Tetrodotoxin differentially blocks peak and steady-state sodium channel currents in early embryonic chick ventricular...

Single ventricular myocytes were dissociated from 3-day-old embryonic chick hearts and maintained in culture for 9-21 h. The whole-cell patch clamp method was used to record tetrodotoxin (TTX)-sensitive fast Na+ currents. The peak Na+ current recorded at -20 mV ranged from 10 to 70 microA/cm2. At more negative potentials, a component of the current decayed very slowly, resulting in a significant steady-state or "late" Na+ current. The origin of the late Na+ current was revealed through the examination of single Na+ channel currents recorded in outside-out membrane patches. The single Na+ channel conductance was 20 pS. A high percentage of the trials (approximately 16%) displayed multiple reopenings of a single Na+ channel, resulting in bursts of current lasting for greater than or equal to 150 ms. The frequency distributions of the Na+ channel open-times were bi-exponential. The burst-like mode of Na+ channel activity (which underlies the slowly- or non-inactivating currents recorded macroscopically), was blocked to a greater degree by TTX, compared to the peak current. The results suggest that differential blockade may occur as a result of the slow binding and increased affinity of TTX to the open Na channel.