Chronic exposure of smooth muscle cells to minimally oxidized LDL results in depressed inositol 1,4,5-trisphosphate receptor density and Ca(2+) transients.
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.
Massaeli
H
H
Division of Stroke and Vascular Disease, St. Boniface General Hospital Research Centre, and Department of Physiology, University of Manitoba, Winnipeg, Manitoba, Canada.
Austria
J A
JA
Pierce
G N
GN
D000818:Animals; D001161:Arteriosclerosis / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007425:Intracellular Membranes / Q000378:metabolism; D008077:Lipoproteins, LDL / Q000494:pharmacology*; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism*; D013997:Time Factors
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008077:Lipoproteins, LDL; D018160:Receptors, Cytoplasmic and Nuclear; C099252:oxidized low density lipoprotein; D002118:Calcium