Chronic Exposure of Smooth Muscle Cells to Minimally Oxidized LDL Results in Depressed Inositol 1,4,5-Trisphosphate Receptor Density and Ca ²⁺ Transients

Abstract —Oxidized LDL (oxLDL) (0.1 mg/mL) increased [Ca 2+ ] i in vascular smooth muscle cells (VSMCs) within 5 to 10 seconds of incubation. This increase was mediated via an inositol 1,4,5-trisphosphate (IP 3 )-dependent release of Ca 2+ from the sarcoplasmic reticulum. However, atherosclerosis is a gradual process in which VSMCs are more likely exposed to low concentrations of oxLDL over extended periods rather than acute exposures. It is very possible, therefore, that lower [oxLDL] and longer exposure times may induce a very different response with regard to regulation of [Ca 2+ ] i . VSMCs were incubated with 4- to 100-fold lower [oxLDL] for up to 6 days. The conditions were not cytotoxic. Basal [Ca 2+ ] i was not altered. Surprisingly, however, after chronic exposure to oxLDL, a brief addition of oxLDL (0.1 mg/mL) or norepinephrine failed to elicit the expected rise in Ca 2+ i . Because the acute effects of oxLDL on control cells were mediated through an IP 3 -dependent pathway, we investigated the integrity of the VSMC IP 3 receptors. Immunocytochemical analysis and Western blots revealed a depression in the density of IP 3 receptors after chronic exposure of VSMCs to oxLDL. These changes in IP 3 receptors have significance under atherosclerotic conditions as well. Immunocytochemical analysis revealed a decrease in IP 3 receptor density in the medial layer under atherosclerotic plaques in situ. Our data, therefore, demonstrate a striking difference between the acute and chronic effects of oxLDL on VSMC calcium. Whereas acute exposure to oxLDL stimulates [Ca 2+ ] i , chronic exposure results in depressed Ca 2+ transients, apparently through a decrease in IP 3 receptor density. These changes have functional implications for the atherosclerotic vessel in vivo, and our data implicates oxLDL in this process.