Tetrodotoxin (10-20 microM) completely blocked Na+ channels when applied to the outside membrane surface of neonatal rat cardiac cells, but had no effect when applied from the inside.
Primary cardiac cell cultures were prepared from the hearts of neonatal rats. The patch-clamp method (Hamill, Marty, Neher, Sakmann & Sigworth, 1981) was applied for studying whole-cell Na+ currents and single-channel Na+ currents, respectively. Whole-cell recordings yielded voltage- and time-dependent Na+ currents which could be blocked by tetrodotoxin. Single-channel Na+ currents were directly compared in cell-attached patches and in inside-out patches. In cell-attached patches the elementary current was about -1 pA at -10 mV and the slope conductance over a 50 mV voltage range was 15.1 +/- 1.6 pS (mean +/- S.D.). Inactivation during depolarization and after conditioning clamp steps, in the steady state, resulted from a reduced opening probability of Na+ channels. In inside-out patches, with identical solutions at both membrane surfaces, there was a large (40-50 mV) shift of channel opening and inactivation kinetics towards more negative potentials. However, for levels of comparable opening probabilities, mean open times of Na+ channels were similar in cell-attached and inside-out patches. Tetrodotoxin (10-20 microM) had no effect on Na+ channels when applied from the inside, but blocked them completely after application to the outside membrane surface.
Cachelin et al. (Fri,) reported a other. Tetrodotoxin was evaluated on Whole-cell and single-channel Na+ currents. Tetrodotoxin (10-20 microM) completely blocked Na+ channels when applied to the outside membrane surface of neonatal rat cardiac cells, but had no effect when applied from the inside.
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