Association between inhibition of arachidonic acid release and prevention of calcium loading during ATP depletion in cultured rat cardiac myocytes

The development of irreversible myocardial ischemic injury is associated with progressive degradation of membrane phospholipids, accumulation of arachidonate and other free fatty acids, and electrolyte derangements, including calcium accumulation. To study the relationship between arachidonate release and calcium loading during adenosine triphosphate (ATP) depletion in cardiac myocytes, the effects of two purported phospholipase inhibitors, mepacrine and U26,384, were evaluated. Cultured neonatal rat ventricular myocytes were pretreated for 90 minutes with 5 to 10 microM U26,384 (a steroidal diamine) or 10 to 50 microM mepacrine (an alkyl acridine) and then treated for 3 hours with 30 microM of the metabolic inhibitor, iodoacetic acid (IAA), with or without an additional dose of drug. IAA treatment resulted in a marked reduction in ATP level and a several-fold increase in free fatty acid radioactivity released from myocytes prelabeled with tritiated arachidonic acid (3H-AA). U26,384 produced substantial inhibition of the increased 3H-AA release, and was effective when given as a single pretreatment dose before IAA exposure or as continuous treatment before and during IAA exposure (for example, with 5 microM U26,384, the percentage of 3H-AA release versus IAA alone was 8% +/- 2% [SEM] [N = 15] for pretreatment only and 13% +/- 4% [N = 10] for continuous treatment). Mepacrine also resulted in significant reduction in 3H-AA release, but was more effective when given as continuous treatment (for example, with 50 microM mepacrine, the percentage of 3H-AA release versus IAA alone was 43% +/- 9% [N = 6] for pretreatment only and 22% +/- 7% [N = 9] for continuous treatment). More detailed analysis showed that U26,384 and mepacrine blocked the IAA-induced redistribution of 3H-AA into free fatty acids from other lipid species. Electron probe x-ray microanalysis of freeze-dried cryosections revealed marked electrolyte derangements in myocytes exposed to IAA, including a 24-fold increase in cellular Ca, a four fold increase in cellular Na, and a seven fold decrease in cellular K, and associated changes in cytoplasm and mitochondria. U26,384 treatment markedly reduced these electrolyte abnormalities, and maintained normal Ca levels in some protocols. Mepacrine treatment was less effective, but did produce normal Ca levels in 50% of myocytes. Prevention of IAA-induced cellular hypercontraction and blebbing also was observed. These data support the hypothesis that reduction of free fatty acid accumulation by inhibition of accelerated phospholipid degradation is associated with protection of myocytes from calcium loading and morphologic damage during inhibition of ene