Chapter 41

After studying this chapter, you should be able to:

• Explain the basic principles of endocrine hormone action, including the determinants of hormone target cell response and the determinants of hormone concentration at target cells.
• Understand the broad diversity and mechanisms of action of endocrine hormones.
• Appreciate the complex steps involved in the production, transport, and storage of hormones.

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 ACTH Adrenocorticotropic hormone ANF Atrial natriuretic factor cAMP Cyclic adenosine monophosphate CBG Corticosteroid-binding globulin CG Chorionic gonadotropin cGMP Cyclic guanosine monophosphate CLIP Corticotropin-like intermediate lobe peptide DBH Dopamine β-hydroxylase DHEA Dehydroepiandrosterone DHT Dihydrotestosterone DIT Diiodotyrosine DOC Deoxycorticosterone EGF Epidermal growth factor FSH Follicle-stimulating hormone GH Growth hormone IGF-I Insulin-like growth factor-I LH Luteotropic hormone LPH Lipotropin MIT Monoiodotyrosine MSH Melanocyte-stimulating hormone OHSD Hydroxysteroid dehydrogenase PNMT Phenylethanolamine-N-methyltransferase POMC Pro-opiomelanocortin SHBG Sex hormone-binding globulin StAR Steroidogenic acute regulatory (protein) TBG Thyroxine-binding globulin TEBG Testosterone-estrogen-binding globulin TRH Thyrotropin-releasing hormone TSH Thyrotropin-stimulating hormone

The survival of multicellular organisms depends on their ability to adapt to a constantly changing environment. Intercellular communication mechanisms are necessary requirements for this adaptation. The nervous system and the endocrine system provide this intercellular, organism-wide communication. The nervous system was originally viewed as providing a fixed communication system, whereas the endocrine system supplied hormones, which are mobile messages. In fact, there is a remarkable convergence of these regulatory systems. For example, neural regulation of the endocrine system is important in the production and secretion of some hormones; many neurotransmitters resemble hormones in their synthesis, transport, and mechanism of action; and many hormones are synthesized in the nervous system. The word “hormone” is derived from a Greek term that means to arouse to activity. As classically defined, a hormone is a substance that is synthesized in one organ and transported by the circulatory system to act on another tissue. However, this original description is too restrictive because hormones can act on adjacent cells (paracrine action) and on the cell in which they were synthesized (autocrine action) without entering the systemic circulation. A diverse array of hormones—each with distinctive mechanisms of action and properties of biosynthesis, storage, secretion, transport, and metabolism—has evolved to provide homeostatic responses. This biochemical diversity is the topic of this chapter.

There are about 200 types of differentiated cells in humans. Only a few produce hormones, but virtually all of the 75 trillion cells in a human are targets of one or more of the <50 known hormones. The concept of the target cell is a useful way of looking at hormone action. It was thought that hormones affected a single cell type—or only a few kinds of cells—and that a hormone elicited a unique biochemical or physiologic action. We now know that a given hormone can affect several different cell types; that more than one hormone can affect a given cell type; and that hormones can exert many different effects in one cell or in different cells. With the discovery of specific cell-surface and intracellular hormone receptors, the definition ...

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