The classic concept that endocrine effects are the result of substances secreted into the blood with effects on a distant target cell has been updated to account for other ways in which hormonal effects occur. Specifically, some hormone systems involve the stimulation or inhibition of metabolic processes in neighboring cells (eg, within the pancreatic islets or cartilage). This phenomenon is termed paracrine. Other hormone effects reflect the action of hormones on the same cells that produced them. This action is termed autocrine. The discoveries of local production of ghrelin, somatostatin, cholecystokinin, incretins, and many other hormones in the brain and gut support the concept of paracrine and autocrine processes in these tissues.
Another significant discovery in endocrine physiology was an appreciation of the role of specific hormone receptors in target tissues, without which the hormonal effects cannot occur. For example, in nephrogenic diabetes insipidus (DI), affected children have defective vasopressin or receptor function, and show the metabolic effects of DI despite more-than-adequate vasopressin secretion. Alternatively, abnormal activation of a hormone receptor leads to the effects of the hormone without its abnormal secretion. Examples of this phenomenon include McCune-Albright syndrome (precocious puberty and hyperthyroidism), testotoxicosis (familial male precocious puberty), and hypercalciuric hypocalcemia.
Hormones are of three main chemical types: peptides and proteins, steroids, and amines. The peptide hormones include the releasing factors secreted by the hypothalamus; the hormones of the anterior and posterior pituitary gland; pancreatic islet cells; parathyroid glands, lung (angiotensin II), heart, and brain (atrial and brain natriuretic hormones); and local growth factors such as insulin-like growth factor 1 (IGF-1). Steroid hormones are secreted primarily by the adrenal cortex, gonads, and kidney (active vitamin D [1,25(OH)2 D3]). The amine hormones are secreted by the adrenal medulla (epinephrine) and the thyroid gland (triiodothyronine [T3] and thyroxine [T4]).
Peptide hormones and epinephrine act through cell surface receptors. The metabolic effects of these hormones are usually stimulation or inhibition of the activity of preexisting enzymes or transport proteins (posttranslational effects). The steroid hormones, thyroid hormone, and active vitamin D, in contrast, act more slowly and bind to cytoplasmic receptors inside the target cell and subsequently to specific regions on nuclear DNA, where they direct a read-out of specific protein(s). Their metabolic effects are generally caused by stimulating or inhibiting the synthesis of new enzymes or transport proteins (transcriptional effects).
Metabolic processes that require rapid response, such as blood glucose or calcium homeostasis, are usually controlled by peptide hormones and epinephrine, while processes that respond more slowly, such as pubertal development and metabolic rate, are controlled by steroid hormones and thyroid hormone. The control of electrolyte homeostasis is intermediate and is regulated by a combination of peptide and steroid hormones (Table 34–1).
Table 34–1.Hormonal regulation of metabolic processes.
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