Nuclear and cytosol steroid hormone receptor levels in the myometrium during pregnancy in the sheep

Methods for the measurement of nuclear receptors for oestradiol and progesterone in sheep myometrium have been established. Scatchard analysis of nuclear receptors gave dissociation constants (nM) on days 0 and 112 of pregnancy of 1.95 and 1.76 for oestradiol and 4.20 and 4.12 for progesterone, respectively. The concentration of nuclear and cytosol high-affinity receptors for oestradiol and progesterone has been determined during the first 112 days of gestation; and possible roles of oestradiol and progesterone in the regulation of myometrial hypertrophy and function are discussed.The rate of hypertrophy, as measured by changes in protein : DNA and RNA : DNA ratios, was maximal during days 56–84 and declined thereafter. The level of cytosol oestradiol receptor decreased rapidly between day 0 (oestrus) and day 28, and then more slowly between days 28 and 112, when expressed per unit of cytosol protein. However, when expressed per unit of DNA the level increased after day 28 to a peak at day 84, then decreased markedly to day 112. The level of nuclear oestradiol receptor declined from a peak at oestrus to very low levels on days 56–84, then increased markedly to day 112. The concentration of cytosol progesterone receptor declined from a peak at oestrus to low levels on days 28–112. The changes in the level of nuclear progesterone receptor were more complex; the level increased between oestrus and day 28, declined markedly to day 56, then increased again to high levels on days 84–112.The data suggest that oestradiol does not have any important role in stimulating myometrial growth, since the level of nuclear receptor for oestradiol was low when the rate of hypertrophy was maximal. The changes in nuclear progesterone receptor level were less clearly separated, temporally, from changes in rate of hypertrophy, and the possible influence of progesterone on myometrial growth remains unclear.