Differences in ionic currents between myocardial and Purkinje cells of canine heart

Canine single ventricular muscle (VM) and Purkinje (P) cells were studied by means of whole cell patch clamp technique. In VM cells, slope conductance is greater and I K1 peak is less negative. In both P and VM cells, quick decay of step current with more negative depolarizing steps is blocked by Ba 2+ and disappears at less negative potentials. In VM cells, sodium currents I Na3 and I Na1 have less negative threshold potential. In both types of cells, I Na3 and I Na1 activations occur at I K1 peak, but in VM cells I Na1 is smaller. Slowly inactivating I Na2 is present in P cells and absent in VM cells, although a small short tail may follow I Na1 in some VM cells. I K1 peak is followed by early and late negative slope (NS) regions. In P cells, the early NS disappears with less negative holding potential (V h ). In VM cells, the late NS region is larger, peaks at positive potentials and persists with slow ramps or less negative V h . The NS region increases in size with faster ramps. In Vm cells, the NS early component is blocked by Ba 2+ and the late component by Ni 2+ . I Ca is greater in VM cells and I to in P cells. During repolarizing ramps, in VM cells the smaller outward current decreases less and in both M and P cells decreases more during faster ramps. A positive slope region is greater in VM cells and may be missing in some P cells. The many differences found are consistent with the different functions of P and VM cells in situ. Supported in part by NIH grant HL56092.