Activation of Phospholipases during Myocardial Ischemia and Their Probable Role in Arrhythmogenesis

Abstract

The isolated perfused cat hearts incorporated radioactive arachidonic (20:4) and stearic (18:0) acids in the rapidly metabolizing pool of membrane phospholipids. Stereospecific analysis of the phospholipid fractions studied revealed that all of the 20:4 was acylated at the sn-2 position and about 80% of 18:0 was acylated at the sn-1 position of the phospholipid fractions studied. Comparison of the radioactivities of 20:4 at 2-position to 18:0 at 1-position of individual phospholipid fractions obtained from ischemic and non-ischemic zones of the same heart after 40 min of LAD occlusion showed a significant decrease in the ratio (20:4/18:0) of radioactivities. This decrease (19–50%) in the ratio for phosphatidylcholine and phosphatidylethanolamine in the ischemic myocardium suggests that phospholipase A2 activity in comparison to A1 is accelerated due to ischemia. The decrease (16%) in the ratio for phosphatidylinositol (PI) probably suggests increased turnover due to stimulated PI-specific phospholipase C activity.Isolated Purkinje fibers or moderator bands containing both Purkinje and muscle fibers were superfused with various concentrations of lysophosphoglycerides (lyso-PL) in tissue bath and electrophysiological characteristics (EP) were monitored. Concentrations of exogenously superfused lyso-PL equivalent to that of total lyso-PL found in ischemic porcine hearts (0.16 mM) or 3 times the amount did not alter EP of either fiber. When lyso-PL (0.4 mM) were produced endogenously in sarcolemma by superfusing with buffer containing phospholipase A2, no changes in EP were observed. When higher endogenous lyso-PL concentrations (0.6–1.0 mM) were produced, action potential deteriorated but normal EP recovered after 6 min of washing with oxygenated normal buffer. At complete recovery, lyso-PL levels in fibers remained high (0.4 mM). Superfusion of fibers with buffer containing phospholipase C which resulted in <1.0% loss of total phospholipids with no lyso-PL production, completely abolished AP in 6 min, the fibers remaining unresponsive to stimulus even after prolonged washings with oxygenated normal buffer. These results suggest that lyso-PL alone may not be responsible for the electrophysiological manifestations of ischemia and that the minor loss of sarcolemmal phospholipids play an important role in membrane dysfunction.KeywordsPhospholipid FractionPurkinje FiberLeft Anterior Descend OcclusionTissue BathNormal BufferThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.