Depressed responsiveness of phospholipase C isoenzymes to phosphatidic acid in congestive heart failure

The cardiac sarcolemmal membrane cis-unsaturated fatty acid-sensitive phosp holipase hydrolyzes phosphalidylcholine to form phosphatidic acid. The Func tional significance of phosphatidic acid is indicated by its ability to inc rease [Ca2+](i) and augment cardiac contractile performance via the activat ion of phospholipase C, Accordingly, we tested the hypothesis that a defect occurs in the membrane level of phosphatidic acid and/or the responsivenes s of cardiomyocytes to phosphatidic acid in congestive heart failure due to myocardial infarction. Myocardial infarction was produced in rats by ligat ion of the left corollary artery while sham-operated animals served as cont rol, At 8 weeks after surgery, the experimental animals were at a stage of moderate congestive heart Failure. Compared to sham controls, phosphatidic acid-mediated increase in [Ca2+](i), as determined by the fura 2-AM techniq ue, was significantly reduced in failing cardiomyocytes. Immunoprecipitatio n of sarcolemmal phospholipase C isoenzymes using specific monoclonal antib odies revealed that the stimulation of phospholipase C gamma (1) and delta (1) phosphatidylinositol 4,5-bisphosphate hydrolyzing activities by phospha lidic acid was decreased in the failing heart. Although the activity of pho spholipase C beta (1) in the failing heart was higher than the control, pho sphatidic acid did not stimulate this isoform in control sarcolemma, and pr oduced an inhibitory action in the failing heart preparation. Furthermore, the specific binding of phosphatidic acid to phospholipase C gamma (1) and delta (1) isoenzymes was decreased, whereas binding to phospholipase beta ( 1) was absent in the failing heart. A reduction in the intramembranal level of phosphatidic acid derived via cis-unsaturated fatty acid-sensitive phos pholipase D was also seen in the failing heart, These findings suggest that a defect in phosphatidic acid-mediated signal pathway in sarcolemma may re present a novel mechanism of heart dysfunction in congestive heart failure. (C) 2001 Academic Press.