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Original Studies |
Interuniversity Project on Reproductive Endocrinology in Women and Exercise, the Department of Applied and Experimental Reproductive Endocrinology (C.D.C.), The Institute for Gyneco-Endocrinological Research, B-3000 Leuven 3, Belgium; the Department of Biochemical and Clinical Endocrinology (P.B., B.S.), Medical University of Lübeck, D-23538 Lübeck, Germany; and the Department of Movement Sciences (G.V.K., P.G., H.A.K.), Faculty of Health Sciences, University of Maastricht, NL-6200 MD Maastricht, The Netherlands
Address all correspondence and requests for reprints to: C. De Crée, M.D., Department of Applied and Experimental Reproductive Endocrinology, The Institute for Gyneco-Endocrinological Research, P.O. Box 134, B-3000 LEUVEN 3 (Belgium).
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Abstract |
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Nine untrained eumenorrheic women (mean percentage of body fat ±SD: 24.8 ± 3.1%) volunteered for an intensive 5-day training program. Resting, submaximal, and maximal (tmax) exercise plasma CE, estrogen, and catecholamine responses were determined pre- and post training in both the follicular (FPh) and luteal phase (LPh).
Acute exercise stress increased total primary estrogens (E) but had little effect on total 2-hydroxyestrogens (2-OHE) and 2-hydroxyestrogen-monomethylethers (2-MeOE) (= O-methylated CE after competition for catechol-O-methyltransferase). This pattern was not significantly changed by training. However, posttraining LPh mean (±SE) plasma E, 2-OHE, and 2-MeOE concentrations were significantly lower (P < 0.05) at each exercise intensity (for 2-OHE: 332 ± 47 vs. 422 ± 57 pg/mL at tmax; for 2-MeOE: 317 ± 26 vs. 354 ± 34 pg/mL at tmax). Training produced opposite effects on 2-OHE:E ratios (an estimation of CE formation) during acute exercise in the FPh (reduction) and LPh (increase). The 2-MeOE:2-OHE ratio (an estimation of CE activity) showed significantly higher values at tmax in both menstrual phases after training (FPh: +11%; LPh: +23%; P < 0.05). After training, NE values were significantly higher (P < 0.05).
The major findings of this study were that: training lowers absolute concentrations of plasma estrogens and CE; the acute exercise challenge altered plasma estrogens but had little effect on CE; estimation of the formation and activity of CE suggests that formation and O-methylation of CE proportionately increases. These findings may be of importance for NE-mediated effects on gonadotropin release.
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Introduction |
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Therefore, it seemed that the original hypothesis, which had claimed that training increases 2-hydroxyestrogen levels, needed to be corrected. Apparently, training mainly increases the proportion of 2-hydroxyestrogen formation from ordinary E and the proportion of O-methylation. Thus, in euestrogenic women, vigorous training will induce a drop in estrogens but also in CE and methylated CE; but, the percent drop in estrogens will outweigh the drop in CE and monomethylethers, suggesting a proportional increase in CE metabolism. Apparently, it is only when the subject becomes hypoestrogenemic and low estrogen levels have stabilized, that not only the proportional formation and methylation of 2-hydroxy CE will increase, but also the absolute levels, as indeed observed by Russel et al.
The present study certainly does not attempt to resolve the debate between those who support or reject a biological role for CE. We also will not focus on any CE other than the 2-hydroxyestrogens. The purpose of this paper is limited to investigating whether, after a brief period of exhaustive training, plasma levels of 2-hydroxyestrogens in normal menstruating women respond any differently to acute exercise, when compared with pretraining conditions. Based on previous findings, our hypothesis is that in normal menstruating women, after training, the acute exercise will produce a proportionately higher formation and O-methylation of 2-hydroxyestrogens, which will not necessarily be accompanied by an increase in absolute plasma levels. It is also hypothesized that such increases will be most prominent during the luteal phase (LPh), because this is the phase when the female reproductive hormone system is most susceptible to exercise effects (1).
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Materials and Methods |
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Nine healthy eumenorrheic untrained women (mean age ± SD: 20.4 yr ± 1.3, range 18–22 yr; age of menarche: 13.2 ± 1.3 yr; cycle length: 28.9 ± 2.9 days) volunteered to participate in this study. Their mean (±SD) physical characteristics were: height: 172 ± 4.7 cm, body mass: 61.4 ± 4.8 kg, percentage of body fat (estimated according to 7 : 24.8 ± 3.1%. All subjects were nulliparous Caucasian women. Each individual gave her written consent, and procedures were carried out in accordance with the Declaration of Helsinki. Subjects then had to fill in a questionnaire to provide information concerning their training and gynecological history. It was a requirement that each individual have a regular ovulatory cycle occurring each 26–32 days, with a documented LH surge in the months immediately before and after the experiments. In addition, they were not allowed to take oral contraceptives and had to be free of any disorder that could interfere with hormone metabolism. Each subject kept a menstrual diary. Ovulation was ascertained by plasma progesterone analysis (data not shown). The LH surge was determined in fasting morning urine using Ovusticks (Samenwerkende Apothekers, The Netherlands). Nutrient and daily caloric intake did not differ among the subjects when menstrual phases or pre- and posttraining cycles were compared. The subjects acted as their own controls. The same subjects have participated previously in another trial (3, 4).
Experimental design
Each subject was exercise-tested four times: two standardized
pretraining incremental exercise tests [one in the follicular phase
(FPh) on a day between days 7–10 and one in the LPh, 10–12 days after
ovulation, i.e. on a day between days 23–25], and two
posttraining exercise tests, again in each menstrual phase and on the
same cycle days as the pretraining tests. After apparatus and gas
calibration, the subject was placed on an electrically braked cycle
ergometer (Lode b.v., Groningen, The Netherlands) and started cycling
at time zero (t0). A detailed description of the
incremental exercise test can be found elsewhere (5). Briefly, it
consisted of a 2-min warm-up period at 25 Watt (W), followed by 4 min
cycling at 50 W. Exercise intensity was increased by bouts of 50 W,
each 4-min interval, up to 150 W. After having cycled for 4 min at 150
W, exercise intensity was lowered for 2 min to 50 W. At this submaximal
level of approximately 60% maximal oxygen consumption
(
O2max) (plasma lactate concentration of
2.0–3.0 mmol/L), named tsubmax, blood samples were
collected. Exercise intensity was then raised again for 1 min at 150 W.
Each following minute, the exercise intensity was increased until total
exhaustion or l00% O2max, labeled
tmax. Exercise intensity was lowered to 50 W during
recuperation time. During the test, the subject breathed through a
breathing mask. The subject’s expired gas was continuously sampled
with a breath-by-breath method (Oxycon ß, Mijnhardt-Jäger
b.v., Bunnik, The Netherlands) to determine metabolic and ventilatory
variables. Recorded variables were: ventilation, oxygen consumption
(O2), and carbon dioxide expiration.
Training protocol
Subjects underwent standardized lab training on a cycle ergometer daily for 5 days immediately preceding the second FPh test and the second LPh test. Each day, training started with 6 min cycling at 100 W. Subsequently, intensity was increased every 2-min interval by 25 W, until the individual’s maximum of that day was reached. The purpose of this maximal training exercise was to establish the individual’s maximal physical working capacity (MPWC), to design her individualized interval training program. After a 10-min recovery period at 50% MPWC, the subject switched to interval training, starting with 2-min series at 90% of the individual’s MPWC, followed by 2-min recovery periods at 50%. When the individual failed to continue the 90% MPWC exercise bouts (i.e. when the number of revolutions decreased below 60 rpm), the intensity of the intervals was lowered by 10%. The same rationale was applied until the intensity of the intervals had to be reduced to 80% and 70%, respectively and the subject reached complete exhaustion, at which point training was stopped. This was usually close to 90 min of training. A more detailed description of the training schedule and standardization procedures can be found elsewhere (6). We selected this type of training because earlier studies from our laboratory had shown that this type of exercise was able to deplete muscle glycogen stores almost completely and to provoke a disturbance of normal gonadotropin pulsatility (8). These and other authors showed that it was the training intensity (even if short-term), rather than the duration of the exercise, that was responsible for interfering with normal hypothalamic gonadotropin release (8, 9).
Blood sample collection, preparation, and storage
All samples were obtained according to the pattern based on both the basal body temperature (BBT) and the Ovustick method on a day between days 7–10 of the FPh and on a day between days 23–25 of the LPh. Half an hour before each test, the subject received a venous indwelling 21-gauge Teflon Quick catheter (Travenol laboratories, Deerfield, IL) in the antecubital vein for blood collection for total protein and hormone determination. A three-way stopcock was attached to the indwelling cannula, and patency was assured by injecting 1 mL sterile saline at fixed intervals. Blood was drawn during rest (t0) and during tsubmax and tmax in disposable syringes (20 mL). Blood samples were collected in precooled lyophilized ethylenediamine tetra-acetate glass tubes stored in an ice bath at 2 C. Blood samples for assays were immediately centrifuged at 2000 x g for 10 min at 4 C.
Blood for determination of 2-hydroxyestrogens and their monomethylethers was prepared as follows: After centrifugation, as described above, 2 mL plasma were pipetted in 5-mL polyethylene Eppendorf cups, in which 1 mL of a 3% aqueous ascorbic acid solution had been added to prevent oxidative decomposition. The samples were then instantly frozen, using liquid nitrogen, and stored at -80 C until assaying, as described above. The plasma for the determination of all other plasma hormones was pipetted in 3-mL Eppendorf cups (without ascorbic acid) and immediately frozen and stored, as described above.
Hormone analyses
Because the free or unconjugated plasma concentrations of CE are
often close to the minimal detectable concentration of normal assaying
procedures, for the sake of the accuracy and validity, we determined
the unconjugated and conjugated fractions. This was done in a
single-assay procedure, with the conjugated CE being measured after hot
acid hydrolysis. The various steps of this relatively complex RIA have
been described previously in detail (5). The results for the CE are
given as total 2-hydroxyestrogens (2-OHE), which consist
of the sum of unconjugated 2-hydroxyestrone (2-OHE1) and
2-hydroxyestradiol (2-OHE2), without 16
- or any other CE
metabolites. Similarly, the values for
2-hydroxyestrogen-monomethylethers (2-MeOE), as mentioned in this
study, indicate the sum of the total conjugated and unconjugated
fractions of both 2-hydroxyestrone-monomethylethers or methoxyestrone
(2-MeOE1) and 2-hydroxyestradiol-monomethylethers or
methoxyestradiol (2-MeOE2). Despite the fact that most
literature on exercise and estrogens has only measured unconjugated
plasma estradiol (E2), we felt we had to determine primary
estrogens as total estrogens (E), if we wanted to derive any
information on the formation of total CE. Thus, E represent the sum of
conjugated + unconjugated estrone (E1) and E2.
Similarly to the CE, E did not contain any 16- or any other estrogen
metabolites. E was determined in one specific RIA. The intraassay
variability for all steroid and nonsteroid hormone analyses was below
10%. The interassay variability was less than 17.2% for 2-OHE and
2-MeOE, and 13.3% for E. The minimal detectable concentration was
approximately 6 pg/mL for all CE assays.
Because CE competitively bind to COMT, the enzyme that decomposes CA,
it was decided to measure NE, epinephrine (EPI), and dopamine (DA). All
CA were assessed by electrochemical determination, after separation,
using HPLC. The procedure and the equipment used have been previously
described in detail (5). The respective interassay coefficients of
variation were 4.5% for NE, 11.3% for EPI, and 11.8% for DA. Plasma
LH concentrations were determined using an immunoradiometric assay
purchased from Serono (Geneva, Switzerland). The cross-reactivity with
hCG, FSH, and TSH was 
1.6, and the sensitivity was 0.4 mIU/mL.
Plasma progesterone (P4) was determined after ether
extraction, using a commercially available kit from Radio-Isotopen
Service (Würlingen, Switzerland; inter-assay coefficient of
variation: 7.2%; minimal detectable concentration: 0.13 ng/mL).
Hematocrit was measured by the microcentrifuge method and total protein
by the Biuret method (data not shown).
Statistical analysis
Anthropometric data, before and after training, were compared using paired t tests. To obtain a rough estimation of CE formation, we calculated the ratio of the end product to its precursor, i.e. the 2-OHE:E ratio. Furthermore, we computed the ratio of O-methylated CE to nonmethylated CE (2-MeOE:2-OHE), as a measure of CE activity. Hormonal data were adjusted for changes in plasma volume during acute exercise (10). Comparisons of hormonal responses, between different menstrual phases, were made by a two-way mixed-model ANOVA for groups with repeated measures. The level of significance for individual contrasts within each of the subjects was adjusted to limit experimental error rate to a maximum of 0.05. The ratios of the steroid responses were evaluated by analyzing the magnitude of absolute and relative changes during submaximal and maximal exercise intensity, compared with resting values. Phase, group, phase group (interactive), and subject effects were evaluated independently by maximum likelihood procedures. Correlations were evaluated by ANOVA. Accounting for individual variability across phases did not alter the correlation. Descriptive data are presented as means ±SD. Comparisons between group means are given as means ± SE.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Menstrual cycles in all subjects were ovulatory both before and after training, although after training the plasma levels of LH and P4 were lower (data not shown). Menstrual cycle length was not significantly different after training. After having participated in an intensive 5-day training program, there was a trend for mean MPWC of the subjects to be higher (from 241 ± 40 W to 250 ± 23 W in the FPh, and from 250 ± 31 W to 258 ± 25 W in the LPh). Pretraining VO2max was lower (P < 0.05) in the LPh, compared with the FPh (41.5 ± 4.1 vs. 45.7 ± 4.5 mL•kg-1•min-1, respectively). After training, the values were 42.5 ± 4.1 (FPh) and 43.5 ± 3.4 mL•kg-1•min-1 (LPh).
Hormonal responses to exercise and training
Hormonal responses to exercise are expressed both as absolute
values (Fig. 1
) and as percent changes
from pretraining resting values (Table 1). The posttraining plasma levels of E
and 2-OHE were significantly reduced (P < 0.05) at all
exercise intensities. These reductions were most pronounced during the
LPh. Primary estrogen levels at maximal exercise intensity showed a
percent increase from resting levels, which was significant during each
of the posttraining trials (P < 0.05). Percent changes
in plasma CE during acute exercise were not significant. The magnitude
of the overall changes in plasma concentrations of E and 2-OHE during
acute exercise was not significantly altered by training in either of
the cycle phases measured. In spite of posttraining plasma
concentrations of E (tsubmax: -56%, tmax:
-45%) and 2-OHE (tsubmax: -24%, tmax:
-21%), which were significantly reduced (P < 0.05)
during the LPh, the O-methylated plasma CE concentrations
only decreased very modestly (tsubmax: -8%,
tmax: -7%).
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Plasma NE and EPI levels progressively rose in response to incremental
exercise (Table 2). Increases were
significant both at submaximal (P < 0.05) and maximal
(P < 0.001) exercise intensity in either phase. Acute
exercise-induced responses in plasma NE were higher after training (2,
811 ± 680 vs. 1, 411 ± 289 pg/mL for the FPh).
We were unable to identify a consistent picture for DA levels, although
after training, there was a trend for plasma levels to increase
proportionally to the exercise intensity, but only in the FPh.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Before training, the O2max was
significantly lower during the LPh, when compared with the FPh. This is
in line with many other studies (11), although some authors have
reported no cycle phase effects (12, 13, 14, 15), or higher
O2max values, in the LPh (16). Lower
O2max values in the LPh have generally been
attributed to the higher progesterone levels, which are postulated to
relatively increase ventilation for a given exercise intensity. After
training, this cycle phase difference in VO2max values
disappeared.
Training effects on primary E responses to acute exercise
Interindividual differences for E concentrations are much higher than for the unconjugated E2 and E1 fractions. Similar to previously documented E2 responses (12, 16, 17, 18), plasma E levels increase with incremental exercise, and this in both menstrual phases, before and after training. Increasing plasma estrogen concentrations during incremental exercise are assumed to be caused by an approximately halved hepatic MCR and splanchnic flow (19). Most studies that have measured estrogens during exercise after physical training have also found that the percent increases in the concentration of female sex hormones during a certain absolute exercise intensity, will be lower than before training (16, 20). This finding is confirmed by the present study, as far as exercise during the LPh is concerned.
Training effects on CE levels during acute exercise
No reports on the effects of training on 2-hydroxy CE during acute exercise have been published before. Our results show that acute exercise stress exerts little effect on plasma 2-OHE and 2-MeOE levels. This was not any different after a period of brief, exhaustive training. Plasma concentrations of both 2-OHE and 2-MeOE were reduced at all intensities after training, but reductions in 2-MeOE (-10%) were smaller than for 2-OHE (-25%). Although, this finding can be interpreted as a decrease in both levels and O-methylation of CE, possibly it could also mean that the proportion of CE that is O-methylated, increased by approximately 15%.
Training effects on CE formation and activity during acute exercise
The ratio of the end product divided by its precursor gives an estimation of the proportion formed, as suggested previously (5). Acute exercise stress had little effect on 2-OHE:E and 2-MeOE:2-OHE ratios before or after training. However, the O-methylation of CE apparently increases in response to training, particularly in the LPh, as suggested by both absolute and percent increases in 2-MeOE:2-OHE ratios. It must be admitted, though, that the use of these ratios to estimate formation and methylation is not without problems. For example, we do not have any firm proof that changes in these ratios are not a nonspecific effect of changes in estrogen levels. We also do not know whether the hydroxylation and methylation reactions are not relatively saturated at a higher substrate concentration, in which case, everything that lowers the substrate concentration would increase the ratio of product to substrate. This is a substantial problem, and introducing radiolabeled compounds might offer additional information on in vivo CE formation and methylation.
Hypothesis 1: effects of training on CE formation
The hypothesis that training facilitates C-2 hydroxylation, largely depends on the results of a single study. Russel and co-authors (2) determined resting levels of plasma (unconjugated) 2-OHE in well-trained oligomenorrheic swimmers. Although, their results must be interpreted with caution (blood treatment procedures, no use of ascorbic acid to prevent oxidative decomposition of CE, large variations in age of the samples, etc.), these authors found significantly increased resting plasma concentrations of 2-OHE in response to (a much longer period of) intense physical training. Our results apparently are difficult to reconcile with their findings, because we found reduced levels of 2-OHE. However, do reduced plasma levels necessarily mean that formation of the product is impaired? Not if, for example, there is either a concomitant increase in MCR, or less available precursor. To help us decide if the results from both studies are indeed irreconcilable, the actual CE formation and O-methylation are crucial. From a close examination of our results, we learn that the magnitude of the posttraining decreases in plasma 2-OHE or 2-MeOE was generally smaller than that of their precursor. This might mean that, in spite of lower posttraining plasma levels, there is still a proportionate increase in both formation and methylation of CE.
What about the previous studies? Although no other authors have reported on formation and activity of 2-hydroxy CE in association with exercise, the second study by Russel et al. (4) provided enough data to allow calculation of the 2-OHE:E ratios. In controls, the ratio was 0.13 ± 0.03; in swimmers training 55 km/week, 0.77 ± 0.30; and in swimmers doing 90 km/week, 1.20 ± 0.21. So, clearly the formation of 2-hydroxy CE seems to increase in response to training. This led us to make the following suggestion for a modification of the initial hypothesis: Based on our own and previous findings, we speculate that, perhaps, training indeed induces an increase in C-2 hydroxylation. But the absolute concentrations of CE possibly may only increase in the presence of a substantially low level of precursor (estrogen), as was the case in the hypoestrogenemic subjects included in the study by Russel et al. (4). We speculate that, as long as the subject is euestrogenemic, a higher formation of CE may not be accompanied by increases in absolute levels of CE. If this indeed is true, then the results of both studies are perfectly reconcilable.
Training effects on CA responses to acute exercise
There is a consensus that sympathoadrenal activity, as indicated
by NE and EPI levels, proportionately increases with the intensity of
the exercise (16, 21, 22, 23). Increases in NE during exercise have been
found to be closely associated to oxygen availability; the higher the
oxygen saturation, the lower plasma NE concentrations (22). It is
suspected that changes in secretion and metabolism of reproductive
hormones, as well as substrate use, are modulated also by the
sympathoadrenal system. It has been demonstrated that training rapidly
reduces acute NE responses to a given absolute exercise intensity [in
men (22, 23), although similarly detailed studies are not available in
women]. Nevertheless, a higher O2max and
maximal exercise intensity, as a result of training, still may be
accompanied by a higher circulating NE level in the end. This is
suggested also by our results.
Hypothesis 2: effects of training on CE formation & menstruation
Although still poorly understood, CE may play a physiological role in the control of the menstrual cycle through various mechanisms (for a review, see 3 . Unfortunately, the activities of CE seem very complex and literature findings inconsistent. A dozen studies, mostly dating back to the 1980’s, suggested that CE mediate the release of LH (3, 24, 25). Recent investigations have revealed that such intracerebral effects of CE seem to involve NE. Reznikov and Rozenko (26) were able to demonstrate that peripheral administration of CE in rats leads to a significant increase in intrahypothalamic NE content. Several authors have shown that LH secretion is directly mediated by NE (27, 28, 29). These findings strengthen the speculations of a physiological role for CE in the control of the menstrual cycle.
As known, the inactivation of CE and CA is catalyzed by the same enzyme, COMT. However, the affinity of COMT for CE is higher than for NE. We postulate that in both cases, eu- and hypoestrogenemic exercising women, there is a proportionately higher O-methylation of CE, which may lead to an increase in NE concentrations and possibly interfere with NE-mediated processes, such as gonadotropin release. Certainly, this hypothesis remains susceptible to criticism. First, there is abundant evidence in favor of the classical theory, assuming that exercise-induced hypoestrogenemia is a consequence of gonadotropin dysfunction (30) and not vice versa. However, it also must be avowed that scientists have mainly investigated the classical pathway, rather than the reverse mechanism. Second, a series of studies by Merriam and collaborators (31, 32, 33) addressed two extremely important points that are relevant to the central hypothesis of the current paper. These authors argued that peripheral concentrations of CE are too low, and their MCR too high, to exert their effects centrally to affect gonadotropin secretion (31). However, one cannot rule out the possibility that during exercise, there may be processes going on in the brain that are parallel to those that we have observed in the periphery. Furthermore, there is evidence that administration of CE, results in a specific decrease in LH in both men (32) and women (33). These studies support a possible role for CE in the mediation of gonadotropin release, irrespective of whether CE function as estrogens in a negative-feedback mode or via noradrenergic modulation of GnRH, as suggested in our hypothesis. Finally, because of the limitations of peripheral measurements for making inferences about intracerebral processes, we advocate additional investigations, using infusions or antagonists, to further elucidate the role of CE in the etiology of exercise-related hypoestrogenemia.
Conclusion
In conclusion, our findings suggest that in previously untrained eumenorrheic women, training leads to decreased plasma levels of estrogens, 2-OHE, and 2-MeOE. This reduction is most pronounced during the LPh. It, however, is not certain that, despite these lower plasma levels, there is also a reduced formation and activity of CE. As far as can be derived from the end product to precursor ratio, we believe that training, irrespective of plasma levels, increases the proportion of CE formed, as well as their activity (O-methylation). These findings may be of significance for NE-mediated effects on gonadotropin release.
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Acknowledgments |
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Footnotes |
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Received October 22, 1996.
Revised March 25, 1997.
Revised June 18, 1997.
Accepted June 27, 1997.
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