Living without oxygen: lessons from the freshwater turtle.

Freshwater turtles, and specifically, painted turtles, Chrysemys picta, are the most anoxia-tolerant air-breathing vertebrates. These animals can survive experimental anoxic submergences lasting up to 5 months at 3 degrees C. Two general integrative adaptations underlie this remarkable capacity. First is a profound reduction in energy metabolism to approximately 10% of the normoxic rate at the same temperature. This is a coordinated reduction of both ATP generating mechanisms and ATP consuming pathways of the cells. Second is a defense of acid-base state in response to the extreme lactic acidosis that results from anaerobic glycolysis. Central to this defense is an exploitation of buffer reserves within the skeleton and, in particular, the turtle's shell, its most characteristic structure. Carbonates are released from bone and shell to enhance body fluid buffering of lactic acid and lactic acid moves into shell and bone where it is buffered and stored. The combination of slow metabolic rate and a large and responsive mineral reserve are key to this animal's extraordinary anaerobic capacity.