Electrophysiological effects of 4-aminoquinoline on frog atrial contractile fibres

The effects of 4-aminoquinoline (4-AQ), a drug structurally related to 4-aminopyridine, were studied on atrial contractile fibres from the amphibian Caudiverbera caudiverbera by means of intracellular electrodes. In concentrations that did not change resting potential, 4-AQ (0.25-1.5 mM) reduced upstroke velocity, action potential amplitude (APA) and overshoot in a dose-dependent manner. 4-AQ also prolonged action potential duration (APD) and the fibre effective refractory period (ERP), making the ratio ERP/APD greater than one. These effects were reversible by washing. The drug-induced reduction of upstroke velocity was frequency-dependent. This effect and increase of APD were more intense at higher values of resting potential. All effects produced by 4-AQ remained constant in either high or low external K+ concentrations. The depressant effects of this compound on upstroke velocity, overshoot, APA as well as increase of APD were antagonized by doubling the extracellular Na+ concentration. Data presented support the view that 4-AQ exerts its effects on the frog heart by blocking both sodium and potassium channels, thus causing quinidine-like actions.