Valproic Acid Stimulates Release of Ca²⁺ from InsP₃-Sensitive Ca²⁺ Stores

Calcium (Ca²⁺)signaling dysfunction is a central contributor to neuronal hyperexcitability and seizure propagation in epilepsy, yet the intracellular mechanisms underlying the actions of valproic acid (VPA) remain incompletely understood. In this study, we investigated whether VPA modulates Ca²⁺ homeostasis at the level of the endoplasmic reticulum (ER) and how this action influences cytosolic Ca²⁺ dynamics associated with epileptiform activity. ER Ca²⁺ levels were directly measured using ER-targeted aequorin in HeLa and PC12 cells, while cytosolic Ca²⁺ signals were monitored by fura-2 fluorescence imaging in bovine chromaffin cells exposed to veratridine, a model of sustained sodium channel activation and Ca²⁺ oscillations. VPA induced a concentration-dependent release of Ca²⁺ from the ER, with an IC₅₀ of approximately 17 µM. This effect was preserved in permeabilized cells and exhibited activation kinetics comparable to those elicited by inositol 1,4,5-trisphosphate (InsP₃). Pharmacological inhibition of InsP₃ receptors (InsP₃Rs), but not ryanodine receptors or SERCA, abolished VPA-induced ER Ca²⁺ release, supporting a selective InsP₃R-mediated mechanism. Functionally, VPA suppressed the repetitive cytosolic Ca²⁺ oscillations induced by veratridine, while simultaneously producing a sustained elevation of cytosolic Ca²⁺ originating from ER stores and facilitating depolarization-evoked catecholamine secretion. Together, these results support the conclusion that VPA induces InsP₃R-mediated Ca²⁺ mobilization from the endoplasmic reticulum and identify ER Ca²⁺ release as a previously unrecognized intracellular mechanism contributing to its modulatory effects on Ca²⁺ signaling and excitability in epilepsy.