The gastrointestinal absorption of paracetamol in the rat.

The absorption of [3H]paracetamol by rat small intestine, colon and stomach was studied in vivo and in vitro. Small intestinal in vivo studies, using a wide range of drug concentrations, showed that absorption was efficient and uniform throughout the small bowel, no site showing preferential absorption. Double reciprocal and direct plots indicated first order kinetics. The pattern was not observed when uptake was occurring from high concentrations of paracetamol in suspension. Gastric and colonic in vivo studies showed that there was appreciable absorption of [3H]paracetamol from these sites. In vitro studies using everted intestinal sacs showed no effect on paracetamol transfer when the incubation temperature was lowered to 10 degrees C or when iodoacetate (5 X 10(-2)M) and 2.4 dinitrophenol (5 X 10(-4)M) was added to the incubation medium. There was, however, a significant reduction in transfer of paracetamol against a concentration gradient of 10:1 applied across the mucosa. These data suggest that the uptake of paracetamol is by a passive transport process and confirm the efficiency of paracetamol absorption observed indirectly by others.