Biliary excretion of drugs: role of ligandin in newborn immaturity and in the action of microsomal enzyme inducers

Microsomal enzyme inducers such as phenobarbital, spironolactone and pregnenolone-16alpha-carbonitrile increase the rate of disappearance of drugs such as sulfobromophthalein and ouabain, from the plasma, to a similar extent by increasing their rate of excretion into bile. However, this effect is not observed after 3-methylcholanthrene. The enhanced biliary excretion is not dependent on increased biotransformation only, since ouabain is not biotransformed before excretion. Newborn rats are immature in their ability to excrete drugs such as BSP and ouabain. Since a hepatic cytosol protein, ligandin, has been shown to bind many drugs and is suggested to be important in the hepatic removal of drugs from the plasma, the correlation between the hepatic content of ligandin and hepatic excretory function was measured. Treatment of adult rats for 4 days with phenobarbital increased the amount of ligandin by 85%, whereas spironolactone increased it by 35% and 3-methylcholanthrene by 17%. The amount of ligandin in the livers of 5-day-old rats was about 10% that of adults and increased until the rats were about 35 days of age. Althoughh ligandin bound sulfobromophthalein, it did not bind ouabain. Since little correlation between the ability of microsomal enzyme inducers to increase ligandin and to increase biliary excretion exists, and since ouabain is excreted at a faster rate after administration of microsomal enzyme inducers and at a slower rate in newborn rats, even thoug ligandin does not bind ouabain, it is suggested that the amount of ligandin in the liver is not related to the increased biliary excretion after micromomal inducers, nor to the decreased hepatic excretory function in newborn rats.

Biliary excretion of colchicine in newborn rats

The 24-hr LD50 of colchicine in newborn rats is 0.24 mg/kg, which is about 1/10 that observed in the adult. The 24-hr LD50 of colchicine was relatively constant in rats over 25 days of age. In an attempt to determine the mechanism of the increased sensitivity of the newborn rat to the toxic action of colchicine, the distribution of 3H after the administration of 3H-colchicine (0.1 mg/kg) was measured in 10- and 35-day-old rats. The concentration of 3H was higher in all tissues of the newborn than the adult after ip administration, suggesting an immaturity in the pathway for colchicine elimination. After iv administration, radioactivity disappeared much more slowly from the plasma of the newborn rat than from the adult. This was due to a lower capacity of the liver of the newborn to concentrate colchicine and to excrete it into the bile. Development of the hepatic excretory mechanism responsible for excretion of colchicine occurred at the same age as did the increase in LD50. These results suggest that colchicine is more toxic in the newborn because the drug remains in the body for a longer time due to immaturity of the liver excretory process.

Biliary excretion of xenobiotics

The biliary route is very important for the elimination of some foreign compounds from the body. For many of these compounds, an increase in the rate at which they are excreted into the bile will decrease their toxicity and vice versa. A number of factors which are known to alter the biliary excretion of xenobiotics, as well as the current concepts of the physiological mechanisms responsible for the excretion of foreign compounds, have been enumerated. However, much remains still to be understood; essentially nothing is known at the subcellular level about the biliary excretion of foreign compounds. It has recently been concluded that our knowledge of the biliary excretion of compounds is about 40 years behind that of the renal excretion mechanism.

Biliary excretion of colchicine

After intravenous administration of 3H-colchicine (0.2 mg/kg) to rats, 68 percent was excreted in the feces in 48 hours suggesting bile might be the major route of excretion for colchicine. Rats with cannulated bile ducts excreted 50 percent of a 2 mg/kg dose into the bile within 2 hours; half of this was colchicine, and the rest was desmethylcolchicine and more polar metabolites. Colchicine was excreted into the bile of the rat against a bile/plasma concentration gradient of 800 which resulted from a liver/plasma ratio of 15 and a bile/liver ratio of 60. The biliary excretion of colchicine varied widely among the hamster, dog and rabbit. Of the administered colchicine, 32, 20 and 16 percent were excreted by the hamster, dog and rabbit, respectively, within 2 hours. There was also a species difference in the percentage of the radioactivity present in bile as the parent drug. In the hamster, dog and rabbit, the percentage of radioactivity excreted into the bile as colchicine was 45, 34 and 72 percent, respectively. These species excreted colchicine into the bile against a bile/plasma gradient ranging from 19 to 870. Partition of these gradients between liver/plasma and bile/liver ratios demonstrated that both ratios were greater than one, suggesting that colchicine is excreted by an active process. The liver/bile gradient was always the larger of the two ratios.

Extrahpetic distribution of sulfobromophthalein

Plasma clearance of sulfobromophthalein (BSP) is widely used as a measure of hepatic function. Its validity depends upon its exclusive elimination from the body via bile. For example, in the present study, when BSP was administered intravenously (i.v.) to rats at four different doses (18.75, 37.5, 75, and 150 mg/kg), less than 0.5% of each dose was excreted into the urine and between 70 and 85% was excreted into the bile within 6 h after administration. It has been assumed that the distribution of BSP is limited to the blood and liver witith very little appearing in other tissues. When we measured the amount of BSP in the plasma, liver, and the bile 10 min after the i.v. administration of either a high (150 mg/kg) or a low (18.75 mg/kg) dose of BSP, only 60% of the dose was accounted for. The concentration of BSP and 12-I-labelled albumin (RISA) was measured in various tissue samples 10 min after administration of 17.5 or 150 mg of BSP or RISA per kilogram. More BSP was found in all tissues than was contained in the plasma entrapped therein. Thus, the distribution of BSP is not limited to the liver and plasma. During excretion BSP leaves other tissue (kidney, spleen, lung, etc.) and is ultimately excreted into the bile.

Bile flow and composition during bile acid depletion and administration

Relatively similar concentrations of the inorganic ions were detected in rat, rabbit, and dog bile; however, dog bile had a higher concentration of protein, cholesterol, phospholipid phosphorous, and percentage solids than rat bile, and rabbit bile had the lowest concentration. The biliary excretion of bile acids was altered in each species by: (1) interruption of the enterohepatic circulation; (2) rapid administration of an exogenous load of bile acids; and (3) constant infusion of an exogenous load of bile acids. Bile acid and phospholipid phosphorous concentration and percentage solids increased after bile acid administration in all three species; however, species differences in bilirubin concentration were observed and a marked decrease was detected in rabbit and dog bile but it markedly increased in rat bile. When the enterohepatic circulation was interrupted in the dog and rat, the bile acid concentration markedly decreased with only minor changes in bile flow. This not only supports the theory that there is a bile salt independent fraction of bile formation, but also demonstrates that canalicular bile formation can be maintained at relatively normal rates with almost no excretion of bile acids. Marked discrepancy between bile acid excretion and bile flow was observed in the rat after bile acid administration, in that a marked increase in bile acid excretion was observed but little or no increase in flow. When bile flow was plotted against bile acid excretion for the three species, the slope of the line was less during bile acid administration than during depletion, indicating that the bile acids are accompanied by less water during bile acid administration than during depletion. Variation in the bile flow intercept with zero bile acid excretion (thought to represent the bile salt-independent fraction) was relatively large, which is probably due in part to alteration in the production of the bile salt independent fraction when bile acid secretion is altered. It appears that both the choleretic property of bile acids varies during various rates of bile acid excretion and the bile salt-independent fraction is not constant. Therefore, calculation of the bile salt independent fraction as previously performed should be interpreted with extreme caution. Thus, it appears difficult to determine the quantitative importance of bile acid excretion in bile formation.

Species differences in the choleretic response to bile salts

1. Cholic, taurocholic and dehydrocholic acids were administered I.V. to rats, rabbits and dogs (3.12-100 mg/kg). With the higher doses of cholic acid, the bile flow was increased sixfold in the dog and only 2 to 2(1/2)-fold in the rat and rabbit. The choleretic response was also maintained for a longer time in the dog (90 min) than in the rat or rabbit (20-30 min).2. Similar species differences in the choleretic response to taurocholic acid and dehydrocholic acid were observed. However, the bile flow returned to control rates more rapidly for cholic acid (10-15 min for rat and rabbit, 45 min for dog) and more slowly for dehydrocholic acid (25-40 min for rat and rabbit, 120 min for dog).3. Cholic acid is conjugated more rapidly by the rabbit and rat than dog.4. An increase in the biliary bile acid concentration was observed in all three species after the I.V. administration of the bile acid excreted by each species.5. Control bile flow was much higher in the rat (50 mul./min.kg) and the rabbit (70 mul./min.kg) than in the dog (5 mul./min.kg).6. Part of the difference in the choleretic response of the three species to the bile acids appears to be due to this difference in basal bile production.7. Administration of taurocholic acid to the rat increases the biliary bile acid concentration in the bile but the bile flow may either increase or decrease, questioning the mechanism by which bile acids increase bile flow.