An in-vivo model to examine the electromyographic activity of isolated myometrial tissue from pregnant sheep

Abstract

Summary. Strips (2\m=.\5\ m=x\3\ m=. \ 5cm) of myometrium alone (MYO) or endometrium/ myometrium (ENDO/MYO) were removed from the pregnant horn of sheep (Day 110 of gestation) and transplanted to sites within the omental fat. These explants developed regular bursts of electromyographic (EMG) activity over a period of 7\p=n-\10 days, as well as a dose-dependent stimulatory response to oxytocin (50\p=n-\200 mU i.v.). The frequency (per 2 h) of EMG bursts in the MYO (5\m=.\3\ m=+-\ 0\m=.\2) and ENDO/MYO (5\m=.\2\ m=+-\ 0\ m=. \ 3) explants was significantly greater (P < 0\m=.\05) than that of the uterine myometrium (3\m=.\0 \ m=+-\ 0\m=.\1), while burst duration (min) in MYO (4\m=.\1\m=+-\ 0\ m=. \ 2) and ENDO/MYO (4\ m=. \ 1 \ m=+-\ 0\m=.\2)explants was significantly (P < 0\m=.\05) less than in the uterine myometrium (7\m=.\3\ m=+-\ 0\m=.\1). The EMG bursts were asynchronous between the explants and uterus, although systemic administration of oxytocin produced a synchronous burst of EMG activity in all three tissues. No differences in EMG activity or responsiveness were apparent between MYO and ENDO/MYO explants. Histological examination of the explant tissue revealed the presence of smooth muscle fibres regularly orientated into two layers; some loss of endometrial tissue was apparent in ENDO/MYO explants. To validate the mechanical integrity of this model we examined the in-vitro contractile activity of myometrial strips prepared from the explants. The strips developed regular spontaneous contractions and demonstrated a dose-dependent stimulation in response to the addition of oxytocin (10\m=-\10 to 10\m=-\4m) to the bath fluid. These results suggest that spontaneous contractures during pregnancy are probably not due to pulsatile release of stimulants into the systemic circulation, or the direct diffusion of stimulants from intrauterine tissues to the myometrium but are probably caused by factors within the myometrium itself.

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.…”

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

“…Under these conditions the expiant develops regular bursts of spontaneous activity within 7-14 days which are very similar in appearance to the uterine contractures. Data obtained using this preparation led us to suggest that this underlying pattern of activity is not due to neural or direct systemic endo¬ crine stimulation but to an intrinsic activity ofthe myometrium itself (Lye & Freitag, 1988), an observation consistent with the report of Sigger et ai (1984).…”

“…Surgery was performed at Days 110-113 of gestation under general anaesthesia (N2/02/halothane). In 8 ofthe animals strips (2-5 3-5 cm) of myometrium alone (MYO explant) and endometrium with attached myometrium (ENDO/MYO expiant) were removed from the pregnant uterine horn attached to stainless-steel frames and transplanted to sites within the omental vasculature as previously described (Lye & Freitag, 1988). In the other 4 sheep only myometrial expiants were prepared.…”

“…In the other 4 sheep only myometrial expiants were prepared. We have previously demonstrated (Lye & Freitag, 1988) The fetal head and neck were then delivered through the uterine incision and polyvinyl catheters (V4 Bolab, Lake Havasu City, AZ, USA) inserted into the jugular vein and carotid artery to allow for infusion and blood sampling. A catheter was also inserted into the amniotic cavity to enaole intrauterine pressure (IUP) to be monitored.…”

“…From around Day 28-60 of gestation until the onset of labour, the uterus is virtually quiescent except for the presence of regular low-amplitude (~2 -5 mmHg), low-frequency (0-5-3/h) contractions which have a duration of 5-8 min (Nathanielsz et ai, 1976; Van der Weyden et ai, 1981;Harding et ai, 1982;Sigger et ai, 1984) and which have been termed contractures by Nathanielsz et ai (1976). We have investigated this pattern of contractile activity in vivo during pregnancy using our expiant model (Lye & Freitag, 1988), in which myometrial tissue is removed from the uterus, attached to a stainless-steel frame and sewn into the omental fat. Under these conditions the expiant develops regular bursts of spontaneous activity within 7-14 days which are very similar in appearance to the uterine contractures.…”