Endocrine glands signal their remote target tissues via physiologically pulsatile release of regulatory molecules. A cardinal assumption of most pathophysiological experiments is that discrete attributes of pulsatile hormone secretion are stable over successive untreated observation intervals; i.e. repeated measurements show serial within-subject reproducibility. To test this hypothesis in the GH axis, we sampled blood every 10 min for 48 h in 14 healthy men (age range, 29-77 yr; body mass index, 21-51 kg/m2). The 2 consecutive 24-h serum profiles were subjected to ultrasensitive GH chemiluminescence assay (sensitivity, 0.002 μg/L) with a new dose-dependent variance model to estimate within-assay precision. We then applied deconvolution analysis to estimate the number, mass, amplitude, and duration of underlying GH secretory bursts as well as simultaneously calculate the apparent GH half-life and any concurrent basal hormone secretion Test-retest consistency was assessed by the Pearson correlation coefficient, and differences were determined by paired non-parametric (Wilcoxon) testing. Comparing successive 24-h profiles, no significant differences existed in any of the foregoing secretion or half-life measures or in a novel estimate of the relative disorderliness of hormone release, namely approximate entropy. Correlation was minimal for secretory burst amplitude and half-duration. In contrast, the calculated mean mass of GH secreted per burst was highly conserved across sessions within subjects, with an r value of 0.932 (P < 10- 6). This correlation equaled or exceeded that of mean and integrated serum GH concentrations on consecutive days (r = 0.920; P = 0.00003). The calculated daily GH production rate was also strongly reproduced (r = 0.784; P = 0.0009). Moreover, the within-subject GH half-life and GH secretory burst frequency estimates were well correlated on successive days (P = 0.034- 0.004; r = 0.568-0.711). Approximate entropy values were consistent at r = 0.837 (P = 0.0019). In addition, basal GH secretion rates correlated at r = 0.622 (P = 0.0176). We conclude that homeostatic control mechanisms within the GH-insulin-like growth factor I axis strongly preserve the day to day mean mass of GH secreted per burst and the serial orderliness of the GH release process in individual healthy men across a wide span of ages and body compositions.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical