Molecular basis of the T- and L-type Ca²⁺ currents in canine Purkinje fibres

This study examines the molecular basis for the T-type and L-type Ca²⁺ currents in canine Purkinje cells. The I_CaT in Purkinje cells was completely suppressed by 200 nm kurtoxin, a specific blocker of both Ca_v3.1 and Ca_v3.2 channels. Since only Ca_v3.2 mRNA is expressed at high levels in Purkinje fibres, being approximately 100-fold more abundant than either Ca_v3.1 or Ca_v3.3 mRNAs, it is concluded that the Ca_v3.2 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 (K^D = 32 μm). For L-type channels, Ca_v1.2 mRNA was most abundant in Purkinje fibres but a significant level of Ca_v1.3 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 Ca_v1.2, suggests that the Ca_v1.3 channels make, at most, a minor contribution to the L-type current in canine Purkinje cells.