Phospholipase D in rat myocardium: formation of lipid messengers and synergistic activation by G-protein and protein kinase C

Abstract

Activation of phospholipase D (PLD) and phosphoinositide-specific phospholipase C (PI-PLC) by fluoride, to stimulate heterotrimeric G-proteins, and by phorbol esters, to stimulate protein kinase C (PKC), was studied in rat atria. Fluoride and 4β-phorbol-12β,13α-dibutyrate (PDB), in contrast to 4β-phorbol-13α-acetate (PAc), activated PLD, catalyzing the formation of [ 3H]-phosphatidylethanol ([ 3H]-PETH), [ 3H]-phosphatidic acid ([ 3H]-PA), choline and sn-1,2-diacylglycerol (DAG). Basal PLD activity was resistant to drastic changes in Ca 2+ and to Ro 31-8220, a PKC inhibitor, but was decreased by genistein, an inhibitor of tyrosine kinase, and increased by vanadate, a tyrosine phosphatase inhibitor; both effects were, however, very small. Fluoride-evoked PLD activity was resistant to Ro 31-8220 and to genistein, but was Ca 2+-dependent. The rate of fluoride-induced PLD activation was maintained for at least 60 min. In contrast, PDB-mediated PLD activity was blocked by Ro 31-8220 and was resistant to extracellular Ca 2+-depletion and desensitized within ca. 15 min. PDB markedly potentiated the fluoride-evoked generation of [ 3H]-phosphatidylethanol and of choline, but inhibited the formation of [ 3H]-inositol phosphates ([ 3H]-IP 1–3). Ethanol (2%) blocked the PDB-evoked generation of both [ 3H]-phosphatidic acid and of sn-1,2-diacylglycerol, whereas fluoride-evoked responses were reduced only to approximately 50%. In conclusion, the trimeric G-protein–PLD pathway in heart tissue did not enclose PKC activation and was long-lasting and Ca 2+-dependent; there was no evidence for an involvement of tyrosine phosphorylation. However, PKC activation modulated G-protein-coupled PLD and PI-PLC activities in opposite directions. PLD activity significantly contributed to the mass production of sn-1,2-diacylglycerol in the heart. The evidence for a pathophysiological role of PLD activation in cardiac hypertrophy and in ischemic preconditioning is discussed.