Altered Ca2+ handling of smooth muscle cells in aorta of apolipoprotein E-deficient mice before development of atherosclerotic lesions

To study the effect of hypercholesterolemia on vascular smooth muscle cell (VSMC) function, atherosclerosis-prone but plaque-free endothelium-denuded aortic rings (width 2 mm) from C57Bl6 Wild Type (WT) and apolipoprotein E-deficient (apoE−/−) mice (age 4 months) were mounted in a myograph and loaded with Fura-2 AM to simultaneously measure free Ca2+ ([Ca2+]i) and force development. In comparison with WT, apoE−/− mice displayed higher basal [Ca2+]i. Moreover, the time constant of the second phase of the biphasic high K+-induced [Ca2+]i response was significantly increased in apoE−/− compared to WT mice. This phase was abolished by treatment with cyclopiazonic acid (CPA), depleting sarcoplasmic reticulum (SR). Further investigation of SR dependent [Ca2+]i handling with CPA and caffeine revealed no alteration of maximal SERCA or ryanodine receptor function. Inositol (1,4,5)-triphosphate receptor (IP3R)-mediated [Ca2+]i release was, however, significantly increased in apoE−/− mice compared to WT mice as established with phenylephrine and ATP. In Ca2+-free conditions the ATP-induced [Ca2+]i was not altered. The ATP-induced store-operated Ca2+ entry was, however, significantly increased in apoE−/− compared to WT mice. The results demonstrate that basal [Ca2+]i levels and IP3R-mediated store-operated [Ca2+]i release over the plasma membrane were elevated in hypercholesterolemic but plaque-free apoE−/− mice.