Gastric, intestinal and colonic absorption of metoprolol in the rat.

The absorption of metoprolol from the stomach, small intestine and colon of anaesthetized rats has been evaluated using an in situ technique. Absorption rates were measured in terms of the rate of disappearance of metoprolol fumarate from the lumen between 5 and 30 min after dosing. Adsorption was estimated from the initial rapid fall in luminal content within the first 5 min after drug administration. The rate of drug absorption from the stomach was low or negligible. In the small intestine, the absorption rate constants, ka, at pH 6.2 and 7.5 were 0.66 and 0.81 h-1, respectively. In the colon, the rate of drug absorption at pH 7.5 was faster (ka = 1.21 h-1) than in other segments of the gut. Drug adsorption in the stomach amounted to 11% of the administered dose. In the small intestine adsorption was greater (16-22%), presumably because of the larger surface area in this segment of the gut, but in the colon adsorption was negligible.