Voltage-clamp studies on the canine Purkinje strand

Purkinje strands were excised from the left and right ventricles of adult mongrel dogs and cut to lengths of less than 2.0 mm in order to apply the two-microelectrodes voltage-clamp technique. A sizeable fraction of these preparations fully recover following dissection, with resting potentials more negative than--80 mV and upstroke velocities faster than 290 V s-1. Analysis of the voltage response to small current pulses shows that the short Purkinje strands can be treated as simple finite one-dimensional cables with ends of infinite resistance. The average length constant is 2.5 mm. In keeping with the relatively long length constant, insertion of a third microelectrode along the strand demonstrates a high degree of longitudinal homogeneity of the voltage clamp. Analysis of the capacity transient gives an estimate of the total capacity, normalized to cylindrical surface area, of 11.5 muF cm-2. The final decay of the capacity transient is a single exponential with an average time constant of 1 ms. A second slower component to the final decay of the capacity transient is absent in solutions of normal conductivity as well as in solutions of reduced (13%) conductivity. This suggests that the extracellular series resistance may be relatively small. The magnitude of the K+ depletion current was estimated by measuring the ratio of depletion current to instantaneous current. This ratio averaged 10%. These two results are consistent with the morphometric data described in the accompanying paper, which show that the canine preparation has wider extracellular clefts than the ungulate preparation. The existence of the full complement of inward and outward currents, including the pacemaker current, is demonstrated. The presence of wide clefts does not affect the potential at which the pacemaker current reverses (about--107 mV in 4 mM [K+] Tyrode solution), since the pacemaker current reverses at approximately the same potential in the canine Purkinje preparation as it does in the ungulate.