Figure 6. I–V relationships for various current components.

Upper panels (A) show I–V curves in the cells which showed I_f, and lower panels (B) show those cells which did not show I_f. The current components are indicated at the top of each column. The amplitude of I_f was determined as the time-dependent increase from the level at 30 ms after the onset of the test pulse to that near the pulse end. I_K1, Ba²⁺-sensitive currents measured 30 ms after the pulse onset. The decrease of current amplitude beyond -130 mV is most probably due to the ionic block of the channel. I_Ca,L, the amplitudes measured from the current level at 100 ms to the peak of transient inward current. I_K,tail, peak current values of the outward tail current on repolarization. The current density (pA pF⁻¹) was determined by dividing the current amplitude by the cell capacitance (pF); these values are indicated for each I–V relationship on the left (or right in I_Ktail) approximately in order of increasing current amplitude. MDP, average maximum diastolic potential of -57.3 mV.