Single sodium channel events were recorded from cell-attached patches on single canine cardiac Purkinje cells at 10-13 degrees C. Data from four patches containing two to four channels and one patch with one channel were selected for quantitative analysis. The channels showed prominent reopening behavior at voltages near threshold, and the number of reopenings declined steeply with depolarization. Mean channel open time was a biphasic function of voltage with the maximum value (1-1.5 ms) occurring between -50 and -40 mV and lower values at more and at less hyperpolarized levels. Inactivation without opening was also prominent near threshold, and this occurrence also declined with depolarization. The waiting time distributions and the probability of being open showed voltage and time dependence as expected from whole- cell current studies. The results were analyzed in terms of a five- state Markovian kinetic model using both histogram analysis and a maximum likelihood method to estimate kinetic parameters. The kinetic parameters of the model fits were similar to those of GH3 pituitary cells (Horn, R., and C. A. Vandenberg. 1984. Journal of General Physiology. 84:505-534) and N1E115 neuroblastoma cells (Aldrich, R. W., and C. F. Stevens. Journal of Neuroscience. 7:418-431). Both histogram and maximum likelihood analysis implied that much of the voltage dependence of cardiac Na current is in its activation behavior, with inactivation showing modest voltage dependence.

Sodium (Na+) currents are responsible for excitation and conduction in most cardiac cells, but their study has been hampered by the lack of a satisfactory method for voltage clamp. We report a new method for low resistance access to single freshly isolated canine cardiac Purkinje cells that permits good control of voltage and intracellular ionic solutions. The series resistance was usually less than 3 omega cm2, similar to that of the squid giant axon. Cardiac Na+ currents resemble those of nerve. However, Na+ current decay is multiexponential. The basis for this was further studied with cell-attached patch clamp recording of single Na+ channel properties. A prominent characteristic of the single channels was their ability to reopen after closure. There was also a long opening state that may be the basis for a small very slowly decaying Na+ current. This rare long opening state may contribute to the Na+ current during the action potential plateau.

Low conductance sodium (Na) channels have been observed in nerve, skeletal muscle, and cardiac cells. In cardiac tissues the higher amplitude, more commonly observed Na channel was first investigated in detail by Cachelin et al. (Cachelin, A.B., J.E. de Peyer, S. Kokubun, and H. Reuter, 1983, J. Physiol. (Lond.), 340:389-402). They also reported low amplitude Na channel events. We have studied this low conductance Na channel in single canine cardiac Purkinje cells using cell-attached patches. Patch pipette solutions contained either 140 or 280 mM NaCl, and cells were bathed in a solution of 150 mM KCl to bring their resting potential close to zero. In 140 mM Na+, during steps to -50 mV, the lower and higher openings had amplitudes of 0.57 +/- 0.2 and 1.2 +/- 0.2 pA (means +/- SD of Gaussian fits). In 280 mM Na+ at -50 mV, amplitudes were 0.72 +/- 0.2 and 1.55 +/- 0.2 pA. Over a substantial voltage range, the lower events had amplitudes of about one-third that of the higher events. The frequency of the low conductance openings varied in different patches from zero to 22% of total openings. Histograms of open durations and latencies at several voltages suggested no difference in kinetics between the two channel events. The behavior of the low conductance channels was more consistent with a second population of channels rather than a second open state.

Open channel properties of canine cardiac Purkinje cell Na+ channels were studied with single channel cell-attached recording and with whole cell macroscopic current recording in internally perfused cells. Single channel currents and membrane currents increased with an increase in Na+ concentration, but showed evidence of saturation. Assuming first-order binding, the Km for Na+ was 370 mM. PCs/PNa was 0.020 and PK/PNa was 0.094. The current-voltage relationship for single channels showed prominent flattening in the hyperpolarizing direction. This flattening was accentuated by 10 mM Ca2+ and was greatly reduced in O mM Ca2+, indicating that the rectification was a consequence of Ca2+ block of the Na+ channels. A similar instantaneous current-voltage relationship was seen for the whole cell membrane currents. These results demonstrate that the cardiac channel shows substantial Ca2+ block, although it is relatively insensitive to tetrodotoxin. The Na+ and Ca2+ binding properties could be modeled by the four-barrier Eyring rate theory model, with similar values to those reported for the neuroblastoma Na+ channel (Yamamoto, D.,J.Z. Yeh, and T. Narahashi, 1984, Biophys J., 45:337-344).