Brain tissue ionic concentrations are indistinguishable from normal after 4 h of reperfusion following a 15-min cardiac arrest. The tissue iron has been recovered into high-molecular-weight species by 8 h of reperfusion. However, after 8 h large shifts of the concentrations of calcium, potassium, and sodium are observed. These shifts most likely reflect equilibration between the cytosol and the interstitial fluid for these ions. Electron microscopic examination of brains fixed in situ after 8 h reperfusion reveals in the vulnerable neurons an obvious general degradation of membrane structure with large holes in the plasmalemma. Nuclear chromatin is densely clumped, with grossly abnormal nuclear architecture. Mitochondrial architecture is well preserved. The ER is dilated and ragged, and normally arranged polyribosomes are virtually nonexistent. Histochemical evidence of lipid peroxidation can be seen by as early as 90 min following a 10-min arrest and involves approximately 30 percent of the neurons in the cortex and most neurons in CA1 and the hilus.14 Membrane injury by lipid peroxidation produces degradation of membrane structure to the point that the membrane becomes freely permeable to ions and the cell is irreversibly injured.
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