1 Department of Obstetrics and Gynaecology, University of Thessalia, Larissa, 2 Department of Obstetrics and Gynaecology, University of Thrace, Alexandroupolis, 3 Clinical Chemistry Laboratory, General Hospital of Athens G. Gennimatas, Athens and 4 Department of Biomathematics, University of Thessalia, Larissa, Greece
5 To whom correspondence should be addressed. Email: messinis{at}med.uth.gr
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Abstract |
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Key words: FSH/LH/GnRH/gonadotrophin surge attenuating factor/pituitary
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Introduction |
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An assessment of the two gonadotrophin pools can be achieved in normal women with the i.v. administration of two submaximal doses of GnRH 2 h apart (Wang et al., 1976). The 30 min point in response to the first injection represents the releasable pool or the pituitary sensitivity, while the whole area under the stimulation curve represents the second pool or the pituitary reserve. Although it would be expected that the enhancement in the pituitary sensitivity to GnRH during the follicular phase of the normal cycle would be a continuous process, recent studies have demonstrated with the use of the model of the two GnRH pulses that a significant increase in pituitary sensitivity takes place only in the late follicular phase, i.e. during the pre-ovulatory period (Messinis et al., 1994
, 1998
). It is reasonable therefore to hypothesize that during the greater part of the follicular phase an unknown factor counteracts the sensitizing effect of E2 on the pituitary.
Up to now, a gonadotrophin surge attenuating factor (GnSAF) has been proposed as a substance that is produced by ovaries undergoing ovulation induction and which attenuates the endogenous LH surge by reducing the pituitary response to GnRH (Messinis and Templeton, 1989, 1991a
). Although data regarding the characterization of this substance are conflicting (Tio et al., 1994
; Danforth and Cheng, 1995
; Pappa et al., 1999
; Fowler et al., 2002
), higher bioactivity of GnSAF has been detected in small growing follicles than in pre-ovulatory follicles (Fowler et al., 2001
) as well as in the circulation of women during the early and mid-follicular phase compared to the late follicular phase of the normal menstrual cycle (Martinez et al., 2002
). It is possible therefore that GnSAF in the early to midfollicular phase of the cycle antagonizes the sensitizing effect of E2 on the pituitary.
The present study was undertaken to test this hypothesis by assessing the pituitary response to GnRH in women with inactive ovaries, such as after menopause, during substitution treatment with exogenous estrogen and progesterone that simulates the hormone profile of the normal menstrual cycle.
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Materials and methods |
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Hormone assays
FSH, LH and E2 were measured in serum using a Chemiluminescent Microparticle Immunoassay (Architect FSH, Architect LH and Architect Estradiol respectively; Abbott Laboratories). The results are expressed as IU/l for FSH and LH, and as pmol/l for E2. Serum progesterone was measured using a Microparticle Enzyme Immunoassay (AxSYM Progesterone, Abbott Laboratories, USA). The results are expressed as nmol/l. The lower limits of detection for FSH, LH, E2 and progesterone were 0.05 IU/l, 0.07 IU/l, 66.06 pmol/l, and 0.636 nmol/l respectively. Inter- and intra-assay coefficients of variation were 3.0 and 3.4%, 2.1 and 3.3%, 4.4 and 6.2%, and 5.9 and 6.7% respectively.
Statistical analysis
In FP-2 (group 1) and in the follicular phase of group 2, the pattern of the time effect in the two groups was investigated using a generalized linear model for repeated measures (Crowder and Hand, 1990; Dafopoulos et al., 2004
) followed by contrast testing (Mead, 1988
). The model has the following form: (y)ijk= (womenxgroup)ik+(time)j+(timexgroup)jk+(error)ijk, where y is the parameter of interest, the (womenxgroup) is the interaction between women (i=17) and groups (k=12), time is the time effect (j=112) and (timexgroup) is the interaction between times and groups. When the (timexgroup) interaction is significant then the pattern of time differences is different for the two groups. The significance of the time effect and the (timexgroup) interaction was assessed using the Huynh and Feldt (1976)
adjustment of degrees of freedom. In the contrast testing, individual time point means or groups of means are compared using the error mean square of the model. For FP-1, LP and FP-2, the differences between the baseline (first time point or first day of each phase) and the consequent time points for each parameter measured were tested by fitting a generalized linear model for repeated measurements followed by contrast testing. The model has the following form: (y)ij=(women)i+(time)j+(error)ij, where y is the parameter of interest. The significance of the time effect was assessed using the Huynh and Feldt adjustment. The contrast testing is based on the error mean square of the model. For comparison of the various corresponding days between the two groups, Student's t-test was used. The statistical analysis was performed using the SAS r6.12 software.
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Results |
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LH values did not differ significantly between the simulated FP-2 (day 32) and the control follicular phase (day 2). For
LH, there was significant (timexgroup) interaction (P<0.01), indicating that the pattern of time effect is different for the two groups (Figure 3, Table II). In particular for group 1, the contrasts of the baseline (day 32) versus day 37 and beyond were significant (P<0.05) (the contrast between baseline and day 43 was significant, P<0.01). For group 2, the contrast of the baseline (day 2) was significant from days 12 and 13 (P<0.05). Therefore, in group 1 there was an earlier increase in
LH values than in the control group (Figure 4).
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Discussion |
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To attain levels of E2 and progesterone exactly the same as those in the normal menstrual cycle is difficult; however, the levels that were eventually achieved in this study were within the normal range for a spontaneous cycle (Messinis and Templeton, 1988). There were only two differences as compared to the control follicular phase, i.e. higher E2 concentrations at the beginning of FP-2 and lower progesterone values during the same phase. These differences could not account for the differential response of the pituitary in the two groups for the following reasons: previous data in normally cycling women have shown that maintenance of high levels of E2 during the immediate post-operative period following ovariectomy did not advance an increase in LH response to GnRH (Messinis et al., 2002
). On the other hand, based on data concerning the administration of the progesterone antagonist mifepristone to normal women during the follicular phase of the cycle (Kazem et al., 1996
), the very low progesterone concentrations in the simulated FP-2 would be expected to result in an attenuated rather than an augmented increase of LH in response to GnRH.
Although the pituitary on a long-term basis after menopause may behave differently from pre-menopausal women, this difference was alleviated in the present study by estrogenizing the women sufficiently for 7 weeks. It is possible therefore that the advanced increase in LH response to GnRH that was seen in the group of post-menopausal women as compared to the normal follicular phase is related to the lack of ovarian function that facilitated an earlier sensitization of the pituitary to GnRH by the rising E2 concentrations. It is suggested that functioning ovaries produce a substance that prevents this sensitization. Such a substance, mainly produced by small growing follicles (Fowler et al., 2001), could be GnSAF, which under specific conditions attenuates the pituitary response to GnRH (Messinis and Templeton, 1989
, 1991a
). It is interesting that the enhanced response of LH to GnRH during the simulated FP-2 was not accompanied by a similar increase in FSH response. This is consistent with the notion that GnSAF controls GnRH-induced LH but not FSH secretion (Fowler and Templeton, 1996
).
Previous studies in normally cycling women seem also to provide some support for the GnSAF hypothesis, although their primary endpoint was the regulation of LH pulse frequency by gonadal steroids (Nippoldt et al., 1989; McCartney et al., 2002
). In experiments in which E2 concentrations were maintained at the midluteal level, although progesterone concentrations were low the increase in LH pulse frequency that was seen for 2 weeks was not accompanied by an increase in LH response to GnRH (McCartney et al., 2002
). This could be interpreted as indicating that the unchanged responsiveness to GnRH was related to the production of GnSAF by the functioning ovaries under the influence of FSH that also increased significantly during the same period of time (McCartney et al., 2002
). Furthermore, in another study the artificial induction of pre-ovulatory concentrations of E2 in the midfollicular phase of normal women stimulated, via a positive feedback mechanism, an LH surge that was attenuated as compared to the midcycle LH surge, while the FSH surge was normal (Taylor et al., 1995
). Although the authors of that study explained their results on the basis of a missing ovarian factor in the midfollicular phase that is required for the normal midcycle surge, they could also easily accept that the attenuation was related to the production of GnSAF by the ovaries at that stage of the cycle.
In terms of basal gonadotrophin secretion, serum FSH values were suppressed during the experimental period, while basal LH values in FP-2 increased gradually and significantly. The inability of E2 to maintain very low levels of LH in the absence of ovarian function indicates that during the follicular phase of the natural cycle this steroid is not the only mediator of the ovarian negative effect on LH secretion. Since in this study serum progesterone concentrations in FP-2 were very low, it is possible that during the normal follicular phase progesterone even at low levels contributes to the ovarian suppressing effect on basal LH secretion. These data demonstrate the differential control of FSH and LH by ovarian steroids as has been suggested previously (Messinis et al., 2002; McNeilly et al., 2003
). Whether inhibin might have an effect on LH secretion is rather unlikely, as administration of inhibin A to rhesus monkeys in the early follicular phase suppressed FSH but not LH levels (Molskness et al., 1996
).
Another interesting finding in the present study is the failure of a positive feedback mechanism to induce an endogenous LH surge either in FP-1 or in FP-2 despite the fact that serum E2 concentrations had exceeded the appropriate threshold level for several days. These data are in disagreement with a previous study in which four out of five post-menopausal women treated with exogenous estrogen and progesterone to simulate the natural menstrual cycle displayed an endogenous LH surge (Lutjen et al., 1986). Although this difference is difficult to explain, in that study all five women had premature menopause and were younger (<37 years old) than in the present study. It is possible that the pituitary behaves in a different way in older compared with younger post-menopausal women. Since, however, post-menopausal women aged >50 years can express a positive feedback effect when treated with high doses of estrogen even for only 3 days (Liu and Yen, 1983
), it may be that in these women the pituitary is less sensitive to the gradual than to the abrupt increase in serum E2 concentrations. Nevertheless, we cannot exclude the possibility that in our study attenuated LH surges occurred during the experimental period that were missed due to the blood sampling frequency or that an LH surge would occur at a later stage.
The lack of an LH surge was noted despite the earlier sensitization of the pituitary to GnRH by E2. This indicates that at a specific point during the normal menstrual cycle the E2-enhanced pituitary sensitivity to GnRH is further augmented by an ovarian factor that plays a key role in timing the onset of the midcycle LH surge. Progesterone may be such a factor since this steroid advances and augments the E2 positive feedback effect experimentally (Liu and Yen, 1983; Messinis and Templeton, 1990
), while the progesterone antagonist mifepristone blocks the endogenous LH surge in normal women despite the presence of high E2 concentrations (Messinis et al., 1997
). In that respect, a small rise in serum progesterone levels in the late follicular phase (Roseff et al., 1989
) may not be necessary for the occurrence of the midcycle LH surge.
The following hypothesis is supported by the results of the present study. During the normal menstrual cycle, E2 sensitizes the pituitary to GnRH, an action that in the early to midfollicular phase is counteracted by GnSAF. In the late follicular phase, the sensitizing effect of E2 is facilitated both by a decrease in the production of GnSAF and by the synergistic action of E2 and progesterone. With these hormonal interactions the secretion of LH up to the midfollicular phase is maintained at a low level and is only markedly enhanced at midcycle when high amounts of this hormone are required for the ovulatory events. In the context of this hypothesis, progesterone secreted in small amounts during the follicular phase of the cycle plays an important physiological role both through the negative and the positive feedback mechanisms.
In conclusion, the present study provides new insights into the ovarian control of LH secretion during the normal menstrual cycle. The results demonstrate for the first time that E2 sensitizes the pituitary to GnRH at an earlier stage in women with inactive ovaries than in the follicular phase of the natural cycle. It is suggested that during the normal follicular phase the ovaries control LH secretion not only through E2 but also via the participation of GnSAF and progesterone.
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Acknowledgements |
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References |
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Submitted on January 7, 2004; accepted on May 28, 2004.