A STUDY OF THE PARTURIENT UTERUS WITH THE MICROELECTRODE TECHNIQUE1

Abstract

The membrane potential of the post-partum uterus (estrogen-dominated) measured in single cells which microelectrodes during prolonged recording of membrane activity is about 45-50 m V, whereas that of late pregnancy is about 55-60 mV. The post-partum uterus periodically discharges trains of action potentials of considerable regularity in contrast to the irregular train discharges of the late pregnant uterus. During and after parturition the membrane activity of the impaled cell is in synchrony with the mechanical response, recorded from the uterine strip as a whole, because activity readily spreads in all directions. In the late pregnant uterus propagation is suppressed and membrane and myoplasmic activity are asynchronous. When labor is delayed by progesterone treatment the electrical and mechanical properties of the uterus are that of late pregnancy. Electrical activity is often preceded by prepotentials or slow depolarization waves which lower the threshold of excitability. These non-propagating cyclic depolarizations are not generated at specific anatomical sites. Oxytocin lowers the membrane potential of the parturient and post-partum uterus and potentiates the train discharge. Ca-deficiency depolarizes the membrane and suspends membrane activity. Oxytocin becomes ineffective. A trace of Ca (0.12-0.25 mM) slightly increases the membrane potential and restores the oxytocin response. However, under these conditions oxytocin hyperpolarizes the membrane. In vitro progesterone treatment of the parturient or post-partum uterus suspends membrane and myoplasmic activity. The membrane is slightly hyperpolarized and the action potentials gradually diminish.