Maternal microvascular calcium sparks are reduced in preeclampsia

Background Maternal microvascular dysfunction is a hallmark of the hypertensive pregnancy disorder preeclampsia, but the mechanism(s) have not been fully defined. Within vascular smooth muscle cells (VSMCs), the principal pressure-induced vasodilatory pathway depends upon localised Ca2+-release events from sarcoplasmic reticulum (SR) ryanodine receptors (‘Ca2+ sparks’) which stimulate large-conductance Ca2+-activated potassium channels (BKCa) to cause spontaneous transient outward currents (STOCs) and vasorelaxation. Here, the Ca2+ sparks–BKCa axis is assessed in preeclampsia for the first time. Methods Pressurised 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 (20-110 mmHg) and caffeine-evoked Ca2+ release (10 mmol/L; 80 mmHg). Electrophysiology was used to assess STOCs (-60 to 0 mV) and BKCa currents (-100 to +100 mV) in isolated VSMCs. A liquid chromatography–selected reaction monitoring–mass spectrometry (LC-SRM-MS) assay measured Sarcoplasmic/Endoplasmic Reticulum Ca2+ ATPase-2 (SERCA2) and unphosphorylated/phosphorylated proteoforms of regulatory phospholamban. Vasoactive effects of the BKCa–dependent vasorelaxant Human β-Defensin-2 (HBD2; 0.01-10 nmol/L) were assessed by wire myography. Results In microvascular arteries from women with preeclampsia there was reduced Ca2+ spark frequency (p=1.1x10-10) and STOC frequency (p=0.023). There was also reduced SR Ca2+ release in response to caffeine (p=0.0013), reduced SR refilling (p=2.0x10-4) and decreased ratio of SERCA2 to inhibitory unphosphorylated phospholamban (p=0.030), implicating increased SERCA2 inhibition. HBD2 (0.01-10 nmol/L) caused BKCa-dependent relaxation (p=4.6x10-10) of omental arteries without affecting Ca2+ spark frequency. Conclusions In microvascular arteries from women with preeclampsia, there is a striking reduction in Ca2+ spark frequency and STOCs, likely secondary to increased inhibition of SERCA2 by unphosphorylated phospholamban and reduced SR Ca2+ load. These defects may provide novel therapeutic targets to restore physiological control of the vasculature. BKCa-mediated relaxation by HBD2 was independent of Ca2+ sparks, supporting vasodilatory strategies bypassing disrupted microvascular mechanisms in preeclampsia.