Histamine 1 receptors and reverse-mode Na+/Ca2+ exchanger drive extracellular Na+-dependent intracellular Ca2+ oscillations in human cerebrovascular endothelial cells

Cerebrovascular endothelial cells represent the core component of the blood-brain barrier, (BBB) which plays a critical role in regulating the local ionic microenvironment around the synapses. Therefore, cerebrovascular endothelial cells experience dramatic changes in the extracellular concentrations of potassium and sodium ions during intense neuronal firing or pathological conditions, such as spreading depression. Herein, we assessed the mechanisms by which a reduction in extracellular sodium concentration ([Na+]o) triggers complex Ca2+ signals in the hCMEC/D3 cell line, which is the most widespread model of human BBB. We demonstrate that lowering the [Na+]o elicits a variety of Ca2+ signals, including monotonic increases in intracellular Ca2+ concentration ([Ca2+]i) and repetitive oscillations in [Ca2+]i, which are triggered by the reverse-mode Na+/Ca2+ exchanger and histamine 1 receptor (H1R). Furthermore, we provide the first evidence that H1R may play a critical role in translating a reduction in [Na+]o into the activation of phospholipase C and following production of inositol triphosphate (InsP3), thereby inducing the rhythmic activation of InsP3 receptors on the endoplasmic reticulum (ER) and progressive depletion of the ER Ca2+ pool. The fall in the ER Ca2+ concentration leads to quick Store-Operated Ca2+ Entry activation, which maintains the intracellular Ca2+ oscillations by rapidly refilling the ER Ca2+ store. The endothelial Ca2+ oscillations induced by the reduction in [Na+]o may then lead to nitric oxide release. These findings, therefore, shed novel light on the mechanisms whereby Gq protein coupled receptors (GqPCRs) can shape endothelial Ca2+ signaling and Ca2+-dependent events at the human neurovascular unit.