MECHANICAL PERFORMANCE OF AN IN-SITU PERFUSED HEART FROM THE TURTLE CHRYSEMYS SCRIPTA DURING NORMOXIA AND ANOXIA AT 5-DEGREES-C AND 15-DEGREES-C

We developed an in situ perfused turtle (Chrysemys scripta) heart prep aration to study its intrinsic mechanical properties at 5 degrees C an d 15 degrees C using normoxic and anoxic perfusion conditions. The in situ preparation proved durable and stable. At 15 degrees C and a spon taneous heart rate of 23.4 beats min(-1), maximum stroke volume was 2. 54 mlkg(-1) body mass, maximum cardiac output was 62.5 mlmin(-1) kg(-1 ) and maximum cardiac myocardial power output was 1.50 mW g(-1) ventri cular mass. There was good agreement between these values and those pr eviously obtained in vivo. Furthermore, since the maximum stroke volum e observed here was numerically equivalent to that observed in ventila ting C. scripta in vivo, it seems likely that C. scripta has little sc ope to increase stroke volume to a level much beyond that observed in the resting animal through intrinsic mechanisms alone. The ability of the perfused turtle heart to maintain stroke volume when diastolic aft erload was raised (homeometric regulation) was relatively poor. At 5 d egrees C, the spontaneous heart rate (8.1 beats min(-1)) was threefold lower and homeometric regulation was impaired, but maximum stroke vol ume (2.25 mlkg(-1)) was not significantly reduced compared with the va lue at 15 degrees C. The significantly lower maximum values for cardia c output (18.9 mlmin(-1) kg(-1)) and power output (0.39 mW g(-1) ventr icular mass) at 5 degrees C were largely related to pronounced negativ e chronotropy with only a relatively small negative inotropy. Anoxia h ad weak negative chronotropic effects and marked negative inotropic ef fects at both temperatures. Negative inotropy affected pressure develo pment to a greater degree than maximum flow and this difference was mo re pronounced at 5 degrees C than at 15 degrees C. The maximum anoxic cardiac power output value at 15 degrees C (0.77 mW g(-1) ventricular mass) was not that different from values previously obtained for the p erformance of anoxic rainbow trout and hagfish hearts. In view of this , we conclude that the ability of turtles to overwinter under anoxic c onditions depends more on their ability to reduce cardiac work to a le vel that can be supported through glycolysis than on their cardiac gly colytic potential being exceptional.