Oxidation of low density lipoprotein enhances its potential to increase intracellular free calcium concentration in vascular smooth muscle cells

Abstract

There have been suggestions that oxidation of low density lipoproteins (LDL) might increase their atherogenic potential. Because changes in intracellular free calcium concentration [Ca2+]i have been linked to atherogenesis, we compared the influence of oxidized LDL (Ox-LDL) and native LDL (N-LDL) on [Ca2+]i in vascular smooth muscle cells cultured from rat aortas. For determination of [Ca2+]i, fura-2 fluorescence was used. LDL was isolated by ultracentrifugation from the sera of human donors (n=17). In N-LDL, oxidation was prevented by addition of antioxidants, whereas Ox-LDL was obtained by auto-oxidation. The extent of oxidation was assessed by measurement of thiobarbituric acid-reactive substances. Addition of Ox-LDL (20 μg protein/ml) to the vascular smooth muscle cells induced a mean increase of 129±13% in [Ca2+]i compared with 81±7% with N-LDL (p<0.01). Dose-response curves from 1 to 20 μg/ml (six experiments) confirmed this difference within the entire dose range. These results indicate that a more pronounced increase in [Ca2+], induced by Ox-LDL might be one of the cellular mechanisms responsible for the higher atherogenic potential of Ox-LDL compared with N-LDL, as [Ca2+]j is an important second-messenger system involved in many atherogenic processes such as hypertrophy, cell migration, and cell damage.