Modulation by continuous oestrogen treatment and by progesterone of oestrogen receptors in the rabbit uterus.

The effect of continuous oestrogen treatment on uterine weight, and on cytoplasmic and nuclear oestrogen receptors was examined in rabbits. Modulation of the effects of oestrogen by progesterone was also studied. Uterine weight increased successively after exposure to oestrogen from 1 to 6 days. The concentration of both cytosolic and nuclear oestrogen receptors decreased in almost parallel fashion, however, irrespective of whether it was expressed per microgram protein or microgram DNA. Treatment with progesterone 5 days after oestrogenization caused no significant change in uterine weight at first. The oestrogen receptor concentration in both cytosolic and nuclear fractions decreased after 1 day but increased again after 3 days of progesterone treatment. The results indicated that there is a reduction in the actual concentration of oestrogen receptors in both cytosolic and nuclear fractions after continuous oestrogen treatment. The antagonism by progesterone of the effects produced by oestrogen appears to be transitory in nature, at least in the rabbit uterus.