While the mammalian heart cannot function without oxygen, this is not true of all vertebrates. Several vertebrate species can survive anoxia, the complete absence of oxygen, for days or even months. These anoxia-tolerant animals include freshwater turtles in the genera Chrysemys and Trachemys, and the crucian carp Carassius carassius. The turtles and fish, however, do not use all of the same adaptations to survive without oxygen. Turtles survive without oxygen by decreasing metabolism by up to 90%, subsisting on the reduced energy provided by anaerobic respiration. The anoxic turtle is essentially comatose, with energy savings provided by greatly reduced brain function and suppressed cardiac output. In contrast to the sluggish turtles, the crucian carp continues to be active for months at a time despite having no access to oxygen, compensating by increasing anaerobic metabolism. The problem is that anaerobic respiration results in the accumulation of toxic waste products (primarily lactic acid) in the fish's tissues. To avoid self-pollution, this lactate is converted to ethanol and excreted across the gills. So to survive in oxygen-less environments, the fish needs a transport system, which depends on a beating heart and functioning circulatory system. But part of the anoxic turtle's energy saving comes from dramatically suppressing its cardiac activity, which results in lowered circulation and corresponding loss of an effective transport system. Goran Nilsson and his group at the University of Oslo wondered if the active carp maintained its cardiac function and nervous control of its heart rate despite the lack of oxygen.
To find out, Nilsson and his team examined cardiac function in the crucian carp over a period of 5 days of anoxia at 8±1°C. In a paper recently published in Science, they report that following an initial 24 h adjustment period of increased stroke volume and cardiac output, the carp's cardiac function returned to normal levels. The team also found that blocking inhibitory heart receptors increased heart rate and cardiac output,while blocking excitatory heart receptors decreased cardiac function. This suggests that, in direct contrast to the turtle, nervous system control of heart rate remains intact in the crucian carp. In fact, the only cardiovascular functions that seemed to have changed over 2-5 days of anoxia were the fish's peripheral vascular resistance and blood pressure in the ventral aorta, which continued to be depressed, indicating that the fish's blood vessels had dilated. This vasodilation allows the carp to transport the glucose required for anaerobic respiration to metabolically active tissues while also enabling the removal of toxic waste products. Nilsson and his team suspect that the heart's ability to continue beating, coupled with an effective vascular transport system, allows the fish to remain active even when oxygen levels plummet.
Amazingly, then, some vertebrate hearts can function without oxygen! It would be of interest to investigate in more detail the biochemical and molecular bases of continued cardiac function during anoxia. As an alternative to the rapidly dying mammalian models, this could reveal new treatments in the face of mammalian heart failure.