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This Article Full Text Full Text (PDF) All Versions of this Article: 92/1/161    most recent expphysiol.2006.035279v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Vassalle, M. Articles by Du, F. Search for Related Content PubMed PubMed Citation Articles by Vassalle, M. Articles by Du, F.

¹ Department of Physiology and Pharmacology, State University of New York, Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, NY 11203, USA

The action potential of Purkinje fibres is markedly shortened by tetrodotoxin, suggesting the possibility that a slowly inactivating sodium current might flow during the plateau. The aim of the present experiments was to investigate, in canine cardiac Purkinje single cells by means of a whole cell patch clamp technique, whether a sodium current slowly inactivates at less negative potentials and (if so) some of its distinctive characteristics. The results showed that a 500 ms depolarizing step from a holding potential of –90 mV to –50 mV induced the fast inward current I_Na (labelled here I_Na1). With steps to –40 mV or less negative values, a slowly decaying component (tentatively labelled here I_Na2) appeared, which peaked at –30 to –20 mV and decayed slowly and incompletely during the 500 ms steps. The I_Na2 was present also during steps to –10 mV, but then the transient outward current (I_to) appeared. When the holding potential (V_h) was decreased to –60 to –50 mV, I_Na2 disappeared even if a small I_Na1 might still be present. Tetrodotoxin (30 µM), lignocaine (100 µM) and cadmium (0.2 mM; but not manganese, 1 mM) blocked I_Na2. During fast depolarizing ramps, the rapid inactivation of I_Na1 was followed by a negative slope region. During repolarizing ramps, a region of positive slope was present, whereas I_Na1 was absent. At less negative values of V_h, the amplitude of the negative and positive slopes became gradually smaller. Gradually faster ramps increased the magnitude of the negative slope, and tetrodotoxin (30 µM) reduced or abolished it. Thus, Purkinje cells have a slowly decaying inward current owing to Na⁺ entry (I_Na2) that is different in several ways from the fast I_Na1 and that appears important for the duration of the plateau.

(Received 26 July 2006; accepted after revision 16 October 2006; first published online 19 October 2006)
Corresponding author M. Vassalle: Department of Physiology, Box 31, SUNY, Downstate Medical Center, 450 Clarkson Avenue, Brooklyn, NY 11203, USA. Email: mario.vassalle{at}downstate.edu