Abstract
Calcium (Ca(2+))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 Ca(2+) homeostasis at the level of the endoplasmic reticulum (ER) and how this action influences cytosolic Ca(2+) dynamics associated with epileptiform activity. ER Ca(2+) levels were directly measured using ER-targeted aequorin in HeLa and PC12 cells, while cytosolic Ca(2+) signals were monitored by fura-2 fluorescence imaging in bovine chromaffin cells exposed to veratridine, a model of sustained sodium channel activation and Ca(2+) oscillations. VPA induced a concentration-dependent release of Ca(2+) from the ER, with an IC(50) 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 (InsP(3)). Pharmacological inhibition of InsP(3) receptors (InsP(3)Rs), but not ryanodine receptors or SERCA, abolished VPA-induced ER Ca(2+) release, supporting a selective InsP(3)R-mediated mechanism. Functionally, VPA suppressed the repetitive cytosolic Ca(2+) oscillations induced by veratridine, while simultaneously producing a sustained elevation of cytosolic Ca(2+) originating from ER stores and facilitating depolarization-evoked catecholamine secretion. Together, these results support the conclusion that VPA induces InsP(3)R-mediated Ca(2+) mobilization from the endoplasmic reticulum and identify ER Ca(2+) release as a previously unrecognized intracellular mechanism contributing to its modulatory effects on Ca(2+) signaling and excitability in epilepsy.