Depressed myofilament co-operativity associated with post-cardioplegic myocardial depression

The mechanism underlying myocardial depression after procedures involving cardioplegia are unknown, We tested the hypothesis that such depression was associated with altered myofilament interactions, using isolated hearts perfused with warm (37°C), oxygenated (95% O₂/5% CO₂) Krebs-Ringer's bicarbonate (KRB) buffer. A latex balloon was inserted into the left ventricle (LV) to monitor LV function. All hearts underwent a 30-min equilibration period. One group of hearts (CPL+RPR) were arrested with St Thomas no 2 cardioplegic solution (4°C; 3 ml followed by 1 ml every 15 min) for 120 min, followed by reperfusion with warm, oxygenated KRB. A second group underwent cardioplegic arrest with no reperfusion (CPL). A third group underwent 60 min of warm, oxygenated perfusion with KRB beyond the equilibration period (60 MIN). The last group only underwent the equilibration period (EQUIL). LV function was assessed at the end of equilibration, and at 30 and 60 min of reperfusion (or 30 and 60 min additional perfusion in the 60 MIN group). All hearts were frozen at the end of the temporal protocol for each group, and stored at -70°C for later measurement of Ca²⁺-stimulated Mg²⁺ ATPase activity after isolation of myofibrils. CPL+RPR hearts demonstrated significant depression of systolic pressure and elevation diastolic pressure at fixed volumes, compared to baseline and 60 MIN group values. There were no significant changes in the amount of constituent myofilament proteins, as assessed by densinometric analyses of Western blots. There were also no changes in the minimal or maximal ATPase activities, nor in the pCa₅₀, indicating no effect of cardioplegic arrest on myofilament sensitivity to calcium. However, all hearts that underwent cardioplegic arrest were found to have significantly lower Hill coefficients (1.85 ± 0.09 and 1.85 ± 0.13 v 2.31 ± 0.13 and 2.34 ± 0.14 in CPL+RPR and CPL v 60 MIN and EQUIL hearts, respectively), suggesting decreased co-operativity of the actomyosin interaction. Such a decrease in co-operativity would contribute to both the systolic and diastolic alterations associated with myocardial depression after cardioplegic arrest. These changes were associated with the cardioplegic event, and appeared to be independent of reperfusion.