2 When a hormone or neurotransmitter is deficient the number of receptors may

2 when a hormone or neurotransmitter is deficient the

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2. When a hormone (or neurotransmitter) is deficient, the number of receptors may increase (up-regulation) , making the target tissue more sensitive to the stimulating effect of the hormone. C. Hormones that pass into the blood to act on distant target cells are called circulating hormones or endocrines ; hormones that act on target cells close to their site of release are called local hormones (paracrines or autocrines). D. Chemically, hormones are classified as steroids, biogenic amines, proteins and peptides, and eicosanoids ( including prostaglandins and leukotrienes ). E. Water-soluble hormones circulate in free form in the blood; lipid-soluble steroid and thyroid hormones are carried attached to transport proteins . 1
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Chapter 18 / The Endocrine System V. MECHANISMS OF HORMONE ACTION A. The response to a hormone depends on both the hormone and the target cell; various target cells respond differently to the same hormone. B. Hormones bind to and activate their specific receptors in two quite different ways. 1. Steroid hormones and thyroid hormones affect cell function by binding to and activating an intracellular receptor (usually in the nucleus), consequently altering gene expression. 2. Water-soluble hormones alter cell function by activating plasma membrane receptors, which initiate a cascade of events inside the cell. a. After a water-soluble hormone is release from an endocrine gland, it circulates in the blood, reaches a target cell, and brings a specific message to that cell; since such a hormone can deliver its message only to the plasma membrane, it is called the first messenger . b. A second messenger is needed to relay the message inside the cell where hormone-stimulated responses can take place; the best known second messenger is cyclic AMP (cAMP) but other substances are known second messengers. c. G-proteins are a common feature of most second messenger systems; the symptoms of cholera are a direct result of the cholera toxin on G-proteins in the intestinal lining. d. Cyclic AMP does not directly produce a particular physiological response, but instead activates one or more enzymes known as protein kinases . 3. The responsiveness of a target cell to a hormone depends on the hormone’s concentration and the number of receptors. The manner in which hormones interact with other hormones is also important: three hormonal interactions are the permissive effect , the synergistic effect , and the antagonistic effect . VI. CONTROL OF HORMONE SECRETIONS A. Most hormones are released in short bursts, with little or no release between bursts. Regulation of hormone secretion normally maintains homeostasis and prevents overproduction or underproduction of a particular hormone; when these regulating mechanisms do not operate properly, disorders result, many of which are discussed below.
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