Daily torpor: When heart and brain go cold —Nonlinear cardiac dynamics in the seasonal heterothermic Djungarian hamster

¹  Max Planck Institute for Experimental Medicine - Göttingen, Germany, EU
²  Center for Neurogenomic and Cognitive Research, Vrije Universiteit - Amsterdam, The Netherlands, EU

Corresponding author: meyer@em.mpg.de

Received: 31 August 2009
Accepted: 24 September 2009

Abstract

Djungarian hamsters (Phodopus sungorus) acclimated to short photoperiod display episodes of spontaneous daily torpor with metabolic rate depressed by ~70%, body temperature (T_b) reduced by ~20 °C, and heart rate (HR) changing from ~70 bpm during torpor to ~570 bpm during arousal from torpor associated with remarkable resistance to arrhythmogenesis. The cardiac dynamics of heartbeat interval fluctuations (RR_i) obtained from high-resolution telemetric ECG recordings during daily torpor were studied using nonlinear techniques of signal processing. The nature of cardiac dynamical properties assessed from pointwise Hölder exponents (h) is shifted from strong irregularity of sinus bradyarrhythmia during low-T_b torpor towards smooth regularity on spontaneous rewarming during arousal. The pattern of h fluctuations indicates that both fractality and multifractal properties of RR_i's were decreased during torpor. The cardiac rhythm exhibits both deterministic and stochastic components with intermittency of alterations of prevalence of one over the other on short time scales. During the low-T_b state, RR_i dynamics exhibt nonlinear properties that temporarily shift to linear characteristics during entry into and arousal from torpor. Sympatho-vagal antagonism of autonomic cardiac control is markedly relaxed but remains active and highly alert when animals are torpid and temporarily out of the mainstream of competition.