Hoàng, Thủy

Hypothermia/Rewarming (H/R) is poorly tolerated by the myocardium; however, the underlying intracellular basis of H/R‐induced cardiac injury remains elusive. We hypothesized that in cardiomyocytes, H/R leads to contractile dysfunction primarily through reduced Ca 2+ sensitivity of the myofilaments. To test this hypothesis, isolated rat cardiomyocytes (13–15 cells from 6 rats per group) were electrically stimulated to evoke both cytosolic Ca 2+ ([Ca 2+ ] cyto ) and contractile (sarcomere shortening) responses that were simultaneously measured using an IonOptix system. These measurements were subjected to the following temperature protocols: 1) hypothermia (15°C) followed by rewarming (35°C) or 2) time‐matched control (35°C). Contractile dysfunction after H/R was indicated by reduced shortening velocity and reduced extent of shortening of sarcomere length (SL) compared to time‐matched controls. Throughout hypothermia, basal [Ca 2+ ] cyto increased and the duration of the evoked [Ca 2+ ] cyto transients was prolonged. Phase‐loop plots of [Ca 2+ ] cyto versus contraction in cells exposed to H/R indicate that Ca 2+ sensitivity of contraction is reduced. Consistent with decreased Ca 2+ sensitivity, we found that cardiac troponin I (cTnI) phosphorylation (measured by Western blot) increased after H/R. Our results support our overall hypothesis and suggest that reduced Ca 2+ sensitivity and increased cTnI phosphorylation contributes to the altered excitation‐contraction coupling of cardiomyocytes following H/R. The clinical importance of our results brings focus to Ca 2+ sensitivity as a plausible therapeutic target in treating patients exposed to accidental hypothermia. Support or Funding Information NIH Grant 5T32HL10535 (NSS)

Hypothermia/Rewarming (H/R) induces myocardial dysfunction although the underlying intracellular mechanisms remain elusive. We hypothesized that in cardiomyocytes, H/R disrupts intracellular Ca 2+ ([Ca 2+ ] cyt ) homeostasis via an effect on both release and clearance of cytosolic Ca 2+ leading to a prolongation of evoked [Ca 2+ ] cyt transients associated with activation with downstream impact on contractility. To test this hypothesis, isolated rat cardiomyocytes (13–15 cells from 6 rats per group) were loaded with the Ca 2+ indicator, Fura‐2AM (0.5 μM), and exposed to H/R with or without electrical pacing at 0.5 Hz. In both groups, evoked [Ca 2+ ] cyt and contractile (sarcomere length (SL) shortening) responses were simultaneously measured at 35°C before and after hypothermia using an IonOptix system. H/R experiments were conducted as follows: experimental temperature was cooled from 35°C to 15°C in 30 min, maintained at 15°C for 2 h, and then rewarmed back to 35°C in 30 min. Time‐matched controls were implemented for 3 h at 35°C. With electrical pacing throughout hypothermia, basal [Ca 2+ ] cyt increased and the kinetics of the evoked [Ca 2+ ] cyt responses were prolonged. In addition, the extent and velocity of SL shortening was reduced in cardiomyocytes that were paced during hypothermia. In contrast, in cardiomyocytes that were not electrically paced during hypothermia, basal [Ca 2+ ] cyt was maintained and the kinetics of evoked [Ca 2+ ] cyt transients were unaffected. In addition, in the absence of pacing during hypothermia, contractility of cardiomyocytes was unaffected. These results suggest that H/R affects the release and clearance of evoked [Ca 2+ ] cyt transients (e.g., RyR mediated Ca 2+ release and SERCA mediated clearance) such that basal [Ca 2+ ] cyt increases. This effect of H/R on [Ca 2+ ] cyt homeostasis is associated with reduced cardiac contractility.

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 an IonOptix system. Cardiomyocytes were divided into two groups: 1) those exposed to hypothermia (15°C for 2 h) followed by rewarming (35°C; H/R); or 2) 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/ .