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The Journal of Clinical Endocrinology & Metabolism Vol. 85, No. 8 2660-2663
Copyright © 2000 by The Endocrine Society

From the Clinical Research Centers

Hypothalamic-Pituitary-Adrenal Axis Activity during Exercise in African American and Caucasian Women1

J. A. Yanovski, S. Z. Yanovski, A. J. Boyle, P. W. Gold, K. N. Sovik, N. G. Sebring and B. Drinkard

Unit on Growth and Obesity, Developmental Endocrinology Branch, National Institute of Child Health and Human Development (J.A.Y., S.Z.Y., A.J.B., K.N.S.); Divisions of Digestive Diseases and Nutrition (S.Z.Y., K.N.S.) and Nutrition Research Coordination (K.N.S.), National Institute of Diabetes and Digestive Disease; Clinical Neuroendocrinology Branch, National Institute of Mental Health (P.W.G.); and Nutrition Department (N.G.S.) and Rehabilitation Medicine Department (B.D.), Warren Grant Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland 20892

Address all correspondence and requests for reprints to: Jack A. Yanovski, M.D., Ph.D., Unit on Growth and Obesity, Developmental Endocrinology Branch, National Institute of Child Health and Human Development, National Institutes of Health, 10 Center Drive, MSC 1862, Building 10, Room 10N262, Bethesda, Maryland 20892-1862. E-mail: JYISi{at}NIH.Gov


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
African American women have a greater prevalence of obesity than Caucasian women, but the reasons for this difference are not known. We have investigated whether activity of the hypothalamic-pituitary adrenal axis plays a role in this phenomenon. Previous studies have shown that plasma ACTH immunoreactivity (ACTH-IR) of African American women, measured after ovine CRH (oCRH) stimulation, is significantly greater than ACTH-IR of Caucasian women, but is not accompanied by greater plasma cortisol concentrations. Analysis by high pressure liquid chromatography has demonstrated that after oCRH stimulation, the plasma ACTH-IR of African American women contains many nonintact ACTH fragments not found in Caucasians. To determine whether these racial differences in ACTH-IR secretion are an artifact of exogenous oCRH administration or are also found after a physiological stimulus for ACTH secretion, we measured hormones of the hypothalamic-pituitary adrenal axis before and after a standardized, maximal exercise treadmill test in 16 African American and 19 Caucasian healthy women matched for age, socioeconomic status, and body mass index.

The intensity of exercise performed was similar in the two groups, as determined by duration of exercise, perceived intensity of exertion, plasma lactate, maximal heart rate, and maximum oxygen uptake. Basal ACTH-IR measured by RIA or immunoradiometric assay and cortisol were similar in African Americans and Caucasians. Plasma ACTH-IR, measured 10 min after completion of exercise, was significantly greater in African Americans than in Caucasians [by RIA: mean ± SD ACTH-IR, 47.1 ± 30.9 vs. 25.4 ± 16.7 pmol/L (P < 0.01); by immunoradiometric assay: ACTH-IR, 45.9 ± 43.2 vs. 21.1 ± 14.6 pmol/L (P < 0.05)]. However, plasma cortisol after exercise was not different (450.2 ± 157.7 vs. 483.6 ± 180.4 nmol/L; P = 0.57).

We conclude that ACTH-IR is significantly greater in African American than in Caucasian women after intense exercise. The ACTH-IR of African Americans and Caucasians does not appear to be equipotent at adrenal melanocortin-2 receptors, because the greater ACTH-IR of African Americans does not lead to greater cortisol secretion. Whether some components of the ACTH-IR detected in African Americans affect signal transduction of the hypothalamic melanocortin-4 receptors implicated in body weight regulation and thus predispose African American women to weight gain without altering plasma cortisol remains to be determined.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
AFRICAN AMERICAN adult women have a greater prevalence of obesity than Caucasian women. Although 23.5% of Caucasian women have a body mass index (BMI) of 30 kg/m2 or more, 36.5% of African American women exceed this BMI (1). African American women are more likely than Caucasians to sustain a major weight gain during adulthood (2) and also appear to have a more difficult time losing weight using behavioral weight loss treatment (3, 4), when consuming very low calorie diets (5), or after obesity surgery (6) and experience more rapid weight regain after dieting (7). These differences do not appear to be explicable solely by environmental or sociocultural factors.

Because of the importance of the hypothalamic-pituitary adrenal (HPA) axis for body weight regulation, we investigated whether there are differences in the activity of the HPA axis in African Americans and Caucasians. We have previously described significant differences in the plasma ACTH immunoreactivity (ACTH-IR) of African American and Caucasian women, unaccompanied by differences in plasma cortisol or urinary free cortisol excretion (8, 9). ACTH-IR, measured after ovine CRH (oCRH) stimulation, was significantly greater in African American than in Caucasian women, whereas plasma cortisol, cortisol-binding globulin, and plasma free cortisol were not different (8, 9). ACTH-IR is also different after oCRH when African American and Caucasian adult men (10) or prepubertal and early pubertal girls (11) are compared. To elucidate why the greater oCRH-stimulated plasma ACTH-IR of African Americans does not lead to greater cortisol secretion, we studied plasma ACTH-IR after high pressure liquid chromatography (9). We found that plasma ACTH-IR in African American and Caucasian women was similar before administration of oCRH (consisting primarily of intact ACTH), but after oCRH, the plasma ACTH-IR of African American women contained many nonintact ACTH fragments not found in Caucasians.

All prior studies demonstrating differences in the ACTH-IR of African Americans and Caucasians (8, 9, 10, 11) have been carried out after the administration of 100 µg/kg oCRH, a dose chosen to stimulate the HPA axis maximally based on studies of Caucasian populations (12, 13). To determine whether racial differences in ACTH-IR secretion are an artifact of exogenous oCRH administration or are also found after a physiological stimulus for ACTH secretion, we measured hormones of the HPA axis before and after a standardized, maximal exercise treadmill test.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Sixteen African American and 19 Caucasian adult women were studied (Table 1Go). Race was self-reported, but all Caucasians reported Caucasian parents and grandparents and were American born, and all African Americans reported African American parents and grandparents. All subjects were healthy, with normal hepatic, renal, thyroid, and reproductive function, and were free from mental illnesses, as assessed by a structured clinical interview (the SCID-R) (14) and a depression inventory (15). No subject had received treatment with glucocorticoids within the past year. Individuals engaging in intensive physical training were excluded. All subjects were premenopausal and were studied in the midfollicular phase. The study was approved by the institutional review board of the NIH, and informed consent was obtained from each subject.


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  		Table 1. Subjects

 
Protocol

After a screening visit in the out-patient clinic during which anthropometric measurements and a baseline electrocardiogram were obtained, subjects completed a 24-h urine collection for free cortisol and creatinine measurements. Subjects were subsequently studied in the Rehabilitation Medicine Department of the NIH Warren Grant Magnuson Clinical Center between 1400–1600 h. After subjects were given 240 mL water to drink, a venous catheter was placed in an antecubital vein. Subjects then stood, at rest, for 1 h, after which each subject performed a maximal treadmill exercise test using the Bruce protocol (16). All subjects were asked to exercise to the limit of their tolerance. Guidelines for test contraindications and termination were observed as published by the American College of Sports Medicine (16). Pulse was monitored electrocardiographically throughout exercise, and blood pressure was measured once during each 3-min stage. Expired ventilatory volume, oxygen uptake (VO2), and carbon dioxide output (VCO2) were measured during each ventilatory cycle using a Sensormedics 2900 metabolic cart (Yorba Linda, CA). Maximal rating of perceived exertion was measured using the Borg 20-point scale (16). Blood for plasma ACTH, cortisol, and lactate determinations was obtained before exercise (preexercise), at the point of physical exhaustion (end- exercise), and 10 min after exercise (postexercise).

Maximum VO2 (VO2 max) was determined when subjects met at least 2 of the following criteria: 1) a plateau in VO2 with 2.0 mL O2/kg·min or less change over a 1-min period during the final increase in work load, 2) respiratory exchange ratio greater than or equal to 1.15, 3) maximal heart rate within ±11 beats of age-predicted maximum heart rate (220 - age), and 4) end-exercise blood lactate greater than or equal to 8.0 mmol/L. The lactate threshold, estimated by gas exchange, was determined using the V-slope method (17).

Assays

Plasma ACTH was measured by a polyclonal RIA and with the Allegro HS-ACTH two-site direct immunoradiometric assay (IRMA; Nichols Institute Diagnostics, San Juan Capistrano, CA) as previously described (9). Intra- and interassay coefficients of variation were 7–12% and 12–25% for the RIA and 7% and 16% for the IRMA. Plasma cortisol was measured by a RIA with 6% intraassay and 13% interassay variations, as previously described (9). Plasma lactate was measured at the NIH Clinical Center using the automatic clinical analyzer instrument (Dade International, Deerfield, IL) as previously described (18). Plasma vasopressin (19) and PRA (20) were measured as previously described.

Statistical analysis

A power calculation indicated that to detect a 15% difference in plasma ACTH with 90% power, at least 16 subjects/group were required. Data were analyzed using SuperAnova (Abacus Concepts, Inc., Berkeley, CA) and StatView 4.5 (SAS Institute, Inc., Cary, NC) on a Macintosh G3 computer. After logarithmic transformation, the data were found to be normally distributed and were then studied by ANOVA with repeated measures. Preplanned comparisons for those variables with significant ANOVAs were carried out using paired and unpaired Fisher’s protected least significant difference tests.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
African American and Caucasian subjects were well matched, with similar age, socioeconomic status, years of education, anthropometric measurements, and urinary free cortisol (Table 1Go). A similar intensity of exercise was also achieved by African American and Caucasian subjects (Table 2Go), as determined by duration of exercise, perceived intensity of exertion, plasma lactate, maximal heart rate, and VO2 max. In addition, lactate threshold was not different between the groups, either relative to body weight or expressed as a percentage of the VO2 max (Table 2Go).


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  		Table 2. Exercise treadmill test

 
ANOVA with repeated measures revealed a significant race by time interaction (P < 0.01). Plasma ACTH-IR, measured by either RIA (Fig. 1AGo) or IRMA (Fig. 1BGo), was similar in African Americans and Caucasians preexercise and immediately after exercise ended. However, 10 min after exercise, at the time of the expected peak in plasma ACTH secretion, plasma ACTH-IR increased by a significantly greater amount in African Americans than in Caucasians whether ACTH-IR was measured by RIA (Fig. 1Go, A and B; mean ± SD, 47.1 ± 30.9 vs. 25.4 ± 16.7 pmol/L) or by IRMA (Fig. 1Go, C and D; ACTH-IR 45.9 ± 43.2 vs. 21.1 ± 14.6 pmol/L; P < 0.05). Plasma cortisol concentrations were not different at any time, either before or after exercise (Fig. 1Go, E and F; postexercise cortisol, 450.2 ± 157.7 vs. 483.6 ± 180.4 nmol/L; P = 0.57). For all subjects, postexercise ACTH-IR was significantly related to BMI for ACTH by RIA (r = 0.81; P < 0.001) and by IRMA (r = 0.74; P < 0.001).



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  		Figure 1. Mean ± SEM plasma ACTH-IR by RIA (A), plasma ACTH-IR by IRMA (C), and plasma cortisol (E) measured just before the start and at two times after exercise (immediately following the end of exercise and 10 min postexercise) in African American ({blacksquare}) and Caucasian women ({square}). Individual data from the 10-min postexercise time point are shown in B, D, and F for African American (•) and Caucasian ({circ}) women. ACTH-IR was greater in African Americans in either assay after exercise, whereas cortisol was not different. *, P < 0.05; **, P < 0.01.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
We have previously shown that after the administration of oCRH, ACTH-IR is significantly greater in African American than in Caucasian females (8, 11). We have also shown that this difference occurs because African American women have circulating ACTH-IR after oCRH stimulation that is not due to intact ACTH; HPLC analysis revealed multiple peaks of ACTH-IR not characteristic of ACTH-(1–39), {alpha}MSH, ßMSH, ß-lipotropin, ß-endorphin, or any other well characterized product that results from the enzymatic cleavage of POMC (9). We have also previously shown that adrenal gland responsiveness to graded doses of ACTH-(1–24) is not different in African Americans and Caucasians, ruling out the possibility of differences in the adrenal melanocortin-2 (MC-2) receptor contributing to this phenomenon (9). However, the formal possibility that the greater ACTH-IR of African Americans was an artifact of oCRH stimulation remained, and we concluded that it was not possible to assess the physiological significance of these differences in secreted ACTH-IR until they were shown to occur under physiological conditions (9).

We now show that African American women have greater ACTH-IR after maximal exercise, achieved with a progressive treadmill test. Exercise was employed as a physiological stimulus for HPA axis activation because it is believed to raise plasma cortisol levels by increasing endogenous hypothalamic human CRH secretion into the hypophyseal portal circulation, and thus inducing ACTH secretion (21). Care was taken to equalize factors that might affect vasopressin levels (such as hydration, recent posture, or time of menstrual cycle), because changes in vasopressin may alter HPA axis activity (22). Because ACTH-IR was greater in African Americans than in Caucasians after a physiological stimulus for ACTH secretion, it appears likely that previous results using oCRH as the stimulus for ACTH secretion did not cause a factitious increase in ACTH-IR in African Americans. Rather, it seems that African American women do have greater ACTH-IR during or after stressful situations. Although we may not have measured plasma cortisol for a sufficiently long time after exercise to be sure that the greater ACTH-IR of African Americans could not cause greater cortisol secretion at a later time point, our prior studies suggesting that the ACTH-IR of African Americans is not equipotent to that of Caucasians at adrenal MC-2 receptors are supported by the finding of similar cortisol increases after exercise in African Americans and Caucasians.

ACTH is only one of the products of the POMC gene. In animals and humans, POMC products, such as {alpha}MSH, act at the hypothalamic MC-4 receptor to regulate body weight (23, 24). The agouti coat color mutation causes obesity in mice because overexpression of the agouti signaling protein inhibits MC-4 receptor activation (25). Humans with mutations in POMC that prevent enzymatic cleavage of POMC into ACTH and {alpha}MSH develop a syndrome of adrenal insufficiency, red hair, and obesity, related, respectively, to insufficient activation of MC-2, MC-1, and MC-4 receptors (26). Humans and animals with MC-4 receptor mutations have also been shown to have greater body weight (27, 28, 29). It is possible that one or more of the peptides that account for the greater ACTH-IR observed in African Americans are MC-4 receptor antagonists that bind to hypothalamic MC-4 receptors, decreasing normal MC-4 receptor signal transduction and thus potentially predisposing African Americans to weight gain.

We conclude that ACTH-IR is significantly greater in African American women than in Caucasian women after intense exercise. Whether some components of the ACTH-IR detected in African Americans affect signal transduction of the hypothalamic MC-4 receptors implicated in body weight regulation remains to be determined.


   Footnotes
 
1 This work was supported by Grant Z01-HD-00641 (to J.A.Y.) and the Office on Research for Minority Health. Back

Received December 29, 1999.

Revised April 19, 2000.

Accepted April 21, 2000.


   References
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 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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