Evidence for an intrinsic control of myometrial contractile periodicity in sheep during pregnancy

Abstract

Reduction in concentration of prostaglandins in plasma by administration of sodium meclofenamate to pregnant sheep failed to alter the frequency or duration of electromyographic activity bursts or the response to oxytocin of myometrial tissue transplanted to the omentum. However, a significant (P < 0.05) delay (8.6 +/- 3.8 versus 1.3 +/- 0.3 min) in the myometrial response to oxytocin was observed when the hormone was administered 1 min after a spontaneous burst of electromyographic activity compared with 15 min after a burst, indicating a period of refractoriness. Similarly, the myometrial threshold for electrical stimulation was higher at 10-25% of the interval between contractions than close to the expected time of the next contraction. Stimulation of the myometrium at intervals of 30 s revealed a cycling of the electrical stimulation threshold: significantly higher voltages were required to elicit responses between spontaneous bursts of electromyographic activity (18.0 +/- 2.2 V) than during bursts (11.3 +/- 1.6 V). In contrast, there was no voltage differential in animals close to labour (< 24 h). These data provide no evidence to support a role for prostaglandins in the generation of contractions during pregnancy, but suggest that periodicity of contractions is associated with inherent changes in myometrial responsiveness to stimulation, which could occur as a result of a cycling of the resting membrane potential.

Uterine Responses to Exogenous Oxytocin Before and After Pre‐partum Luteolysis in the Cow

“…This is in agreement with previously published data in late pregnant sheep, in which contractures were induced by intravascular administration of 70 mU (Woudstra et al. 1991), or 50–200 mU (Lye 1992) of oxytocin. It supports the concept of a physiological role for the myometrial oxytocin receptors during late pregnancy.…”

Multiple mechanisms involved in oxytocin-induced modulation of myometrial contractility

“…Towards the end of pregnancy, there is a relatively gradual change from myometrial quiescence to a more contractile state. The transformation of the uterine muscle from quiescent to contractile was termed "activation" and is thought to involve increased synthesis of contraction-associated proteins [1,2] . Activation prepares the uterus for labor and alters it from a relatively insensitive organ to a sensitive, pro-contractile organ.…”

Ringwomb in ewes and does

“…Εκτός από τις παραπάνω αλλαγές του μυομητρίου, υ πάρχουν επίσης αλλαγές στην ηλεκτροφυσιολογία των λείων μυϊκών ινών του μυομητρίου, καθώς κατά την πριν τον τοκετό περίοδο, η μείωση της συγκέντρωσης προγε στερόνης και η αύξηση της συγκέντρωσης των οιστρογό νων οδηγούν σε διαφοροποίηση των ενεργών δυναμικών και στην έναρξη των συσπάσεων του μυομητρίου (Lye 1992).…”