Steroid secretion rates and plasma binding activity in androstenedione-immune ewes with an autotransplanted ovary

Abstract

Summary. Mature Merino ewes in which the left ovary and its vascular pedicle had been autotransplanted to the neck were divided into control (N = 5) and immunized groups (N = 6). The immunized ewes were treated (2 ml s.c.) with Fecundin\s=r\1 and 4 weeks before the start of blood sampling. Ovarian and jugular venous blood was collected every 10 min at two stages of the follicular phase (21\p=n-\27 h and 38\p=n-\42h after i.m. injection of 125 \g=m\g of a prostaglandin (PG) analogue) and during the mid-luteal phase (8 h at 15-min intervals). The ewes were monitored regularly for luteal function and preovulatory LH surges. Hormone concentrations and anti-androstenedione titres were assayed by RIA and ovarian secretion rates of oestradiol-17\g=b\,progesterone and androstenedione were determined.After the booster immunization, progesterone increased simultaneously with titre in immunized ewes, reaching 30 ng/ml at the time of PG injection when median titre was 1:10 000. All ewes responded to PG with LH surges 42\p=n-\72h later: 2 of the immunized ewes then had a second LH surge within 3\p=n-\4 days at a time when peripheral progesterone values were 2\p=n-\3ng/ml. The frequency of steroid and LH pulses was greater in immunized ewes (P < 0\m=.\05)during the luteal phase but not the follicular phase. The secretion rate of androstenedione was 6\p=n-\10 times greater (19\p=n-\37ng/min; P < 0\m=.\001) in immunized ewes at all sampling stages. Progesterone secretion rates were 3 times greater (16 \g=m\g/min; P < 0\m=.\001)during the luteal phase in immunized ewes. The amplitude of oestradiol pulses was significantly reduced in immunized ewes (4\m=.\8 vs 2\ m=. \ 1ng/min at +24 h and 6\m=.\5 vs 2\ m=. \ 8ng/min at +40 h in control and immunized ewes, respectively: P < 0\m=.\05)during the follicular phase. However, the mean secretion rate of oestradiol at each phase of the cycle was not significantly different between treatment groups. Analysis of bound and free steroid using polyethylene glycol showed that >98% of peripheral and ovarian venous androstenedione and 86% of peripheral progesterone was bound in immunized ewes but there was no appreciable binding (<0\m=.\1%) in control ewes. Similarly, 50% of ovarian venous oestradiol was bound in immunized ewes compared to 15% in control ewes.We conclude that immunization against androstenedione increases the secretion rate of androstenedione and progesterone but not of oestradiol. The enhanced plasma binding of progesterone and oestradiol in androstenedione-immunized ewes may interfere with the biological action of these steroids, thus providing an explanation for the elevated LH pulse frequently observed in immunized ewes.

Effects of active immunization of sheep against an amino terminal peptide of the inhibin alpha C subunit on intrafollicular levels of activin A, inhibin A and follistatin

“…These rises have been attributed to a compensatory increase in steroid production rate combined with a decrease in metabolic clearance rate (Nieschlag & Wickings 1978). Campbell et al (1990) reported similar findings of raised ovarian androstenedione secretion rates in ewes actively immunized against androstenedione. In the same manner, since inhibin A is a direct target of the inhibin antibodies generated, an increase in its total intrafollicular level is not surprising and could be attributed to (i) a compensatory increase in its production rate, or (ii) the formation of immune complexes which decrease its exit rate from the follicle and/or stabilize the dimer against proteolysis.…”

The effect of a direct arterial infusion of insulin and glucose on the ovarian secretion rates of androstenedione and oestradiol in ewes with an autotransplanted ovary

“…The antisera for androstenedione (2119), oestradiol-17 (727) and progesterone (7943) were each raised in sheep against androstenedione-7-human serum albumin (HSA), 17 -oestradiol-6-BSA and progesterone-11-BSA respectively. The cross-reactivities of each of the three steroid antisera have been reported (Campbell et al 1990). The sensitivities (minimum detectable mass) of the androstenedione, oestradiol-17 and progesterone radioimmunoassays were 4·0, 2·7 and 2·5 pg/tube respectively.…”

Active Immunization Against 5.ALPHA.-androstane-3.ALPHA.,17.BETA.-diol Increases Ovulation Rate in Merino Ewes.

“…However, it was assumed that the secretion of these f e e d b a c k h o r m o n e s w a s i n c r e a s e d b y immunization against 3α-diol as the number of ovulatory follicles was nearly doubled in these ewes. The immunization induced a low antibody titer against E2 that was comparable to that reported in A4 immunized ewes [23]. Although it was shown that very low levels of E2 antibody significantly reduced concentrations of free E 2 in A 4 -immunized ewes [23], it is doubtful if this was sufficient to suppress negative feedback allowing a rise in FSH.…”

“…The immunization induced a low antibody titer against E2 that was comparable to that reported in A4 immunized ewes [23]. Although it was shown that very low levels of E2 antibody significantly reduced concentrations of free E 2 in A 4 -immunized ewes [23], it is doubtful if this was sufficient to suppress negative feedback allowing a rise in FSH. In ewes passively immunized against E2, Mann et al [22] reported that an antibody titer higher than 1:400 was required to increase levels of FSH up to 140% of the pre-treated value, a level comparable to that observed in the present study.…”