THE STAIRCASE PHENOMENON IN THE MYOMETRIUM OF THE RAT

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Abstract
Three types of staircase phenomena were observed in vitro in the pregnant and post partum rat myometrium by varying the frequency of electrical stimulation. A positive staircase (positive correlation between frequency of stimulation and amount of tension developed) was occasionally observed on day 1 of pregnancy. This response rapidly changed to a transient and then to a negative staircase within a few hours. The negative staircase was observed throughout the remainder of pregnancy and for the first few hours following parturition. At approximately 8–12 hours post partumi.e the predominant type of staircase was of the transient type which changed to the positive type at about 24 hours post partum. A few positive staircase responses were recorded on day 19 of pregnancy, i.e. as early as three days prior to parturition. Uteri of ovariectomized rats treated with estrogen, or estrogen and progesterone, showed only the negative type of staircase.