Myometrial strips isolated from uterus of rabbits under either estrogen or progesterone domination were impaled with glass capillary microelectrodes for measurement of resting potential. This was found to be 49.81 ± 0.34 mv for estrogen-dominated myometrium (mean ±se of 616 samples), and 48.86 ± 0.43 mv for progesterone-dominated myometrium (514 samples), the difference between the resting potentials being insignificant. Therefore, the validity of the so-called "progesterone-block" hypothesis is questioned. Collagen was present in 14 g/kg of wet tissue in both hormonal states. These amounts did not hold sufficient chloride to appreciably lower the intracellular chloride concentrations, which were high enough to suggest either a complex distribution pattern or some active uptake for chloride. The relative cation permeabilities (Pna/Pk) were estimated as 0.11 for the estrogen-dominated myometrium, and 0.17 for the progesterone-dominated myometrium.
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Kao et al. (1964) studied this question.