Age dependence of the density current in freshly isolated single atria and ventricular cells. (a) was measured as the bidirectional Ni²⁺ (5 mM) sensitive current from a freshly isolated single cells in culture (see methods for details). Top representative traces from an atria and middle ventricle cells (st. 26). Black line control current, red line Ni²⁺ sensitive current. Bottom Slow-ramp pulses (+60 mV to −120 mV; 0.09 V/sec; holding potential −30 mV) at 0.25 Hz were applied. (b) Changes on in atria (black line) and ventricles (red line) st. 14 (), 21 (), 26 (), and 29 () (Mean ± SE).