The Effects of Changes in NaCl Concentration on the Action Potential of Rat Uterine Smooth Muscle

1. The electrical activities of pregnant rat uterus near or term were investigated by intracellular microelectrode. 2. During spontaneous activity the membrane potential was very unstable and varied from 40 to 50mV. The spikes accompanied by spontaneous contraction ranged from 50 to 67mV. The overshoot recorded was usually 5 to 17mV. 3. Several types of slow fluctuation of membran potentials were recorded during spontaneous activities which were similar to that previously reported by Bulbring et al. 4. The pace maker potential was found by chance all over the whole uterus, which seemingly was not localized in any particular area. 5. The behavior of the smooth muscle in the solution of low sodium concentration was examined. Little changes were detectable in spike height and configuration when the external sodium concentration was reduced to 20mM but further reduction to 10mM or less affected the spike height and configuration gradually. 6. The spontaneous activity was maintained in Na free solution for 25 min. or more though the spikes degenerated into small peaked negativities. The findings in our experiments seem to be important for explaining the electrical activity of the uterine muscle cell membrane from the stand point of the sodium theory.