| Summary: | In humans, cortisol and GH are secreted in a pulsatile manner, and an interaction between GH and the hypothalamic-pituitary-adrenal axis has been established. In view of the sexually dimorphic pattern in GH secretion, we investigated the GH-cortisol bihormonal secretory dynamics in male and female healthy older individuals. We studied the GH and cortisol secretory patterns in 83 healthy subjects (45 men and 38 women; age range, 59.4-73.0 yr) by determining serum GH and cortisol concentrations at 20-min intervals for 24 h. The irregularity of GH and cortisol secretion was assessed using approximate entropy (ApEn), a scale- and model-independent statistic. The synchrony of joint GH-cortisol spontaneous secretion was quantified using the cross-ApEn statistic. Cross-correlation analysis of GH and cortisol patterns was computed at various time lags covering the 24-h period. Mean 24-h serum GH concentrations were significantly higher in females (mean, 1.31 mU/L; SD, 0.87) than in males (mean, 0.88 mU/L; SD, 0.42; P = 0.009), whereas mean 24-h serum total cortisol concentrations were higher in males (mean, 9.0 microg/dL; SD, 1.4) than in females (mean, 7.3 microg/dL; SD, 1.4; P = 0.0001). GH secretion was more irregular in females (mean ApEn, 0.81; SD, 0.23) than in males (mean ApEn, 0.60; SD, 0.20; P < 0.001). No significant difference in the regularity of cortisol secretion was noted between sexes. Cross-ApEn values of paired GH-cortisol were higher in females (mean, 1.15; SD, 0.18) than in males (mean, 1.01; SD, 0.16; P = 0.0003). Stepwise multiple linear regression analysis indicated that estradiol and insulin-like growth factor-binding protein-3 concentrations were independently related to GH ApEn values (r(2) = 0.14; P = 0.01), whereas cross-ApEn values of paired GH-cortisol were best predicted by FSH concentrations (r(2) = 0.37; P = 0.003). Cross-correlation analysis revealed a significant positive correlation between GH and cortisol, peaking at lag time of 4.7 h in males (r = 0.30; P < 0.0001) and 4.3 h in females (r = 0.14; P < 0.0001), with GH leading cortisol by these time intervals. In addition, a significant negative correlation between the two hormones was noted over time, peaking at 4.7 h in males (r = -0.21; P < 0.0001) and 6.3 h in females (r = -0.25; P < 0.0001), with cortisol leading GH by these time intervals. The above results indicate that in the elderly, females have a more disordered GH secretory pattern and a more asynchronous joint GH-cortisol secretion than their male counterparts. These observations most likely reflect bidirectional interactions between the GH and hypothalamic-pituitary-adrenal axis in humans as well as diminution of subsystem integrity and synchronous control of interconnected hormonal systems with advancing age.
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