Hypothermia/rewarming disrupts excitation-contraction coupling in cardiomyocytes

Hypothermia/rewarming (H/R) is poorly tolerated by the myocardium; however, the underlying intracellular basis of H/R-induced cardiac dysfunction remains elusive. We hypothesized that in cardiomyocytes, H/R disrupts excitation-contraction coupling by reducing myofilament Ca²⁺sensitivity due to an increase in cardiac troponin I (cTnI) phosphorylation. To test this hypothesis, isolated rat cardiomyocytes (13–15 cells from 6 rats per group) were electrically stimulated to evoke both cytosolic Ca²⁺([Ca²⁺]_cyto) and contractile (sarcomere shortening) responses that were simultaneously measured using anIonOptixsystem. Cardiomyocytes were divided into two groups:1) those exposed to hypothermia (15°C for 2 h) followed by rewarming (35°C; H/R); or2) time-matched normothermic (35°C) controls (CTL). Contractile dysfunction after H/R was indicated by reduced velocity and extent of sarcomere length (SL) shortening compared with time-matched controls. Throughout hypothermia, basal [Ca²⁺]_cytoincreased and the duration of evoked [Ca²⁺]_cytotransients was prolonged. Phase-loop plots of [Ca²⁺]_cytovs. contraction were shifted rightward in cardiomyocytes during hypothermia compared with CTL, indicating a decrease in Ca²⁺sensitivity. Using Western blot, we found that H/R increases cTnI phosphorylation. These results support our overall hypothesis and suggest that H/R disrupts excitation-contraction coupling of cardiomyocytes due to increased cTnI phosphorylation and reduced Ca²⁺sensitivity. Listen to this article’s corresponding podcast at http://ajpheart.podbean.com/e/hypothermiarewarming-disrupts-e-c-coupling/.