Absorption and metabolism of procaine by the rat small intestine.

The aim of this study was to obtain information about the absorption of procaine in the rat small intestine (Fisher-Parsons preparation). In the range from 0.25-10 mmol.l-1 procaine in the luminal perfusate, much more of the unchanged drug was absorbed in segments of the ileum than of the duodenum and jejunum. Besides procaine, two metabolites, p-aminobenzoic acid (PABA) and acetylated p-aminobenzoic acid (AABA), formed in the intestinal mucosa, appeared in the absorbate. With increasing substrate concentration in the perfusate the PABA in the absorbate increased considerably in all three segments; from 0.75 mmol.l-1 procaine upwards the PABA produced was highest in the jejunum. AABA formed in the mucosa and measured in the absorbate did not increase in the same manner with increasing substrate concentration; in the absorbate of jejunal segments the amount of AABA was significantly higher than in duodenal and ileal segments. Taking into account that in rats the microclimate of the ileum differs considerably from that of the upper part of the small intestine, the marked difference observed in the absorption of procaine between ileal segments on the one side, and duodenal and jejunal segments on the other, can be explained on the basis of the "non-ionic diffusion" theory.