Methylxanthines and intestinal drug absorption.

1. Jejunal loops of anaesthetized rats were filled with buffered isotonic solutions of tritiated water (HTO), urea, antipyrine and salicylic acid at pH 6-8. The venous outflow and the appearence rate of the substances in the intestinal venous blood were determined. Blood pressure was kept constant by adjustable supply of blood from donor rats throughout the experiment. 2. The absorption of urea, antipyrine and salicylic acid was, in concentrations from 0.001 to 1.0 mg/ml found to be directly proportional to the intraluminal concentration. 3. Theophylline and caffeine (2 mg/ml), when injected into the lumen, increased the blood flow to 188% and 166% of controls. 4. The theophylline induced increase in blood flow caused an enhancement in the absorption of antipyrine to 153%, HTO and urea to 135% and salicylic acid 123% of controls. 5. Caffeine influenced the absorption of HTO and salicylic acid similar to theophylline.