Hypothalamic-pituitary-adrenal axis activity during exercise in African American and Caucasian women

African American women have a greater prevalence of obesity than Caucasian women, but the reasons for this difference are not known. We have investiga ted whether activity of the hypothalamic-pituitary adrenal axis plays a rol e in this phenomenon. Previous studies have shown that plasma ACTH immunore activity (ACTH-IR) of African American women, measured after ovine CRH (oCR H) stimulation, is significantly greater than ACTH-IR of Caucasian women, b ut is not accompanied by greater plasma cortisol concentrations. Analysis b y high pressure liquid chromatography has demonstrated that after oCRH stim ulation, the plasma ACTH-IR of African American women contains many noninta ct ACTH fragments not found in Caucasians. To determine whether these racia l differences in ACTH-IR secretion are an artifact of exogenous oCRH admini stration or are also found after a physiological stimulus for ACTH secretio n, we measured hormones of the hypothalamic-pituitary adrenal axis before a nd after a standardized, maximal exercise treadmill test in 16 African Amer ican 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 deter mined by duration of exercise, perceived intensity of exertion, plasma lact ate, maximal heart rate, and maximum oxygen uptake. Basal ACTH-IR measured by RIA or immunoradiometric assay and cortisol were similar in African Amer icans and Caucasians. Plasma ACTH-IR, measured 10 min after completion of e xercise, was significantly greater in African Americans than in Caucasians [by RIA: mean +/- so ACTH-IR, 47.1 +/- 30.9 vs. 25.4 +/- 16.7 pmol/ (P < 0. 01); by immunoradiometric assay: ACTH-IR, 45.9 +/- 43.2 vs. 21.1 +/- 14.6 p mol/L (P < 0.05)]. However, plasma cortisol after exercise was not differen t (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 t o greater cortisol secretion. Whether some components of the ACTH-IR detect ed in African Americans affect signal transduction of the hypothalamic mela nocortin-4 receptors implicated in body weight regulation and thus predispo se African American women to weight gain without altering plasma cortisol r emains to be determined.