Plasma membrane, description

At present, the only available drug that stimulates glucose transport is insulin. Insulin increases the abundance of the GLUT4 in plasma membranes of adipose and muscle cells by its recruitment from intracellular storage sites (for a detailed description of its mechanism, see Chapter Diabetes Mellitus). 

The flow of ions across the plasma membrane is recorded as current. An extensive description of the patch-clamp technique is given in ref (30). 

In Chapter 10 the basic principles of oxidative phosphorylation, the complex mechanism by which modem aerobic cells manufacture ATP, are described. The discussion begins with a review of the electron transport system in which electrons are donated by reduced coenzymes to the electron transport chain (ETC). The ETC is a series of electron carriers in the inner membrane of the mitochondria of eukaryotes and the plasma membrane of aerobic prokaryotes. This is followed by a description of chemiosmosis, the means by which the energy extracted from electron flow is captured and used to synthesize ATP. Chapter 10 ends with a discussion of the formation of toxic oxygen products and the strategies that cells use to protect themselves. 

After a brief overview of signal transduction, the text describes the structure of the seven-helix transmembrane P-adrenergic receptor and indicates how it transmits to the intracellular side of the plasma membrane a signal arising from binding the hormone epinephrine on the extracellular surface of the cell. The common features of the G proteins are presented next. The description of the information-transmission pathway from hormone stimulus to G proteins to adenylate cyclase is completed by a discussion of how cAMP activates specific protein kinases to modulate the activities of the phosphorylated target proteins. A small number of hormone molecules outside the cell results in an amplified response because each activated enzyme in the triggered cascade forms numerous products. There are many distinct seven-helix transmembrane hormone receptors. 

This discovery was followed by the description of the Ca ATPases, found both in intracellular membranes, such as the sarcoplasmic reticulum (SERCA ATPases), and on the plasma membrane (PM Ca ATPases). These are also considered to be electrogenic, exchanging 2Ca for 2H. The PM Ca ATPases are uniquely regulated by the binding of calmodulin to the C-terminal region of this single-subunit enzyme. 

Membrane-bound hormone receptors were detected in the late 1960s. The binding of insulin, glucagon, and epinephrine to isolated plasma membranes of the rat liver or to isolated fat cells and fat cell membranes has been reported (Tomasi et al., 1970 Rodbell et al., 1971 Cuatrecasas, 1971a,b Freychet et al., 1971 Dunnick and Marinetti, 1971). Species-specific interaction between growth hormones and erythrocyte membranes has been shown by Cambiaso et al. (1971). Lef-kowitz et al. (1971) have published a detailed description of the interaction of adrenocorticotropic hormone with its receptors in the adrenal cortex, which appears to be a membrane-associated interaction (Finn et al., 1972). The modes of action for polypeptide hormones and their receptors have been the subject of intense investigation, and a number of reviews on this subject have been published (Cuatrecasas, 1974 Kahn, 1975 Catt and Dufau, 1977). 

Every cell possesses a plasma (or cell) membrane which isolates its contents from its surroundings. This membrane consists of a double layer of phospholipid molecules with proteins attached or dispersed within. The uneven distribution of proteins and their ability to move in the plane of the membrane led to the description of this structure as a fluid mosaic (Figure 1.2) (Chapter 5). Some of these proteins facilitate the transport of molecules and ions through the membrane, while others are receptors for extracellular molecules which provide information about conditions in adjacent cells, blood and elsewhere in the body. Physical or chemical damage to these membranes can render them leaky so that, for example, Na and Ca ions, the concentrations of which are much higher in the extracellular fluid, can enter the cell causing damage. On the outer surface of... 

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