Relative changes in LH pulsatility during the menstrual cycle: using data from hypogonadal women as a reference point

The basic premise of this study is that the GnRH-LH pulsatile activity, particularly its frequency characteristics, constitutes, in the absence of any considerable ovarian feedback, the intrinsic rhythm of the hypothalamic-pituitary unit at its maximal rate. Thus, LH pulse attributes determined in postpubertal hypogonadal subjects may be used as a reference in assessing the degree of influence exerted by endocrine factors that modulate GnRH-LH pulses. Accordingly, serum LH levels were determined in samples obtained at 15-min intervals for 24 h in 20 hypogonadal women: 13 postmenopausal women (PMW) and seven women with premature ovarian failure (POF). Similar measurements were performed in 60 normally cycling women: 25 in the early follicular phase (EFP), 13 in the late follicular phase (LFP), seven at midcycle surge (LH surge) and 15 in the midluteal phase (MLP). Significant pulses were identified by the cluster algorithm utilizing factors appropriate for 24 h data series of a sampling frequency of 15-min intervals. The results show a 24-h mean (+/- SE) LH pulse frequency of 78.2 +/- 2.8 and 85.5 +/- 2.4 min per pulse for young (POF) and older (PMW) hypogonadal women, respectively. During the follicular phase of the cycle, the LH pulse frequency is not significantly different from that of hypogonadal women, but there is a significant (P less than 0.05) increase from early to late follicular phases (95.4 +/- 3.3 vs 78.8 +/- 2.2 min per pulse). However, when the sleep periods are excluded from the 24-h data series because of the associated decrease of LH pulse frequency in EFP women, the resulting pulse frequencies are almost identical for EFP, LFP and PMW. An elevation beyond the basic pulse rhythm determined in PMW or POF is not observed in any phase of the menstrual cycle studied, including the midcycle surge. The decrease in LH pulse frequency during the luteal phase of the cycle (151.8 +/- 8.0 min per pulse, P less than 0.001 vs hypogonadal women) beyond the reference pulse frequency of hypogonadal women is unequivocal. By contrast, the pulse amplitude varies markedly among the groups with the largest found in POF (36.6 +/- 4.5 IU/l). It follows, in descending order, PMW (22.7 +/- 3.1 IU/l), midcycle surge (17.3 +/- 2.8 IU/l), MLP women (7.0 +/- 1.3 IU/l) and the EFP (4.9 +/- 0.3 IU/l) and LFP (4.0 +/- 0.4 IU/l).(ABSTRACT TRUNCATED AT 250 WORDS)