SUMMARY

Oestrogen receptors were identified in both the cytoplasmic and the particulate fractions of the mammary glands of lactating rats. The quantities of cytoplasmic receptor and nuclear receptor—oestrogen complex in the particulate fraction were measured by the charcoal adsorption assay and the [³H]oestradiol exchange assay respectively. The quantity of cytoplasmic oestrogen receptor increased markedly between day 1 and day 10 and reached a maximum value on the 21st day of lactation. Although the quantity of cytoplasmic receptor increased markedly during lactation, the concentration of nuclear receptor—oestrogen complex that resulted from endogenous concentrations of blood oestrogen changed only slightly, indicating that the blood level of oestrogen was quite low during lactation. One hour after a subcutaneous injection of 25 μg oestradiol, the concentration of nuclear receptor—oestrogen complex increased dramatically and a translocation process similar to that for the uterine receptor—oestrogen complex was observed. The increased quantity of particulate receptor—oestrogen complex after the administration of exogenous oestradiol indicated that the low levels of endogenous nuclear receptor—oestrogen complex observed during lactation were not the result of failure of the translocation process. The concentration of cytoplasmic receptor on day 21 of lactation in rats that were ovariectomized on day 1 of lactation was not significantly different from the concentration in intact control animals. The translocation process was also similar in ovariectomized and intact rats on day 21 of lactation. These results suggest that the increased quantity of oestrogen receptors in lactating mammary tissue is not dependent on ovarian oestrogens.

Recent advances in comparative genomics allow a new paradigm for hormonal research. At the centennial of the first use of the term hormone by Ernest Starling, we reflected on the changing approaches in elucidating hormonal signaling mechanisms and highlighted the inadequacy of the term endocrinology, implying remote activation, to describe the diverse modes of hormone actions. Several examples were presented to underscore the power of comparative genomics in the identification of new polypeptide hormones, receptors, and signaling pathways. We propose the use of the term hormonology to more accurately reflect the expanding boundaries of the discipline.

Alterations in the concentrations of oestrogen receptors in the uterus, pituitary gland and hypothalamus during the 2 weeks following a single administration of clomiphene citrate (Clomid) to immature, bilaterally ovariectomized rats were investigated. Examination of the uterine wet weight at 1, 7 and 14 days following a single injection of Clomid (100 μg, 250 μg or 10 mg) indicated significant time- and dose-related increments from a control value of 45 ± 2 (s.e.m.) mg to a maximum of 123 ± 3 mg (250 μg dose at 14 days). In contrast, a single injection of oestradiol led to a transient increase in the uterine weight on day 1 to 94 ± 6 mg, but was without effect by days 7 and 14. Analysis of the uterine DNA content 7 and 14 days after treatment with Clomid revealed significant increments from control values of 390 ± 10 μg to a high level of 558 ± 8 μg (10 mg dose at 7 days). There was a transient retention of nuclear oestrogen receptors and rapid replenishment of cytoplasmic oestrogen receptors in less than 24 h in the uteri of animals treated with oestradiol (25 μg), but determinations of receptor content in Clomid-treated animals revealed prolonged retention of nuclear receptors and delayed replenishment of cytoplasmic receptors. The duration and extent of retention of nuclear receptors and depletion of cytoplasmic receptors after treatment with Clomid were found to be dose-dependent. Fourteen days after Clomid treatment, levels of oestrogen receptors in nuclei from the uterus were still raised in all treatment groups, whereas replenishment of cytoplasmic receptors was complete in animals treated with the lower doses (100 and 250 μg) of Clomid.

A single injection of Clomid (250 μg) induced similar prolonged retention of nuclear receptors and delayed depletion of cytoplasmic receptors in pituitary tissue. In contrast, changes in the content of oestrogen receptors in the hypothalamus following Clomid treatment were minimal. The limited effect of Clomid on hypothalamic tissue may mean that the pituitary gland is a more important target for this compound than is the hypothalamus. The findings have confirmed earlier reports on the long-term uterotrophic effect of Clomid and have suggested that under these long-term, in-vivo conditions, Clomid acts in the uterus and pituitary gland as a long-acting oestrogen characterized by prolonged retention of oestrogen receptors in the nucleus and delayed, but otherwise effective, replenishment of the oestrogen receptors in the cytoplasm.