Pulsatile LH Release During the Ovulatory LH Surge on Proestrus in the Rat1

Abstract

The present experiments were carried out to determine if LH release during the ovulatory LH surge in the rat was pulsatile. Unanesthetized rats with 4-day estrous cycles were bled through jugular vein cannulae at a rate of 10 µl whole blood/2-3 min from 1400 to 1930 h on proestrus during the ovulatory LH surge. Ovulation occurred in all rats that had surges of LH release. These surges consisted of ascending, plateau, and descending periods. LH release was pulsatile during the plateau portion as indicated by a significant increase in the percent coefficient of variation for individual bleeding periods when compared with assay variation, and during the ascending and descending phases as evidenced by pronounced nadir-to-peak changes in blood LH levels which were greater than the minimum significant change detected by radioimmunoassay. Blood LH levels increased during the ascending phase and then plateaued because of high amplitude LH pulses (538 ng/ml in the ascending phase and 572 ng/ml in the plateau phase), which occurred with LH interpulse intervals of 16 and 23 min, respectively. Blood LH levels declined during the descending phase since the pulse amplitude (386 ng/ml) significantly decreased, and in comparison with the ascending phase the LH interpulse interval lengthened significantly to 29 min. These studies indicate that pulsatile LH release occurs during the ovulatory LH surge on proestrus in the rat, and that the rise, plateau, and fall in blood LH levels can be accounted for by changes in the LH pulse amplitude and frequency during different phases of the surge.