Prolonged hypoxia can lead to remodeling of tissue functions, in part via the altered expression of cellular proteins. Of these, we have shown that numerous ion channel types display altered functional expression (Peers, 2002), which can have marked effects on cell function. The mechanisms underlying hypoxic modulation of channels are poorly understood, but we have provided evidence that hypoxic up-regulation of L-type Ca²⁺ channels is mediated by amyloid peptides of Alzheimer’s disease in both PC12 cells and cerebellar granule neurones (Taylor et al., 1999) and unpublished observations). Thus we have established a potential mechanism to account for the known increased incidence of Alzheimer’s disease following hypoxic / ischemic episodes (Moroney et al., 1996). Here, we describe the up-regulation by chronic hypoxia (CH) of L-type Ca²⁺ channels in HEK 293 cells stably expressing the α_1C subunit of the human cardiac L-type Ca²⁺ channel (Scragg et al., 2004). Whole-cell patch clamp recordings revealed that 24h exposure to 2.5% O₂ dramatically increased Ca²⁺ current density in this recombinant expression system. CH also increased the levels of endogenous Alzheimer’s amyloid β peptides (AβPs), determined immunocytochemically. Pharmacological prevention of AβP production (via exposure to inhibitors of secretase enzymes that are required to cleave AβP from its precursor protein) prevented hypoxic augmentation of currents, as did inhibition of vesicular trafficking with bafilomycin A1, whereas application of exogenous AβPs mimicked the effects of CH. The enhancing effect of exogenous AβPs or CH were abolished following incubation with the monoclonal 3D6 antibody, raised against the extracellular N’ terminus of AβP. Immunolocalization and immunoprecipitation studies provided compelling evidence that AβPs physically associated with the α_1C subunit, and this association was promoted by hypoxia. Current augmentation in response to CH was absent in cells in which the mitochondrial electron transport chain (ETC) was depleted following treatment with ethidium bromide (ρ⁰ cells) or in control (ρ⁺) cells in the presence of 1μM rotenone. Hypoxic augmentation of currents could be mimicked in ρ⁰ cells by the exogenous production of O₂^– by xanthine / xanthine oxidase. In ρ⁰ cells, exogenously applied AβPs enhanced currents by a similar degree 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 AβP mediated augmentation of Ca²⁺ current in ρ⁰ cells. Thus oxidant production in complex I of the mitochondrial ETC is a critical factor, acting upstream of AβP production in the up-regulation of Ca²⁺ channels in response to chronic hypoxia. These data suggest an important role for AβPs in mediating the increase in Ca²⁺ channel activity following CH, and show that AβPs act post-transcriptionally to promote L-type Ca²⁺ channel insertion into (and / or retention within) the plasma membrane. Furthermore, they indicate that mitochondrially-derived reactive oxygen species are intimately involved in this process. Such an action will likely contribute to the Ca²⁺ dyshomeostasis of Alzheimer’s disease (LaFerla, 2002).