The cardiovascular responses of the red-eared slider (Trachemys scripta) acclimated to either 22 or 5 degrees C. I. Effects of anoxic exposure on in vivo cardiac performance

Abstract

The extreme anoxia-tolerance of freshwater turtles under cold conditions is well documented, but little is known about their cardiac performance in such situations. Using chronic catheterization techniques, we measured systemic cardiac power output (PO(sys)), systemic cardiac output (Q(.)(sys)), heart rate (fh), systemic stroke volume (V(s,sys)), systemic resistance (R(sys)) and mean arterial pressure (P(sys)) in red-eared sliders (Trachemys scripta). The effects of cold acclimation and anoxic exposure were studied. Turtles were acclimated to either 22 degrees C or 5 degrees C, and the anoxic exposure was either acute (6 h) at 22 degrees C or chronic (3 weeks) at 5 degrees C. Cold acclimation alone decreased PO(sys) by 15-fold, representing a Q(10) of 8.8. In addition, fh and V(s,sys) decreased significantly, while R(sys) increased and moderated the arterial hypotension. Acute and chronic anoxic exposures significantly decreased PO(sys), V(s,sys), fh and P(sys) and increased R(sys). But the changes were qualitatively much larger with chronic anoxia. For example, acute anoxia in 22 degrees C-acclimated turtles decreased PO(sys) by 6.6-fold, whereas chronic anoxia in 5 degrees C-acclimated turtles decreased PO(sys) by 20-fold. The remarkable cardiovascular down-regulation that accompanies long periods of cold anoxia in these turtles was characterized by comparing cardiovascular status during chronic anoxia at 5 degrees C with that during normoxia at 22 degrees C. Cardiac PO(sys) was reduced 330-fold, through decreases in Q(.)(sys) (120-fold), fh (24.2-fold), V(s,sys) (5.7-fold) and P(sys) (2. 2-fold), while R(sys) was increased 64.6-fold. We also compared cardiac glycolytic rates by assuming that PO(sys) was proportional to ATP supply and that glycolysis yielded 18 times less ATP per mole of glucose than oxidative metabolism. At 22 degrees C, the 6.6-fold decrease in PO(sys) with anoxia suggests that a Pasteur effect was needed in cardiac tissues during acute anoxia. However, this would not be so with chronic anoxia at 5 degrees C because of the 22-fold decrease in PO(sys). We propose that the suppression of the Pasteur effect and the large Q(10) values for cold acclimation would conserve glucose stores and enable turtles to withstand anoxia much longer under cold than under warm conditions.