Enterohepatic recycling of phenolphthalein, morphine, lysergic acid diethylamide (LSD) and diphenylacetic acid in the rat Hydrolysis of glucuronic acid conjugates in the gut lumen

In female Wistar albino rats, biliary elimination of phenolphthalein, morphine, LSD, and diphenylacetic acid occurred predominantly as glucuronides, with 90%, 45%, 75%, and 57% of injected doses eliminated in bile within three hours. Infusing this bile into the duodena of other rats showed enterohepatic circulation: 85%, 41%, 28%, and 66% of the infused conjugates were reabsorbed and re-excreted over 24 hours. Suppressing intestinal bacteria with antibiotics reduced recirculation to 22%, 8.6%, and 21% for phenolphthalein, morphine, and diphenylacetic acid glucuronides, but did not affect absorption of free aglycones. This demonstrates that bacterial beta-glucuronidase hydrolysis of glucuronides is essential for enterohepatic circulation. Intestinal absorption of released aglycones correlated with their lipid-solubility, measured by octan-1-ol:phosphate buffer partition ratios.

Study at a glance

Abstract

1. Biliary elimination in female Wistar albino rats 3 h after i.p. injection of [3H]phenolphthalein, [3H]morphine, 14C-LSD and [14C]diphenylacetic acid was 90%, 45%, 75% and 57% respectively, predominantly as glucuronides. 2. Infusion of 3 h bile from the previous experiments into the duodena of bile-duct-cannulated animals demonstrated enterohepatic circulation, amounting in 24 h to 85%, 41%, 28% and 66% of the infused doses of the conjugates of phenolphthalein, morphine, LSD and diphenylacetic acid respectively. 3. Pretreatment with antibiotics to suppress intestinal microflora decreased this enterohepatic recirculation to 22%, 8.6% and 21% in 24 h for phenolphthalein, morphine and diphenylacetic acid glucuronides respectively. Antibiotic pretreatment did not influence the absorption and re-excretion of infused doses of the free aglycones, thus demonstrating the importance of bacterial beta-glucuronidase hydrolysis of the biliary conjugates. 4. The extent of intestinal absorption of the aglycones after bacterial beta-glucuronidase hydrolysis of the conjugates is related to their lipid-solubility as estimated by octan-1-ol:0.1 M phosphate buffer partition ratios (P-values). 5. The persistence of compounds in the enterohepatic circulation is determined by the faecal and urinary elimination of the circulating compounds. Faecal elimination is governed by the extent of intestinal absorption of the circulating compounds, which is influenced by the efficacy of intestinal hydrolysis of the conjugates and the relative lipophilicity of the aglycones released.