Anoxia and ischemia tolerance in turtle hearts.

It has been known since ancient times that turtle hearts exhibit extraordinary tolerance to anoxia or ischemia. The mechanisms by which they accomplish this remain obscure. The most important adaptation in anoxic turtles is a rapid and dramatic decrease in metabolic rate. Nuclear magnetic resonance measurements indicate that painted turtle (Chrysemys picta) hearts respond to anoxia with a rapid decrease in phosphocreatine (PCr; to 50% of control) after which PCr remains constant for at least 4 h. ATP is defended and does not decrease while intracellular pH (pHi) decreases by 0.2 pH units early in anoxia and is then maintained constant. Softshelled turtles (Trionyx spinifer) have been demonstrated to be far more sensitive than painted turtles to anoxia in vivo. However, isolated hearts from softshelled turtles appear to be as anoxia tolerant as those of Chrysemys. During ischemia there is also little difference in cardic performance, high energy phosphates, or pHi between these two species. A peculiar feature of turtle hearts is an extremely high concentration of phosphodiesters (PDE). The role of cytosolic PDEs remains controversial but they may function as lysophospholipase inhibitors and thereby limit phospholipid turnover (Burt CT and Ribolow H, Comparative Biochemistry and Physiology, 108B: 11-20, 1994). Whether PDEs promote anoxia/ischemia tolerance is unknown but these stresses can result in membrane lipid dysfunction in mammals. Metabolic control, acid-base, and phospholipid homeostasis all play a role in anoxia and ischemia tolerance in turtle hearts. These physiologic processes are interdependent, and how they interact in these animals is unknown, but they are experimentally accessible by modern analytical methods.