Electrophysiological effects of diclofurime on rabbit and frog atrial heart muscle

The effects of diclofurime on the electrical activity of the rabbit sinus node, rabbit atria and frog atrial fibres were studied using microelectrode and the double sucrose gap voltage-clamp techniques respectively. In rabbit sinus node, diclofurime (10(-7) M to 10(-6) M) decreased the action potential (AP) amplitude and maximum rate of depolarization (Vmax), increased the AP duration and slowed the sinus rate. In rabbit atria, the drug reduced the amplitude of the depolarizing phase and Vmax, lengthened the AP duration and decreased the resting membrane potential. In frog atrial fibres, the drug (10(-5) M) depolarized the resting membrane potential, decreased Vmax as well as the plateau amplitude. It inhibited the sodium current (INa) with a dissociation constant of 3.7 X 10(-6) M and a one to one relationship between the drug molecule and the Na channel. Diclofurime did not alter the apparent reversal potential for the fast Na current (ENa) but it inhibited the sodium conductance (GNa) in a frequency-dependent manner. Diclofurime also blocked the slow inward current (Islow) without alteration of Eslow. The block of Islow occurred with a dissociation constant of 2 X 10(-5) M and unity stoichiometry. The data suggest that diclofurime might be effective in the control of cardiac arrythmias since it exhibited both local anaesthetic-like and calcium antagonistic properties.