Effects of anti-estrogens on early pregnancy in guinea pigs.

OBJECTIVE: To compare effects of various anti-estrogens on early pregnancy. METHODS: Fifty regularly cycling guinea pigs were divided into five groups (five animals/group), of which three groups received 3 mg/kg body weight Nafoxidine, Centchroman, or Tamoxifen, respectively, on the 1st, 2nd, or 3rd day of pregnancy. The rest of the animals were kept as controls, and received either saline or propylene glycol. Autopsies were done on the 8th day of pregnancy in all the groups. Presence or absence of implantation sites was observed by using a stereomicroscope. Plasma levels of 17 beta-estradiol and progesterone and their receptors, as well as enzyme levels in the uterine tissue, were estimated. RESULTS: During normal early pregnancy there was an increase in the concentration of peripheral plasma levels of steroid hormones with respect to enzyme and steroid hormone receptor levels in the uterus. Administration of anti-estrogens showed a decreased trend in all the variables studied, especially in the tamoxifen-treated animals. CONCLUSIONS: The results of the study suggest that anti-estrogens, if administered during early pregnancy, can prevent the process of ovum implantation in this species.

Changes in the levels of protein and steroid hormones in the plasma and steroid hormone receptors in the uterus of normal cycling guinea pigs.

This study deals with the estrous cycle of guinea pigs in relation to sexual behavior, uterine weight, levels of gonadotropins, steroid hormones, and steroid hormone receptors in the uterus. The guinea pigs in this study showed cyclic changes in various reproductive functions broadly similar to other laboratory species studied. The increase in the uterine weight coincided with high concentration of steroid hormones (estradiol and progesterone) secreted during proestrus and estrus. The elevated levels of steroid hormone receptor concentrations in the uterus during these periods also confirm the role of these hormones. The rise in progesterone level from day 14 of the cycle was associated with lordosis and its related behavior. It was noted that the "estrus behavior" is the most accurate external marker for ovulation and sexual receptivity to males. It was also observed that there is an association between follicle-stimulating hormone and luteinizing hormone during the preovulatory period that was not demonstrated in previous studies.