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

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 (Q̇_sys), heart rate (f_H), 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 °C or 5 °C, and the anoxic exposure was either acute (6 h) at 22 °C or chronic (3 weeks) at 5 °C. Cold acclimation alone decreased PO_sys by 15-fold, representing a Q₁₀ of 8.8. In addition, f_H 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, f_H and P_sys and increased R_sys. But the changes were qualitatively much larger with chronic anoxia. For example, acute anoxia in 22 °C-acclimated turtles decreased PO_sys by 6.6-fold, whereas chronic anoxia in 5 °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 °C with that during normoxia at 22 °C. Car.diac PO_sys was reduced 330-fold, through decreases in Q_sys (120-fold), f_H (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 °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 °C because of the 22-fold decrease in PO_sys. We propose that the suppression of the Pasteur effect and the large Q₁₀ values for cold acclimation would conserve glucose stores and enable turtles to withstand anoxia much longer under cold than under warm conditions.