Effects of changing the ionic environment on passive and active membrane properties of pregnant rat uterus.

1. In pregnant rat myometrium electrotonic potentials, produced by externally applied current, were recorded intracellularly.2. The space constant, lambda, was 1.8 mm, the time constant, tau(m), 120 msec. The values obtained on the 7th day and on the 20th day of pregnancy were the same.3. The magnitude of the electrotonic potential and the time constant of the membrane were increased in the absence of potassium from the external solution and decreased by excess potassium.4. The magnitude of the electrotonic potential and the time constant of the membrane were increased by the replacement of chloride with C(6)H(5)SO(3) (-) or SO(4) (2-), and decreased with NO(3) (-) or I(-) replacement.5. When the sodium chloride was replaced with sucrose (16.7 mM sodium remaining in the buffers) the spontaneous spikes deteriorated and activity stopped within 30 min. However, for periods up to 4 hr, a spike of larger amplitude and faster rate of rise than in normal solution could be evoked when a depolarizing current was applied.6. When the external calcium concentration was raised (5 and 10 mM) the amplitude and the rate of rise of the evoked spike were increased. They were decreased by reducing calcium. In zero calcium spontaneous activity stopped within 15 min.7. The effects of calcium deficiency were much less marked and slower in onset when, simultaneously, the sodium concentration was reduced to 16.7 mM.8. When calcium was replaced with strontium (2.5 mM), the membrane was depolarized and the duration of the spontaneous and evoked action potential was prolonged, mainly due to a slowed rate of repolarization. When the concentration of strontium was raised to 7.5 or 12.5 mM the membrane was hyperpolarized, the duration of the action potential became short and the amplitude of the spike was increased.9. Addition of barium or the replacement of calcium with barium caused depolarization and oscillatory membrane activity. However, a spike could be evoked by applying conditioning hyperpolarization.10. Manganese abolished the spontaneous and evoked spike. Tetrodotoxin had no effect.11. The results show that rat uterus has cable-like properties. The action potential may be due to calcium entry, while sodium, by influencing the membrane potential in competition with calcium, may be involved in the spontaneous spike generation and the spread of excitation.