Comparative Analysis of the Action of Class I Antiarrhythmic Drugs (Lidocaine, Quinidine, and Prajmaline) in Rabbit Atrial and Ventricular Myocardium

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Abstract
Effects of three class I antiarrhythmic drugs (quinidine, lidocaine, and prajmaline) on transmembrane resting (RMP) and action potentials (AP) of isolated rabbit atrial and ventricular myocardium were studied at different stimulation rates. The frequency-dependent depression of the maximal upstroke velocity (Vmax) of the AP (sodium channel block) was analyzed according to the "guarded receptor" hypothesis. The resting block (Vmax depression after a resting period) induced by prajmalaine (10-6 M) quinidine (2.2 .times. 10-5 M), and lidocaine (4.3 .times. 10-5 M) was more expressed in the atrium (44, 28, and 19%, respectively) than in the ventricle (32, 9, and 0%, respectively). There were also significant (p < 0.05) atrioventricular differences in the frequency-dependent extra block (Vmax reduction on stimulation at 3.3 Hz) for quinidine (39 vs. 26%) and lidocaine (4 vs. 25%). From the analysis, according to the guarded receptor hypothesis, it follows that the three compounds bind preferentially to inactivated sodium channels with about the same affinity to the atrium and ventricle, except for quinidine which shows a significantly smaller dissociation constant in the atrium (5 .times. 10-6 M vs. 2.7 .times. 10-5 M; p < 0.001). We conclude that the atrioventricular differences in the resting block are mainly due to atrioventricular differences in the RMP, whereas the differences in the frequency-dependent extra block are based on the shorter atrial AP duration (lidocaine) or are due to higher affinity to atrial sodium channels (quinidine).

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