The action of chlormadinone acetate (CAP) on the uptake of [6,7-3H]-17β-oestradiol in the castrated rat was studied.
It was shown that when CAP is injected, the uptake of 3H-oestradiol diminishes significantly in the uterus and in the anterior pituitary (P < 0.01), whereas the uptake is not modified in the hypothalamus, posterior pituitary, skeletal muscle and plasma.
A significantly (P < 0.01) smaller dose of CAP was required to inhibit the uptake of 17β-oestradiol by the anterior pituitary (27.2 ng) than by the uterus (1000 ng).
Inhibition of oestradiol uptake can be demonstrated when CAP is administered before the oestrogen as well as when it is given subsequent to it (P < 0.01). This suggests the existence of a competitive mechanism between 17β-oestradiol and CAP for specific oestradiol receptors.
When the action of CAP on different fractions of the uterine cell was studied, it was observed that the inhibition of oestradiol uptake occurs exclusively in the nuclear fraction (P < 0.01).
Competition with oestrogens for specific protein receptors might be the common mechanism of action of the different manifestations of the effects of CAP.
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