All cells in living organisms have a limited life span, and when the time set by their biological clocks has run out, they will die , their highly orga­ nized macromolec ular structure will disintegrate, and the low-molec ul ar degrad ation products be­ come part of the disorder of the surroundings . In thermodynamic terms, this series of events consti­ tutes what has been c alled the entropic doom. We know little about those molec ul ar mechanisms that limit the life span of cells in the absence of dise ase. For the clinician, therefore, the important task is to prevent or tre at dise ases that jeopardize the proper functioning and viability of cells whose biologic al clocks have not yet run out . Measures instituted to prevent brain cell death have a special importance . This i s partly due t o the fact that w e equate human life with the functioning of the brain. However, this importance also resides in the v ulnerability of brain cells to conditions that allow cells of most other tissue to survive , and to continue or res ume their activitie s . The clinically most important conditions leading to brain cell death are those associated with cere­ brovascul ar dise ase, particularly stroke, and with head trauma. We will begin, though, by rec alling the traditional view of the occ urrence and loc alization of neuronal damage in four conditions that lead to more disseminated alterations, e specially since they have been considered to c ause nerve cell injury of a

https://journals.sagepub.com/doi/pdf/10.1038/jcbfm.1981.18

Cell Damage in the Brain: A Speculative Synthesis

✓ The mechanisms that give rise to ischemic brain damage have not been definitively determined, but considerable evidence exists that three major factors are involved: increases in the intercellular cytosolic calcium concentration (Ca++i), acidosis, and production of free radicals. A nonphysiological rise in Ca++i due to a disturbed pump/leak relationship for calcium is believed to cause cell damage by overactivation of lipases and proteases and possibly also of endonucleases, and by alterations of protein phosphorylation, which secondarily affects protein synthesis and genome expression. The severity of this disturbance depends on the density of ischemia. In complete or near-complete ischemia of the cardiac arrest type, pump activity has ceased and the calcium leak is enhanced by the massive release of excitatory amino acids. As a result, multiple calcium channels are opened. This is probably the scenario in the focus of an ischemic lesion due to middle cerebral artery occlusion. Such ischemic tissues ca...

Pathophysiology and treatment of focal cerebral ischemia. Part II: Mechanisms of damage and treatment.

During renal ischemia, ATP is degraded to hypoxanthine. When xanthine oxidase converts hypoxanthine to xanthine in the presence of molecular oxygen, superoxide radical (O-2) is generated. We studied the role of O-2 and its reduction product OH X in mediating renal injury after ischemia. Male Sprague-Dawley rats underwent right nephrectomy followed by 60 min of occlusion of the left renal artery. The O-2 scavenger superoxide dismutase (SOD) was given 8 min before clamping and before release of the renal artery clamp. Control rats received 5% dextrose instead. Plasma creatinine was lower in SOD treated rats: 1.5, 1.0, and 0.8 mg/dl vs. 2.5, 2.5, and 2.1 mg/dl at 24, 48, and 72 h postischemia. 24 h after ischemia inulin clearance was higher in SOD treated rats than in controls (399 vs. 185 microliter/min). Renal blood flow, measured after ischemia plus 15 min of reflow, was also greater in SOD treated than in control rats. Furthermore, tubular injury, judged histologically in perfusion fixed specimens, was less in SOD treated rats. Rats given SOD inactivated by prior incubation with diethyldithiocarbamate had plasma creatinine values no different from those of control rats. The OH X scavenger dimethylthiourea (DMTU) was given before renal artery occlusion. DMTU treated rats had lower plasma creatinine than did controls: 1.7, 1.7, and 1.3 mg/dl vs. 3.2, 2.2, and 2.4 mg/dl at 24, 48, and 72 h postischemia. Neither SOD nor DMTU caused an increase in renal blood flow, urine flow rate, or solute excretion in normal rats. The xanthine oxidase inhibitor allopurinol was given before ischemia to prevent the generation of oxygen free radicals. Plasma creatinine was lower in allopurinol treated rats: 2.7, 2.2, and 1.4 mg/dl vs. 3.6, 3.5, and 2.3 mg/dl at 24, 48, and 72 h postischemia. Catalase treatment did not protect against renal ischemia, perhaps because its large size limits glomerular filtration and access to the tubular lumen. Superoxide-mediated lipid peroxidation was studied after renal ischemia. 60 min of ischemia did not increase the renal content of the lipid peroxide malondialdehyde, whereas ischemia plus 15 min reflow resulted in a large increase in kidney lipid peroxides. Treatment with SOD before renal ischemia prevented the reflow-induced increase in lipid peroxidation in renal cortical mitochondria but not in crude cortical homogenates. In summary, the oxygen free radical scavengers SOD and DMTU, and allopurinol, which inhibits free radical generation, protected renal function after ischemia. Reperfusion after ischemia resulted in lipid peroxidation; SOD decreased lipid peroxidation in cortical mitochondria after renal ischemia and reflow. We concluded that restoration of oxygen supply to ischemic kidney results in the production of oxygen free radicals, which causes renal injury by lipid peroxidation.

/pdf/oxygen-free-radicals-in-ischemic-acute-renal-failure-in-the-508vnqgtu0.pdf

Oxygen free radicals in ischemic acute renal failure in the rat.

This article examines the pathophysiology of lesions caused by focal cerebral ischemia. Ischemia due to middle cerebral artery occlusion encompasses a densely ischemic focus and a less densely ischemic penumbral zone. Cells in the focus are usually doomed unless reperfusion is quickly instituted. In contrast, although the penumbra contains cells "at risk," these may remain viable for at least 4 to 8 hours. Cells in the penumbra may be salvaged by reperfusion or by drugs that prevent an extension of the infarction into the penumbral zone. Factors responsible for such an extension probably include acidosis, edema, K+/Ca++ transients, and inhibition of protein synthesis. Central to any discussion of the pathophysiology of ischemic lesions is energy depletion. This is because failure to maintain cellular adenosine triphosphate (ATP) levels leads to degradation of macromolecules of key importance to membrane and cytoskeletal integrity, to loss of ion homeostasis, involving cellular accumulation of Ca++, Na+, and Cl-, with osmotically obligated water, and to production of metabolic acids with a resulting decrease in intra- and extracellular pH. In all probability, loss of cellular calcium homeostasis plays an important role in the pathogenesis of ischemic cell damage. The resulting rise in the free cytosolic intracellular calcium concentration (Ca++) depends on both the loss of calcium pump function (due to ATP depletion), and the rise in membrane permeability to calcium. In ischemia, calcium influx occurs via multiple pathways. Some of the most important routes depend on activation of receptors by glutamate and associated excitatory amino acids released from depolarized presynaptic endings. However, ischemia also interfers with the intracellular sequestration and binding of calcium, thereby contributing to the rise in intracellular Ca++. A second key event in the ischemic tissue is activation of anaerobic glucolysis. The main reason for this activation is inhibition of mitochondrial metabolism by lack of oxygen; however, other factors probably contribute. For example, there is a complex interplay between loss of cellular calcium homeostasis and acidosis. On the one hand, a rise in intracellular Ca++ is apt to cause mitochondrial accumulation of calcium. This must interfere with ATP production and enhance anaerobic glucolysis. On the other hand, acidosis must interfere with calcium binding, thereby contributing to the rise in intracellular Ca++.

Pathophysiology and treatment of focal cerebral ischemia: Part I: Pathophysiology

Increasing evidence suggests that glutamate neurotoxicity is partly mediated by reactive oxygen species, formed as a consequence of several processes, including arachidonic acid metabolism and nitric oxide production. Here we used an oxidation-sensitive indicator, dihydrorhodamine 123, in combination with confocal microscopy, to examine the hypothesis that electron transport by neuronal mitochondria may be an important source of glutamate-induced reactive oxygen species (ROS). Exposure to NMDA, but not kainate, ionomycin, or elevated potassium stimulated oxygen radical production in cultured murine cortical neurons, demonstrated by oxidation of nonfluorescent dihydrorhodamine 123 to fluorescent rhodamine 123. Electron paramagnetic resonance spectroscopy studies using 5,5-dimethyl-1- pyrroline-N-oxide (DMPO) as a radical-trapping agent, also showed production of ROS by cortical neurons after NMDA but not kainate exposure. NMDA-induced ROS production depended on extracellular Ca2+, and was not affected by inhibitors of nitric oxide synthase or arachidonic acid metabolism. The increased production of ROS was blocked by inhibitors of mitochondrial electron transport, rotenone or antimycin, and mimicked by the electron transport uncoupler, carbonyl cyanide p-trifluoromethoxyphenylhydrazone. These data support the possibility that NMDA receptor-mediated, Ca(2+)-dependent uncoupling of neuronal mitochondrial electron transport may contribute to the oxidative stress initiated by glutamate exposure.

https://www.jneurosci.org/content/jneuro/15/10/6377.full.pdf

Mitochondrial production of reactive oxygen species in cortical neurons following exposure to N-methyl-D-aspartate

The possibility that cerebral ischemia may initiate a series of pathological free radical reactions within the membrane components of the CNS was investigated in the cat. The normally occurring electron transport radicals require adequate molecular oxygen for orderly transport of electrons and protons. A decrease in tissue oxygen removes the controls over the electron transport radicals, and allows them to initiate pathologic radical reactions among cell membranes such as mitochondria. Pathologic radical reactions result in multiple products, each of which may be present in too small a concentration to permit their detection at early time periods. It is possible to follow the time course, however, by the decrease of a major antioxidant as it is consumed by the pathologic radical reactions. For this reason, ascorbic acid was measured in ischemic and control brain following middle cerebral artery occlusion. There was a progressive decrease in the amount of detectable ascorbic acid ranging from 25% at 1 hour to 65% at 24 hours after occlusion. The reduction of this normally occurring antioxidant and free radical scavenger may indicate consumption of ascorbic acid in an attempt to quench pathologic free radical reactions occurring within the components of cytomembranes.

Free radicals in cerebral ischemia.

Studies of the role of free radical damage to the spinal cord following a 400 g-cm impact have suggested an increase in at least one free radical product, malonaldehyde, 24–36 hr post injury. To investigate further the role of free radical lipid peroxidation in degeneration of the spinal cord following injury, a study of specific lipid fluorescence (SLF) indicative of the double Schiff-base adduct formed by a reaction between malonaldehyde and cellular components was carried out in the presence of ethanol, a known potentiator of free radical lipid peroxidation. The study was carried out in cats who received a 200 g-cm impact 3 hr to 23 days prior to sacrifice. Half of the impacted animals received ethanol, 5 ml/kg, prior to injury. These animals were rendered paraplegic, whereas the nonethanol treated animals were neurologically intact. Controls consisting of laminectomies alone or laminectomies with ethanol but without injury were also studied. Spinal cord segments at the impact or laminectomy site were minced and extracted with chloroform-methanol, cleared by centrifugation, and examined in a scanning fluorometer with excitation maximum at 360 nm and emission maxima at 420, 440, 450, and 460 nm. SLF was minimal in cats 3 hr and 1 day post injury, but markedly increased at 3 days. By 5 days, background levels were again found in all groups. SLF in the alcohol-pretreated impact animals rose to a peak at 7 days, followed by a decline to background by 10 days. The presence of SLF supports a role for free radical lipid peroxidation in the degenerative changes in the spinal cord following injury. The findings of two peaks of SLF activity suggest two different sites of damage. One site, found acutely after injury, appears in all groups and was associated with reversible changes, while the other site is associated with later changes and chronic paraplegia only. The two sites could be the gray and white matter.

Richard G. Poser

Papers

Free radicals in cerebral ischemia.

Spectrofluorescent detection of malonaldehyde as a measure of lipid free radical damage in response to ethanol potentiation of spinal cord trauma.