Quinidine-digoxin interaction: effect of quinidine on 86Rb-uptake of human erythrocytes.

Co-administration of quinidine results in a marked increase in serum digoxin concentration (SDC). The implication of this increase in SDC in regard to an increased digoxin effect on the heart is controversial. The 86Rb-erythrocyte-assay represents a perfect model to study whether quinidine interferes with digoxin at the Na-K-ATPase (the so called glycoside receptor) and thus presumably with the inotropic effect of the glycoside. The inhibitory effect of digoxin (0-150 ng/ml) on the 86Rb-uptake was measured in the absence and presence of quinidine in therapeutic or higher concentrations (0-60 microgram/ml). Addition of quinidine produced no effect on digoxin-induced inhibition of Na-K-ATPase activity. Together with our clinical observations these results strongly suggest that the increased SDC during concomitant quinidine therapy actually reflects an increased digoxin effect on the heart.