To compare profiles of hypothalamic-pituitary-adrenal (HPA) responsiveness, healthy, moderately trained men (n = 15) were classified as high (n = 7) or low responders (n = 8) on the basis of plasma adrenocorticotropic hormone (ACTH) responses to strenuous treadmill exercise 4 h after 4 mg of dexamethasone (Dex). These groups were then evaluated to compare 1) HPA and growth hormone responses to exercise at 90% maximal oxygen uptake 4 h after placebo, Dex (4 mg), and hydrocortisone (100 mg); 2) pituitary-adrenal responses to infusion of arginine vasopressin (AVP); 3) plasma cortisol after a Dex suppression test (1 mg); and 4) behavioral characteristics. In comparison to low responders, high responders exhibited significantly 1) higher plasma ACTH responses to exercise after placebo and Dex; 2) higher plasma AVP secretion with exercise after placebo and marked Dex- and hydrocortisone-induced enhancement of exercise-induced AVP secretion; 3) lower Dex-induced increases in basal and stimulated growth hormone secretion; 4) higher plasma ACTH responses to infusion of AVP; and 5) a trend (P = 0.09) for higher trait anxiety ratings. Similar suppression of plasma cortisol was noted after 1 mg Dex. We conclude that subgroups of healthy male volunteers exhibit unique profiles of HPA responsiveness. We also believe that glucocorticoid pretreatment combined with strenuous exercise allows functional HPA responsiveness to be distinguished between subgroups of healthy controls and may be useful in the determination of susceptibility to disorders characterized by hyper- and hypo-HPA activation.

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Studies of the growth hormone (GH) secretory dynamics of children with normal and idiopathic short stature (ISS) have revealed that the regulation of the GH-somatomedin (GHS) axis can differ significantly among normal individuals. Information on the GH secretion in idiopathic tall stature (ITS) is scarce. We previously showed that the GH response to stimulation with GH-releasing hormone (GHRH) in male, late adolescents and young adults with ITS is significantly greater than that of their sex and age-matched controls of average height. In the present study, we studied the 24-hour (hr) GH, insulin-like growth factor-I and -II (IGF-I and -II), prolactin (PRL) and thyroid-stimulating hormone (TSH) secretion by every 30 minutes (min) sampling in 12 young, healthy male Greek army recruits. Group I [n = 6, age 22 + 1.4 years (y.), mean + standard deviation (SD)] had a height of 198.5 + 4.2cm, at least 3 SD's above the mean of the Greek male population. Group II (n = 6, age 20.5 + 1.05 y.) had a height of 169.2 + 3.4cm, within 2SD's of the normal mean. Serum IGF-I levels were determined in both unextracted and extracted samples. Our results indicated that the number of secretory bursts and the circadian panel of GH, IGF-I and -II, PRL and TSH were similar in the two groups. Both the amplitude of the secretory GH peaks (5.08 + 3.07 vs. 3.3 + 0.8 ng/ml, p = 0.19,) and the area under the curve (AUC) of the 24-hour GH secretion (9.8 + 5.5 vs. 6.6 + 1.3 ng/ml/hr, p = 0.2) were higher in group I than in group II, but the difference was not significant. A significant nocturnal increase of both IGF-I and -II levels was found only in extracted human plasma (p < 0.001), whereas measurements of IGF-I in unextracted samples failed to reveal circadian variation (p < 0.1). We conclude that no significant differences were found in this pilot study of the neurosecretory regulation of the GHS axis between individuals of tall and normal stature. A tendency for greater amount of GH secretion per secretory peak was found in persons with tall stature; however, this finding needs to be confirmed in a larger study. IGF-I and -II levels had a significant circadian variation with a large nocturnal surge, when measured in extracted plasma. The latter, might be explained by circadian variation of the circulating IGF-binding proteins and its detection appears to be method of extraction-dependent.

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