Short term hypothyroidism affects ovarian function in the cycling rat

Abstract

Background: Rats made hypothyroid with propilthyouracil start showing abnormal cycling on the second cycle after the start of the treatment, with a high proportion of spontaneous pseudopregnancies and reduced fertility.

Methods: To investigate some of the mechanisms involved in these reproductive abnormalities, hypothyroidism was induced in virgin rats by propilthyouracil (0.1 g/L in the drinking water) and we determined circulating hormones by radioimmunoassay and whole ovary expression of ovarian hormone receptors, growth factors and steroidogenic enzymes using semi-quantitative RT-PCR.The study was performed on days 6 to 9 of treatment, corresponding to diestrus I (at 20.00-22.00 h), diestrus II (at 20.00-22.00 h), proestrus and estrus (both at 8.00-10.00 h and 20.00-22.00 h) of the second estrous cycle after beginning propilthyouracil treatment. Another group of rats was mated on day 8 and the treatment continued through the entire pregnancy to evaluate reproductive performance.

Results: Hypothyroidism increased circulating prolactin and estradiol on estrus 5 to 7-fold and 1.2 to 1.4-fold respectively. Growth hormone and insulin-like growth factor 1 diminished 60 and 20% respectively on proestrus morning. Hypothyroidism doubled the ovarian mRNA contents of estrogen receptor-beta on proestrus and estrus evenings, cyp19A1 aromatase mRNA on estrus evening and of growth hormone receptor on proestrus evening. Hypothyroidism did not influence ovulation rate or the number of corpora lutea at term, but a diminished number of implantation sites and pups per litter were observed (Hypothyroid: 11.7 +/- 0.8 vs.

Control: 13.9 +/- 0.7).

Conclusions: Short term hypothyroidism alters normal hormone profile in the cycling rat increasing the expression of estrogen receptor-beta and cyp19A1 aromatase on estrus, which in turn may stimulate estradiol and prolactin secretion, favouring corpus luteum survival and the subsequent instauration of pseudopregnancy.

Figure 1

Circulating hormone concentrations during the estrous cycle in control and HypoT rats during the 2^nd estrous cycle after initiation of PTU treatment (0.1 g/l in the drinking water). See Materials and Methods section for further details. The results represent the means ± SEM of groups of 6-9 rats. * p < 0.05 compared with respective control group.

Figure 2

Ovarian mRNA content relative to S16 (see Materials and Methods section for further details) of PRLR_long, ERα, ERβ and cyp19A1 aromatase estimated by semi-quantitative RT-PCR in control and HypoT rats during the 2^nd estrous cycle after initiation of PTU treatment. The results represent the means ± SEM of groups of 6-9 rats. * p < 0.05 compared with respective control group; § p < 0.05 compared with D 1 20.00 h; ‡ p < 0.05 compared with E 8.00 h.

Figure 3

Ovarian mRNA content relative to S16 of IGF 1, IGF R, IGF BP-3, IGF BP-5 and GHR estimated by semi-quantitative RT-PCR in control and HypoT rats during the 2^nd estrous cycle after initiation of PTU treatment. The results represent the means ± SEM of groups of 6-9 rats.* p < 0.05 compared with respective control group; § p < 0.05 compared with D 1 20.00 h.

Figure 4

Number of oocytes, corpora lutea and implantation sites in control and HypoT rats at the end of gestation. The results represent the means ± SEM of groups of 6-9 rats. * p < 0.05 compared with respective control group; § p < 0.05 compared with corpora lutea counted in the HypoT group.