Characterization of rat hypothalamic progestin binding by spheroidal hydroxylapatite chromatography

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Abstract
The progestin-high-affinity-binding components in rat target tissues were assayed by a simple and precise procedure by using spheroidal hydroxylapatite. The progestin receptors in the uterus and hypothalamus of female rats are highly specific for progestins, receptors in the uterus and hypothalamus of female rats are highly specific for progestins, which they bind with high affinity (Kd for [3H]progesterone in hypothalamus is 1.9 nM and in uterus is 3.7 nM). The dissociation of [3H]progesterone from receptor in vitro is rapid: t1/26.degree.C [half-life at 6.degree.C] = 45 min in uterine cytosol; t1/226.degree.C = 160 min in hypothalamic cytosol. The binding is destroyed by proteinase. In the cytosol of hypothalamus and cortex of developing rats, progestin receptors were present in both male and female rats by 2-3 days after birth; subsequent changes in concentration of these receptors appeared to be independent of sex. Concentrations of progestin receptor were close to adult values by 8-9 days, and thereafter changed relatively little.