Ultimately, there is irreversible damage to mitochondrial and lysosomal membranes, and the cell undergoes necrosis. Membrane damage is a late event in cell injury caused by diverse mechanisms, and is discussed later. Although necrosis is the principal form of cell death caused by hypoxia, apoptosis by the mitochondrial pathway is also thought to contribute.
The functional consequences of hypoxia and ischemia depend on the severity and duration of the deficit. For instance, the heart muscle ceases to contract within 60 seconds of coronary occlusion. If hypoxia continues, worsening ATP depletion causes further deterioration, undergoing the sequence of changes illustrated in
Fig. 2.3 and described earlier.
Under certain circumstances, the restoration of blood flow to ischemic but viable tissues results, paradoxically, in increased cell injury. This is the reverse of the expected outcome of the restoration of blood flow, which normally results in the recovery of reversibly injured cells. This so-called “ischemia-reperfusion injury” is a clinically important process that may contribute significantly to tissue damage, especially after myocardial and cerebral ischemia.
Several mechanisms may account for the exacerbation of cell injury resulting from reperfusion of ischemic tissues:
New damage may be initiated during reoxygenation by increased generation of ROS (described in more detail below). Some of the ROS may be generated by injured cells with damaged mitochondria that cannot carry out the complete reduction of oxygen, and at the same time cellular anti-oxidant defense mechanisms may be compromised by ischemia, exacerbating the situation. ROS generated by infiltrating leukocytes also may contribute to the damage of vulnerable injured cells.
The inflammation that is induced by ischemic injury may increase with reperfusion because it enhances the influx of leukocytes and plasma proteins. The products of activated leukocytes may cause additional tissue injury (Chapter 3). Activation of the complement system also may contribute to ischemia-reperfusion injury. Complement proteins may bind to the injured tissues, or to antibodies that are deposited in the tissues, and subsequent complement activation generates byproducts that exacerbate the cell injury and inflammation.
Oxidative stress refers to cellular abnormalities that are induced by ROS, which belong to a group of molecules known as free radicals. Free radical-mediated cell injury is seen in many circumstances, including chemical and radiation injury, hypoxia, cellular aging, tissue injury caused by inflammatory cells, and ischemia-reperfusion injury. In all these cases, cell death may be by necrosis, apoptosis, or the mixed pattern of necroptosis.
Free radicals are chemical species with a single unpaired electron in an outer orbit. Such chemical states are extremely unstable, and free radicals readily react with inorganic and organic molecules; when generated in cells, they avidly attack nucleic acids as well as a variety of cellular proteins and lipids. In addition, free radicals initiate reactions in which molecules that react with the free radicals are themselves converted into other types of free radicals, thereby propagating the chain of damage.
Generation and Removal of Reactive Oxygen Species
The accumulation of ROS is determined by their rates of production and removal (
Fig. 2.17 ). ROS are produced by two major pathways.