Characterization of the slowly inactivating sodium current I Na2 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.