Thrombin activates a membrane-associated calcium-independent PLA₂ in ventricular myocytes

Activation of phospholipase A₂ (PLA₂) and accumulation of lysophosphatidylcholine contribute importantly to arrhythmogenesis during acute myocardial ischemia. We examined thrombin stimulation of PLA₂ activity in isolated ventricular myocytes. Basal and thrombin-stimulated cardiac myocyte PLA₂ activity demonstrated a distinct preference for sn-1 ether- linked phospholipids with arachidonate esterified at the sn-2 position. The majority of PLA₂ activity was calcium independent and membrane associated. Thrombin stimulation of membrane-associated PLA₂ occurs in a time- and concentration-dependent fashion. An increase in PLA₂ activity was also observed using the synthetic peptide SFLLRNPNDKYEPF (the tethered ligand generated by thrombin cleavage of its receptor). Bromoenol lactone, a selective inhibitor of calcium-independent PLA₂, completely blocked thrombin-stimulated increases in PLA₂ activity and arachidonic acid release. No significant inhibition of thrombin-induced PLA₂ was observed following pretreatment with mepacrine or dibucaine. These data confirm the presence of high-affinity thrombin receptors on isolated cardiac myocytes and demonstrate the specific activation of a unique membrane-associated, calcium-independent PLA₂ following thrombin receptor ligation.