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Simultaneous administration of digoxin and calcium channel blockers results in increased plasma levels of digoxin. One of the possible mechanisms of this interaction could be a changed enteral absorption of digoxin under the influence of calcium channel blockers. The study of possible involvement of calcium channel blockers in the enteral absorption of digoxin showed that calcium channel blockers inhibited the intestinal absorption of digoxin. In further studies this interaction was investigated in vivo in guinea pigs given racemic form of nicardipine and its negative and positive enantiomers. Minimal lethal dose of digoxin determined intraveniously 30 minutes after intraduodenal administration was increased in animals pretreated with either racemic form of nicardipine or negative enantiomer, but not in those given positive enentiomer of nicardipine. These results indicate that the effectiveness of nicardipine in the described interaction depends upon its optical activity.