Hypoxic augmentation of Ca2+ channel currents requires a functional electron transport chain

Abstract

The incidence of Alzheimer disease is increased following ischemic episodes, and we previously demonstrated that following chronic hypoxia (CH), amyloid β (Aβ) peptide-mediated increases in voltage-gated L-type Ca2+ channel activity contribute to the Ca2+ dyshomeostasis seen in Alzheimer disease. Because in certain cell types mitochondria are responsible for detecting altered O2 levels we examined the role of mitochondrial oxidant production in the regulation of recombinant Ca2+ channel α1C subunits during CH and exposure to Aβ-(1-40). In wild-type (ρ+) HEK 293 cells expressing recombinant L-type α1C subunits, Ca2+ currents were enhanced by prolonged (24 h) exposure to either CH (6% O 2) or Aβ-(1-40) (50 nM). By contrast the response to CH was absent in ρ0 cells in which the mitochondrial electron transport chain (ETC) was depleted following long term treatment with ethidium bromide or in ρ+ cells cultured in the presence of 1 μM rotenone. CH was mimicked in ρ0 cells by the exogenous production of O 2.- by xanthine/xanthine oxidase. Furthermore Aβ-(1-40) enhanced currents in ρ0 cells to a degree similar to that seen in cells with an intact ETC. The antioxidants ascorbate (200 μM) and Trolox (500 μM) ablated the effect of CH in ρ+ cells but were without effect on Aβ-(1-40)-mediated augmentation of Ca2+ current in ρ0 cells. Thus oxidant production in the mitochondrial ETC is a critical factor, acting upstream of amyloid β peptide production in the up-regulation of Ca2+ channels in response to CH. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.