ORIGINAL
E n d o c r i n e
ARTICLE
R e s e a r c h
Basal, Pulsatile, Entropic (Patterned), and Spiky
(Staccato-like) Properties of ACTH Secretion: Impact
of Age, Gender, and Body Mass Index
Johannes D. Veldhuis, Ferdinand Roelfsema, Ali Iranmanesh, Bernard J. Carroll,
Daniel M. Keenan, and Steven M. Pincus
Endocrine Research Unit (J.D.V.), Mayo Medical School, Mayo School of Graduate Medical Education,
Center for Translational Science Activities, Mayo Clinic, Rochester, Minnesota 55905; Department of
Endocrinology and Metabolic Diseases (F.R.), Leiden University Medical Center, 2333 ZA Leiden, The
Netherlands; Endocrine Section (A.I.), Medical Service, Salem Veterans Affairs Medical Center, Salem,
Virginia 24153; Pacific Behavioral Research Foundation (B.J.C.), Carmel, California 93922; Department
of Statistics (D.M.K.), University of Virginia, Charlottesville, Virginia 22904; and Independent
Mathematician (S.M.P.), Guilford, Connecticut 06437
Background: Age, gender, and BMI determine ultradian modes of LH and GH secretion, viz., pulsatile, basal, pattern-defined regularity [approximate entropy (ApEn)] and spikiness (sharp, brief
excursions). Whether the same determinants apply to ACTH secretion is not known.
Setting: The study was conducted at a tertiary medical center.
Subjects: We studied normal women (n ⫽ 22) and men (n ⫽ 26) [ages, 23–77 yr; body mass index
(BMI), 21–32 kg/m2].
Methods: Volunteers underwent 10-min blood sampling to create 24-h ACTH concentration
profiles.
Outcomes: Dynamic measures of ACTH secretion were studied.
Results: Mean ACTH concentrations (R2 ⫽ 0.15; P ⫽ 0.006) and both pulsatile (R2 ⫽ 0.12; P ⫽ 0.018) and
basal (nonpulsatile) (R2 ⫽ 0.16; P ⫽ 0.005) ACTH secretion correlated directly with BMI (n ⫽ 48). Men
had greater basal (P ⫽ 0.047), pulsatile (P ⫽ 0.031), and total (P ⫽ 0.010) 24-h ACTH secretion than
women, including when total secretion was normalized for BMI (P ⫽ 0.019). In men, both ACTH-cortisol
feedforward and cortisol-ACTH feedback asynchrony (cross-ApEn) increased with age (R2 ⫽ 0.20 and
0.22; P ⫽ 0.021 and 0.018). ACTH spikiness rose with age (P ⫽ 0.046), principally in women. Irregularity
of cortisol secretion (ApEn) increased with age (n ⫽ 48; P ⫽ 0.010), especially in men. In both sexes,
percentage pulsatile ACTH secretion predicted 24-h mean cortisol concentrations (R2 ⫽ 0.14; P ⫽ 0.009).
Conclusion: Valid comparisons of ultradian ACTH dynamics will require cohorts matched for age,
gender, and BMI, conditions hitherto not satisfied in most physiological studies of this axis. (J Clin
Endocrinol Metab 94: 4045– 4052, 2009)
A
CTH secretion comprises an admixture of pulsatile
(burst-like) and basal (nonpulsatile) hormone release
(1, 2). Ultradian pulse patterns of ACTH secretion are superimposed upon or driven by underlying circadian rhythms
(3, 4). Ultradian and circadian rhythmicity of cortisol has
been appraised thoroughly in adults (5–7). However, little is
known about the impact of age, gender, and relative adiposity on basal and pulsatile ACTH secretion.
ISSN Print 0021-972X ISSN Online 1945-7197
Printed in U.S.A.
Copyright © 2009 by The Endocrine Society
doi: 10.1210/jc.2009-1143 Received May 28, 2009. Accepted July 17, 2009.
First Published Online September 15, 2009
Abbreviations: ApEn, Approximate entropy; BMI, body mass index; CVs, coefficients of
variation.
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
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Veldhuis et al.
Regulated Pulsatile ACTH Secretion
The relatively high frequency of ACTH pulses (12–30
events per day) and short half-life of plasma ACTH (variously 14 –35 min) have made pulsatility studies more difficult by necessitating intensive blood sampling to resolve
secretory bursts accurately (8 –10). Earlier studies of
ACTH secretion often used insufficiently frequent sampling, such as blood withdrawal every 20, 30, 60, 120, or
even 240 min (5, 11–13). On empirical and theoretical
grounds, insufficiently frequent measurements may severely bias analytical estimates of pulsatile hormone secretion (10). Accurate quantification of ACTH pulsatility
is important to provide a window into the putative brain
CRH pulse-generating system (14), the nature of stimulatory ACTH signals delivered to the adrenal cortex (15),
and the effects of inhibitory cortisol signals on the hypothalamo-pituitary unit (16).
ACTH secretion is characterized by not only pulsatility,
but also pattern regularity (a barometer of feedback control) and spikiness (sharp brief fluctuations) (17). Pulsatile
ACTH secretion is quantifiable by deconvolution methods (10, 18), whereas pattern orderliness is evaluable by
the approximate entropy (ApEn) statistic (17, 19). Spikiness is a measure of the extent of delimited abrupt excursions in data values, such as occur in serial mood ratings
or tumoral hormone secretion profiles (20). Recent studies
have demonstrated that validated deconvolution algorithms and normalized ApEn achieve at least 90% sensitivity and 90% specificity in discriminating altered from
normal hormone secretion using 10-min sampling over
24 h (10, 19).
Despite significant advances in analytical tools to quantify regulated hormone secretion patterns (10), how major
endocrine covariates, such as age, gender, and body mass
index (BMI), influence the dynamics of ACTH secretion
remains unknown. The present study addresses this fundamental issue.
Subjects and Methods
Subjects
Healthy, unmedicated volunteers participated in the study
(26 men and 22 women). Each subject provided written informed consent, and the protocol was approved by the Institutional Review Board. The age range was 18 –77 yr. Participants
maintained conventional work and sleeping patterns and reported no recent (within 10 d) transmeridian travel, weight
change (⬎2 kg in 6 wk), shift work, psychosocial stress, prescription medication use, substance abuse, neuropsychiatric illness, or acute or chronic systemic disease. A complete medical
history, physical examination, and screening tests were normal.
No subject had been exposed to glucocorticoids within the preceding 3 months.
Volunteers were admitted to the Study Unit the evening before sampling for adaptation. Premenopausal women were stud-
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
ied in the follicular phase. No women were receiving estrogens,
and no men were receiving androgens. Ambulation was permitted to the lavatory only. Vigorous exercise, daytime sleep,
snacks, caffeinated beverages, and cigarette smoking were disallowed. Meals were provided at 0800, 1230, and 1730 h, and
room lights were turned off between 2200 and 2400 h, depending upon individual sleeping habits. Blood samples (2.0 ml) were
withdrawn at 10-min intervals for 24 h beginning either at
2400 h (17 subjects) or at 0900 h (31 subjects). Blood was collected in prechilled siliconized tubes containing EDTA (ACTH)
or heparin (cortisol), centrifuged at 4 C to separate plasma, and
frozen at ⫺20 C within 30 min of collection. Total blood loss was
less than 360 ml. Volunteers were compensated for the time spent
in the study.
Technical overview
The goal was linear regression of ultradian ACTH secretory
measures on age, gender, and BMI as primary covariates. Plasma
was collected every 10 min over 24 h in healthy women (n ⫽ 22)
and men (n ⫽ 26) after informed consent was obtained with local
institutional review-board approval. The two centers comprised
Duke University (Durham, NC) and the University of Leiden
(Leiden, The Netherlands) (2, 4). Samples were frozen at ⫺70 C
and assayed in the same ACTH immunoradiometric assay after
a single thaw (Allegro; Nichols Institute, San Juan Capistrano,
CA). Assay sensitivity was 3 ng/liter ACTH (3 SD values above the
zero-dose tube), with intraassay coefficients of variation (CVs) of
4.5–7.8% and interassay CVs of 6.5–10.3%. ACTH measurements cross-reacted less than 1% with ␤-endorphin or ␣-melanocyte stimulating hormone (8, 9). Cortisol was measured by
solid-phase RIA with a sensitivity of 2 ␮g/dl and with median
intra- and interassay CVs of 5.4 and 7.7%, respectively (21).
Deconvolution analysis
ACTH and cortisol concentration time series were analyzed
using a recently developed automated deconvolution method,
which was mathematically verified by direct statistical proof and
empirically validated using hypothalamo-pituitary sampling and
simulated pulsatile time series (18, 22). The Matlab-based algorithm first detrends the data and normalizes concentrations to
the unit interval [0, 1]. Second, the program creates multiple
successive potential pulse-time sets, each containing one fewer
burst via a smoothing process (a nonlinear adaptation of the
heat-diffusion equation). Third, a maximum-likelihood expectation estimation method computes all secretion and elimination
parameters simultaneously conditional on each of the candidate
pulse-time sets. Deconvolution parameters comprise basal secretion (␤0), two half-lives (␣1, ␣2), secretory-burst mass (␩0, ␩1),
random effects on burst mass (␴A), measurement error (␴␧), and
a three-parameter flexible ␥-secretory-burst waveform (␤1, ␤2, ␤3).
For ACTH, the fast half-life was represented as 3.5 min constituting 37% of the decay amplitude (9). The slow half-life was
represented as an unknown variable between 14 and 35 min. For
cortisol, the rapid half-life was fixed at 2.4 min (37% of decay
amplitude), and the slow half-life was estimated as a variable
within the range 40 – 85 min (23). All candidate pulse-time sets
were deconvolved. Statistical model selection was then performed to distinguish among the independently framed fits of the
multiple candidate pulse-time sets using the Akaike information
criterion (24). Observed interpulse intervals were described by a
two-parameter Weibull process (more general form of a Poisson
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
process, which uncouples the mean from the variance). The parameters (and units) are frequency (number of bursts per total
sampling period, ␭ of Weibull distribution), regularity of interpulse intervals (unitless ␥ of Weibull), slow half-life (minutes),
basal and pulsatile secretion rates (concentration units/session),
mass secreted per burst (concentration units), and waveform
shape (mode, or time delay to maximal secretion after objectively
estimated burst onset, in minutes).
ApEn
ApEn (1, 20%) was used as a scale- and model-independent
regularity statistic to quantify the orderliness (regularity) of
ACTH release. Higher ApEn denotes greater disorderliness (irregularity) of the secretion process. Mathematical models and
clinical experiments establish that changes in irregularity signify
alterations in feedforward and feedback control with high sensitivity and specificity (both ⬎90%) (19).
Cross-ApEn
Analogous to univariate ApEn, bivariate cross-ApEn is a
scale- and model-independent two-variable regularity statistic used to quantitate the relative pattern synchrony of coupled time series (25). Clinical experiments establish that
changes in cross-ApEn (1, 20%) reflect feedback and/or feedforward adaptations within an interlinked axis with high sensitivity and specificity (19).
Spikiness
Spikiness was defined as the ratio of the SD of the first-differenced (incremental) series to the SD of the original series (20). The
intent is to quantify sharp, brief, staccato-like unpatterned fluctuations, which would heighten the uncertainty of any single
measurement. This is important because inferences are often reported from single (morning) hormone measurements.
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Mean concentrations
Supplemental Table 1 (published as supplemental data
on The Endocrine Society’s Journals Online web site at
http://jcem.endojournals.org) gives 24-h mean and peak
ACTH and cortisol concentrations in the combined cohorts (n ⫽ 48) and separately in women and men. Mean
ACTH (P ⫽ 0.003) concentrations were higher in men
than women. There was a nonsignificant trend in the same
direction for mean cortisol concentration (P ⫽ 0.052).
Gender did not affect the cortisol/ACTH ratio (10.8 ⫾ 1.1
in men and 12.6 ⫾ 1.2 in women; P ⫽ 0.23).
In the combined groups, 24-h mean ACTH concentrations correlated positively with BMI (R2 ⫽ 0.15; P ⫽
0.006) (Fig. 1, top). On the other hand, mean ACTH concentrations were not related to age. Cortisol/ACTH concentration ratios declined with BMI in the combined cohorts (R2 ⫽ 0.14; P ⫽ 0.008) and were unaffected by age
or gender.
Deconvolution outcomes
Supplemental Table 2 summarizes key parameters of
ACTH secretion and elimination in men, women, and the
combined groups. Pulsatile and basal ACTH secretion
rates were positively related to BMI in the 48 subjects (R2 ⫽
0.12, P ⫽ 0.018; and R2 ⫽ 0.16, P ⫽ 0.005, respectively)
(Fig. 1, middle and bottom). By multivariate analysis, BMI
positively determined pulsatile ACTH secretion (P ⫽
0.014). Pulsatile ACTH secretion in turn positively preImpact of BMI on ACTH Concentration and Secretion
Men (N = 26)
50
Statistical analysis
Women (N = 22)
2
Mean ACTH Concentration
P = 0.006; R = 0.15
(ng/L)
40
30
20
10
0
18
1250
(ng/L/24 h)
Comparisons between means in men and women were made
via an unpaired two-tailed Student’s t test. Univariate linear regression analysis was performed using Systat (Richmond, CA),
with Pearson’s product-moment coefficient to test for significance. Stepwise forward-selection multivariate regression analysis was applied to adjust for possible correlations among age,
BMI, and/or gender.
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20
22
24
26
28
30
32
34
2
P = 0.018; R = 0.12
Pulsatile ACTH Secretion
1000
750
500
250
0
18
Results
Age averaged (⫾ SEM) 46 ⫾ 3.0 (range, 24 –77) yr in
women (n ⫽ 22) and 45 ⫾ 2.5 (range, 23–72) yr in men
(n ⫽ 26). BMI averaged 24 ⫾ 0.85 (range, 21–32) kg/m2
in women and 26 ⫾ 0.60 (range, 21–30) kg/m2 in men. Age
and BMI did not differ by gender. All subjects had at least
six serum cortisol concentrations no greater than 138
nmol/liter (divide by 27.6 for ␮g/dl) overnight, indicating
a relatively unstressed state. ACTH concentrations analogously declined to 7.5 ng/liter (divide by 4.5 for pmol/
liter) for 60 min or more during the night in all individuals.
(ng/L/24 h)
2500
20
22
24
26
28
30
32
34
2
Basal ACTH Secretion
P = 0.005; R = 0.16
2000
1500
1000
500
0
18
20
22
24
26
28
30
32
34
Body Mass Index (kg/m2)
FIG. 1. BMI is a positive linear determinant of mean ACTH
concentrations (top) and pulsatile (middle) and basal (bottom) ACTH
secretion in 48 healthy adults. ACTH secretion was estimated by
variable-waveform deconvolution analysis of 10-min plasma ACTHconcentration profiles collected over 24 h. Open and closed circles
reflect data in women (n ⫽ 22) and men (n ⫽ 26), respectively.
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J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
Impact of Gender on Facets of ACTH Secretion
Total
Pulsatile
1500
1000
500
P = 0.010
(ng/L/24 h)
(ng/L/24 h)
1500
1000
500
P = 0.031
0
0
Men N = 26
Women N = 22
Basal
Mass Per Burst
45
(ng/L)
(ng/L/24 h)
1500
1000
500
30
15
P = 0.047
0
P = 0.010
0
M
W
M
W
FIG. 3. Gender contrasts in total ACTH secretion (top left), pulsatile
ACTH secretion (top right), basal (nonpulsatile) ACTH release (bottom
left), and the mass (amount) of ACTH secreted per burst (bottom
right). M, Men; W, women.
FIG. 2. Multivariate relationships among daily pulsatile ACTH
secretion (top) or the size (mass) of ACTH secretory bursts (bottom)
and BMI and mean (24-h) cortisol concentrations in 48 adults (26 men
and 22 women). Overall and partial P values with corresponding slopes
(⫾ SEM) are given numerically. BMI and cortisol concentrations
correlated positively with pulsatile (top) and burst-like (bottom) ACTH
secretion. The single boxed datum value was a statistical outlier at P ⬍
0.001 based upon Studentized residuals.
dicted cortisol concentrations (P ⫽ 0.041) (overall, R2 ⫽
0.20; P ⫽ 0.008) (Fig. 2, top). This was explained by positive joint associations of ACTH secretory-burst mass with
BMI (P ⫽ 0.041) and (via expected forward by ACTH)
with mean cortisol concentrations (P ⫽ 0.015) (overall, R2
⫽ 0.20; P ⫽ 0.008) (Fig. 2, bottom). The slow-phase halflife of ACTH averaged 21 ⫾ 0.91 min in the combined
cohorts, and varied weakly inversely with BMI (R2 ⫽ 0.09;
P ⫽ 0.036). In contrast, gender, age, and BMI did not
affect ACTH secretory-burst frequency or duration (mode
of burst), percentage pulsatile ACTH secretion, or interpulse regularity (␥ of Weibull distribution: see Subjects
and Methods). In all 48 adults, daily pulsatile (0.55 ␮g/
liter) and basal (0.51 ␮g/liter) ACTH secretion summed to
1.1 ␮g/liter (3.8 ␮g per person for a mean distribution
volume of 3.5 liters). Basal (P ⫽ 0.047), pulsatile (P ⫽
0.031), and total (sum of basal and pulsatile) ACTH secretion (P ⫽ 0.010) were each higher in men than women
(Fig. 3). Total daily ACTH secretion (ng/liter/24 h) normalized for BMI (kg/m2) was higher in men (47 ⫾ 3.1)
than women (37 ⫾ 3.6) (P ⫽ 0.019). This was not true for
cortisol (P ⫽ 0.55). Greater pulsatile ACTH secretion in
men reflected larger (greater mass per burst) rather than
more ACTH secretory bursts (P ⫽ 0.010). Unlike ACTH,
cortisol secretion and elimination parameters manifested
no age or gender effects (Supplemental Table 3).
By univariate linear regression analyses, mean ACTH
concentrations in men were determined negatively by pulsatile cortisol secretion (R2 ⫽ 0.18; P ⫽ 0.033) and negatively by cortisol secretory-burst mass (but not basal cortisol secretion) (R2 ⫽ 0.16; P ⫽ 0.042). In women but not
men, mean ACTH concentrations positively predicted the
duration (mode) of ACTH secretory bursts (R2 ⫽ 0.20;
P ⫽ 0.033). In the combined groups, percentage pulsatile
ACTH secretion positively predicted 24-h mean concentrations of cortisol (n ⫽ 48; R2 ⫽ 0.14; P ⫽ 0.009).
ApEn analysis
ApEn of ACTH did not differ by sex or vary with age
or BMI, whereas cortisol ApEn (irregularity, disorderliness) increased with age in the combined groups (n ⫽ 48;
R2 ⫽ 0.13; P ⫽ 0.01) (Fig. 4, top). Asynchrony of cortisol-ACTH feedback increased (cross-ApEn rose) with
age in the combined cohorts (R2 ⫽ 0.16; P ⫽ 0.005)
(Fig. 4, bottom). In men only, both the synchrony of
ACTH-cortisol feedforward (P ⫽ 0.018) and the synchrony of cortisol-ACTH feedback (P ⫽ 0.021) declined (cross-ApEn rose) significantly with age (R2 ⫽
0.22 and 0.20, respectively) (Fig. 5). These effects were
not observed in women.
Spikiness
ACTH but not cortisol spikiness (a metric of brief sharp
fluctuations) was higher in women than men (P ⫽ 0.047).
Spikiness of ACTH release increased with age in women
(R2 ⫽ 0.18; P ⫽ 0.051) and in the combined group (P ⫽
0.046) (Fig. 6). High spikiness increases the incidence of
abrupt changes in ACTH measurements compared with
that predicted by typical smoothed estimators.
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
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Impact of Age on ACTH-Cort and Cort-ACTH Synchrony
Age Elevates Cortisol ApEn (degree of irregularity)
ACTH-Cortisol Feedforward Asynchrony
2.0
1.5
1.0
1.0
Asynchrony (Synchrony Loss)
Cortisol Irregularity
(ApEn)
1.5
0.5
Men (N = 26)
Women (N = 22)
P = 0.011
R2 = 0.13
0.0
20
30
40
50
60
70
80
Age (yr)
Age Heightens Cortisol-ACTH Feedback Asynchrony
2.0
0.5
P = 0.018
2
R = 0.22
N = 26 men
0.0
20
2.0
30
40
50
60
70
80
Cortisol-ACTH Feedback Asynchrony
1.5
1.0
1.5
0.5
1.0
P = 0.021
2
R = 0.20
0.0
20
30
40
50
60
70
80
Age (yr)
0.5
P = 0.005
R2 = 0.16
0.0
20
30
40
50
60
70
80
Age (yr)
FIG. 4. Top, Age is accompanied by loss of regular (orderly) cortisol
secretion in the combined cohorts, as assessed by ApEn analysis (n ⫽
48). Higher ApEn denotes greater process randomness (less subpattern
reproducibility). Bottom, Erosion of synchronous cortisol-ACTH
feedback coupling with age, quantified by cross-ApEn (n ⫽ 48).
Discussion
Novel outcomes of this appraisal of potential covariates of
ACTH dynamics include the following inferences: 1) all
four of mean ACTH concentrations (P ⫽ 0.003) and basal,
pulsatile, and total 24-h ACTH secretion rates are higher
in men than women (0.010 ⱕ P ⱕ 0.047); 2) men maintain
greater ACTH secretion rates normalized per unit BMI
than women (P ⫽ 0.019); 3) by stepwise forward-selection
multivariate analysis, pulsatile ACTH secretion and the
amount of ACTH secreted per burst are directly associated
with BMI and also (based upon presumptive ACTH feedforward) mean cortisol concentrations, together explaining 20% of interindividual variability (P ⫽ 0.008); 4) percentage pulsatile ACTH secretion is a positive determinant
of mean 24-h cortisol concentrations (P ⫽ 0.009); 5) cortisol, but not ACTH, secretory patterns are more irregular
with age (P ⫽ 0.011), as defined by ApEn; 6) in men,
ACTH-cortisol feedforward synchrony (P ⫽ 0.018) and
cortisol-ACTH feedback synchrony (P ⫽ 0.021) both decline with age, as quantified by cross-ApEn; and 7) cortisol-ACTH feedback synchrony also deteriorates with age
in the combined cohorts (P ⫽ 0.005). Accordingly, BMI,
gender, and age all modulate selected facets of ultradian
ACTH secretion in healthy adults. This means that prior
studies not matched expressly for BMI, gender, and age
FIG. 5. Age is associated with greater asynchrony (less joint pattern
coordination) of ACTH-cortisol feedforward (top) and cortisol-ACTH
feedback (bottom) in men (n ⫽ 26). Asynchrony was quantified from
paired 10-min ACTH-cortisol concentration time series using crossApEn (Subjects and Methods), a model- and scale-independent
measure of joint pattern synchrony. The single boxed datum is a
statistical outlier at P ⬍ 0.001 by Studentized residuals.
carry a significant risk of confounding by one or more of
these three major covariates.
The exact bases for the gender differences in ACTH
secretion identified here are not yet known and may be
complex. For example, higher ACTH concentrations have
been recognized in men than in women both before and
after exposure to a psychological stressor (26 –28). Studies
of CRH expression in the human brain also indicate that
CRH-positive neurons are more abundant in older men
than older women (29). The present analyses demonstrate
greater ACTH secretion in men than women after adjustment for ACTH half-lives.
Age Accentuates ACTH Spikiness in Men and Women
0.8
ACTH Spikiness
Feedback Asynchrony
(Synchrony Loss)
4049
0.6
0.4
0.2
Men (N = 26)
Women (N = 22)
slope 0.0018 ± 0.0017
slope 0.0023 ± 0.0011
N = 48
P = 0.046
2
R = 0.084
0.0
20
30
40
50
60
70
Age (yr)
FIG. 6. ACTH spikiness (a measure of sharp brief fluctuations)
increases with age (n ⫽ 48).
80
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From a univariate perspective, pulsatile, basal, and total 24-h ACTH secretion and mean ACTH concentrations
all increased with BMI. When analyzed jointly, cortisol
and BMI were together associated with greater pulsatile
ACTH secretion due to a greater mass (amount) of ACTH
secreted per burst per unit distribution volume. In contrast, ACTH secretory-burst frequency and waveform
(mode of burst) were independent of BMI. The mechanisms mediating the positive relationship between ACTH
secretion and BMI are not known. Plausible hypotheses
would include augmented hypothalamic peptide drive of
and/or diminished cortisol negative feedback on pulsatile
ACTH release (18). In this regard, CRH and/or arginine
vasopressin stimulation tests point to BMI-associated amplification of ACTH responses, especially in viscerally
obese men (30 –33). On the other hand, glucocorticoid
feedback on ACTH secretion has been reported variously
as increased, decreased, and unchanged in obese individuals (34 –37). Other considerations entail increased adrenergic outflow, hyperinsulinism, lower birth weight, relative hyperestrogenemia, and higher concentrations of
inflammatory cytokines or adipokines as factors potentiating ACTH secretion in association with greater BMI.
Age did not significantly influence mean ACTH concentrations; basal, pulsatile, or total 24-h ACTH secretion; ACTH secretory-burst frequency, mass, or duration;
percentage pulsatile ACTH secretion; or the pattern regularity (ApEn) of ACTH release. However, increasing age
was associated with reduced synchrony (diminished pattern coordination) of both ACTH feedforward and cortisol feedback in men. Decreased joint synchrony, which is
quantified by higher cross-ApEn, denotes deterioration of
coordinate control of ACTH and cortisol secretion patterns in a directionally selective fashion. Declining cortisol-ACTH feedback synchrony with age was evident in the
combined cohorts, also suggesting that factors associated
with aging may impair pulsatile cortisol-mediated inhibition of the hypothalamo-corticotrope unit (16). Although
few data exist on whether age alters endogenous pulsatile
cortisol feedback, several but not all investigations have
identified age-related declension of exogenously manipulated glucocorticoid and mineralocorticoid inhibition of
ACTH secretion (36, 38 – 42). Our model provides a new
means of estimating feedback alterations.
ApEn analysis revealed that greater age is associated
with less orderly (more irregular) patterns of cortisol secretion. Stepwise forward-selection multivariate analysis
revealed no additional effect of BMI or gender. Because
ACTH secretory regularity was independent of both age
and BMI, more disorderly cortisol secretion patterns in
older adults might reflect erosion of intraadrenal paracrine and/or neurogenic mechanisms that modulate
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
ACTH-driven or ACTH-independent glucocorticoid
secretion (43). Thus, the present data provide a motivation for investigating possible age-associated loss of
adrenal autoregulation, as well as hypothalamo-pituitary regulation.
Less prominent findings included a longer duration of
cortisol secretory bursts with higher ACTH concentrations in women, and greater spikiness of ACTH secretion
with age especially in women. Because these effects were
smaller, further studies will be needed to verify or refute
the outcomes. If confirmed, more prolonged cortisol secretory bursts in response to higher ACTH concentrations
in women would provide a mechanism for potential estrogenic modulation of the amount of cortisol secreted per
pulse. This concept is important, inasmuch as systemic
corticosteroid pulses appear to be transmitted rapidly into
brain interstitial fluids and thence as glucocorticoid receptor-bound complexes into neuronal nuclei (44). Increased spikiness indicates the existence of occasional
marked variations between successive ACTH concentration measurements. Spikiness is severalfold larger than
assay variation because the former includes the latter plus
any brief sharp pathophysiological excursions (high or
low) in the secretion process per se. Recognition of increased spikiness in older adults is important in planning
investigations in this age group because greater spikiness
would decrease statistical power by accentuating singlesample variability in aging individuals.
The clinical implication of sex differences is that factors
associated with gender exert distinct quantifiable effects
upon ACTH secretion. This means that 1) many prior
clinical studies which failed to account for sex effects may
have come to untenable conclusions; and 2) further studies
can now examine which specific gender-related factors
mediate sex differences.
Caveats include the need to verify inferred age-related
changes by way of longitudinal population-based studies,
clarify the biochemical bases of augmentative effects of
BMI on ACTH secretion, and evaluate the robustness of
gender differences in larger cohorts. Massive weight loss
unlike aging or moderate obesity appears to lower,
whereas exogenous estrogens elevate, corticosteroidbinding globulin and thereby total cortisol concentrations
(34, 37). Massive obesity (BMI ⬎40 kg/m2), weight loss,
and exogenous sex steroids were exclusion criteria in this
study.
In summary, pulsatile and basal modes of ACTH secretion are statistically positively determined by BMI and
are greater in men than women. These contrasts introduce
a critical need to control for both BMI and sex when
ACTH dynamics are compared. Percentage pulsatile
ACTH secretion predicted average daily cortisol concen-
J Clin Endocrinol Metab, October 2009, 94(10):4045– 4052
trations, suggesting the importance of agonist (ACTH)
pulses to the target organ (adrenal). The same relationship
between agonist pulses and tissue responses is recognized
for the GH-IGF-I system in muscle (45), albeit not for
LH-testosterone drive in the testis (46). The regularity of
cortisol (but not ACTH) secretion and the synchrony of
cortisol-ACTH feedback both decrease with age. In men,
ACTH-cortisol feedforward synchrony additionally declines with age. Accordingly, a crucial caveat in evaluating
corticotropic hormone dynamics is that all three factors—
age, BMI, and gender—should be matched systematically.
Acknowledgments
We thank Donna Scott for capable support of manuscript preparation and Ashley Bryant for data analysis and graphics.
Address all correspondence and requests for reprints to:
Johannes D. Veldhuis, Endocrine Research Unit, Mayo Medical
School, Mayo School of Graduate Medical Education, Center for
Translational Science Activities, Mayo Clinic, Rochester, Minnesota 55905. E-mail: [email protected].
These studies were supported in part by the National Center
for Research Resources (Rockville, MD) Grant M01 RR00030
(to Duke University Medical Center) and Grant P30 MH40159
(to Clinical Research Center Study of Depression in Late Life);
the National Institutes of Health (Bethesda, MD) Grants R01
DK73148, K01 AG19164, and R21 AG29215; and the National
Sciences Foundation (Washington, DC) Interdisciplinary Grant
DMS-0107680.
Disclosure Summary: The authors have nothing to declare.
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