THE ANTAGONISTIC EFFECTS OF ESTROGEN AND PROGESTERONE ON THE STAIRCASE PHENOMENON IN UTERINE MUSCLE

Abstract

The occurrence of a staircase phenomenon in uterine muscle in vitro was demonstrated. Development and slope of the staircase are detd. by the ovarian hormones. The staircase is elicited by successive electrical stimulations after rest, or by varying the frequency of stimulation after a steady state of muscle response is established. After ovariectomy no staircase occurs. Uterine strips taken from rabbits in natural estrus or from castrates treated with estrogen and studied 24 or 96 hours after the last injn. exhibit a typical staircase phenomenon. Strips taken from rabbits in early pregnancy or from castrates treated with progesterone following a course of estrogen exhibit a negative staircase, i. e., increase in tension with decreasing frequency of electrical stimulation. The ovarian hormones were also found to alter the duration of a single contraction-relaxation cycle of a uterine strip in vitro Taking the castrate as a base line, estrogen increases whereas progesterone decreases the duration of a single contraction cycle.