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1 The John B. Pierce Laboratory, Departments of 2 Epidemiology and Public Health, and 3 Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06519; and 4 Women and Infants Hospital, Brown University School of Medicine, Providence, Rhode Island 02905
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We tested the physiological reliability of
plasma renin activity (PRA) and plasma concentrations of arginine
vasopressin
(P[AVP]), aldosterone
(P[ALD]), and atrial
natriuretic peptide (P[ANP]) in the
early follicular phase and midluteal phases over the course of two
menstrual cycles (n = 9 women, ages 25 ± 1 yr). The reliability (Cronbach's 
0.80) of
these hormones within a given phase of the cycle was tested
1) at rest,
2) after 2.5 h of dehydrating
exercise, and 3) during a
rehydration period. The mean hormone concentrations were similar within
both the early follicular and midluteal phase tests; and the mean
concentrations of
P[ALD] and PRA for the
three test conditions were significantly greater during the midluteal
compared with the early follicular phase. Although Cronbach's for
resting and recovery P[ANP] were high (0.80 and 0.87, respectively), the resting and rehydration values for
P[AVP],
P[ALD], and PRA were
variable between trials for the follicular ( from 0.49 to 0.55) and
the luteal phase ( from 0.25 to 0.66). Physiological reliability was
better after dehydration for
P[AVP] and PRA but
remained low for
P[ALD]. Although
resting and recovery
P[AVP],
P[ALD], and PRA were not consistent within a given menstrual phase, the differences in the
concentrations of these hormones between the different menstrual phases
far exceeded the variability within the phases, indicating that the low
within-phase reliability does not prevent the detection of menstrual
phase-related differences in these hormonal variables.
aldosterone; renin; atrial natriuretic peptide; arginine vasopressin; estrogen; progesterone
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE BODY'S WATER- AND SODIUM-regulating hormones vary considerably over the course of the menstrual cycle (7-9, 14, 15, 18, 19). For example, during the midluteal phase of the menstrual cycle, plasma aldosterone concentration (P[ALD]) and plasma renin activity (PRA) are greater at rest (9) and during exercise (14, 15) than in the follicular phase. In addition, resting plasma arginine vasopressin concentration (P[AVP]) is higher (8) during the preovulatory and midluteal phases of the cycle when plasma estrogen concentration (P[E2]) is high. In lower animals, estrogen administration increases osmotic stimulation of AVP (1, 4, 5) and water retention (3), and both estrogen and progesterone exhibit important effects on sodium regulation and the sodium-regulation hormones (10-12, 21); this supports the hypothesis that the gonadal steroids have important modulatory effects on body fluid and electrolyte balance.
No studies exist that examine the physiological reliability of the fluid-regulating hormones within a given phase and over the course of two or more menstrual cycles. Reported plasma concentrations of these hormones across different menstrual cycles differ due to natural physiological variations, due to selection of an inappropriate day to conduct physiological testing, due to variations in water and/or sodium intake, or due to inaccurate hormone-analysis techniques. The purpose of this study was to eliminate variability caused by the latter three reasons to determine the natural physiological variability of the responses of fluid- and sodium-regulating hormones over two menstrual cycles. Accordingly, we tested women twice during the early follicular phase (when estrogen and progesterone are low) and twice during the midluteal phase of the menstrual cycle (when estrogen and progesterone are high).
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Study Design
Subjects were nine healthy, nonsmoking women (age, 25 ± 1 yr; range, 22-31 yr). To drive the fluid-regulation system, each woman participated in a series of dehydration experiments in which the study hormones were measured 1) at rest, 2) during dehydration, and 3) during rehydration in both the early follicular and the midluteal menstrual phases. The study design employed four dehydration experiments: two conducted in the early follicular phase (2-4 days after the beginning of menstrual bleeding) and two in the midluteal phase of the menstrual cycle (conducted 7-10 days after the luteinizing hormone peak), as determined individually by the use of ovulation-prediction kits (OvuQuick; Quidel, San Diego, CA). The tests were conducted during nonconsecutive menstrual phases, 12-16 wk apart. To verify the phase of the menstrual cycle, plasma levels of 17
-estradiol and
progesterone were assessed from a basal blood sample.
Dehydration Experiments
On the day of the dehydration test, volunteers arrived at the laboratory between 7:00 and 8:00 AM, after they had eaten only a prescribed low-fat breakfast (~300 kcal). The subjects refrained from alcohol and caffeine for 12 h before the experiment. Subjects were asked to drink 7 ml/kg body weight of tap water at home before arrival at the laboratory. On arrival at the laboratory, the subjects gave a baseline urine sample; they were weighed and then sat on the contour chair of a cycle ergometer in the test chamber [27°C, 30% relative humidity (RH)] for 60 min of control rest. During the control period, an indwelling catheter was placed in an arm vein. At the end of the control period, a 20-ml blood sample was drawn and urine was collected. Consistency of the pretest hydration state was assessed from the specific gravity of the basal urine sample (mean = 1.001), which did not differ across trials.After the control period, the chamber temperature was increased to 36°C. The subjects then exercised at 50% maximal power output for 150 min, with 5-min rest periods every 25 min, during which time they were deprived of fluids. Blood samples (10-20 ml) were drawn and body weight was assessed at 60, 120, and 150 min during exercise. At the end of exercise, the chamber temperature was reduced to 27°C. After dehydration, subjects rested for 30 min in a contour chair, without access to fluids; after 30 min, they drank water ad libitum for 180 min. Blood samples (20 ml) were taken just before drinking (time 0) and at 30, 60, 120, and 180 min of rehydration. Urine was collected at the end of exercise and at hourly intervals during rehydration, and the urine samples were analyzed for volume and sodium excretion.
Blood samples. Subjects were semirecumbent during placement of the catheter (21 gauge) and were seated for 60 min before samples were taken to ensure a steady state in plasma volume and constituents. Free-flowing venous blood was obtained for the measurement of hematocrit (Hct), plasma osmolality (POsm), PRA, P[AVP], P[ALD], P[E2], and plasma concentrations of atrial natriuretic peptide (P[ANP]) and progesterone (P[P4]). An aliquot (0.5 ml) was removed for immediate assessment of Hct in triplicate by microhematocrit. Second and third aliquots were transferred to a heparinized tube and a tube without additive, and all other aliquots were placed in tubes that contained EDTA. The tubes were centrifuged, and the plasma taken off the heparinized sample was analyzed for aldosterone. P[E2] and P[P4] were measured by using serum from the tube without additive. The EDTA samples were analyzed for P[AVP], P[ANP], and PRA. All blood samples were analyzed for Hct, POsm, P[ALD], P[AVP], P[ANP], and PRA; only the basal blood samples were also analyzed for P[E2] and P[P4].
Blood Analysis
POsm was measured by freezing-point depression (Advanced Instruments 3DII); P[ALD], P[AVP], P[ANP], P[E2], and P[P4] were measured by radioimmunoassay. Intra- and interassay coefficients of variation for the midrange standards were, respectively, as follows: P[AVP] (4.52 pg/ml), 6.0 and 3.4% [Immuno Biological Laboratories (IBL), Hamburg, Germany]; PRA (4.5 ng · ml
1
ANG · h1),
2.3 and 2.9% (Diasorin, Stillwater, MN);
P[ALD] (132 pg/ml),
3.4 and 3.6% (Diagnostic Products, Los Angeles, CA);
P[ANP] (63.3 pg/ml),
5.1 and 5.2% (Diasorin);
P[E2]
(64.3 pg/ml), 3.7 and 4.0% (Diagnostic Products); and
P[P4]
(3.7 pg/ml), 2.1 and 2.5% (Diagnostic Products). The assay for AVP has
a sensitivity of 0.8 pg/ml; this sensitivity is necessary to detect
small, but important, changes in this hormone.
Statistical Analysis
Pearson's product-moment correlation on individual data was used to assess the slope and abscissal intercepts of the P[AVP]-POsm relationship during dehydration (6). The within-phase reliability of our most important dependent variables (fluid-regulating hormones and osmotic regulation of AVP, as measured at rest, dehydration, and rehydration) was determined with Cronbach's, assuming a value
0.80 as an acceptable level of reliability (2). Areas under the curve
(AUC; trapezoid method) were calculated during the rehydration period
(starting 30 min postexercise) for PRA, P[ALD], and
P[ANP], and their
reliability was determined within a given menstrual cycle by using
Cronbach's . We used repeated measures ANOVA models, followed by
Bonferroni's t-test to test
differences in the dependent variables both within and between
menstrual phases. Data were analyzed by using BMDP statistical software
(BMDP Statistical Software, Los Angeles, CA) and were expressed as
means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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All subjects were tested during the first 5 days (4 ± 1 days) after the start of menstrual bleeding for early follicular-phase tests, and between 20 and 25 days (22 ± 2 days) for the midluteal-phase tests. Specifically, the subjects were tested between days 7 and 10 after the LH peak, and, therefore, ~6-9 days after ovulation.
Between-Phase Measurements
At rest, Hct, P[E2], P[P4], P[ALD], and PRA were higher, and POsm and P[ANP] were lower, in the luteal phase compared with the follicular phase (P < 0.05); however, there were no differences in body weight or P[AVP] (Tables 1 and 2). During dehydrating exercise, body water loss (1.5 ± 0.2 kg, or 2.3% of preexercise body weight) was comparable between the follicular and midluteal phases. Similarly, despite the baseline variability, P[AVP] and PRA responses to exercise (i.e., change from baseline) were similar between the two phases (Table 2). However, this was not the case for P[ALD], in which the exercise response was greater during the midluteal phase. Linear regression analysis of the individual subjects' data during dehydration indicated significant correlations between P[AVP] and POsm, with r values ranging from 0.82 to 0.98. The abscissal intercept of the linear P[AVP]-POsm relationship, or "theoretical osmotic threshold" for AVP release, was lower in the midluteal phase (278 ± 1 and 279 ± 1 mosmol/kgH2O; Table 1 and Fig. 1) compared with the follicular phase (282 ± 1 and 283 ± 1 mosmol/kgH2O; P < 0.05). The slopes of this relationship were unaffected by menstrual phase. During rehydration, the AUCs for P[ALD] and PRA were significantly greater in the luteal compared with the follicular phase.
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Within-Phase Measurements
Early follicular phase.
Within the follicular phase, there were no significant differences
among the means of any of the variables during rest, dehydration, and
rehydration. However, with the exception of
P[ANP], none of the
resting values of the fluid-regulating hormones attained sufficiently
high Cronbach's to be considered reliable (Table 3). Reliability for
P[AVP] and PRA was
better after dehydrating exercise, although reliability remained low
for P[ALD] ( = 0.66) and remained high for
P[ANP] ( = 0.90). During dehydration, both the slope and abscissal intercept of the
POsm-P[AVP]
relationship were highly reliable within the follicular phase,
attaining Cronbach's of 0.96 and 0.90, respectively. Again,
P[AVP],
P[ALD], and PRA were
not reliably reproduced during rehydration, whereas Cronbach's for
P[ANP] was 0.93. P[E2]
was highly reproducible within the follicular-phase tests, attaining
Cronbach's of 0.85, but
P[P4]
attained a Cronbach's value of only 0.62 between tests in the
follicular phase.
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Midluteal phase.
As in the follicular phase, there were no differences in mean hormonal
concentrations at rest, after dehydration, or during rehydration within
the midluteal phase. Again, resting values for
P[AVP],
P[ALD], and PRA were
not highly reproducible between the two midluteal phase tests (Table
3). Reliability for
P[ANP] was greater,
compared with the other fluid-regulating hormones, at rest and during
exercise and rehydration. Despite high levels of reliability for
osmotic regulation of AVP (Table 3), resting and
rehydration levels of
P[AVP] were not
consistently correlated within the luteal-phase tests. In contrast to
the follicular phase, however, both
P[E2] and
P[P4]
were highly consistent between the two luteal-phase tests, yielding
Cronbach's values of 0.93 and 0.93, respectively.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We examined the within-phase physiological reliability of the fluid- and sodium-regulating hormone concentrations in the plasma over two nonconsecutive menstrual cycles (12-16 wk apart) during the early follicular and midluteal phases. P[AVP], P[ALD], and PRA varied within each of the different menstrual phases; however, there were no statistical differences among the means of any of these hormone concentrations. This indicates that the within-subject variability remains undetected when only the means are tested or reported. Nonetheless, our data indicate that between-phase differences in the hormone concentrations far exceed the variability within the phases, and, therefore, the low within-phase reliability does not prevent the detection of menstrual-phase-related changes in these variables. In contrast, P[AVP], PRA, and P[ANP] responses to dehydration were highly reliable within each menstrual phase; this indicates that hormonal responses to stress are more consistent, despite the variability in baseline values.
Although there were no significant within-phase differences between the means of the sodium-regulating hormones, only P[ANP] values were consistently reliable during rest, exercise, and rehydration within either of the two phases. Resting P[AVP], P[ALD], and PRA were quite variable across the two trials within both the follicular and luteal menstrual phases. Indeed, this baseline variability exists even with careful control of predehydration water and sodium intake, posture, and timing of the experiments to coincide with specific events during the menstrual cycle (such as ovulation and menses). Resting or basal variations in P[AVP] may be exaggerated further by the fact that values were close to the lowest level of sensitivity of our assay technique (i.e., 0.8 pg/ml). Also, because the rehydration was ad libitum, hydration-recovery rates may have been different among the test days. Therefore, although total fluid intake was similar over the four tests, changes in drinking patterns or drinking rates may substantially affect AVP release at a given blood sampling point (17) and, consequently, affect our ability to observe repeatable P[AVP].
In any case, despite the low within-phase reliability of P[AVP] at rest and during rehydration, osmotic regulation of P[AVP] (i.e., slopes and intercepts) during dehydration was highly reproducible. This indicates that, although individual values may vary, the regulation of this hormone in response to environmental stress (e.g., exercise) remains constant. This is an important finding, because small shifts in the regulation of AVP lead to large changes in renal water retention (13). Moreover, although a number of studies have demonstrated changes in osmotic regulation of P[AVP] over the course of a single menstrual cycle (18, 19), the menstrual-phase effects on the POsm threshold for AVP release are only ~5-6 mosmol/kgH2O, making essential a precise and consistent measurement of the POsm intercept within a given menstrual phase.
Interestingly, the shifts in osmotic regulation of AVP and the fluid regulation hormones over the course of the menstrual cycle do not seem to impact overall body fluid and sodium retention. Despite the shift in osmotic AVP regulation, fluid loss during exercise was similar in both menstrual phases. In the luteal phase, a progesterone-induced inhibition of aldosterone-dependent sodium reabsorption at distal sites in the nephron causes transient natriuresis (12). This natriuresis is followed by a compensatory stimulation of the renin-aldosterone system (9, 16, 20), resulting in a slight attenuation of sodium excretion during the luteal phase (7.2 ± 1.4 vs. 11.5 ± 2.0 meq). Nonetheless, overall water and sodium balance appear unaffected by the shifts in either progesterone or the sodium regulation hormones (9). This leads us to speculate that estrogen and progesterone have their primary impact on body water regulation through changes in body water and sodium distribution rather than through retention.
We also tested the reliability of the female sex hormones
17-estradiol and progesterone.
P[E2] was highly reproducible between the two trials in both the follicular and midluteal phases. P[P4], although reproducible during the
luteal phase, was somewhat variable between the two trials in the
follicular phase.
P[P4]
is normally low during the follicular phase of the menstrual cycle, so
even small variations lead to large error values and may thus
exaggerate the variability of
P[P4] during the follicular phase. Nonetheless, despite the low reliability, P[P4]
values were consistent and low enough to indicate the subjects were in
the follicular phase of the menstrual cycle.
The variability in the fluid-regulating hormones was not substantial enough either to create significant statistical differences in means between trials within the same menstrual phase or to obscure the large differences in these hormone concentrations between menstrual phases. Nonetheless, these findings suggest that there is a natural variability in these hormone responses, which may be undetected when only grouped mean values are presented.
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ACKNOWLEDGEMENTS |
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We gratefully acknowledge the technical support of Tamara S. Morocco, John R. Stofan, and Richard Wemple, and the cooperation of the volunteer subjects. We also thank Lou A. Stephenson for contributions to the research design.
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FOOTNOTES |
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This work is supported by the US Army Medical Research and Materiel Command under contract No. DAMD17-96-C-6093. The views, opinions, and/or findings contained in this report are those of the authors and should not be construed as an official Department of the Army position, policy, or decision unless so designated by other documentation.
In conduct of research where humans are the subjects, the investigators adhered to the policies regarding the protection of human subjects as prescribed by 45 CFR 46 and 32 CFR 219 (Protection of Human Subjects).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: N. S. Stachenfeld, The John B. Pierce Laboratory, 290 Congress Ave., New Haven, CT 06519 (E-mail: nstach{at}jbpierce.org).
Received 18 September 1998; accepted in final form 23 November 1998.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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