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This study was performed to examine the metabolism and absorption of intragastrically administered capsaicinoids in the anaesthetized rat. [3H]-dihydrocapsaicin ([3H]-DHC) and unlabelled capsaicin were readily absorbed from the gastrointestinal tract but were almost completely metabolized before reaching the general circulation. A certain degree of biotransformation already took place in the intestinal lumen. Unchanged compounds (identified by chromatography) were present in portal vein blood. There seems to be a saturable absorption and degradation process in the gastrointestinal tract and a very effective metabolism in the liver. Less than 5% of the total amount of extracted radio-activity consisted of unchanged [3H]-DHC in trunk blood and brain 15 min after gastrointestinal application. On the other hand, approximately 50% unchanged [3H]-DHC was detected in these tissues 3 min after i.v. or 90 min after s.c. application of the capsaicinoids. Dihydrocapsaicin (DHC) or [3H]-DHC were metabolized when incubated in vitro with liver tissue but not with brain tissue. The metabolic product(s) did not show capsaicin-like biological activity. It can be concluded that rapid hepatic metabolization limits systemic pharmacological effects of enterally absorbed capsaicin.

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