Membrane potential and ionic content in pregnant and non‐pregnant rat myometrium

It is well known that the membrane potential and excitability of uterine muscle as well as the production of tension are under hormonal influence. Thiersch, Landa & West (1959) observed that, during the progress of gestation, the membrane potential of the rat myometrium gradually increased and that the placental site had a higher membrane potential than non-placental sites. Goto & Csapo (1959) extended the observations to the post-partum uterus, using rabbits, and they found that the membrane potential rapidly fell within 2 days after delivery to the same level as that before pregnancy. Similar observations have been made by other workers on different animal species (for references see Kuriyama, 1961). Membrane-potential changes in non-pregnant uterus resembling those of pregnant and post-partum uterus can also be produced by treatment with oestradiol and progesterone (rat: Marshall, 1959, 1962; Jung, 1960, 1964; rabbit: Goto & Csapo, 1959; Kuriyama & Csapo, 1961; guinea-pig: Bulbring & Kuriyama, unpublished observations). These changes in membrane potential may be due either to a change in internal ionic contents or to a change in ionic permeability of the cell membrane. Ionic contents of myometrium have been measured by many authors in different animal species after different hormonal treatment (Rat: Cole, 1950; Kalman, 1957; rabbit: Horvath, 1954; Daniel & Daniel, 1957; Daniel, 1958; Kao, 1961; Bitman, Cecil, Hawk & Sykes, 1959; Daniel & Robinson, 1960a, b; cat: Daniel & Daniel, 1957; Daniel, 1958; human: Daniel & Boyes, 1957; Daniel, 1958). On the other hand, data about the ionic content of the myometrium during the oestrus cycle and pregnancy are not available. Bitman et al. (1959) found that endometrium of the progesterone-treated rabbit uterus had a high potassium and low sodium concentration compared with the oestrogen-treated rabbit endometrium. However, the myometrium showed only slight changes throughout. Kao (1961) also found only a slight change of the ionic contents in the rabbit myometrium after treatment with oestradiol and progesterone. In the present experiments a systematic study was undertaken in order

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