Hydrogen Peroxide Potentiates the EDHF Phenomenon by Promoting Endothelial Ca 2+ Mobilization

OBJECTIVE: The purpose of this study was to test the hypothesis that H(2)O(2) contributes to the EDHF phenomenon by mobilizing endothelial Ca(2+) stores. METHODS AND RESULTS: Myograph studies with rabbit iliac arteries demonstrated that EDHF-type relaxations evoked by the SERCA inhibitor cyclopiazonic acid (CPA) required activation of K(Ca) channels and were potentiated by exogenous H(2)O(2) and the thiol oxidant thimerosal. Preincubation with a submaximal concentration of CPA unmasked an ability of exogenous H(2)O(2) to stimulate an EDHF-type response that was sensitive to K(Ca) channel blockade. Imaging of cytosolic and endoplasmic reticulum Ca(2+) in rabbit aortic valve endothelial cells with Fura-2 and Mag-fluo-4 demonstrated that H(2)O(2) and thimerosal, which sensitizes the InsP(3) receptor, both enhanced CPA-evoked Ca(2+) release from stores, and that the potentiating effect of H(2)O(2) was suppressed by the cell-permeant thiol reductant glutathione monoethylester. CPA-evoked relaxations were attenuated by exogenous catalase and potentiated by the catalase inhibitor 3-aminotriazole, and were abolished by the connexin-mimetic peptide (43)Gap26, which interrupts intercellular communication via gap junctions constructed from connexin 43. CONCLUSIONS: H(2)O(2) can enhance EDHF-type relaxations by potentiating Ca(2+) release from endothelial stores, probably via redox modification of the InsP(3) receptor, leading to the opening of hyperpolarizing endothelial K(Ca) channels and an electrotonically-mediated relaxant response.