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SLEEP AND TEMPERATURE REGULATION

Cardiac dynamics during daily torpor in the Djungarian hamster (Phodopus sungorus)

¹Fractal Physiology, Max Planck Institute for Experimental Medicine, Göttingen, Germany; ²Center for Neurogenomics and Cognitive Research, Vrije Universiteit, Amsterdam, The Netherlands; ³Department of Zoology, University of Veterinary Medicine, Hannover, Germany

Submitted 10 July 2007 ; accepted in final form 14 November 2007

Djungarian or Siberian hamsters (Phodopus sungorus) acclimated to short photoperiod display episodes of spontaneous daily torpor with metabolic rate depressed by 70% and body temperature (T_b) reduced by 20°C. To study the cardiovascular adjustment to daily torpor in Phodopus, electrocardiogram (ECG) and T_b were continuously recorded by telemetry during entrance into torpor, in deep torpor, and during arousal from torpor. Minimum T_b during torpor bouts was 21°C, and heart rate, 349 beats/min at euthermy, displayed marked sinus bradyarrhythmia at 70 beats/min. Arousal was typically completed within 40 min, followed by a sustained posttorpor inactivity tachycardia (540 beats/min). The absence of episodes of conduction block, tachyarrhythmia, or other forms of ectopy throughout the torpor cycle demonstrates a remarkable resistance to arrhythmogenesis. The ECG morphology lacks a distinct isoelectric interval following the QRS complex, and the ST segment resembles the ECG pattern in mice, with a prominent fast transient outward K⁺ current (I_to,f) determining the early phase of ventricular repolarization. During low-temperature torpor, the amplitudes of the QRS complex substantially increased, suggesting that in the euthermic state the terminal portion of ventricular depolarization is fused with the beginning of repolarization, low T_b acting to decorrelate the superposition between depolarization and repolarization by delaying the repolarization onset. Atrioventricular and ventricular conduction times were prolonged as function of T_b. In contrast, the QT vs. T_b relationship showed marked hysteresis indicating the operation of nonlinear control mechanisms whereby the rapid QT shortening during arousal results from additional mechanisms (probably sympathetic stimulation) other than temperature alone.

hibernation; electrophysiology; conduction; arrhythmia

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