Myocyte Contractility With Caspase Inhibition and Simulated Hyperkalemic Cardioplegic Arrest

BACKGROUND: Exposure of left ventricular (LV) myocytes to simulated hyperkalemic cardioplegic arrest (HCA) has been demonstrated to perturb ionic homeostasis and adversely affect myocyte contractility on rewarming. Altered ionic homeostasis can cause cytosolic activation of the caspases. While caspases participate in apoptosis, these proteases can degrade myocyte contractile proteins, and thereby alter myocyte contractility. Accordingly, this study tested the hypothesis that caspase inhibition during HCA would attenuate the degree of myocyte contractile dysfunction upon rewarming, independent of a loss in myocyte viability.

METHODS: Porcine (n = 8) LV myocytes were isolated and assigned to the following treatment groups: normothermic control: incubation in cell culture media for 2 hours at 37°C; HCA only: incubation for 2 hours in hypothermic HCA solution (4°C, 24 mEq K+); or incubation in hypothermic HCA solution supplemented with 10 µM of the caspase inhibitor, z-VAD (z-Val-Ala-Asp-fluoromethyl-ketone, HCA+zVAD). Myocyte viability, assayed as a function of mitochondrial function, was determined to be similar in the normothermic and both HCA groups.

RESULTS: The HCA caused a significant reduction in myocyte shortening velocity compared with normothermic control values (41 ± 6 versus 86 ± 8 µm/s, p < 0.05). The HCA+zVAD group had significantly improved myocyte shortening velocity compared with the HCA only group (63 ± 7 µm/s, p < 0.05). CONCLUSIONS: Independent of changes in viability, caspase inhibition attenuated myocyte contractile dysfunction after HCA and rewarming. Thus, caspase activation during HCA contributes, at least in part, to impaired myocyte contractility with rewarming. Supplementation of HCA with caspase inhibitors may provide a means to preserve myocyte contractile function after cardioplegic arrest.