Interaction between verapamil and beta-methyldigoxin. Study by 86Rubidium uptake in erythrocytes

Digoxin and Verapamil may often be used in association in heart diseases; we tested whether this coadministration increases digitalis activity. Ten patients with failing heart were given 0.10 mg of beta-methyldigoxin, twice a day, for nine days; then they received in addition 80 mg of Verapamil, four times a day, for seven days. Digoxin activity was measured on the eighth, ninth and 16th days of treatment by 86Rubidium uptake in erythrocytes. The steady-state concentration was 0.91 +/- 0.13 ng/ml; during the coadministration the concentration rose to 1.23 +/- 0.16 ng/ml (an increase of 35.12%). The change was significant (p less than 0.001). The mechanism of this increase is unknown: it is important that there exists an increase in digitalis activity and not only a haematic elevation.