Maintained Cardiac Pumping in

Humans, like most vertebrates, die within minutes when deprived of molecular oxy-gen (anoxia), in part because of cardiac fail-ure. In contrast, freshwater turtles (genera Chrysemys and Trachemys) have the ability to survive anoxia for months at low temper-atures, but to do so, they drastically suppress cardiac activity and autonomic cardiovascu-lar control (1, 2). Although the crucian carp (Carassius carassius) shares this anoxia toler-ance, we show here that this fish can retain normal cardiac performance and autonomic cardiovascular regulation for at least 5 days of anoxia at 8-C. In contrast, its cousin the common carp (Cyprinus carpio) survives only 24 hours of severe hypoxia, even though cardiac activity is strongly depressed (3). Prolonged anoxic survival requires balanc-ing energy supply and demand, as well as coping with the acidosis associated with anaerobic end-product accumulation. There-fore, anoxic freshwater turtles reduce metabo-lism by 90%, precluding an up-regulation of glycolysis (4), and they buffer lactic acid accumulation with their bone and shell (5). Correspondingly, cardiac output (Q) is reduced by 92 % (1), and autonomic cardio-vascular control is blunted (2). Conversely, anoxic crucian carp remain active, up-regulate glycolysis to maintain their adenosine triphos-phate supply, and avoid acidosis by convert-ing lactate into ethanol (6). Thus, we hypothesized that their cardiovascular status and control might be maintained during an-oxia. We examined this hypothesis by mea-suring Q, heart rate ( f H), stroke volume (