Fig. 4. (A) Upper trace: maxi 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, 0.2 nA (top) and 60 mV (bottom). (B) Effect of charybdotoxin (CHTX) on the spontaneous beating rate of atrial cultured cells from oyster. Vertical: beating frequency. Horizontal: CHTX addition in Petri dishes corresponding to a final concentration of: 5x10^–6, 10^–5, 1.5x10^–6 mol l^–1. All the CHTX values are significantly different from the control.