Summary:The exceptional ability of the turtle brain to survive prolonged anoxia makes it a unique model for studying anoxic survival mechanisms. We have used epi illumination microscopy to record blood flow rate in ve nules on the cortical surface of turtles (Trachemys scripta). During anoxia, blood flow rate increased 1.7 times after 45-75 min, whereupon it fell back, reaching preanoxic values after 115 min of anoxia. Topical super fusion with adenosine (50 /-LM) during normoxia caused a 3.8-fold increase in flow rate. Superfusing the brain with Freshwater turtles of the genera Trachemys and Chrysemys possess the exceptional ability among vertebrates to tolerate prolonged anoxia, surviving days of anoxia at room temperature and several months at temperatures close to ooe (Ultsch, 1989).In vertebrates, from mammals (Hansen, 1985; Siesjo, 1992) to fish (Nilsson et aI., 1993), the brain is particularly sensitive to anoxia, rapidly losing ATP levels-the primary cause of neuronal death. To maintain brain ATP levels, turtles utilize two cooperating strategies. An initial enhancement of glycolytic ATP production is followed by deep hy pometabolism (Lutz, 1992).An increased glycolytic rate would probably de mand increased cerebral blood flow and/or an ele vated blood glucose level. Previous studies on tur tles using the microsphere technique have shown an approximately threefold increase in cerebral blood Received January 19, 1994; final revision received March 7, 1994; accepted March 8, 1994.Address correspondence and reprint requests to Dr. P.Hylland at Vertebrate Physiology and Behaviour Unit, Depart ment of Limnology, Norbyvagen 20, S-752 36 Uppsala, Sweden.
877the adenosine receptor blocker aminophylline (250 /-LM)totally inhibited the effects of both adenosine and anoxia, while aminophylline had no effect on normoxic flow rate. None of the treatments affected systemic blood pressure. These results indicate an initial adenosine-mediated in crease in cerebral blood flow rate during anoxia, probably representing an emergency response before deep meta bolic depression sets in.