Hypothermia induces postrepolarization refractoriness in the atrial myocardium of the hibernating and active ground squirrel Citellus undulatus

The heart of the hibernating mammals demonstrates tolerance to the cold-induced arrhythmias and the electrophysiological mechanisms that underlie this phenomenon expecially in the atrial myocardium is still not elucidated. This study is aimed to the investigation of the hypothermia-induced changes of the atrial action potentials (AP) and refractoriness of the hibernating ground squirrel Citellus undulatus. APs were recorded with usage of standard sharp-electrode technique in the isolated, perfused multicellular continuously paced atrial myocardium preparations obtained from hibernating (HS) and summer active (SAS) squirrels. Action potentials duration (APD) and refractoriness duration (RD) were estimated at 37-17 °С and at various pacing cycle length (200-500 ms). It has been demonstrated that hypothermia causes prolongation of both APD and RD similarly in HS and SAS animals. However, the duration of the refractoriness in the atrial myocardium significantly exceeds the duration of the APs during hypothermia (27-17 °C). Revealed phenomenon may be considered as postrepolarization refractoriness (PRR). Proposed hypothermia-induced PRR probably substantially contributes to the insusceptibility of the hibernators to the cold induced arrhythmias by preventing the afterdepolarizations.