Clinical Note

cAMP and Growth Hormone Regulation

The importance of the cAMP system in the regulation of GH secretion is highlighted by the finding that about 40% of GH-secreting tumors of acromegalics express a somatic mutation in Gas that interferes with its ability to hydrolyze GTP and therefore renders it constitutively active. A similar activating mutation is seen in McCune-Albright disease and is manifested by excessive GH secretion, accompanied by hyperthyroidism and precocious puberty.

concentration ([Ca +];) triggers exocytosis of GH. Increased [Ca2 + ]; also limits the secretory event by inhibiting voltage-sensitive calcium channels and restores membrane polarity by activating potassium channels. Somatostatin acts through the inhibitory guanine nucleotide binding protein (G;) to antagonize activation of adenylyl cyclase. Somatostatin receptors also inhibit calcium channels and activate potassium channels through a G-protein-mediated mechanism. Activation of potassium channels hyperpolarizes the plasma membrane and thereby prevents activation of voltage-sensitive calcium channels. Somatostatin also activates a protein phosphatase through a G-protein-dependent mechanism and thereby antagonizes activation of the secretory apparatus by protein kinase A.

The negative feedback effects of IGF-I are slower in onset than the G-protein-mediated effects of GHRH and somatostatin and require tyrosine-phosphorylation-initiated changes in gene expression that downregulate GHRH receptors and GH synthesis. Somatotropes also express G-protein-coupled receptors for ghrelin (Chapter 38). In cultured cells, the activated ghrelin receptor signals through the IP3-DAG second messenger system. Release of calcium from intracellular stores in response to IP3 and DAG-dependent activation of protein kinase C complement the actions of GHRH and enhance GH secretion. Because the physiology of ghrelin remains to be elaborated, it not included in Fig. 12.

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