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Department of Physiology, School of Medicine, Hiroshima University, Japan.

1. The generation of action potentials elicited from enzymatically dispersed ventricular cells from the frog, Rana catesbeiana, has been shown to be due to the influx of both Na+ and Ca2+. The maximum rate of rise, the amplitude and the duration at 50% repolarization of the action potential were estimated to be 26.4 +/- 5.1 V/s (n = 8), 110 +/- 2.7 mV (n = 8) and 601 +/- 180 ms (n = 8) at 15 degrees C, respectively. 2. Inward Na+ current (INa) was studied in these ventricular cells by the whole-cell patch clamp technique in a medium where Ca2+ current was eliminated by substituting extracellular Mg2+ for Ca2+ and K+ current was suppressed by applying Cs+ intracellularly. All the voltage clamp experiments were carried out at 4 degrees C. 3. INa elicited by single depolarizing steps from a holding potential (VH) of -80 mV had a threshold of -50 mV and was maximal at -20 mV. Peak currents in normal Ringer solution containing 113.5 mM-Na+ were of the order of 0.01-0.02 mA/cm2. Maximum Na+ conductance (gNa) was calculated to be 5.9 mS/cm2. 4. Under normal conditions the reversal potential for INa was determined to be 50 mV, which is close to the value predicted from the Nernst equation. The reversal potential changed by 59 mV per tenfold change in the activity of extracellular Na+ (aNa). 5. The instantaneous relation between INa tail currents and membrane potential is linear, crossing the abscissa at the reversal potential for INa. 6. Reconstructions of INa were made in terms of the parameters of the Hodgkin-Huxley model for the squid axon, using constants obtained from the frog ventricular cells. 7. The falling phase of INa and the development of inactivation measured by the double-pulse method could be well fitted by a single-exponential function. 8. The time course for recovery of INa from inactivation exhibited a single time constant.

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