This study examines the molecular basis for the T-type and L-type Ca2+ currents in canine Purkinje cells. The ICaT in Purkinje cells was completely suppressed by 200 nm kurtoxin, a specific blocker of both Cav3.1 and Cav3.2 channels. Since only Cav3.2 mRNA is expressed at high levels in Purkinje fibres, being approximately 100-fold more abundant than either Cav3.1 or Cav3.3 mRNAs, it is concluded that the Cav3.2 gene encodes the bulk of the T-type Ca2+ channels in canine Purkinje cells. This conclusion is consistent with the sensitivity of the current to blockade by Ni2+ ions (KD = 32 μm). For L-type channels, Cav1.2 mRNA was most abundant in Purkinje fibres but a significant level of Cav1.3 mRNA expression was also found. A comparison of the sensitivity to blockade by isradipine of the L-type currents in Purkinje cells and ventricular epicardial myocytes, which only express Cav1.2, suggests that the Cav1.3 channels make, at most, a minor contribution to the L-type current in canine Purkinje cells.
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