A comparison of the effects of oestrogen and tamoxifen on the synthesis of uterine RNA in immature rats

The anti-oestrogen, tamoxifen, at an optimal dose of 1–2 mg/kg, has been shown to exert a profound stimulating effect on the synthesis of immature rat uterine RNA. This effect occurs over a totally different time course to that induced by oestradiol. A single administration of oestradiol-17β causes a stimulation in uterine RNA synthesis which peaks, at 7-fold control levels, 2 h after hormone injection. Tamoxifen has no effect at 2 h but induces a broad peak of stimulated RNA synthesis which lasts from 12 h to 32 h after administration and peaks at 7-fold control levels 24 h after administration. If implants were used to ensure its continuous presence, oestradiol-17β was able to stimulate RNA synthesis by 4-fold at 24 h after its administration. Unlike tamoxifen, however, these long term effects could be largely accounted for by changes in precursor pool size.