Porth's Essentials of Pathophysiology, 4e

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Mechanisms of Endocrine Control

C h a p t e r 3 1

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Hormone

Nuclear Receptors. Steroid hormones, vitamin D, thyroid hor- mones, and other lipid-soluble hormones diffuse across the cell membrane into the cytoplasm of the target cell. Once inside, they bind to an intracellular receptor that is acti- vated by the interaction. The acti- vated hormone–receptor complex then moves to the nucleus, where the hormone binds to a hormone response element (HRE) in the pro- moters on a target gene or to another transcription factor. Attachment to the HRE results in transcription of a specific messenger RNA (mRNA). The mRNA then moves into the cytoplasm, where the “transcribed message” is translated and used by cytoplasmic ribosomes to produce new cellular proteins or changes in the production of existing proteins. These proteins promote a specific cellular response or, in some cases, the synthesis of a structural protein that is exported from the cell. 2

Extracellular fluid

Cell membrane

Intracellular fluid

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Nucleus

Receptor

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Receptor

DNA

Transcription

mRNA

mRNA

Translation

Ribosome

New proteins

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Pituitary Hormones. The pituitary is a pea-sized endocrine gland that is located at the base of the brain, where it lies in a saddle-shaped depression in the sphe- noid bone called the sella turcica . The pituitary gland has two functional units: an anterior lobe, or adeno- hypophysis, and a posterior lobe, or neurohypophysis . Embryologically, the anterior lobe of the pituitary gland develops from glandular tissue and the posterior lobe from neural tissue. The pituitary gland has been called the master gland because its hormones control the functions of many target glands and cells. The anterior pituitary gland contains five cell types: (1) thyrotrophs, which pro- duce thyrotropin, also called TSH ; (2) corticotrophs, which produce corticotropin, also called ACTH ; (3) gonadotrophs, which produce the gonadotropins, LH, and FSH; (4) somatotrophs, which produce GH;

and (5) lactotrophs, which produce prolactin. The pos- terior pituitary gland stores and releases antidiuretic hormone (ADH) and oxytocin, which are synthesized in the hypothalamus. Hormones produced by the ante- rior pituitary control body growth and metabolism (GH), function of the thyroid gland (TSH), glucocor- ticoid hormone levels (ACTH), function of the gonads (FSH and LH), and breast growth and milk production (prolactin). Melanocyte-stimulating hormone (MSH), which is involved in the control of pigmentation of the skin, is produced by the pars intermedia (the region between the two lobes of the pituitary gland). The func- tions of many of these hormones are discussed in other parts of this book (e.g., thyroid hormone, GH, and the corticosteroids in Chapter 32; the sex hormones in Chapters 39 and 40; and ADH from the posterior pitu- itary in Chapter 8).