Oestradiol-17  both inhibits and stimulates myometrial activity in ewes in vivo

Relaxin and progesterone are myometrial inhibitors in the ovariectomized non-pregnant mini-pig

“…However, it is clear that complete uterine quiescence, as reported for rats (Burns, 1972;Fuchs, 1974;Downing, Lye, Bradshaw & Porter, 1978), ewes (Lye et al, 1983) and sows (Taverne, 1982), was only observed in a minority of treatments in our animals. This may reflect the limited times that we were able to record from our animals, but there was no evidence in our gilts that phases of enhanced uterine activity, as reported for the ewe (Lye et al, 1983), followed periods of quiescence whether observed or missed. Plasma concentrations of oestradiol were not measured in this study but the dose used was comparable to that employed by Mercado-Simmen, Goodwin, Ueno, Yamamoto & Bryant-Greenwood (1982) which was adequate to maintain the population of relaxin receptors normally lost after ovari¬ ectomy.…”

“…It is possible that the presence of an intrauterine balloon may have induced a higher level of activity than would have occurred in the intact uterus, and this may have interfered with the inhibitory action of oestradiol. However, the technique employed was the same as that used for the ewe in which the inhibitory action is reproducible (Lye et al, 1983;Lye, Sprague & Challis, 1982). Moreover, an electromyographic study of the guinea-pig (Porter, Yoshinaga & Ford, 1974) suggested that the presence of a balloon produces minimal if any disturbance of myometrial activity.…”

“…With regard to the action of oestradiol the picture is no clearer. There are no definitive studies and although Taverne (1982) claimed that oestradiol induced phasic myometrial activity and quiescence in ovariectomized sows, similar to that subsequently demon¬ strated in ewes (Lye, Wathes & Porter, 1983), it is not clear that this is a reproducible phenomenon in the sow. Although in an isolated report McLennan (1983) noted that relaxin inhibits in vitro the spontaneous contractions of myometrial strips obtained from a pregnant pig, no information is available on the action of this hormone on uterine muscle in vivo in this species.…”

The Initiation and Inhibition of Labor - Toward a Molecular Understanding

“…Administraion of E 2 to pregnant ewes can lead to increased oxytocin re sponse 129 and premature parturition. 130 In non pregnant sheep, E 2 stimulates myometrial activity, 131 increases responsiveness to agonists, increases propa gation, 132 and reverses the inhibitory effects of pro gesterone on the myometrium 133 over a period of 12 to 24 hr. These effects may be due to a direct action of E 2 on the myometrium via increased expression of contraction-associated proteins or indirectly through increased prostaglandin production and oxytocin release.…”

The effects of combined intravenous infusions of oestradiol 17ß and progesterone on uterine activity of ovariectomized ewes

“…I t is generally assumed that these hormones have opposite effects on the myometrium, with progesterone inhibiting and oestrogens enhancing uterine activity (Ryan, 1969). The administration of exogenous progesterone t o ewes has a 'blocking action' on the myometrium (Naaktegeboren et al, 1973;Crocket & Shelton, 1973;Rousseau & Prud'homme, 1974;Lye & Porter, 1978), whilst exogenous oestradiol 17p has a biphasic effect initially inhibiting and then stimulating myometrial activity (Robalo Silva, 1982;Lye et al, 1983).…”