Augmentation and suppression of action potentials by estradiol in the myometrium of pregnant rat

The purpose of this study was to investigate the actions of estradiol on sp ontaneous and evoked action potentials in the isolated longitudinal smooth muscle cells of the pregnant rat. Single cells were obtained by enzymatic d igestion from pregnant rat longitudinal myometrium. Action potentials and c urrents were recorded by whole-cell current-clamp and voltage-clamp methods , respectively. The acute effects of 17 beta-estradiol on action potentials and inward and outward currents were investigated. The following results w ere obtained. The average resting membrane potential of single myometrial c ells was -54 mV (n = 40). In many cells, an electrical stimulation evoked a membrane depolarization, and action potentials were superimposed on the de polarization. In some cells, spontaneous action potentials were observed. E stradiol (30 mu M) slightly depolarized the membrane (ca. 5 mV) and attenua ted the generation of action potentials by reducing the frequency and ampli tude of the spikes. Afterhyperpolarization was also attenuated by estradiol (30 mu M). On the other hand, in 5 of 35 cells, estradiol increased the fi rst spike amplitude and action potential duration, while frequency of the s pike generation and afterhyperpolarization were inhibited. In voltage-clamp ed muscle cells, estradiol inhibited both inward and outward currents. Acut e inhibition or augmentation of spike generation by estradiol is due to the balance of inhibition of inward and outward currents. Inhibition of both c urrents also prevented afterhyperpolarization, causing potential-dependent block of Ca spikes.