Overactivation of
glutamate receptors and subsequent deregulation of the intraneuronal
calcium ([Ca2+]i) levels are critical components of the injurious pathways initiated by
cerebral ischemia. Another hallmark of
stroke is parenchymal
acidosis, and we have previously shown that mild
acidosis can act as a switch to decrease NMDAR-dependent neuronal loss while potentiating the neuronal loss mediated by AMPARs. Potentiation of AMPAR-mediated neuronal death in an acidotic environment was originally associated only with [Ca2+]i dyshomeostasis, as assessed by Ca2+ imaging; however, intracellular dyshomeostasis of another divalent
cation, Zn2+, has recently emerged as another important co-factor in ischemic neuronal injury. Rises in [Zn2+]i greatly contribute to the fluorescent changes of Ca2+-sensitive
fluorescent probes, which also have great affinity for Zn2+. We therefore revisited our original findings (Mcdonald et al., 1998) and investigated if AMPAR-mediated
fura-2 signals we observed could also be partially due to [Zn2+]i increases.
Fura-2 loaded neuronal cultures were exposed to the AMPAR agonist,
kainate, in a physiological
buffer at pH 7.4 and then washed either at pH 7.4 or pH 6.2. A delayed recovery of
fura-2 signals was observed at both pHs. Interestingly this impaired recovery phase was found to be sensitive to chelation of intracellular Zn2+. Experiments with the Zn2+ sensitive (and Ca2+-insensitive)
fluorescent probe FluoZin-3 confirmed the idea that AMPAR activation increases [Zn2+]i, a phenomenon that is potentiated by mild
acidosis. Additionally, our results show that selective Ca2+ imaging mandates the use of intracellular
heavy metal chelators to avoid confounding effects of endogenous metals such as Zn2+.