Discontinued stimulation of cardiomyocytes provides protection against hypothermia-rewarming-induced disruption of excitation-contraction coupling.

After exposure of spontaneously beating hearts or electrically stimulated isolated cardiomyocytes to hypothermia-rewarming (H/R), cardiac dysfunction or alteration in excitation-contraction coupling, respectively, is a consequence. In contrast, hypothermic cardiac arrest, as routinely applied during cardiac surgery, will not impose any hazard to cardiac function after rewarming. We hypothesize that by maintaining asystole during H/R, cardiomyocytes will avoid Ca2+ overload attributable to the transient stimulation-evoked elevation of [Ca2+ ]i and thus, H/R-induced elevation of phosphorylated cardiac troponin I and reduced Ca2+ sensitivity after rewarming. To test this hypothesis, the aim of the study was to determine whether discontinued electrical stimulation (to imitate hypothermic cardiac arrest) versus stimulation during 3 h of H/R prevents disruption of excitation-contraction coupling in our established cardiomyocyte H/R model. Cytosolic Ca2+ and the contractile response (sarcomere length shortening) were measured using an IonOptix system, and the dynamic assessment of Ca2+ sensitivity of contraction was conducted using a phase-loop plot. Cardiomyocytes were divided into three groups. Group 1 (time-matched control) was continuously stimulated at 0.5 Hz for 3 h at 35°C. Group 2 was continuously stimulated during H/R at 0.5 Hz, whereas in group 3 stimulation was discontinued during H/R and thus the cells remained quiescent until the resumption of stimulation after rewarming. The results demonstrate that discontinued stimulation of cardiomyocytes during H/R, imitating hypothermic cardiac arrest during cardiac surgery, provides protection against H/R-induced disruption of excitation-contraction coupling. We suggest that protective effects are caused by preventing the protein kinase A-induced elevation of phosphorylated cardiac troponin I, which is a key mechanism to reduce myofilament Ca2+ sensitivity of contraction.