Absorption of digoxin in man after oral and intrasigmoid administration studied by portal vein catheterization

The absorption of digoxin has been studied in fasting patients undergoing transumbilical, portal catheterization for diagnostic purposes. A purely aqueous solution was administered orally in 4 patients and in the sigmoid colon in 2 patients. Blood samples were taken simultaneously from the portal and a peripheral vein. Plasma digoxin concentrations were determined by radioimmunoassay. Digoxin appeared early in the blood after oral administration; the average peak of porto-peripheral concentration differences was at 18 min. After intrasigmoid administration, absorption was slower, and no distinct peaks were found. Calculation of the mean amounts absorbed showed that half the dose had been absorbed via the portal vein during 2 hours in the oral study and during6 hours in the intrasigmoid test. Approximately 2/3 of the dose had been absorbed during 6 hours after oral dosing. The absorption rate was estimated taking into account the decreasing amount of drug left to be absorbed at different times. After cral administration, the mean peak rate was found to correspond to an absorption half-time of 0.78 h, which was more than 20 times faster than the rate after 6 h. The mean rate after intrasigmoid administration appeared to be about 1/3 of that after oral dosing. Physiological factors that might account for these differences in absorption rate are discussed.