Effects of 4-aminoquinoline on action potentials of the frog sinus venosus

The electrophysiological effects of 4-aminoquinoline (4-AQ), a drug structurally related to 4-aminopyridine, were studied on the sinus venosus of the frog Caudiverbera caudiverbera with standard microelectrode techniques. 4-AQ in concentrations (0.01-1.5 mM) that did not modify membrane resting potential, reversibly depressed pacemaker activity and the rate of diastolic depolarization. These effects were not prevented by atropine. 4-AQ also reduced maximum diastolic potential, whereas action potential amplitude, overshoot and dV/dtmax remained unaffected. Action potential duration was prolonged by 4-AQ in a concentration-related manner by slowing down the repolarization phase. The effects upon repolarization and on diastolic depolarization were abolished by doubling extracellular calcium concentration. They were not suppressed, however, by electrically pacing sinus preparations at the pre-drug frequency. A plausible interpretation of the data is that 4-AQ effects are the consequence of a direct inhibition of potassium conductance at the cell membrane.