Abstract
Age and gender impact the full repertoire of neurohormone systems, including most prominently the somatotropic, gonadotropic and lactotropic axes. For example, daily GH production is approximately 2-fold higher in young women than men and varies by 20-fold by sexual developmental status and age. Deconvolution estimates of 24-h GH secretion rates exceed 1200 microg/m2 in adolescents and fall below 60 microg/m2 in aged individuals. The present overview highlights plausible factors driving such lifetime variations in GH availability, i.e., estrogen, aromatizable androgen, hypothalamic peptides and negative feedback by GH and IGF-I. In view of the daunting complexity of potential neuromodulatory signals, we underline the utility of conceptualizing a simplified three-peptide regulatory ensemble of GHRH, GHRP (ghrelin) and somatostatin. The foregoing signals act as individual and conjoint mediators of adaptive GH control. Regulation is enforced at 3-fold complementary time scales, which embrace pulsatile (burst-like), entropic (orderly) and 24-h rhythmic (nycthemeral) modes of GH release. This unifying platform offers a convergent perspective of multivalent control of GH outflow.
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