Background. In isolated hearts, the potassium-channel opener pinacidil is an effective cardioplegic agent. This study tested the hypothesis that pinacidil is superior to St. Thomas' solution in the more clinically relevant intact animal.Methods. Sixteen pigs were placed on full cardiopulmonary bypass. Hearts underwent 2 hours of global ischemia (10°to 15°C). Either St. Thomas' or 100 μmol/L pinacidil was administered every 20 minutes (10 mL/kg). Preischemic and postreperfusion slopes of the preload-recruitable stroke work relationship were determined. Changes in myocardial adenine nucleotide levels and cellular ultrastructure were analyzed.Results. Pinacidil cardioplegia resulted in an insignificant change in the slope of the preload-recruitable stroke work relationship (40.6 ± 2.1 mm Hg/mm before ischemia and 36.5 ± 3.7 mm Hg/mm after ischemia; p = 0.466). In contrast, St. Thomas' solution resulted in a significant decrease in the slope after reperfusion (34.3 ± 5.5 mm Hg/mm and 13.5 ± 2.3 mm Hg/mm; p = 0.003). Adenine nucleotide levels, myocardial tissue water, and ultrastructural changes were similar between groups.Conclusions. Pinacidil ameliorated myocardial stunning associated with traditional hyperkalemic cardioplegia without causing significant differences in cellular metabolism. Copyright (C) 1999 The Society of Thoracic Surgeons.