Basal, pulsatile, entropic (patterned), and spiky (staccato-like) properties of ACTH secretion: impact of age, gender, and body mass index.

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/m(2)].

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 (R(2) = 0.15; P = 0.006) and both pulsatile (R(2) = 0.12; P = 0.018) and basal (nonpulsatile) (R(2) = 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 (R(2) = 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 (R(2) = 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.