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The patch-clamp method was applied to single atrioventricular (a.v.) node cells of the rabbit heart to study the characteristics of the K+ channel. When the electrode contained 5.4 mM-K+, depolarizations of the cell-attached patch membrane induced outward single channel currents characterized by burst-like openings; the open-state probability increased from 0.005-0.01 at -40 mV to 0.07-0.1 at +20 mV of membrane potential. The reversal potentials of the current at K+ concentrations of 5.4, 20 and 130 mM in the electrode agreed with those given by the Nernst equation, indicating that this channel is selective for K+ ions. The slope conductance of the channel decreased beyond 60-90 mV positive to the reversal potential (inward-going rectification). The conductance near the reversal potential increased with increasing K+ concentrations on either side of the membrane: from 31-32 pS at 5.4 mM-K+ to 41-42 pS at 20 mM-K+ on the outside, and from 19 pS at 90 mM-K+ to 29.3 pS at 130 mM-K+ on the inside. Superfusion of the cell with 5.4 mM-CN-, glucose-free Tyrode markedly increased the number of channel openings in the cell-attached patch. In the inside-out patch, application of 1 mM-adenosine-5'-triphosphate (ATP) at the inner surface of the patch membrane blocked reversibly the channel activity, while 1 mM-adenosine-5'-diphosphate (ADP) failed to block it. The conductance and kinetics of the channel were not modified by increasing the Ca2+ concentration from 10(-8) M to 5 X 10(-6) M on the inner side of the membrane, while a further increase in Ca2+ to 10(-4) M decreased the open-state probability. The probability density for the open time fitted well with an exponential distribution (time constant of 5.4 ms at 60 mV positive to the resting potential), while that for the closed time was separated into a fast and a slow component (time constants of 4.0 and 132.0 ms, respectively). The time constant of the slow component decreased significantly with depolarization in some preparations. However, neither the time constant of the fast component of the closed-time histogram nor that of the open-time histogram was voltage-dependent.
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