Abstract

Vascular endothelium, which produces endothelium-derived relaxing and constricting factors, plays an important role in regulating the vascular tone. We recently demonstrated that oxidized low density lipoprotein inhibited endothelium-dependent relaxation and that lysophosphatidylcholine accumulated during the oxidative modification of low density lipoprotein was the essential substance for the inhibition of endothelium-dependent relaxation. To clarify the mechanisms of the inhibitory effect of lysophosphatidylcholine, we used a bioassay system to investigate the effect of lysophosphatidylcholine on the production and/or release of endothelium-derived relaxing factor and its effect on the cytosolic Ca2+ level ([Ca2+]i) and phosphoinositide hydrolysis in cultured bovine aortic endothelial cells. [Ca2+]i was monitored by the fura 2 method, and the accumulation of inositol phosphates in cells labeled with myo-[2-3H]inositol was measured. Bioassay experiments showed that lysophosphatidylcholine inhibited the production and/or release of endothelium-derived relaxing factor from cultured endothelial cells. Lysophosphatidylcholine (5-20 micrograms/ml) induced a biphasic increase in [Ca2+]i, which consisted of a rapid increase followed by a sustained increase, and the initial component was a result of mobilization from intracellular Ca2+ stores without detectable synthesis of inositol 1,4,5-trisphosphates. Furthermore, lysophosphatidylcholine (5-20 micrograms/ml) dose-dependently inhibited both phosphoinositide hydrolysis and the increases in [Ca2+]i evoked by bradykinin. These results indicate that the impairment of endothelium-dependent relaxation induced by lysophosphatidylcholine is due to the inhibition of phosphoinositide hydrolysis and the subsequent increases in [Ca2+]i in endothelial cells. Lysophosphatidylcholine that accumulates in oxidized low density lipoprotein and atherosclerotic arteries may play an important role in the modification of endothelial function.