Cardiovascular Autonomic Control During Hibernation in American Black Bears

The dramatic early decrease in HR upon entry into torpor in small hibernators has recently been shown to be associated with a marked increase in cardiac parasympathetic activity as indicated by increased short term beat to beat heart rate variability (RMSSD) normalized to average R-R interval (RMDSSD/RR, Zanetti et al 2023). At steady state torpor, as their body temperature drop to ~2°C, small hibernators benefit from the Q 10 effect on cardiac pacemaker cells in maintaining a low HR with a reduced parasympathetic activity. Black bears in hibernation maintain body temperatures between 30-35°C while suppressing metabolism by 75% and with a marked decrease in HR from non-hibernating 55 bpm to hibernating 6-12 bpm including extreme respiratory sinus arrhythmia (Tøien et al 2011). As hibernating bears would not significantly benefit from the Q 10 effect on cardiac pacemaker cells to sustain a low HR, we hypothesized that an elevated cardiac parasympathetic tone is needed to suppress HR. Continuous electrocardiogram (ECG) and blood pressure (BP) were recorded with implantable telemetry transmitters (DSI D70-CCP/EET and Konigsberg T28F-14B) in American black bears. A Pan-Tomkins algorithm modified for R-wave detection with long R-R intervals followed by visual inspection was used for the analysis. RMDSSD/RR for a 2 hour period in mid hibernation with no activity was 3 times higher than that during the resting phase in non-hibernation (0.68 vs. 0.23, n=10 and 7 respectively, p<0.001), confirming the presence of a high parasympathetic tone during hibernation. HR decreased from 68.3 ± 5.8 bpm to 12.9 ± 0.9 bpm (±SE, p<0.0001). RMSSD/RR during a 10 min period recovering from anesthesia (Telazol with supplemental Ketamin) during hibernation in mid March was only 0.019 ± 0.008 vs 0.58 ± 0.14 during a 10 min period before disturbance to induce anesthesia and HR 87.0 ± 8.7 b/min vs. 16.3 ± 1.4 b/min, demonstrating a parasympatholytic effect (n=6, p< 0.0006). The longest R-R intervals during extreme sinus arrhythmia detected in mid hibernation ranged from 11.8 to 33.0 s (Mean 19.0 ± 2.7, n=21). BP typically increased to above 200 mmHg during periods of rapid heart beats, and decayed slowly to 50-60 mmHg at the end of the longest R-R intervals. Analysis of data from a single bear from 2 hour periods at 2 week intervals throughout hibernation and recovery indicates that a high parasympathetic activity is maintained throughout hibernation and strongly correlated to the maximum observed R-R interval. We conclude that opposed to small hibernators, hibernating bears sustain low HR in hibernation with high parasympathetic activity controlling the extreme sinus arrhythmia. We suggest that BP is maintained during long R-R intervals by peripheral vasoconstriction. Supported by NIH COBRE grant number P20GM130443. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.