Oestriol receptor interactions: their biological importance and therapeutic implications.

An investigation into certain aspects of the interaction between oestriol and oestrogen receptors (specificity and kinetics) shows the specificity of the receptors to be very similar in various target tissues-human breast tumour and myometrium, rat myometrium, and rabbit myometrium, pituitary, and thymus-and a preferential binding of oestriol to specific oestrogen receptors to be unlikely. Competition for the oestrogen receptors in the uterus of the three species was analysed in vitro under equilibrium and non-equilibrium conditions, and oestriol and epimestrol were found to compete with oestradiol-17 beta more strongly in the latter. In rat uterus a late oestrogenic effect, the synthesis of the progesterone receptor, induced by ethinyloestradiol, is shown to be inhibited by oestriol. Oestriol emerges as a short-action agonist when administered in a single dose. It is also concluded that oestriol may prevent eostradiol-17 beta from inducing a full uterotrophic response.