Effects of Anoxia and Aglycemia on Cytosolic Calcium Regulation in Rat Sensory Neurons

Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca(2+) regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca(2+)](i) by evoking Ca(2+) release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca(2+) content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca(2+) uptake when neurons were depolarized but substantially delayed mitochondrial Ca(2+) release and subsequent Ca(2+) clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity and Ca(2+) extrusion [probably via plasma membrane Ca(2+)-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca(2+)](i) homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca(2+) pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca(2+)](i), which was associated with complete ER Ca(2+) store emptying and progressive, and eventually complete, inhibition of Ca(2+) clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca(2+) regulation, and this may play an important role in modulating neuronal responses to ischemia.