LH-secretory responses caused by continuous infusion of LH-RH in pseudopregnant rats

The LH-secretory response (LH-S.R.), caused by (standard) infusion of LH-RH, was estimated throughout pseudopregnancy (PSP) as well as on dioestrus - 1 and - 2 of the 4-day oestrous cycle. The LH-S.R. was judged according to 3 parameters: (1) mean maximal plasma LH concentration ("height", expressed in ng LH-RP-1/ml plasma), (2) mean area under the curve (AUC, expressed in "area units"), and (3) a composite, dimensionless, parameter: RQ ("response quotient"), defined as height/AUC. This latter parameter informs on the shape of the curve. The LH-S.R.'s on day 1 of PSP and dioestrus-1 were both relatively low (about 350 ng LH-RP-1/ml plasma) and characterized by the same RQ about 12). On days 2, 5, 7 and 9 of PSP the LH-S.R.'s were essentially the same and appeard to have characteristics of the LH-S.R.'s of both PSP-1 and dioestrus-1 (RQ) and dioestrus-2 (height; about 1000 ng LH-RP-1/ml plasma). From day 10 on the LH-S.R.'s became increasingly higher (PSP 11: about 1900 ng LH-RP-1/ml plasma; PSP 12: about 3300 ng LH-RP-1/ml plasma) and steeper (RQ equals about 22). At the end of PSP the spontaneous LH-surges were measured. These appeared to be very high if compared to those which are generated in 4-day cyclic rats (about 5800 ng LH-RP-1/ml plasma). The results indicate that a period of PSP is not simply an ovulatory cycle of which the dioestrous phase is extended by about a week and it is suggested that "active" corpora lutea secrete, next to progesterone, substances which play a key role in the regulation of the secretion of LH in the course of PSP.