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 (INa2) found in the plateau range in canine cardiac Purkinje single cells. The INa2 was separated from the fast-activating and -inactivating INa (labelledhere INa1)by applying a two-step protocol.Thefirst step, froma holding potential (Vh) of −90 or −80 mV to −50 mV, led to the quick activation and inactivation of INa1. 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 INa2. The INa2 was fittedwith a double exponential functionwith time constants of tens and hundreds milliseconds, respectively. After the activation and inactivation of INa1 at−50 mV, the slope conductance was very small and did not change with time. Instead, during INa2, the slope conductance was larger and decreased as a function of time. Progressively longer conditioning steps at−50 mVresulted in a progressive decrease in amplitude of INa2 during the subsequent test steps. Gradually longer hyperpolarizing steps (increments of 100 ms up to 600 ms) from Vh −30 mV to −100 mV were followed on return to −30 mV by a progressively larger INa2, as were gradually more negative 500 ms steps from Vh −30 mV to−90 mV. At the end of a ramp to−20 mV, a sudden repolarization to approximately−35 mV fully deactivated INa2. The INa2 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 INa2 and the transient outward current, Ito, as well as the calcium current, ICa. In the absence of Ito and ICa (blocked by means of 4-aminopyridine and nickel, respectively), INa2 reversed at 60mV. In conclusion, INa2 is a sodium current that can be initiated after the inactivation of INa1 and has characteristics that are quite distinct from those of INa1. 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.