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 [Ca(2+)]. 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.