Endothelial cells nitric oxide formation

Another central element of the Grishko hypothesis is the upregulation of inducible nitric oxide synthase (iNOS). Since the induction of iNOS is often associated with extensive nitric oxide formation, it is commonly a toxic event. However, nitric oxide formation is not detrimental in all cell types. Dimmeler et al. (1997) found that nitric oxide protects human endothelial cells against angiotensin II-mediated apoptosis. A key variable appears to be the amount of nitric oxide produced. In the cardiomyocyte, sufficient levels of nitric oxide are generated to cause the formation of toxic levels of peroxynitrite, while in endothelial cells, ROS, rather than reactive nitrogen species, appear to be the toxic mediators of apoptosis. 

Beckman, J.S., Beckman, T.W., Chen, J., Marshall, P.A. and Freeman, B.A. (1990). Apparent hydroxyl radical formation by peroxynitrite implications for endothelial cell injury from nitric oxide and superoxide anion. Proc. Natl Acad. Sci. USA 87, 1620-1624. 

A. Pailleret, J. Oni, S. Reiter, S. Isik, M. Etienne, F. Bedioui, and W. Schuhmann, In situ formation and scanning electrochemical microscopy assisted positioning of NO-sensors above human umbilical vein endothelial cells for the detection of nitric oxide release. Electrochem. Commun. 5, 847-852 (2003). 

FIGURE 14. Proposed scheme of formation of NOC by bacterial activated macrophages and endothelial cells through intermediate nitric oxide... 

Figure 22.4 Injury to endothelial cells can lead to vasospasm. Normal endothelial cells release nitric oxide (NO) which relaxes smooth muscle this is achieved by nitric oxide increasing the concentration of cyclic GMP within smooth muscle fibres and cyclic GMP relaxing the smooth muscle. Injured endothelial cells secrete very little nitric oxide but secrete more endothelin. The latter increases the formation of inositol trisphosphate (IP3), which binds to the sarcoplasmic reticulum (SR) where it stimulates the Ca ion channel. The Ca ion channel in the plasma membrane is also activated. Both effects result in an increase in cytosolic Ca ion concentration, which then stimulates contraction (vasospasm). This reduces the diameter of the lumen of the artery.

The diverse actions of AM are mediated by the 7-transmembrane G protein-coupled calcitonin receptor-like receptor (CRLR) which coassembles with subtypes 2 and 3 of a family of receptor-activity-modifying proteins (RAMPs), thus forming a receptor-coreceptor system. Binding of AM to CRLR activates Gs and triggers cAMP formation in vascular smooth muscle cells, and increases nitric oxide production in endothelial cells. Other signaling pathways are also involved. 

The above examples point out at the direct stimulation of apoptosis by nitric oxide. At the same time, the exclusively rapid reaction of NO with superoxide always suggests the possibility of peroxynitrite participation in this process [141] correspondingly, the role peroxynitrite in the stimulation of apoptosis has been considered. Bonfoco et al. [144] has found that the producers of low peroxynitrite concentrations during the exposure of cortical neurons to the low level of NMDA or the use of peroxynitrite donors resulted in an apoptosis in neurons, while the high concentrations of peroxynitrite induced necrotic cell damage. The formation of peroxynitrite is apparently responsible for NO-stimulated apoptosis in superoxide-generating transformed fibroblasts because nontransformed cells, which do not produce superoxide, were not affected by nitric oxide [145]. It is of interest that proapoptotic effect of peroxynitrite may depend on the cell type. Thus, the formation of peroxynitrite enhanced the NO-induced apoptosis in glomerular endothelial cells, while superoxide inhibited the formation of ceramide and apoptosis in these cells exposed to nitric oxide probably due to peroxynitrite formation... 

A study has been undertaken to clarify whether glucocorticoid excess affects endothelium-dependent vascular relaxation in glucocorticoid treated patients and whether dexamethasone alters the production of hydrogen peroxide and the formation of peroxynitrite, a reactive molecule between nitric oxide and superoxide, in cultured human umbilical endothelial cells (7). Glucocorticoid excess impaired endothelium-dependent vascular relaxation in vivo and enhanced the production of reactive oxygen species to cause increased production of peroxynitrite in vitro. Glucocorticoid-induced reduction in nitric oxide availability may cause vascular endothelial dysfunction, leading to hypertension and atherosclerosis. 

Janssens S, Flaherty D, Nong Z, et al. Human endothelial nitric oxide synthase gene transfer inhibits vascular smooth muscle cell proliferation and neointima formation after balloon injury in rats. Circulation 1998 97 1274-1281. 

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