Analysis of Na+/Ca2+ exchanger (NCX) function and current in murine cardiac myocytes during heart failure

Na+/Ca2+ exchanger (NCX) plays important roles in cardiac electrical activity and calcium homeostasis. NCX current (I-NCX) shows transmural gradient across left ventricle in many species. Previous studies demonstrated that NCX expression was increased and transmural gradient of I-NCX was disrupted in failing heart, but the mechanisms underlying I-NCX remodeling still remain unknown. In present study, we used patch clamp technique to record I-NCX from subepicardial (EPI) myocytes and subendocardial (ENDO) myocytes isolated from sham operation (SO) mice and heart failure (HF) mice. Our results showed that I-NCX was higher in normal EPI cells compared with that in ENDO, whatever for forward mode or reverse mode. In HF group, I-NCX was significantly up-regulated, but EPI-ENDO difference was disrupted because of a more increase of I-NCX in ENDO myocytes. In order to explore the molecular mechanism underlying remodeling of I-NCX in failing heart, we detected the protein expression of NCX1 and Ca2+/calmodulin-dependent protein kinase II (CaMKII) by Western blot. We found that CaMKII activity was dramatically enhanced and parallel with the expression of NCX1 in failing heart. Our study demonstrated that transmural gradient of I-NCX existed in murine left ventricle, and increased activity of CaMKII should account for I-NCX remodeling in failing heart.