SECRETORY DYNAMICS OF OESTRADIOL (E2) AND PROGESTERONE (P4) DURING PERIODS OF RELATIVE PITUITARY LH QUIESCENCE IN THE MIDLUTEAL PHASE OF THE MENSTRUAL CYCLE

Although the temporal relationship between pulsatile pituitary luteinizing hormone (LH) secretion and steroid hormone release from the corpus luteum has been investigated, the secretory profiles of oestradiol (E2) and progesterone (P4) during periods without any discernible LH pulsatile activity remain unknown. Consequently, blood was sampled at 15-min intervals for 24 h from 16 women during the midluteal phases (6-8 days after midcycle LH surge) of their cycles. LH was measured in all samples and analysed for significant pulses by the Cluster pulse algorithm. Nine studies showing the lowest LH pulse frequencies and large LH pulse amplitudes were also assessed for E2 and P4 in all samples. All three hormones were released in pulsatile fashions. Pulses of E2 and P4 were found to be synchronous. While the release frequencies for E2 (mean +/- SEM: 8.9 +/- 0.7 pulses/24 h) and P4 (8.5 +/- 0.7 pulses/24 h) were comparable, the LH pulse frequency (4.6 +/- 0.4 pulses/24 h) was found to be significantly (P less than 0.001) lower than the ovarian steroid pulse frequencies. Maximum (P less than 0.01) cross-correlation coefficients were determined at positive time lags of 28.1 +/- 7.7 min for LH/E2 and 31.7 +/- 5.8 min for LH/P4, indicating that changes in E2 or P4 levels tended to occur within approximately 30 min following LH concentration changes. Further, the degree of concomitance between a steroid pulse and an LH peak was much higher (P less than 0.001) than by chance. Maximum (P less than 0.01) cross-correlation coefficients between E2 and P4 hormonal data series were found at zero time lag, suggesting that these sex steroids were secreted simultaneously. The pulse amplitudes, pulse durations and areas under the peaks of those E2 or P4 pulses preceded by large (greater than 5 IU/l) amplitude LH pulse were significantly greater (P less than 0.05 or less for all comparisons) than for steroid pulses not associated with preceding LH pulses. Thus, two populations of steroid pulses were observed; one associated with preceding LH pulses and having greater magnitude of all pulse attributes (duration, amplitude, area under the peaks), and another, not associated with preceding LH pulses and having pulse characteristics of lower magnitude. This observation suggests that the pulsatile release of ovarian steroids is a result of the episodic modulating influence of LH and that pulsatile steroid hormone secretion pertains with smaller magnitude during periods of relative pituitary quiescence of LH pulsatility.