The H2O2-Generating Enzyme,Xanthine Oxidase,Decreases Luminal Ca2+ Content of the IP3-Sensitive Ca2+ Store in Vascular Endothelial Cells

Objective: Xanthine oxidase inhibits agonist-stimulated Ca²⁺ signaling in calf pulmonary artery endothelial cells by an H₂O₂-dependent mechanism. We investigated the effect of xanthine oxidase on luminal Ca²⁺ content of the inositol-1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ store. Methods: Luminal Ca²⁺ content was estimated from the net release of Ca²⁺ activated by 2,5-di-t-butylhydroquinone (BHQ), an inhibitor of microsomal Ca²⁺ pumps. Results: Initially, xanthine oxidase depleted the IP₃-sensitive Ca²⁺ store of releasable Ca²⁺, but with more prolonged incubation, the enzyme also depleted non-IP₃-sensitive stores. In addition, xanthine oxidase inhibited capacitative Ca²⁺ influx. Similar results were observed when thapsigargin was substituted for BHQ. Conclusions: Depletion of luminal Ca²⁺ content within the IP₃-sensitive Ca²⁺ store contributes to xanthine oxidase inhibition of Ca²⁺ signaling in vascular endothelial cells.