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. Contrast membrane bound and intracellular hormone receptors and understand the principles of desensitization, down-regulation, and inactivation.
Define how hormones are synthesized and secreted by cells of endocrine glands, including how peptide hormones are cleaved from longer precursors.
Understand the effects of plasma hormone binding proteins (thyroid and steroid hormones) on the availability of hormones to their sites of action, and on the mechanisms that regulate hormone secretion.
Explain the principles of feedback control (negative and positive) of hormone secretion.
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 physiologic 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 or the autonomic nervous system, or both.
HORMONAL SPECIFICITY OF ACTION ON TARGET CELLS
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.
Steroids and thyroid hormones are distinguished by their predominantly intracellular sites of action, since they can diffuse freely through the cell membrane. They bind to a family of largely cytoplasmic proteins known as nuclear receptors. Upon ligand binding, the receptor–ligand complex translocates to the nucleus where it either homodimerizes, or associates with a distinct liganded nuclear receptor to form a heterodimer. In either case, the dimer binds to DNA to either increase or decrease gene transcription in the target tissue. Individual members of the nuclear receptor family have a considerable degree of homology, and share many functional domains, such as the zinc fingers that permit DNA binding. However, sequence variations allow for ligand specificity as well as binding to specific DNA motifs. In this way, the transcription of distinct genes is regulated by individual hormones.