Metabolic arrest and its regulation in anoxic eel hepatocytes

Some vertebrates depress overall metabolism in an abrupt and reversible fashion when challenged with anoxia, ensuring stabilization of cellular [ATP] and long-term survival, but little is known about the eliciting stimuli (e.g., change in O2, adenylates) and downstream effectors responsible for metabolic arrest. Accordingly, eel (Anguilla anguilla) hepatocytes were treated with inhibitors of putative components of the oxygen/metabolite-sensing pathway(s) and exposed to anoxia (Po2=0 mmHg). Anoxia in untreated cells caused a remarkable 85-fold decrease in ATP production rate, but cellular ATP levels stabilized following an initial steep drop. Reoxygenation of cells after 4 h of anoxia caused a fast metabolization of accumulated lactate and reestablishment of preanoxic ATP levels. Unlike physiological anoxia, pharmacological inhibition of the electron transport chain in the presence of oxygen caused extensive cellular ATP depletion, though no loss in viability. In contrast, cellular lactate (i.e., ATP) production rate was affected similarly by either treatment, suggesting that anaerobic glycolysis is regulated by a stimulus other than oxygen tension per se, whereas the continuous matching of ATP consumption and a rapidly ceasing mitochondrial ATP supply require a physiological relevant change in oxygen tension. Protein kinases, notably kinase C (PKC) and A (PKA), have been proposed as key downstream regulators of stress-induced defense mechanisms, but anoxic cell viability, metabolic rate, and [ATP] were not significantly affected by inhibitors of PKC and PKA. Likewise, inhibition of the upstream PKC-activating enzymes phospholipase C (PLC) and phosphatidylinositol 3-kinase (PI 3-K) had no effect on recorded parameters. Anoxic cell survival in complex organisms may, in vivo, also depend on stress hormones released from distant oxygen-sensing cells. Accordingly, adrenaline elevated anaerobic energy production but, apparently, also elevated ATP consumption because cellular ATP levels during oxygen deprivation were slightly lowered by adrenergic stimulation.