Physiological variation in abundance of oestrogen specific high-affinity binding sites in hypothalamus, pituitary and uterus of the rat.

High-affinity binding of [2,4,6,7-3-H]oestradiol-17 beta has been studied in cytosols prepared from hypothalami, pituitaries and uteri of female rats killed at different stages of the oestrous cycle, and in cytosols prepared from the hypothalami and pituitaries of male rats. In all cases the equilibrium dissociation constant of reaction was of the order of 10- minus 10 mol/1. The number of available high affinity sites per tissue (n) varied with physiological state. In females, n fluctuated with the oestrus cycle. In hypothalamus and pituitary, n fell by about 60 and 40% respectively in pro-oestrus, replenishment occurred during oestrus but could be delayed by phenobarbitone administration during the afternoon of pro-oestrus. In the uterus, n varied biphasically, there being peaks during dioestrus and oestrus, and troughs at pro-oestrus and metoestrus. The numbers of available sites at metoestrus were 12-5 times 10-9, 10-6 times 10-10 and 24-4 times times 10-10 for hypothalamus, pituitary and uterus respectively. In male rats, values for n were similar to those obtained for females at pro-oestrus (n/hypothalamus equals 6-8 times 10-9, n/pituitary equals 4-2 times 10-10). Binding was oestrogen specific in all the tissues studied.