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 2mm) 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 Ca(2+) (Ca(2+)](i)) and force development. In comparison with WT, apoE(-/-) mice displayed higher basal Ca(2+)](i). Moreover, the time constant of the second phase of the biphasic high K(+)-induced Ca(2+)](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 Ca(2+)](i) handling with CPA and caffeine revealed no alteration of maximal SERCA or ryanodine receptor function. Inositol (1,4,5)-triphosphate receptor (IP(3)R)-mediated Ca(2+)](i) release was, however, significantly increased in apoE(-/-) mice compared to WT mice as established with phenylephrine and ATP. In Ca(2+)-free conditions the ATP-induced Ca(2+)](i) was not altered. The ATP-induced store-operated Ca(2+) entry was, however, significantly increased in apoE(-/-) compared to WT mice. The results demonstrate that basal Ca(2+)](i) levels and IP(3)R-mediated store-operated Ca(2+)](i) release over the plasma membrane were elevated in hypercholesterolemic but plaque-free apoE(-/-) mice.