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 G_q protein coupled receptors (G_qPCRs) can shape endothelial Ca²⁺ signaling and Ca²⁺-dependent events at the human neurovascular unit.