We all breathe air to supply our organs and cells with sufficient oxygen,but what happens when oxygen becomes scarce? To get an answer to this question we could learn a thing or two from eels that experience hypoxic and anoxic periods routinely when buried in mud. Eels seem to get over anoxia without any difficulties, as they profit from a mechanism that instantaneously and reversibly downregulates their metabolic rate. This process extends the duration of stored energy reserves and, in turn, postpones ATP fuel depletion,lengthening an eel's survival time. Yet relatively little is known about the cellular oxygen-sensing mechanisms that induce this so-called `metabolic arrest'. To fill this gap, Morten Busk from Aarhus University, Denmark and Bob Boutilier from The University of Cambridge, UK studied liver cells from European eels.
Firstly, they exposed isolated hepatocytes to anoxia by turning off the cellular oxygen supply. Doing so, they found that anoxia caused an amazing 85-fold decrease in ATP production. Accordingly, they quantified the accumulation of cellular lactate, produced when ATP is generated anaerobically from intracellular glycogen, and found that lactate production peaked when ATP levels were at their lowest. Reoxygenating the same hepatocytes 4 hours later,they were surprised to find how reversible anoxic hypometabolism is; the cells quickly returned to their previous cellular ATP levels by metabolizing lactate. The eels survive anoxia by rapidly dropping their metabolism to conserve ATP and switching to glycolytic ATP synthesis to regenerate ATP as soon as the episode has ended.
To discover the nature of the signal that induces metabolic suppression during anoxia, the team compared cell survivability and energy metabolism in cells experiencing physiological anoxia with oxygenated cells that had been treated with respiratory blockers to stimulate pharmacological anoxia.
Busk and Boutilier quickly concluded that the presence of oxygen itself does not regulate anaerobic ATP generation when they showed that lactate levels were similar in cells experiencing both pharmacological and physiological anoxia. However, the team found that oxygen appears to coordinate ATP consumption rates with the reduced mitochondrial ATP production caused by metabolic arrest. Busk and Boutilier explain that protein kinases have been suggested to be responsible for the control of anoxic downregulation of metabolism in turtles. But when the team specifically tested the role of protein kinase A and C in eel hepatocytes, it turned out that both kinases had no effect on survivability, metabolic rate or energy equilibrium during anoxia, suggesting that kinases play no role in metabolic suppression in hepatocytes.
Next, the two scientists wondered whether anoxic cell survival could depend on stress hormones released from distant oxygen-sensing cells. To test this idea, they treated the isolated hepatocytes with adrenaline just before exposure to anoxia. They discovered that adrenaline in fact elevated glycolytic ATP production during anoxia while simultaneously dropping total cellular ATP availability, probably due to increased glucose synthesis and release, or unidentified metabolic costs arising from adrenergic stimulation. Thus, after treatment with adrenaline, total cellular ATP supply during experimental anoxia was substantially lower than in unstressed hepatocytes.
Whilst the entire mechanism of fully reversible anoxic downregulation of metabolic rate still remains to be resolved, Busk and Boutilier have successfully shown the importance of certain stimuli that elicit metabolic arrest in eel hepatocytes. Finally, from the eel perspective, surviving anoxia will always be a capability envied by us humans.
