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Plasma membrane transport mechanisms

Garcia Ruiz, C., Fernandez Checa, J. and Kaplowitz, N. (1992). Bidirectional mechanism of plasma membrane transport of reduced glutathione in intact rat hepatocytes and membrane vesicles. J. Biol. Chem. 267, 2256-2264. [Pg.70]

Although there are a number of mechanisms in cells for buffering or sequestering Ca2+ to prevent untoward or inappropriate rises in [Ca2+]i( in the long term, it is the activity of plasma membrane transport processes that determines the steady-state [Ca2+]j. This is because the plasma membrane acts as a Ca2+ buffer of essentially infinite capacity. This results from in vivo clamping of the extracellular concentration of Ca2+ by dietary and endocrine mechanisms. In in vitro experiments, this results from incubation volumes very much larger than the cell volume. [Pg.380]

Substrate availability to the cell is affected by the supply of raw materials from the environment. The plasma membranes of cells incorporate special and often specific transport proteins (translocases) or pores that permit the entry of substrates into the cell interior. Furthermore pathways in eukaryotic cells are often compartmentalized within cytoplasmic organelles by intracellular membranes. Thus we find particular pathways associated with the mitochondria, the lysosomes, the peroxisomes, the endoplasmic reticulum for example. Substrate utilization is limited therefore by its localization at the site of need within the cell and a particular substrate will be effectively concentrated within a particular organelle. The existence of membrane transport mechanisms is crucial in substrate delivery to, and availability at, the site of use. [Pg.57]

The aquaporins are a family of small, integral membrane proteins that function as plasma membrane transporters of water and in some cases small polar solutes. There are at least 10 distinct aquaporins in mammals with specific expression patterns in epithelial, endothelial, and other tissues. Studies in aquaporin-null mice indicated a key role for aquaporins in the urinary concentrating mechanism, fluid secretion of glands, brain swelling, skin moisture, hearing and vision, and gastrointestinal absorption.62... [Pg.235]

Intracellular fluids (also called the cytosol) are quite different compositionally from plasma and interstitial fluids (Table 4, Figures 4 and 5). The internal pH of many cells is maintained near 6.9-7.0. via various membrane transport mechanisms such as Na" /H and CP/HCO exchangers, and various phosphate and protein buffers. In contrast to the plasma, the intracellular fluids have substantially lower concentrations of sodium, calcium, chloride, and bicarbonate and higher to substantially higher concentrations of potassium, magnesium,... [Pg.4827]

Release of Asp and Glu from nerve terminals may occur through exocytosis of vesicular content or through reversal of plasma membrane transporter proteins for Asp and Glu. Alternatively, Asp could escape from nerve terminals through this transporter system by exchange with synaptically released Glu. Such transporters (see Chapter VIII), besides localization on glial membranes (see Danbolt, 1994), are situated on excitatory nerve terminals (Gundersen et al., 1993, 1996) and they transport Asp and Glu with similar affinities (Balcar and Johnston, 1972 Arriza et al., 1994). In favor of an exocytotic release mechanism... [Pg.47]

MEMBRANE TRANSPORT Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be carefully regulated to meet each cell s metabolic needs. For example, a cell s plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes. Several examples of these structures, referred to as transport proteins or permeases, are discussed. [Pg.364]

A FIGURE 18-1 Overview of synthesis of major membrane lipids and their movement into and out of cells. Membrane lipids (e.g., phospholipids, cholesterol) are synthesized through complex multienzyme pathways that begin with sets of water-soluble enzymes and intermediates in the cytosol (D) that are then converted by membrane-associated enzymes into water-insoluble products embedded in the membrane (B), usually at the interface between the cytosolic leaflet of the endoplasmic reticulum (ER) and the cytosol. Membrane lipids can move from the ER to other organelles (H), such as the Golgi apparatus or the mitochondrion, by either vesicle-mediated or other poorly defined mechanisms. Lipids can move into or out of cells by plasma-membrane transport proteins or by lipoproteins. Transport proteins similar to those described in Chapter 7 that move lipids (0) include sodium-coupled symporters that mediate import CD36 and SR-BI superfamily proteins that can mediate... [Pg.744]

Most members of classes II aud in have beeu ideutified iu geuomewide database searches. The fuuctiou of these newly discovered transporter isoforms is not yet well defined. Several of them (GLUT-6 and 8) contain motifs that lead to their retention inside the cell, and therefore this prevents their involvement in glucose transport at the plasma membrane. Whetho" mechanisms exist to invoke ceU surface insafion of these transporta is unknown, but it has been established that insulin does not promote translocation of GLUT-6 and GLUT-8 to the ceU surface. [Pg.339]

Stimulation of efflux is an important component of auxin action in many plant tissues [26]. A recent report indicates that active auxins directly stimulate a plasma membrane NADH oxidase from soybean hypocotyls. Enzyme from tissue which had completed elongation was no longer stimulable [27]. NADH oxidase is implicated in an electron transport-driven -efflux across the plasma membrane, a mechanism which co-exists with -pumping ATPases and, for cultured carrot cells, predominates specifically in young cells [28]. Because BA hyperpolarized the cell membrane and stimulated efflux in squash cotyledons, it was suggested that... [Pg.163]

A variety of non-lipid soluble small molecules, e.g. sugars and amino acids, can nevertheless pass through the plasma membrane. The mechanism involved is facilitated (or mediated) diffusion in which transport is enhanced by specific integral membranous proteins called carriers. Carrier... [Pg.105]

Care should be exercised when attempting to interpret in vivo pharmacological data in terms of specific chemical—biological interactions for a series of asymmetric compounds, particularly when this interaction is the only parameter considered in the analysis (10). It is important to recognize that the observed difference in activity between optical antipodes is not simply a result of the association of the compound with an enzyme or receptor target. Enantiomers differ in absorption rates across membranes, especially where active transport mechanisms are involved (11). They bind with different affinities to plasma proteins (12) and undergo alternative metaboHc and detoxification processes (13). This ultimately leads to one enantiomer being more available to produce a therapeutic effect. [Pg.237]

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). [Pg.551]

A second mechanism that impinges on the localization of transporters is through the association with proteins, the most prominent example being syntaxin. Syntaxin is a t-SNARE protein necessary for the fusion of vesicles with the plasma membrane (see the chapter on exocytosis). On the cell surface syntaxin consistently stabilizes the localization of GABA, noradrenaline, glycine, and 5HT transporters the PKCa isoform can sever the interaction with syntaxin suggesting a general mechanism for transporter internalization. [Pg.840]

Protein trafficking is the transport of proteins to their correct subcellular compartments or to the extracellular space ( secretory pathway ). Endo- and exocytosis describe vesicle budding and fusion at the plasma membrane and are by most authors not included in the term protein trafficking. Protein quality control comprize all cellular mechanisms, monitoring protein folding and detecting aberrant forms. [Pg.1015]

Most proteins that are synthesized on membrane-bound polyribosomes and are destined for the Golgi apparatus or plasma membrane reach these sites inside transport vesicles. The precise mechanisms by which proteins synthesized in the rough ER are inserted into these vesicles are not known. Those involved in transport from the ER to the Golgi apparatus and vice versa—and from the Golgi to the plasma membrane— are mainly clathrin-free, unlike the coated vesicles involved in endocytosis (see discussions of the LDL receptor in Chapters 25 and 26). For the sake of clarity, the non-clathrin-coated vesicles will be referred to in... [Pg.508]


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