Apoptosis versus oncotic necrosis in hepatic ischemia/reperfusion injury

AbstractWarm and cold hepatic ischemia followed by reperfusion leads to necrotic cell death (oncosis), which often occurs within minutes of reperfusion. Recent studies also suggest a large component of apoptosis after ischemia/reperfusion. Here, we review the mechanisms underlying adenosine triphosphate depletion—dependent oncotic necrosis and caspase-dependent apoptosis, with emphasis on shared features and pathways. Although apoptosis causes internucleosomal DNA degradation that can be detected by terminal deoxynucleotidyl transferase—mediated deoxyuridine triphosphate nick-end labeling and related assays, DNA degradation also occurs after oncotic necrosis and leads to pervasive terminal deoxynucleotidyl transferase—mediated deoxyuridine triphosphate nick-end labeling staining far in excess of that for apoptosis. Similarly, although apoptosis can occur in a physiological setting without inflammation, in pathophysiological settings apoptosis frequently induces inflammation because of the onset of secondary necrosis and stimulation of cytokine and chemokine formation. In liver, the mitochondrial permeability transition represents a shared pathway that leads to both oncotic necrosis and apoptosis. When the mitochondrial permeability transition causes severe adenosine triphosphate depletion, plasma membrane failure and necrosis ensue. If adenosine triphosphate is preserved, at least in part, cytochrome c release after the mitochondrial permeability transition activates caspase-dependent apoptosis. Mitochondrial permeability transition-dependent cell death illustrates the concept of necrapoptosis, whereby common pathways lead to both necrosis and apoptosis. In conclusion, oncotic necrosis and apoptosis can share features and mechanisms, which sometimes makes discrimination between the 2 forms of cell death difficult. However, elucidation of critical cell death pathways under clinically relevant conditions will show potentially important therapeutic intervention strategies in hepatic ischemia/reperfusion injury

Distribution of electrical potential, pH, free Ca2+, and volume inside cultured adult rabbit cardiac myocytes during chemical hypoxia: a multiparameter digitized confocal microscopic study.

Exploiting the optical sectioning capabilities of laser scanning confocal microscopy and using parameter-specific fluorescent probes, we determined the distribution of pH, free Ca2+, electrical potential, and volume inside cultured adult rabbit cardiac myocytes during ATP depletion and reductive stress with cyanide and 2-deoxyglucose ("chemical hypoxia"). During normoxic incubations, myocytes exhibited a cytosolic pH of 7.1 and a mitochondrial pH of 8.0 (delta pH = 0.9 units). Sarcolemmal membrane potential (delta psi) was -80 mV, and mitochondrial delta psi was as high as -100 mV, yielding a mitochondrial protonmotive force (delta p) of -155 mV (delta P = delta psi - 60 delta pH). After 30 min of chemical hypoxia, mitochondrial delta pH decreased to 0.5 pH units, but mitochondrial delta psi remained essentially unchanged. By 40 min, delta pH was collapsed, and mitochondrial and cytosolic free Ca2+ began to increase. Mitochondrial and sarcolemmal delta psi remained high. as Ca2+ rose, myocytes shortened, hypercontracted, and blebbed with a 30% decrease of cell volume. After hypercontraction, extensive mitochondrial Ca2+ loading occurred. After another few minutes, mitochondrial depolarized completely and released their load of Ca2+. After many more minutes, the sarcolemmal permeability barrier broke down, and viability was lost. These studies demonstrate a sequence of subcellular ionic and electrical changes that may underlie the progression to irreversible hypoxic injury