Myometrial activity during natural and dexamethasone-induced parturition in the cow.

Myometrial activity was monitored during natural and dexamethasone-induced parturition in 8 Holstein dairy cattle, using strain gauge transducers. Four gauges were attached to the serosal surface of the gravid uterine horn, dividing it into thirds. Parturition was induced in 2 of 4 heifers and 2 of 4 cows (group 1); the remaining animals were allowed to calve spontaneously (group 2). Chains of low-amplitude contractions (repeated small deviations from base line) were detected before parturition was induced, and these were more common at distended parts of the uterus. Uncharacteristically sharp peaks followed by small rhythmic contractions, during preinduction recording, indicated that the myometrium was responsive to fetal movement even several days before parturition. By 18 hours before parturition, discrete single contractions appeared independently of contraction chains, and the first tubocervical peristaltic contraction waves were detected. The mean area under recorded contraction curves (uterine work) increased quadratically and the frequency of contractions decreased linearly from 12 hours before parturition to 2 hours after parturition. There was also an increase in the proportion of tubocervical waves over this period, and contraction chains were no longer present. During the second stage of labor, distended and undistended parts of the uterus were equally active, and forceful maternal straining was associated with larger sustained contractions. Fetal membrane rupture was accompanied by a doubling in the rate of passage of contraction waves along the length of the uterus. After the calf was expelled, contractions became extremely regular, and the majority progressed in a tubocervical direction.(ABSTRACT TRUNCATED AT 250 WORDS)