Hormones Are Organized into a Hierarchy

The synthesis of many hormones is regulated by a cascade of hormones (fig. 24.20). Frequently, a hypothalamic hormone impinges on the pituitary to stimulate synthesis of a hormone that activates hormone synthesis in yet another organ. The end products of these cascades generally feed-back-inhibit the production of hormones at the beginning of the cascade (fig. 24.21). 

Another common mechanism for modulating hormonal response involves two (or more) hormonal inputs with both positive and negative effects (see fig. 24.21). The hypothalamic peptides, somatostatin and GRF, have opposite effects on GH synthesis and secretion. Similarly, glucagon and insulin have opposite effects on gluconeogenesis in the liver (see the discussion earlier in this chapter), and some of the effects of ecdysone on gene expression in insects are blocked by juvenile hormone (a terpene derivative fig. 24.22). 

Considerable cross-talk also occurs between different hormones at the level of receptor function. For instance, the diacylglycerol-activated protein kinase C phosphory-lates and thereby inhibits the activity of insulin and epidermal growth factor receptor kinases (epidermal growth factor is discussed in a later section). Likewise, when cAMP-dependent protein kinases are active they can inhibit receptors for epinephrine by phosphorylating them. 

Cells also have mechanisms that tend to prevent chronic stimulation. Exposure of cells to epinephrine leads to an initial sharp rise in cAMP levels however, cAMP levels fall nearly to basal levels within an hour or so, despite the continuous presence of saturating amounts of epinephrine. Furthermore, if the hormone is removed and the cells 

Regulation of Specific Genes by Steroid and Thyroid Hormones 

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Hormones Are Organized into a Hierarchy Diseases Associated with the Endocrine System... 

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