Porth's Essentials of Pathophysiology, 4e

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Endocrine System

U N I T 9

Hormones bring about their effects on cell activity by binding to specific cell receptors.There are two general types of receptors: (1) cell surface receptors that exert their actions through cytoplasmic second messenger systems, and (2) intracellular nuclear receptors that modulate gene expression by binding to DNA or promoters of target genes. U N D E R S T A N D I N G Hormone Receptors

Hormone

Cell Surface Receptors. Water- soluble peptide hormones, such as parathyroid hormone and glucagon, which cannot penetrate the lipid layer of the cell plasma membrane, exert their effects through intracel- lular second messengers. They bind to a portion of a membrane receptor that protrudes through the surface of the cell. This produces a struc- tural change in the receptor mol- ecule itself, causing activation of a hormone-regulated signal system located on the inner aspect of the cell membrane. This system allows the cell to sense extracellular events and pass this information to the intracellular environment. There are several types of cell surface recep- tors, including G-protein–coupled receptors that mediate the actions of catecholamines, prostaglandins, thyroid-stimulating hormone, and others. Binding of the hormone to the receptor activates a G protein, which in turn acts on an effector (such as adenyl cyclase) to generate a second messenger (such as cyclic adenosine monophosphate, cAMP). The second messenger, in turn, acti- vates other enzymes that participate in cellular secretion, gene activation, or other target cell responses. 1

(First messenger)

Extracellular fluid

Effector

G-protein

Receptor

(Transducer)

Adenyl cyclase

Intracellular fluid

Adenosine triphosphate (ATP)

Cyclic AMP (cAMP)

Second messenger

Target cell response