Cytosolic contents

Death by apoptosis is much less spectacular and is usually quicker than necrosis. Cells shrink, possibly to a quarter of their previous size, and vesicles appear in the cytosol and in the nucleus. As the cytosol contracts, the chromatin fragments into a number of distinct particles and the endoplasmic reticulum fuses with the plasma membrane. The dying cell breaks up into small vesicles (apop-totic bodies) which are removed by phagocytosis (Figure 20.34). Since little if any of the cytosolic contents escapes, there is little or no inflammation. That is, death by apotosis rather than reduces the risk of local or general inflammation. In fact, death by apoptosis is so inconspicuous that it was not discovered for many years. 

The cell membrane serves as a protective barrier in renal cells. It is the initial site which p-lactams encounter in their journey to the cellular environment from the blood or tubular fluid, p-lactams may disrupt the functional organization of the membrane through peroxidation of membrane lipids, which, in turn, leads to the inability of membrane to serve as an osmotic barrier and causes the cytosol contents to leak. As a result of the cephalosporins disruptive effect on cell membrane, increased leakage of the cytosolic enzyme lactate dehydrogenase (LDH) occurs. The increased LDH concentration was from the cytosol of the renal cortex [49,71] or from isolated proximal and distal tubular cells [39] or in the urine of experimental animals [39]. The results of these studies indicate that plasma membrane became permeable to large molecules such as LDH. After cephalosporin treatment, cephaloridine caused the greatest decrease of LDH concentration in cytosol [49]. Whereas, cephaloridine induced a greater release of LDH from proximal tubular cells than cepha-lothin and cephalexin, distal cells were not affected by any of these cephalosporins [38,39]. 

By contrast, in necrosis, severe physical, chemical, or bacterial damage causes a cell membrane to burst, releasing apoptotic mediators (Sect. 13.4.3) and proinflammatory cytokines into the stroma (Sect. 13.2.2). The cytosolic enzymes continue to make lactic acid in the absence of mitochondrial function, making the necrotic environment strongly acidic and activating lysosomal enzymes (Sect. 13.2.1) to digest the released cytosolic contents. Necrosis is discussed in relation to aggressive periodontitis (Chap. 14). 

The detoxification isoenzyme a-GST is mainly located in cen-trilobular hepatocytes and is a kidney proximal renal tubular cytosolic enzyme (Harrison et al., 1989 de Geus et al., 2012). When proximal renal tubular epithelial cells are damaged, their cytosolic contents enter the tubular fluid and appear in the urine for short periods around acute cellular injury. The a-GST is not stable in acid urine, and this may limit its utility in preclinical and clinical studies (Vaidya et al., 2008). 

Little is known about the NADH aitd NAD content of the cytosol. In order to estimate the NADH/NAD ratio, the cytosolic contents of malate, aspartate, glutamate and 2-oxoglutarate were determined by nonaqueous fractionation of spinach leaves (Table l). From these values the NADH/NAD ratio was calculated on the reasonable assumption that the reactions catalyzed by the cytosolic malate dehydrogenase and glutamate oxaloacetate transaminase are near to equilibrium. Introducing the equilibrium constants of these enzymes 2.8. 10 at pH 7.0 (4), Kqqt... 

Osmotic pressure from high concentrations of dissolved solutes is a serious problem for cells. Bacterial and plant cells have strong, rigid cell walls to contain these pressures. In contrast, animal cells are bathed in extracellular fluids of comparable osmolarity, so no net osmotic gradient exists. Also, to minimize the osmotic pressure created by the contents of their cytosol, cells tend... 

Synthesis of the transferrin receptor (TfR) and that of ferritin are reciprocally linked to cellular iron content. Specific untranslated sequences of the mRNAs for both proteins (named iron response elements) interact with a cytosolic protein sensitive to variations in levels of cellular iron (iron-responsive element-binding protein). When iron levels are high, cells use stored ferritin mRNA to synthesize ferritin, and the TfR mRNA is degraded. In contrast, when iron levels are low, the TfR mRNA is stabilized and increased synthesis of receptors occurs, while ferritin mRNA is apparently stored in an inactive form. This is an important example of control of expression of proteins at the translational level. 

Metallothioneins are a group of small proteins (about 6.5 kDa), found in the cytosol of cells, particularly of liver, kidney, and intestine. They have a high content of cysteine and can bind copper, zinc, cadmium, and mercury. The SH groups of cysteine are involved in binding the metals. Acute intake (eg, by injection) of copper and of certain other metals increases the amount (induction) of these proteins in tissues, as does administration of certain hormones or cytokines. These proteins may function to store the above metals in a nontoxic form and are involved in their overall metaboHsm in the body. Sequestration of copper also diminishes the amount of this metal available to generate free radicals. 

Proulx [30] summarized the published lipid compositions of BBM isolated from epithelial cells from pig, rabbit, mouse and rat small intestines. Table 3.1 shows the lipid make-up for the rat, averaged from five reported studies [30], On a molar basis, cholesterol accounts for about 50% of the total lipid content (37% on a weight basis). Thus, the cholesterol content in BBM is higher than that found in kidney epithelial (MDCK) and brain endothelial cells (Table 3.1). Slightly different BBM lipid distribution was reported by Alcorn et al. [31] here, the outer (luminal) leaflet of the BBM was seen to be rich in sphingomyelin content, while the inner leaflet (cytosol) was rich in PE and PC. Apical (brush border) and basolateral lipids are different in epithelia. The basolateral membrane content (not reported by... 

There is considerable evidence that the release of 5-HT occurs by exocytosis, i.e. by the discharge from the cell of the entire content of individual storage vesicles. First, 5-HT is sufficiently ionized at physiological pH so that it does not cross plasma membranes by simple diffusion. Second, most intraneuronal 5-HT is contained in storage vesicles and other contents of the vesicle including SPB are released together with serotonin. By contrast, cytosolic proteins do not accompany electrical stimulation-elicited release of 5-HT. Third, the depolarization-induced release of 5-HT occurs by a calcium-dependent process indeed, it appears that the influx of extracellular calcium ions with or without membrane depolarization can increase the release of 5-HT. Calcium stimulates the fusion of vesicular membranes with the plasma membrane (see Chs 9,10). 

NMDA and AMPA receptors are spread across the post-synaptic density (PSD), whereas metabotropic glutamate receptors (except mGluR7) are located along the periphery of the PSD (Fig. 15-2). NMDA receptors appear to be present at most or all glutamatergic synapses whereas the content of AMPA receptors is variable - from zero to about 50 receptors per PSD [33]. Some synapses are silent , meaning that activation of them does not elicit AMPA receptor currents when the plasma membrane is hyperpolarized and Mg2+ blocks NMDA receptors. Such silent synapses contain only NMDA receptors. However, AMPA receptors are recruited from the cytosol to the PSD to activate such silent synapses in LTP. 

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