Uterine Motility of the Estrogenized Rabbit

The myometrial response to rapid distension consists of a quick contraction, the early stretch response, followed by an increase in the rhythmic activity. The latency and amplitude of the early contraction has been studied during increasing degrees of distension and during repetitive distensions. The frequency of the rhythmic contractions increases with increasing distension; the amplitude and the external work performed by each contraction at first increases and later decreases. Data obtained from isometric and isotonic recordings indicate that the pacemaker activity is not directly related either to the intra‐uterine volume or to the intra‐uterine pressure but rather to the tension in the myometrial cells. The independency with which the two horns respond to distension, the lack of effect of local and spinal anesthesia and of autonomic blocking agents indicates that the active response to distension is of myogenic origin. The intra‐uterine pressure is not maintained during constant distensicn but decreases at first rapidly, later more slowly.