OBSERVATIONS ON SUCCESSIVE PRO-OESTROUS LH SURGES IN INDIVIDUAL 4-DAY CYCLING AND POST-PSEUDOPREGNANT RATS

Abstract

ABSTRACT The internal variations (i. e. in timing, duration and amplitude) of the LH surge mechanism within individual rats were examined by monitoring from 3 to 7 successive pro-oestrous LH surges in each of 5 regular 4-day cycling rats fitted with chronic intravenous cannulas. On each successive pro-oestrus blood was collected (0.5–0.6 ml hourly from 14.00–21.00) for radioimmunoassay of LH. The surgery of cannulation had no long-term effect on the regularity of the oestrous cycle. Two rats did, however, show briefly irregular cycles, including one with a 9-day period of anoestrus (pseudopregnancy). In three of the five rats successive pro-oestrous plasma LH curves (4 in one and 3 each in the other two) were internally very consistent in timing, shape and amplitude, However, between each 2 of these 3 animals there were distinct differences in the LH secretory patterns, by as much as 2 h in timing of the onset of the surge and its peak amplitude. The first two surges of the other two rats were atypical of their subsequent surges, which were mostly consistent in timing and amplitude. The pro-oestrous LH surge following the 9-day period of anoestrus was advanced by 2 h and elevated to twice the mean peak amplitude of the cyclic LH surges in that rat. Subsequently, post-PSP surges were studied in rats made pseudopregnant by mechanical stimulation of the cervix. In all cases the immediate post-PSP surge occurred earlier in the afternoon and with a greater peak amplitude than the subsequent cyclic LH surge in the same rat.