Freshwater turtles are the most anoxia-tolerant vertebrate species known, capable of surviving hours of anoxia at high temperatures and weeks to months at low temperatures. There is tremendous interest in understanding the cellular mechanisms underlying anoxia-tolerance in this group because they are anoxia-tolerant vertebrates and because of the far-reaching medical benefits that would be gained. It has become clear that a pre-condition of prolonged anoxic survival must involve the matching of ATP production with ATP utilization to maintain stable ATP levels during anoxia. In most vertebrates anoxia leads to a severe decrease in ATP production without a concomitant reduction in utilization, which inevitably leads to the catastrophic events associated with cell death or necrosis. Anoxia-tolerant organisms do not increase ATP production when faced with anoxia but rather decrease utilization to a level that can be met by anaerobic glycolysis alone. Protein synthesis and ion movement across the plasma membrane are the two main targets of regulatory processes that reduce ATP utilization and promote anoxic survival However, the oxygen sensing and biochemical signaling mechanisms that achieve a coordinated reduction in ATP production and utilization remain unclear. One candidate signaling compound whose extracellular concentration increases in concert with decreasing oxygen availability is adenosine. Adenosine is known to have profound effects on various aspects of tissue metabolism, including protein synthesis, ion pumping and permeability of ion channels. In this review we will investigate the role of adenosine in the naturally anoxia-tolerant freshwater turtle and give an overview of pathways by which adenosine is produced and removed from the extracellular space.
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