The coordination of contractions of circular and longitudinal muscle layers of rat uterus in late pregnancy and during delivery.

Although it is well known that, in visceral smooth muscle, gap junctions between muscle fibers of the same muscle layer are responsible for synchronized strong contractions, much less attention has been paid to the interaction of muscle layers. An L-shaped preparation of the myometrium composed of circular and longitudinal muscle layers was adapted to study the relationship between the activities of these two muscle layers of pregnant rats. After careful separation, each of the two arms of the preparation would consist of only one muscle layer, while both layers remained connected at the angle of the L. Thus, contractions of both muscle layers could be monitored simultaneously and their relationship could be investigated. It was found that coordination of the contractions of longitudinal and circular muscles appeared in the morning of gestation day 21 and continued toward delivery. The coordination was expressed as a special sequence of the contractions of the two muscle layers which occurred repetitively for an hour or longer. In most of the cases, pacemaker was located in the longitudinal layer. Dissociation of one of the arms would make both arms contract independently. A few specimens from animals in their early third trimester also showed interaction of the muscle layers. Nevertheless, the correlation was not as strong as that in specimens prepared from animals close to term.