LOSS OF ASYMMETRIC DISTRIBUTION OF SARCOLEMMAL PHOSPHATIDYLETHANOLAMINE DURING SIMULATED ISCHEMIA IN THE ISOLATED NEONATAL RAT CARDIOMYOCYTE

In the present study we have investigated the reorganization of the sa rcolemmal phospholipids during the first 60 minutes of simulated ische mia (''ischemia'') as induced by anoxia, volume restriction, and nutri ent deprivation. Experiments were carried out on [H-3]acetate-labeled neonatal rat cardiomyocytes and isolated (nonradiolabeled) sarcolemmal membranes obtained from the same culture system. After 60 minutes of ''ischemia,'' cellular high-energy phosphate (ATP) levels had decrease d to approximately 40% of the control values, but no significant phosp holipid hydrolysis was detected. Labeling experiments using the nonper meant (primary amine-containing phospholipid) probe trinitrobenzenesul fonic acid and nonlytic treatment with (different) exogenous phospholi pases A2 were both indicative of a shifted transbilayer distribution o f the hexagonal(II) phase-preferring and fusion-promoting sarcolemmal phosphatidylethanolamine in favor of the outer membrane leaflet. This specific change in sarcolemmal phospholipid asymmetry preceded the los s of integrity of the sarcolemma, monitored by the release of lactate dehydrogenase as well as by scanning electron microscopy. It is propos ed that, in addition to the previously reported lateral phospholipid r eorganization, uncontrolled transbilayer movement of the non-bilayer-p referring phosphatidylethanolamine from the inner to the outer leaflet of the sarcolemma is an additional factor in destabilizing the lipid bilayer, eventually leading to the irreversible membrane damage seen a fter a prolonged period of ischemia.