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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