Late post-myocardial infarction induces a tetrodotoxin-resistant Na+ current in rat cardiomyocytes

Left ventricular remodeling after myocardial infarction is accompanied by electrical abnormalities that might predispose to rhythm disturbances. To get insight into the ionic mechanisms involved, we studied myocytes isolated from four different regions of the rat ventricles, 4-6 months after ligation of the left coronary artery. Using the whole-cell patch-clamp technique, we never observed T-type Ca2+ current in both diseased and control hearts, In contrast, in 41 out of 78 cells isolated from 16 post-myocardial infarcted rats, analysed in the presence of 30 mM Na+ ions, we found a tetrodotoxin (TTX)-resistant Na+ current with quite variable amplitude in every investigated region, Albeit being resistant to 100 mu M TTX, this Na+-dependent current was highly sensitive to lidocaine since 3 mu M lidocaine induced about 65% tonic block. It was also inhibited by 5 mu M nifedipine and 2 mM Co2+, but was insensitive to 100 mu M Ni2+. The TTX-resistant Na+ channel availability was shifted rightward by 25-30mV with respect to TTS-sensitive Na+ current; therefore, a large "window current" might flow in the voltage range from -70 to -20 mV. In conclusion, in late post-myocardial infarction, a Na+ current with specific kinetics and pharmacology may provide inward charges in a critical range of membrane voltages that are able to alter action potential time course and trigger ventricular arrhythmia, These apparent new characteristics of the Na+ channel might result in part from environmental changes during heart remodeling. (C) 2000 Academic Press.