Anomalous growth hormone response to vasoactive intestinal peptide and peptide histidine methionine in patients with prolactinoma or hypothalamic hyperprolactinemia

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Stimulation by neuropeptide Y of growth hormone secretion in prolactinoma in vivo

We examined the effects of intravenous bolus injection of human neuropeptide Y (NPY, 100 micrograms) on the plasma growth hormone (GH) and prolactin (PRL) responses in 15 patients with prolactinoma (PRLoma). The GH and PRL responses to NPY were considered positive (a paradoxical increase) when an increase over baseline of at least 100% occurred. Although NPY did not affect PRL secretion in any of the patients examined, 60% (9 of 15) of the patients showed a significant rise in GH secretion after NPY. The presence or absence of the positive paradoxical increase in GH after NPY was not related to the age of the patients, basal PRL levels, the size of the pituitary adenoma (macro- or microadenoma), or the presence or absence of suprasellar extension of the adenoma. Although the underlying mechanism of the NPY stimulation of GH secretion and also its pathophysiological significance in PRLoma are open to question, the present observation may represent another example of paradoxical hormone responses which are occasionally found in functioning pituitary adenomas.