Fig. 1. (A) Upper trace: sum of the currents recorded in cultured atrial cells from oyster. Inward current, mainly the fast sodium current, is classically represented downwards, and outward currents, mainly the potassium currents, upwards. Lower trace: voltage steps imposed to the membrane. Depolarizing steps are upwards. Horizontal bar, 1 ms; vertical bars, 0.1 nA (top) and 40 mV (bottom). (B) Upper trace: sum of the fast inactivating and of the delayed potassium current. Lower trace: voltage steps applied to the membrane. Horizontal bar, 10 ms; vertical bars, 0.1 nA (top) and 60 mV (bottom). (C) Upper trace: delayed potassium current recorded after inhibition of the fast inactivating potassium current by 4-aminopyridine. Lower trace: voltage steps applied to the membrane. Horizontal bar, 10 ms; vertical bars, 0.1 nA (top) and 60 mV (bottom). (D) Current–voltage relationship of the delayed potassium current. The maximum conductance is calculated from the slope of the linear part.