Characterization of the slowly inactivating sodium current INa2 in canine cardiac single Purkinje cells

The aim of our experiments was to investigate by means of a whole cell patch-clamp technique the characteristics of the slowly inactivating sodium current (I-Na2) found in the plateau range in canine cardiac Purkinje single cells. The I-Na2 was separated from the fast-activating and -inactivating I-Na (labelled here I-Na1) by applying a two-step protocol. The first step, from a holding potential (V-h) of -90 or -80 mV to -50 mV, led to the quick activation and inactivation of I-Na1. The second step consisted of depolarizations of increasing amplitude from -50 mV to less negative values, which led to the quick activation and slow inactivation of I-Na2. The I-Na2 was fitted with a double exponential function with time constants of tens and hundreds milliseconds, respectively. After the activation and inactivation of I-Na1 at -50 mV, the slope conductance was very small and did not change with time. Instead, during I-Na2, the slope conductance was larger and decreased as a function of time. Progressively longer conditioning steps at -50 mV resulted in a progressive decrease in amplitude of I-Na2 during the subsequent test steps. Gradually longer hyperpolarizing steps (increments of 100 ms up to 600 ms) from V-h -30 mV to -100 mV were followed on return to -30 mV by a progressively larger I-Na2, as were gradually more negative 500 ms steps from V-h -30 mV to -90 mV. At the end of a ramp to -20 mV, a sudden repolarization to approximately -35 mV fully deactivated I-Na2. The I-Na2 was markedly reduced by lignocaine (lidocaine) and by low extracellular [Na+], but it was little affected by low and high extracellular [Ca2+]. At negative potentials, the results indicate that there was little overlap between I-Na2 and the transient outward current, I-to, as well as the calcium current, I-Ca. In the absence of I-to and I-Ca (blocked by means of 4-aminopyridine and nickel, respectively), I-Na2 reversed at 60 mV. In conclusion, I-Na2 is a sodium current that can be initiated after the inactivation of I-Na1 and has characteristics that are quite distinct from those of I-Na1. The results have a bearing on the mechanisms underlying the long plateau of Purkinje cell action potential and its modifications in different physiological and pathological conditions.