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J Physiol. 1983 Jul;340:389-401.

Sodium channels in cultured cardiac cells.


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.

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