Tetrodotoxin-sensitive Na+ channels in isolated single cultured rat myocardial cells

We studied the existence of tetrodotoxin (TTX)-sensitive fast Na+ channels in isolated single (verified by dye injection) myocardial cells compared with small multiple-cell groups from 1- to 3-day-old rat ventricles in cell culture. For single cells, average values were -63 mV maximum membrane potential, 32 mV overshoot, and 65 V/s maximum rate of rise of the action potentials (+Vmax). These values were comparable to values from groups of multiple (2-10) cells. TTX strongly depressed +Vmax dose dependently, with no difference between single and multiple cells. +Vmax was also decreased by lowering extracellular Na+ concentration but not by D 600 or lowering extracellular Ca2+ concentration. These results suggest that 1) isolated single cells possess TTX-sensitive fast Na+ channels, 2) culturing per se does not alter TTX sensitivity, and 3) TTX sensitivity is not modified by cell density.