Chronic exposure of smooth muscle cells to minimally oxidized LDL results in depressed inositol 1,4,5-trisphosphate receptor density and Ca2+ transients

Oxidized LDL (oxLDL) (0.1 mg/mL) increased [Ca2-](i) in vascular smooth mus cle cells (VSMCs) within 5 to 10 seconds of incubation. This increase was m ediated via an inositol 1,4,5-trisphosphate (IP3)-dependent release of Ca2 from the sarcoplasmic reticulum. However, atherosclerosis is a gradual pro cess in which VSMCs are more likely exposed to low concentrations of oxLDL over extended periods rather than acute exposures. It is very possible, the refore, that lower [oxLDL] and longer exposure times may induce a very diff erent response with regard to regulation of [Ca2+](i). VSMCs were incubated with 4- to 100-fold lower [oxLDL] for up to 6 days. The conditions were no t cytotoxic. Basal [Ca2+](i) was not altered. Surprisingly, however, after chronic exposure to oxLDL, a brief addition of oxLDL (0.1 mg/mL) or norepin ephrine failed to elicit the expected rise in Ca-i(2+). Because the acute e ffects of oxLDL on control cells were mediated through an IP3-dependent pat hway, we investigated the integrity of the VSMC IP3 receptors. Immunocytoch emical analysis and Western blots revealed a depression in the density of I P3 receptors after chronic exposure of VSMCs to oxLDL. These changes in IP3 receptors have significance under atherosclerotic conditions as well. Immu nocytochemical analysis revealed a decrease in IP3 receptor density in the medial layer under atherosclerotic plaques in situ. Our data, therefore, de monstrate a striking difference between the acute and chronic effects of ox LDL on VSMC calcium. Whereas acute exposure to oxLDL stimulates [Ca2+](i), chronic exposure results in depressed Ca2+ transients, apparently through a decrease in IP3 receptor density. These changes have functional implicatio ns for the atherosclerotic vessel in vivo, and our data implicates oxLDL in this process.