Measurements with flexible intracellular microelectrodes show that the rabbit myometrium when insufficiently stimulated by estrogen has a low membrane potential of about 30 to 35 mv. Estrogen (endogenous or exogenous) increases this value up to 43 to 48 mv., whereas progesterone increases it still further up to 55 to 60 mv.

Between the 20th and 29th days of rabbit pregnancy the membrane potential at placental implantation sites is about 10 mv. higher than between these sites. Progesterone treatment increases the value at the interplacental sites to the level of the placental sites. The onset of labor is preceded by a drop in membrane potential at the placental sites to the level of the interplacental sites. This change results in a more or less uniform potential of 45–50 mv. along the length of the uterine horn at term.

The membrane potential of the uterus is a log function of the [K]0 at values greater than 12 mM/liter, as is the case in other excitable tissues. In Ca-free Krebs the membrane potential of the estrogen-dominated uterus drops to a low level; the progesterone-dominated myometrium exhibits only a transient and lesser decrease.

The membrane potential of the myometrial cell is influenced by endocrine regulation. The correlation between membrane potential and threshold only holds for the estrogen-dominated uterus, however, in other endocrine conditions the relationship breaks down.

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