Effect of oestradiol — 17β and oxytocin treatment on prostaglandin F alpha release in the anoestrous ewe

Summary. Seven bilaterally ovariectomized heifers were used in 4 experiments and received: (1) saline injections, as control; (2) one injection of oestradiol (3 mg; i.v.); (3) two i.v. injections of oxytocin (100 i.u.) 6 h apart; or (4) one oestradiol injection 30 min after the first oxytocin injection and a second oxytocin injection 6 h later. All experiments were performed without progesterone and then after 7, 14 and 21 days of progesterone treatment. Frequent blood samples were taken for 1 h before and 7 h after the first injection of oxytocin or oestradiol for the measurement of 13,14-dihydro-15-keto-PGF-2\g=a\ (PGFM) by radioimmunoassay. After 7, 14 and 21 days of progesterone priming, oestradiol caused a significant increase (P < 0\m=.\001) in plasma PGFM after 6 h but not before. After 7, 14 and 21 days of progesterone, there was a significant increase (P < 0\m=.\005) in PGFM after the first oxytocin injection and a similar increase following the second. The oxytocin-induced increase in PGFM after 14 and 21 days of progesterone was significantly higher (P < 0\m=.\001) 6h after oestradiol injection than before the oestradiol injection. There was no significant effect of oestradiol on the response to oxytocin in animals that received no progesterone or in those animals that received progesterone for only 7 days. These results show that, under the influence of progesterone, oestradiol enhances the oxytocin-induced release of PGF-2\g=a\, and suggest a possible synergistic action of these hormones for the induction of luteolysis in heifers.

Section: Resultsmentioning

confidence: 99%

Effects of progesterone and oestradiol-17 on oxytocininduced release of prostaglandin F-2 in heifers

Lafrance

Goff²

1988

“…In fact, our data suggest that oestradiol and progesterone may act together to downregulate the receptor, which may explain why low concen¬ trations of the receptor are found during most of the luteal phase of the oestrous cycle in sheep and cattle (Roberts et al, 1976;Sheldrick & Flint, 1985;Fuchs et ai, 1990). Oestradiol may be expected to increase receptor density because oestradiol increases oxytocin-induced secretion of PGF,U in anoestrous (Sharma & Fitzpatrick, 1974) and ovariectomized ewes (McCracken et ai, 1981). However, these effects of oestradiol may not be entirely mediated by changes in receptor density since Vallet et al, (1990) found that the administration of oestradiol changed the pattern of PGFM secretion associated with the administration of oxytocin without an increase in receptor density.…”

Section: Discussionmentioning

confidence: 99%

“…In ruminants endometrial concentrations of the oxytocin receptor are sensitive to the action of steroids, the density of receptors generally being lowest during the luteal phase of the cycle and highest at oestrus (Roberts et ai, 1976;Sheldrick & Flint, 1985;Fuchs et ai, 1990 (Sharma & Fitzpatrick, 1974;McCracken et ai, 1981) and that the administration of a luteolytic dose of oestradiol benzoate results in an increased density of the uterine oxytocin receptor (Hixon & Flint, 1987). The role of progesterone may be more complex.…”

Section: Introductionmentioning

confidence: 99%

“…The pulsatile pattern in the secretion of PGF2u, which begins on day 12 or 13 of the cycle (Thorburn et ai, 1973;Flint & Sheldrick, 1983;Hooper et ai, 1986), appears to be an important component of its luteolytic activity (Schramm et al, 1983). Because the secretion of PGF2a is an established response of the uterine endometrium to oxytocin (Sharma & Fitzpatrick, 1974;Mitchell et al, 1975;Roberts et al, 1976), interest has focused on the role of oxytocin and its uterine receptor in regulating the pulsatile secretion of PGF2(I (Flint et al, 1990).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Role of progesterone and oestradiol in the regulation of uterine oxytocin receptors in ewes

Zhang

Weston²,

Hixon³

1992

Summary. The interaction between oestrogen and progesterone in the regulation of the uterine oxytocin receptor in sheep was evaluated by measuring the binding of oxytocin to membrane preparations of caruncular and intercaruncular endometrium and myometrium. Ovariectomized ewes were assigned in groups of five to each cell of a 4 \m=x\2 factorial design. The four treatments were (a) vehicle (maize oil) for 12 days, (b) progesterone (10 mg day\m=-\1) for 9 days, (c) progesterone for 9 days followed by maize oil until day 12 and (d) progesterone for 12 days. The two oestradiol treatments consisted of the administration of implants in the presence or absence of oestradiol. The ewes were killed on day 10 (group b) or day 13 (groups a, c and d) for collection of uterine tissues. The response of the caruncular and intercaruncular endometrium to the treatments was similar. In the absence of oestradiol, treatment with progesterone continuously for either 9 or 12 days reduced the concentration of the oxytocin receptor in comparison with both the control and the progesterone withdrawal group (in which values were similar). The presence of oestradiol reduced the receptor concentrations in control and both 9-and 12-day continuous progesterone treatment groups, but enhanced the concentration in the progesterone withdrawal group. The myometrial oxytocin receptors responded in a similar way to those in the endometrium to progesterone treatment alone, but the addition of oestradiol produced no further effect. In conclusion, progesterone and oestradiol caused downregulation of the endometrial oxytocin receptor. On the other hand, progesterone withdrawal, similar to that which occurs during luteolysis, increased receptor density in the presence of oestradiol. Progesterone may influence the response of the myometrium to oxytocin by causing a reduction in receptor density.

“…One suggestion is that luteal secretion of oxytocin, which occurs in pulses synchronous with those of PGF-2a (Flint & Sheldrick, 1983), ensures that PGF-2a is secreted in an episodic manner; it has been proposed that oxytocinstimulated PGF-2a secretion by the uterus and PGF-2a-stimulated oxytocin secretion by the ovary form a positive-feedback loop which stimulates maximal PGF-2a secretion during periods of short duration (Flint & Sheldrick, 1983;Sheldrick & Flint, 1984). Evidence in favour of such a mechanism includes the rapid release of PGF-2a from the uterus after administration of oxytocin (Sharma & Fitzpatrick, 1974;Mitchell, Flint & Turnbull, 1975), the ability of PGF-2a and its analogues to stimulate secretion of oxytocin from the corpus luteum (Flint & Sheldrick, 1982), and the fact that almost all the oxytocin contained in the corpus luteum is released in response to a single dose of a PGF-2a analogue (Flint & Sheldrick, 1983). To account for the cessation of each episode of oxytocin and PGF-2a secretion, and the existence of a 6-12 h interval between pulses, it has been proposed that operation of such a positive-feedback loop may be terminated when oxytocin stored in the corpus luteum becomes depleted, and that it takes 6-12 h for the synthesis of sufficient oxytocin to support release of a further pulse (Flint & Sheldrick, 1983;Sheldrick & Flint, 1984 .…”

Section: Introductionmentioning

confidence: 99%

Transient uterine refractoriness after oxytocin administration in ewes

Sheldrick¹,

Flint²

1986