Local and systemic control of myometrial contractile activity during labour in the sheep

Abstract

Summary. The relative contribution of systemic versus local (intrauterine) factors in the activation and stimulation of the sheep myometrium during labour was examined using an in-vivo myometrial explant preparation. Myometrial tissue alone (MYO) or with attached endometrium (ENDO/MYO) was removed from the pregnant uterine horn, sutured to a stainless-steel frame and placed into the omental fat. After 7\p=n-\10 days the explants developed a pattern of electromyographic activity qualitatively similar to that of the uterine myometrium. Induction of preterm labour by infusion of ACTH (66\m=.\6ng/min for 15 min every 2 h) to the fetus resulted in a reduction in plasma progesterone concentrations and increases in values of oestradiol-17\g=b\ and 13,14-dihydro 15-keto PGF-2\g=a\in maternal plasma. The onset of labour, which followed these endocrine changes, was characterized by an increase in EMG burst frequency and reduction in burst duration occurring simultaneously in both the uterine myometrium and in the explants. The response of the uterine and explant myometrium to oxytocin also exhibited a parallel significant increase over the 24-h period leading to delivery. No differences were apparent between the explants containing myometrial tissue alone or those comprising endometrial and myometrial tissue. There was no significant change in uterine or explant EMG activity, or oxytocin responsiveness, after saline administration to the fetus. The pattern of EMG activity changes during spontaneous labour were not distinguishable from those during ACTH-induced labour. As with oxytocin, the responsiveness of the explants to electrical stimulation increased significantly at labour compared to pre-labour. These data suggest that factors within the systemic circulation play a major role in both the onset of labour contractions and the increased response to electrical or hormonal (oxytocin) stimulation during parturition in sheep.

Cervical diameter in relation to uterine and cervical EMG activity in early postpartum dairy cows with retained placentas after PGF2alpha induced calving

“…This agrees with a study in parturient cows that showed a decline in the frequency of contractions from the uterine horn towards the uterine body [31]. Although the parturient uterus may be considered as a functional syncitium [35,36], not all contractions that originate at the tip of the uterine horn will reach the uterine body and the cervix. This results in lower contraction frequencies at more caudal sites of the genital tract.…”

Adaptive Responses of the Maternal Hypothalamic‐Pituitary‐Adrenal Axis during Pregnancy and Lactation

“…In several species (including rodents, sheep, pigs, and humans), maternal ACTH and ⁄ or cortisol levels are increased on the day of parturition (13,32,35,(57)(58)(59)(60)(61)(62). Despite this, the HPA axis is inhibited during the birth process in rodents (35,63).…”

The Initiation and Inhibition of Labor - Toward a Molecular Understanding

“…In sheep this begins 6-18 hours before delivery when the frequency and amplitude of contractures begins to increase, while the dura tion decreases. 68 With time single contractions with associated EMG bursts appear and increase in fre quency and amplitude as labor progresses reaching maximal levels of 20-30 per hour and 15-20 mm Hg amplitude prior to delivery. 62,68 Data obtained using the explant model suggest that in the sheep these events are at least in part due to endocrine stimula tion (rather than paracrine or autocrine events.…”