In vitro tolerance to anoxia and ischemia in isolated hearts from hypoxia sensitive and hypoxia tolerant turtles

Abstract

Although freshwater turtles as a group are highly anoxia tolerant, dramatic interspecific differences in the degree of anoxia tolerance have been demonstrated in vivo. Painted turtles (Chrysemys picta bellii) appear to be the most hypoxia-tolerant species thus far studied, while softshelled turtles (Trionyx spinifer) are the most hypoxia-sensitive. We have assumed that this dichotomy persists in vitro but have not, until now, directly tested this assumption. We, therefore, directly compared the responses of isolated, perfused, working hearts from these two species to either 240 min of anoxia, 90 min of global ischemia, or 240 min of global ischemia followed by reoxygenation/reperfusion. Isolated hearts were perfused at 20°C and monitored continuously for phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (P₁), and intracellular pH (pH₁) by ³¹P-nuclear magnetic resonance spectroscopy as well as for ventricular developed pressure and heart rate. Contrary to our expectations, we observed few significant differences in any of these parameters between painted and softshelled turtle hearts. Hearts from both species tolerated 240 min of anoxia equally well and both restored PCr, pH₁, and P₁ contents to control levels during reoxygenation. We did observe some significant interspecific differences in the 90 min (pH₁ and P₁) and 240 min (PCr) ischemia protocols although these seemed to suggest that Trionyx hearts might be more tolerant to these stresses than Chrysemys hearts. We conclude that: (a) the observed in vivo differences in anoxia tolerance between painted and softshelled turtles must either be due to differences in organ metabolism in organs other than the heart (e.g., brain) or to some integrative physiologic differences between the species; and (b) isolated hearts from a species known to be relatively anoxia sensitive in vivo can exhibit an apparent high degree of anoxia and ischemia tolerance in vitro.