1 Department of Obstetrics and Gynaecology, University of Thessalia, Larissa and 2 Department of Biological Chemistry, University of Ioannina, Ioannina, Greece.
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Abstract |
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Key words: estradiol/GnRH/gonadotrophins/ovary/progesterone
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Introduction |
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In-vivo experiments have shown marked changes in the responsiveness of the pituitary to GnRH during the normal menstrual cycle, with a significant increase from the early follicular phase to mid-cycle and a progressive decline thereafter (Wang et al., 1976). Although estradiol is the primary factor that sensitizes the pituitary to GnRH during the follicular phase (Lasley et al., 1975
), the role of ovarian steroids in the control of pituitary sensitivity to GnRH during the luteal phase has not been investigated. In a recent study in women, we have demonstrated that following ovariectomy in the luteal phase of the cycle, the response of FSH to GnRH increased progressively, while that of LH declined markedly. This indicates a differential control of FSH and LH by the ovaries (Alexandris et al., 1997
), but the mechanism is not clear.
The present study was undertaken to investigate the mechanism through which the ovaries control GnRH-induced LH and FSH secretion during the luteal phase of the menstrual cycle by treating normal premenopausal women with estradiol and progesterone in order to prevent the ovariectomy-induced decline in the concentrations of these two steroids.
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Materials and methods |
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Hormone assays
For the measurement of FSH and LH in serum, a microparticle enzyme immunoassay (MEIA) was used (AxSYM FSH and AxSYM LH respectively; Abbott Laboratories, Abbott Park, IL, USA). The lower limits of detection for FSH and LH were 0.37 and 0.50 IU/l respectively. Estradiol was measured using MEIA (AxSYM Estradiol; Abbott Laboratories). The lower limit of detection for estradiol was 73 pmol/l. For progesterone measurement, a solid-phase chemilluminescent enzyme immunoassay (Immulite progesterone; DPC, Los Angeles, CA, USA) was used. The lower limit of detection for progesterone was 0.6 nmol/l. The inter- and intra-assay coefficients of variation for FSH, LH, estradiol and progesterone were 3.2 and 4.1, 2.6 and 4.2, 2.3 and 5.5, and 8.0 and 4.1% respectively.
Statistical analysis
Before the statistical analysis the results were transformed into logarithms, but the arithmetic means of values are presented. For comparisons within the same group statistical analysis of the results was performed using one-way analysis of variance (ANOVA) followed by Dunnet's post-hoc test, while for comparisons between groups two-way ANOVA was used. The Statview 5 program (Abacus Concepts Inc., Berkley, CA, USA) was used.
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Results |
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In each GnRH experiment, the response of LH and FSH to GnRH was calculated as the net increase at 30 min above the basal value (LH and
FSH respectively). Serum
LH and
FSH values before the operation did not differ significantly between the three groups (Figure 2
). In group 1,
FSH values after the operation increased progressively up to post-operative day 7 (P < 0.001). In contrast, in groups 2 and 3,
FSH values remained unchanged up to days 5 and 6 respectively, after which the values increased significantly (P < 0.05).
FSH values were significantly higher in group 1 than in the other two groups on days 17 and in group 2 significantly higher than in group 3 on day 7 (Figure 2
). In contrast to
FSH,
LH values showed the same pattern of changes in all three groups during the post-operative period (Figure 2
). In particular,
LH values declined gradually up to day 4 (P < 0.01), increasing significantly thereafter up to day 7 (P < 0.05). At all points,
LH values did not differ significantly between the three groups.
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Discussion |
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The present study is the first to investigate the effect of estradiol and progesterone on pituitary sensitivity to GnRH in premenopausal women following bilateral ovariectomy. In terms of changes in GnRH-induced FSH secretion in the untreated (control) group of women, the pattern was similar to that previously reported, i.e. a continuous rise following ovariectomy (Alexandris et al., 1997), thus illustrating a suppressing effect of the ovaries on the pituitary at that stage of the cycle. We infer that estradiol contributes to, but is not solely responsible for, this suppressing effect, since in the women who were treated with estradiol alone the increase in
FSH values was delayed but not abolished. Although with the addition of progesterone the period of the estradiol-induced suppression was extended, the rise in
FSH eventually occurred, suggesting that the two steroids together are not sufficient to mediate completely the ovarian suppressing effect on FSH and that other ovarian factors also play a role. A factor that could negatively affect GnRH-induced FSH secretion, at least in vitro, is inhibin A (Burger, 1992
), the levels of which are normally high in the mid-luteal phase of the cycle (Groome et al., 1996
).
Our data confirm previous findings that following ovariectomy the pattern of changes in LH response to GnRH is different from that of FSH response (Alexandris et al., 1997). The decreasing values of
LH in the women who did not receive hormonal treatment could be interpreted as indicating that the ovaries exerted a sensitizing effect on LH secretion before the operation. However, the fact that the pattern of changes in
LH values was unaffected by treatment with the steroids suggests that estradiol and progesterone are not mediators of such an ovarian effect on LH response to GnRH in the mid-luteal phase. It is possible, therefore, that either a sensitizing effect of the ovaries on the pituitary is exerted through unspecified substances, or that the decrease in
LH values following ovariectomy is controlled by extra-ovarian mechanisms. Such mechanisms could be related to depleted stores of pituitary gonadotrophins as a result of the preceding mid-cycle LH surge that affected LH reserves more than those of FSH. The latter possibility is more likely based on previous data that a declining pattern of LH response to GnRH during the luteal phase of the cycle has been also reported in women with intact ovaries (Messinis et al., 1993
). The fact, however, that following ovariectomy the decline in
LH was interrupted shortly after the operation, i.e. ~4 days from the mid-luteal stage (Figure 2
), while in women with intact ovaries the decline continues until the end of the luteal phase (Messinis et al., 1993
), indicates an earlier recovery of the pituitary in the ovariectomized than in the non-ovariectomized women. This suggests that GnRH-induced LH secretion in the luteal phase is not entirely unaffected by the ovaries. It is possible that a factor, different from estradiol and progesterone, maintains a low responsiveness of LH to GnRH towards the end of the cycle. Such a factor that specifically reduces LH response to GnRH is gonadotrophin surge attenuating factor (GnSAF) (Messinis and Templeton, 1989
), but its role at that stage of the cycle needs to be further investigated.
So far, this factor has been purified from human follicular fluid as a protein of 12.5 kDa that shows identity to the carboxyl terminal fragment of human serum albumin (HSA) (Pappa et al., 1999). More recently, we have demonstrated that human luteinized granulosa cells express the mRNA of HSA, thus supporting the initial characterization of GnSAF (Karligiotou et al., 2001
). Previous studies have shown that exogenous FSH stimulates the production of GnSAF in vivo both in the follicular and the luteal phases of the cycle (Messinis et al., 1996
, 1998
). The evidence from these studies is that in the luteal phase GnSAF is produced by small antral follicles rather than by the corpus luteum. From a physiological point of view, it is possible that the activity of GnSAF increases in the late luteal phase of the cycle as a result of the effect of the inter-cycle rise of FSH.
Although estradiol and progesterone do not seem to play an important role in GnRH-induced LH secretion in the early to mid-luteal phase, an alternative approach to the explanation of the present data, particularly regarding basal gonadotrophin secretion, could be through a regulating effect of estradiol on progesterone receptors in the pituitary (Sprangers et al., 1991). Such an effect might be mediated via estrogen receptor
, based on data in mice demonstrating the presence of high levels of receptor
mRNA and the absence of receptor ß mRNA in the pituitary (Couse et al., 1997
).
In conclusion, the present study provides evidence that in the early to mid-luteal phase of the cycle, estradiol and progesterone are important components of the suppressing effect of the ovaries on basal FSH and LH secretion. However, in terms of gonadotrophin response to GnRH, the study demonstrates for the first time that these two steroids participate in the control of FSH, but not of LH, secretion. It is possible that in the luteal phase the response of LH to GnRH is partly controlled by GnSAF.
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Acknowledgements |
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Notes |
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References |
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accepted on October 6, 2001.