Myocardial dysfunction and arrhythmias may be induced by congenital heart defects, but also be the result of heart surgery with cardiopulmonary bypass (CPB), potentially caused by differential expression of connexin40 (Cx40) and connexin43 (Cx43). In 16 pediatric patients undergoing corrective heart surgery, connexin mRNA expression was studied in volume overloaded (VO group, n = 8) and not overloaded (NO group, n = 8) right atrial myocardium, excised before and after CPB. Additionally, in eight of these patients ventricular specimens were investigated. The atrial Cx43 expression decreased during CPB, which was restricted to the VO group (p = 0.008). In contrast, atrial Cx40 mRNA did not change during CPB. In ventricular myocardium compared to atrial mRNA levels, Cx40 was lower (p = 0.006) and Cx43 higher (p = 0.017) expressed, without significant change during CPB. This study revealed a significant influence of CPB and the underlying heart defect on Cx43 expression. (c) 2006 Elsevier Inc. All rights reserved.