Direct and interactive effects of cardioplegic arrest and protamine on myocyte contractility

Background. Cardioplegic arrest with rewarming and protamine administration have been implicated in causing transient left ventricular dysfunction perioperatively. However, whether interactive effects between cardioplegic arrest and rewarming with protamine occur with respect to myocyte contractile processes remains unclear. Accordingly, using an isolated myocyte model, the present study tested the hypothesis that simulated cardioplegic arrest with rewarming and protamine would have direct and interactive effects on myocyte contractile function. Methods. Left ventricular isolated myocyte contractile function was examined using computer-aided videomicroscopy under normothermic conditions (37°C, cell medium; n = 183) and after simulated hypothermic, hyperkalemic cardioplegic arrest with rewarming (4°C, 24 mEq/L K+, 2 hours; then 37°C, cell medium, 5 minutes; n = 268). Myocyte function was then examined in the presence of protamine (10 to 40 μg/mL) under normothermic conditions (n = 102) and after cardioplegic arrest with rewarming (n = 175). Results. Myocyte contractile function decreased by 43% from baseline after simulated cardioplegic arrest with rewarming. Under normothermic conditions, protamine (20 μg/mL) reduced myocyte contractile function by 43.9% ± 4.3%, whereas myocyte contractile function decreased by only 31.1% ± 2.7% with protamine (20 μg/mL) after cardioplegic arrest with rewarming. Thus, the negative effects of protamine on myocyte contractility were attenuated after cardioplegic arrest when compared with normothermic conditions. Conclusions. The present study demonstrated that simulated cardioplegic arrest with rewarming and protamine have direct and interactive effects on myocyte contractile function, which are not additive or synergistic.