Biochemical events associated with rapid cellular damage during the oxygen-and calcium-paradoxes of the mammalian heart

Summary The O_2– and Ca²⁺-paradoxes have a number of features in common and it is suggested that release of cytosolic proteins in both paradoxes is initiated by the activation of a sarcolemma NAD(P)H dehydrogenase which can generate a transmembrane flow of H⁺ and e^– and also oxygen radicals or recox cycling which damage ion channels and membrane proteins (phase I). Entry of Ca²⁺ through the damaged ion channels then exacerbates the damage by further activating this system, either directly or indirectly, and the redox cycling and/or oxygen radicals cause further damage to integral and cytoskeletal proteins of the sarcolemma resulting in microdamage to the integrity of the membrane (phase II) and the consequent release or exocytosis of cytoplasmic proteins and, under specialised condition, the blebbing of the sarcolemma. The system may be primed either by removal of extracellular Ca²⁺ or by raising Ca²⁺]_i by a variety of measures, these two actions being synergistic. The system is initially activated in the Ca²⁺-paradox by the membrane perturbation associated with removal of extracellular Ca²⁺; prolonged anoxia in the metabolically active cardiac muscle causes a depletion of the ATP supply, particularly in the absence of glucose, and hence a rise in Ca²⁺]_i in phase I of the oxygen paradox with the consequent activation of the NAD(P)H oxidase at the sarcolemma. Oxygen radicals are probably generated in both paradoxes and may have a partial role in the genesis of damage, but are not essential in the Ca²⁺-paradox which continues under anoxia. Massive entry of Ca²⁺ also activates an intracellularly localised dehydrogenase (probably at the SR) which produces myofilament damage by redox cycling.