ON THE MECHANISM OF THE LOSARTAN-MEDIATED INHIBITION OF PHOSPHATIDYLCHOLINE BIOSYNTHESIS IN H9C2 CELLS

Phosphatidylcholine is the major phospholipid in mammalian tissues and the biosynthesis of phosphatidylcholine in H9c2 cells was previously shown to be stimulated by angiotensin II. In this study, we used the p otent AT(1) receptor antagonist, losartan, to determine if the angiote nsin II-mediated stimulation of phosphatidylcholine biosynthesis was m ediated by AT(1) receptors. H9c2 cells were incubated with angiotensin LT in the absence or presence of various concentrations of losartan. The cells were then incubated with [methyl-H-3]choline for an addition al 60 min and the radioactivity incorporated into phosphatidylcholine and its choline-containing metabolites determined. Losartan at concent rations which block AT(1) receptors did not effect phosphatidylcholine biosynthesis mediated by angiotensin II. In contrast, higher concentr ations of losartan inhibited radioactivity incorporated into phosphati dylcholine and its metabolites and this was due to a losartan-mediated reduction in choline uptake. Kinetic studies revealed that the losart an-mediated inhibition of choline uptake was competitive. High concent rations of losartan caused a translocation of CTP:phosphocholine cytid ylyltransferase from the cytosolic (inactive) to the membrane (active) fraction likely as a compensatory mechanism for the losartan-mediated reduction in new phosphatidylcholine biosynthesis. Incubation of cell s with PD123319, a potent AT(2)-receptor antagonist, did not block the angiotensin II-mediated stimulation of phosphatidylcholine biosynthes is. The results suggest that angiotensin II stimulates phosphatidylcho line biosynthesis independent of AT(1)- and AT(2)-receptor activation and losartan inhibits phosphatidylcholine biosynthesis by reducing cho line uptake in H9c2 cells. (C) 1997 Elsevier Science B.V.