Changes in electrical properties of rat myometrium during gestation and following hormonal treatments.

Abstract

The membrane properties of the rat myometrium, during gestation and following ovarian hormone treatment, were investigated with the microelectrode technique. Spontaneously generated bursts of electrical activity alternating with silent periods were recorded from non-pregnant, pregnant and postpartum myometria. The membrane potential was highest during the middle stage of gestation, but the spike amplitude within a burst was not uniform. In the final stage of gestation and during parturition, the membrane potential was low and the spikes within a burst were of low frequency and uniform amplitude. During parturition and postpartum, a gradual depolarization of the membrane, accompanied by an increase in membrane resistance, occurred before the generation of a burst. Excitability of the membrane fluctuated from a peak just before the generation of a burst to a low after the cessation of a burst. Displacement of the membrane potential by electrical current or by lowering the temperature modified the slope of the slow spontaneous depolarization, but the fluctuations of excitability persisted. The Q10 value for the frequency of spontaneous bursts, measured between 36-30.degree. C, was 3.8. Hyperpolarization of the membrane increased the maximum rate of rise of the spike, but beyond -70 mV, the rate of rise was reduced. Half-inactivation of spike generation occurred at a membrane potential less negative than the interburst potential, indicating that the current carrying system was not fully activated during parturition. In both normal and spayed rats, estradiol hyperpolarized the membrane and the burst of spikes was generated on a sustained depolarization. Progesterone slightly hyperpolarized the membrane and burst discharges occurred without a sustained depolarization. Simultaneous treatment with progesterone and estradiol produced a plateau potential of long duration during burst discharges. The thickness of the muscle layer, length constant of the tissue and time constant of the membrane were measured during gestation and from spayed rats under various hormonal conditions. The length constant of the tissue was increased by estradiol and was further increased by simultaneous treatment with estradiol and progesterone. The increase in tissue thickness to have the most marked influence on the length constant. The resting and active membrane properties of the progesterone treated myometrium were similar to those observed during the middle stages of gestation. The estradiol-treated myometrium did not resemble that during the last stages of gestation and parturition, which was simulated by combination of the 2 hormones, estradiol preceding progesterone.