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The mechanism of enzyme release from Langendorff-perfused rat hearts was studied under the injury conditions of the Ca2+ paradox and 2,4-dinitrophenol poisoning. During perfusion with Krebs-Ringer buffer or in buffered sucrose sarcoplasmic enzymes were massively released when Ca2+ was reintroduced to the perfusion medium (Ca2+ paradox). Mitochondrial matrix enzymes were released to a very small extent. Only the cytoplasmic isoenzyme of the bilocular enzyme malate dehydrogenase was released. The release kinetics of various enzymes with greatly differing molecular weights showed no significant differences. Qualitatively the same results were obtained under 2,4-dinitrophenol poisoned conditions in Ca2+ -free sucrose media. Sarcoplasmic enzymes were massively released, mitochondrial enzymes did not appear in the perfusate. 2,4-Dinitrophenol poisoning alone was not sufficient to cause enzyme release. An additional swelling under these conditions was necessary. ATP from the extracellular space was able to enhance the enzyme release, which was brought about by 2,4-dinitrophenol and cell swelling. A hypothesis is presented that enzyme release is produced by initiating a membrane blebbing process. An elevated intracellular Ca2+ concentration is a necessary prerequisite. In the presence of ATP, active membrane blebbing is caused by contractions of the membrane-anchored cytoskeleton. In the absence of ATP passive membrane blebbing is induced by cell swelling, provided that the cytoskeleton has been crosslinked by Ca2+.

Enzyme Release from the Perfused Rat Heart. The Functions of the Cytoskeleton under Cell-Pathological Conditions