Chapter 16

After studying this chapter, you should be able to:

• Describe hormones and their contribution to whole body homeostatic mechanisms.
• Understand the chemical nature of different classes of hormones and how this determines their mechanism of action on target cells.
• Define how hormones are synthesized and secreted by cells of endocrine glands, including how peptide hormones are cleaved from longer precursors.
• Explain the relevance of protein carriers in the blood for hydrophobic hormones, and the mechanisms that determine the level of free circulating hormones.
• Understand the principles of feedback control for hormone release and its relevance for homeostasis.
• Understand the principles governing disease states that result from over- or under-production of key hormones.

This section of the text deals with the various endocrine glands that control the function of multiple organ systems of the body. In general, endocrine physiology is concerned with the maintenance of various aspects of homeostasis. The mediators of such control mechanisms are soluble factors known as hormones. The word hormone was derived from the Greek horman, meaning to set in motion. In preparation for specific discussions of the various endocrine systems and their hormones, this chapter will address some concepts of endocrine regulation that are common among all systems.

Another feature of endocrine physiology to keep in mind is that, unlike other physiological systems that are considered in this text, the endocrine system cannot be cleanly defined along anatomic lines. Rather, the endocrine system is a distributed system of glands and circulating messengers that is often stimulated by the central nervous system and/or autonomic nervous system.

As noted in the introduction to this section, hormones comprise steroids, amines, and peptides. Peptide hormones are by far the most numerous. Many hormones can be grouped into families reflecting their structural similarities as well as the similarities of the receptors they activate. However, the number of hormones and their diversity increases as one moves from simple to higher life forms, reflecting the added challenges in providing for homeostasis in more complex organisms. For example, among the peptide hormones, several are heterodimers that share a common α chain, with specificity being conferred by the β-chain. In the specific case of thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH), there is evidence that the distinctive β-chains arose from a series of duplications of a common ancestral gene. For these and other hormones, moreover, this molecular evolution implies that hormone receptors also needed to evolve to allow for spreading of hormone actions/specificity. This was accomplished by co-evolution of the basic G-protein coupled receptors (GPCR) and receptor tyrosine kinases that mediate the effects of peptide and amine hormones that act at the cell surface (see Chapter 2). The underlying ancestral relationships sometimes re-emerge, however, in the cross-reactivity that may be seen when hormones rise to unusually high levels (eg, endocrine tumors).

Steroids and thyroid hormones are distinguished by their predominantly intracellular sites ...

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