Adenosine prevents K⁺-induced Ca²⁺ loading: Insight into cardioprotection during cardioplegia

In clinical practice, hyperkalemic cardioplegia induces sarcolemmic depolarization, and therefore is used to arrest the heart during open heart operations. However, the elevated concentration of K⁺ that is present in cardioplegic solutions promotes intracellular Ca²⁺ loading, which could aggravate ventricular dysfunction after cardiac operations. This review highlights recent findings that have established, at the single cell level, the protective action of adenosine against hyperkalemia-induced Ca²⁺ loading. When it was added to hyperkalemic cardioplegic solutions, adenosine, at millimolar concentrations and through a direct action on ventricular cardiomyocytes, prevented K⁺-induced Ca²⁺ loading. This action of adenosine required the activation of protein kinase C, and it was effective only in cardiomyocytes with low diastolic Ca²⁺ levels. Of importance, adenosine did not diminish the magnitude of K⁺-induced membrane depolarization, allowing unimpeded cardiac arrest. Taken together, these findings provide direct support for the idea that adenosine is valuable when used as an adjunct to hyperkalemic cardioplegia. This idea has emerged from previous clinical studies that have shown improvement of the clinical outcome after cardiac operations when adenosine or related substances were used to supplement cardioplegic solutions. Further studies are required to define more precisely the mechanism of action of adenosine, and the conditions that may determine the efficacy of adenosine as a cytoprotective supplement to cardioplegia.