Abstract Body: Introduction:
During hibernation, 13-lined ground squirrels cycle between torpor and intermittent bouts of arousal (IBA). Bouts of IBA last 12-48 hours, during which heart rates rise from 2-4 bpm in torpor to 300 bpm during IBA before returning to torpor. Thus, rates of contractility and relaxation and their mechanisms must rapidly change. Phosphorylation of cardiac troponin I (cTnI) at serine 23/24 (pSer23/24) has been shown to decrease calcium sensitivity and increase the rate of myocardial relaxation, enabling faster heart rates. Heat shock protein 27 (HSP27) has been shown to protect contractility by stabilizing troponin.

Hypothesis:
We hypothesized that cTnI Ser23/24 phosphorylation would be higher in summer and IBA and that HSP27 would be found in higher quantities during torpor and IBA.

Methods and Results:
Left ventricular tissue (summer, n=8; torpor, n=8; IBA, n=8) was solubilized, separated by SDS-PAGE, and blotted with antibodies for quantification.

The pSer23/24 to total troponin ratios were higher in summer compared to torpor (1.22±0.75 vs 0.32±0.19, p=0.003) and IBA (0.57±0.19, p=0.03), but not different between IBA and torpor. No significant season-dependence was observed for the serine 43 cTnI phosphorylation to total troponin ratio (torpor 1.45±0.69, summer 1.05±0.34, IBA 1.15±0.84).

No significant season-dependence was observed in HSP27 (normalized to GAPDH) (summer 0.09±0.02, IBA 0.14±0.10, torpor 0.16±0.08). Nor was there a significant season-dependence observed in HSP90 (normalized to GAPDH) (summer 1.57±0.66, IBA 1.13±0.19, torpor 1.22±0.21).

Conclusions:
None of these mechanisms fully explain the rapid change in contractility observed during the hibernating periods of 13-lined ground squirrels. Other post-translational modifications of troponin or modifiers of HSP activity may modify the reported results. Such regulation may identify new treatments for humans where rapid changes in heart rate or contractility occur.