregnancy and the early postpartum period in ewes. Six ewes with ovarian autotransplants were transplanted with three day 6 embryos and three gave birth on day 147. Ovarian and jugular blood sampled were collected on three different occasions. On each occasion a 4 h period of sampling was followed by a 6 or 8 h period during which the ewes were challenged with 150 ng of gonadotropin-releasing hormone (GnRH). Basal secretion of oestradiol and androstenedione was 0.3 +/- 0.1 and 10.5 +/- 3.0 ng min-1, respectively, on day 120 of pregnancy. Oestradiol secretion remained low on days 7 and 21 postpartum (0.4 +/- 0.3 and 0.3 +/- 0.1 ng min-1, respectively). Androstenedione secretion (ng min-1) on days 7 and 21 postpartum was 2.5 +/- 0.5 and 4.1 +/- 1.8, respectively. The injection of GnRH on day 121 of pregnancy produced luteinizing hormone (LH) release with a peak concentration of 0.6 +/- 0.1 ng mL-1, that did not stimulate steroid secretion. On day 8 postpartum GnRH injection induced LH release with a peak concentration of 3.9 +/- 1.1 ng mL-1 that stimulated secretion of oestradiol (0.2 +/- 0.1 to 2.1 +/- 0.9 ng min-1; P < 0.01) and androstenedione (2.3 +/- 0.6 to 17.1 +/- 6.9 ng min-1; P < 0.001). Similar effects were seen on day 22 postpartum; GnRH injection induced LH release with a peak concentration of 4.7 +/- 1.4 ng mL-1 that stimulated secretion of oestradiol (0.2 +/- 0.1 to 3.7 +/- 1.1 ng min-1; P < 0.001) and androstenedione (4.2 +/- 2.6 to 29.5 +/- 9.2 ng min-1; P < 0.01). These results suggest that the suppression of ovarian function during late pregnancy in the ewe is reversed by 7 days postpartum.