Effect of oestrogen on the responsiveness of the pituitary gland to LRH: The role of corpora lutea

The hypothesis that corpora lutea (CL) secrete substances which prevent oestrogens from influencing the state of responsiveness of the pituitary gland to LRH (RESP) was tested in rats rendered persistently oestrous (PO) by exposure to permanent illumination. In these rats ovulation and hence the presence of a single set of CL, was induced by exogenous gonadotrophin, hCG. The RESP of the animals was judged on 3 parameters based on the surge-like LH-secretory responses which were induced by 21-h long constant rate LRH infusions (104 ng/LRH/h). These 3 parameters were: a) the maximal height (MH) of the responses; b) the 'area under the curve' (AUC) of the LH values; and c) the constant β (or alternatively the t½) which characterizes the rate of decrease of the plasma LH concentrations after 2 h of infusion. Four experiments were performed, all with PO rats: 1) rats were injected with either oestradiol benzoate (OeB; 3 μg sc) or oil on day 0 (the day treatments were started was always denoted as day 0); 2) rats were treated similarly, but they also received an ovulatory dose of hCG (50 IU/100 g b.w. ip) on day 0; 3) rats were injected with hCG on day 0 and with OeB or oil on day 3; and 4) rats were injected with hCG on days 0 and 4, on which latter day they also received OeB or oil. In all 4 experiments the LRH infusions were started 21 h after administration of OeB or oil. Blood samples for LH determinations were taken at times apparent from 'Results and Discussion'. It is observed that: 1) after administration of OeB the MH and thereby the AUC increased significantly but the β of the LH-secretory responses was unchanged; 2) after administration of an ovulatory dose of hCG the LH-secretory responses changed also; of these responses, however, the β had decreased and the AUC had increased, whilst the MH remained unchanged; 3) when given together, OeB and hCG exhibited their effect simultaneously: the two effects are additive; 4) in the presence of 3-day old CL OeB is ineffective; and 5) in the presence of 4-day old CL neither OeB nor hCG is able to affect the RESP. It is concluded: 1) OeB and 'hCG' probably influence a different substrate of the LH-release mechanism; and 2) that these results confirm the hypothesis that CL secrete substances which prevent oestrogen from affecting the RESP for at least 4 days. If this hypothesis is extended to the cyclic rat, CL, arisen after the previous ovulation, may still be of importance on the day of the next pro-oestrus by exerting a significant influence on the RESP