Vascular smooth muscle calcium sparks and sarcoplasmic reticulum calcium load are reduced in women with preeclampsia

BackgroundMaternal microvascular dysfunction is a hallmark of the hypertensive pregnancy disorder preeclampsia, but the mechanism(s) have not been defined. Historically, attention has focused on the vascular endothelium, but microvascular function is largely determined by the downstream effector, vascular smooth muscle cells (VSMCs). Within VSMCs, the principal pressure-induced vasodilatory pathway depends upon stimulation of large-conductance Ca2+-activated potassium channels (BKCa) by localised Ca2+-release events from sarcoplasmic reticulum (SR) ryanodine receptors ( Ca2+ sparks). Here, Ca2+ sparks are assessed in preeclampsia for the first time. MethodsPressurised omental resistance arteries from normotensive pregnant women and women with preeclampsia were imaged by high-speed spinning-disk laser confocal microscopy to assess Ca2+ sparks (20-120 mmHg) and caffeine-evoked Ca2+ release (10 mmol/L; 80 mmHg). A liquid chromatography-selected reaction monitoring-mass spectrometry (LC-SRM-MS) assay was developed to measure SR Ca2+ pump Sarcoplasmic/Endoplasmic Reticulum Ca2+ ATPase 2 (SERCA2) and unphosphorylated/phosphorylated proteoforms of regulatory phospholamban. Vasoactive effects of the BKCa-dependent vasorelaxant Human {beta}-Defensin-2 (HBD2; 0.01-10 nmol/L) were assessed by wire myography. ResultsCa2+ spark frequency was decreased at all intraluminal pressures in women with preeclampsia compared to normotensive pregnancy (p=1.1x10-10). In preeclampsia there was reduced SR Ca2+ released in response to caffeine (p=5.1x10-4) and the ratio of SERCA2 to inhibitory unphosphorylated phospholamban was decreased (p=0.030), indicating increased SERCA2 inhibition. In arteries from women with preeclampsia, HBD2 (0.01-10 nmol/L) caused BKCa-dependent relaxation (p=4.6x10-10) without affecting Ca2+ spark frequency. ConclusionsIn microvascular arteries from women with preeclampsia, there is a striking reduction in Ca2+ spark frequency, likely secondary to increased inhibition of SERCA2 by unphosphorylated phospholamban and reduced SR Ca2+ load. These defects may contribute to vascular dysfunction in preeclampsia and present novel therapeutic targets to restore physiological control of the vasculature. Meanwhile, BKCa-mediated relaxation by HBD2 independent of Ca2+ sparks supports vasodilatory strategies bypassing disrupted microvascular mechanisms in preeclampsia.