The orderliness of the gh release process and the mass of gh secreted per burst are highly conserved within individual humans studied on successive days

Endocrine glands signal their remote target tissues via physiologically pulsatile hormone release. 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 widely held hypothesis, we sampled blood every 10 min for 48 h in 14 healthy men (age range 29 -77, body mass index 21-51). The two consecutive 24-h profiles were subjected to ultrasensitive GH chemiluminescence assay (sensitivity 0.002 ug/L) with a new dosedependent variance model to estimate within-assay precision (Meth. Enzym. 240:121,1994), followed by deconvolution analysis in order to estimate the number, mass, and duration of underlying GH secretory bursts, as well as simultaneously calculate the apparent GH half-life and any concurrent basal hormone secretion (PNAS 84:7686, 1987). Test-retest consistency was assessed by the Pearson correlation coefficient, and 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 relative disorderiiness of hormone release, namely approximate entropy (ApEn, PNAS 88:2297,1991). Correlation was minimal for mean secretory burst amplitude or half-duration. In contrast, the calculated 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 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 within-subject GH half-life and burst frequency estimates were correlated on successive days (P values 0.034 to 0.004, R=+0.568 to + 0.711). ApEn values correlated 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-IGF-I axis strongly preserve the day-to-day mass of GH secreted per burst and the serial orderliness of the GH release process in individual healthy humans across a wide span of ages and body compositions.