Increase of myometrial activity correlated with variations in 17 beta-estradiol and progesterone uterine concentrations in mid-term pregnant rat: estrogen agonist effect of 4-hydroxytamoxifen.

The objective of this investigation was to examine the effects of 4-hydroxytamoxifen on the uterine activity. For this, we evaluated the electrical activity of the myometrium, chronically, in conscious unrestrained rats at mid-pregnancy. We also examined the tissular progesterone and 17 beta-estradiol concentrations in the myometrium and uterus 6 hours after administration of 4-hydroxytamoxifen. Comparison of myometrial electrical activities recorded during the control period with those obtained during the two periods (6 and 24 hours) after administration of 4-hydroxytamoxifen (80 micrograms.kg-1, s.c.) showed an increase in simultaneity of uterine contractions (P < 0.01). Tissular steroid hormone measurement by radioimmunoassay shows a fall of progesterone in the myometrium (P < 0.001) and of 17 beta-estradiol in the uterus (P < 0.01), 6 hours after administration of 4-hydroxytamoxifen. In the myometrium, for 50% of animals, 17 beta-estradiol concentration decreased (P < 0.01) and for 50% of animals it increased (P < 0.05). The decrease in progesterone is significant in the myometrium and in the whole uterus (respectively P < 0.001 and P < 0.01), 24 hours after administration of 4-hydroxytamoxifen. The 17 beta-estradiol concentration significantly decreased for all animals in the myometrium (P < 0.01) and in the uterus (P < 0.01), after this time. It appears that variation in progesterone induces the activation of uterine motility and exerts an effect on some factors involved in the regulation of the rat myometrium at mid-pregnancy.