In human hypercholesterolemia increased reactivity of vascular smooth muscle cells is due to altered subcellular Ca2+ distribution

There is evidence that, besides an attenuated endothelium-dependent relaxation, functional changes in smooth muscle contractility occur in experimental hypercholesterolemic animals. Unfortunately, little is known of the situation in human arteries, and the intracellular mechanisms involved in the modulation of vascular smooth muscle function in human hypercholesterolemia are still unclear. Thus, besides acetylcholine-induced endothelium-dependent relaxation, smooth muscle reactivity to KCl, norepinephrine (NE) and phenylephrine (PE) was evaluated in uterine arteries from 34 control individuals (CI) and 22 hypercholesterolemic patients (HC). Contractions to KCl, norepinephrine and phenylephrine were enhanced by 1.3-, 2.1- and 3.5-fold in vessels from HC. Furthermore, the Ca2+ signaling in the perinuclear cytosol, which promotes cell contraction, and that of the subplasmalemmal region, which contributes to smooth muscle relaxation, were examined in freshly isolated smooth muscle cells. In cells from HC, increases in perinuclear Ca2+ concentration ([Ca2+]peri) in response to 30 mM KCl and 300 nM NE were increased by 67 and 93%, respectively. In contrast, the increase in the subplasmalemmal Ca2+ concentration ([Ca2+]sub) to 10 μM NE was reduced in cells from HC by 33%. No further differences in perinuclear and subplasmalemmal Ca2+ signaling were found in cultured smooth muscle cells from CI and HC (primary culture 4–6 weeks after isolation). These data indicate a significant change in the subcellular Ca2+ distribution in smooth muscle cells from HC. In addition, production of superoxide anions (O2−) was increased 3.8-fold in uterine arteries from HC. Treatment of smooth muscle cells with the O2−-generating mixture xanthine oxidase/hypoxanthine mimicked hypercholesterolemia on smooth muscle Ca2+ signaling. From these findings, we conclude that during hypercholesterolemia, besides a reduced endothelium-dependent relaxation, changes in smooth muscle reactivity take place. Thereby, smooth muscle contractility is increased possibly due to the observed changes in subcellular Ca2+ signaling. The observed increased O2− production in HC might play a crucial role in the alteration of smooth muscle function in hypercholesterolemia.