The UDP-glucuronyltransferase inducers, phenobarbital and pregnenolone-16alpha-carbonitrile, enhance thyroid-follicular cell apoptosis: association with TGF-beta1 expression.

Exposure to certain UDP-glucuronosyltransferase (UDP-GT) inducers leads to follicular cell hyperplasia, and ultimately thyroid gland tumors. These compounds decrease thyroid hormones, which increases serum concentrations of thyroid stimulating hormone (TSH). This induction of TSH enhances thyroid-follicular cell proliferation. In addition, treatment with classical goitrogenic compounds, such as propylthiouracil (PTU) and methimazole (MMI), induces TGF-beta1 in thyroid-follicular cells, presumably through increased TSH. In other tissues, increases in TGF-beta1 induce apoptosis, a particular form of programmed cell death. In this experiment, we sought to determine whether the UDP-GT inducers, phenobarbital (PB) and pregnenolone-16alpha-carbonitrile (PCN) modulate thyroid-follicular cell apoptosis. If so, are the induction of apoptosis and TGF-beta1 possibly linked? An additional group of rats treated with the thyroid goitrogen, PTU was included. Male Sprague-Dawley rats were treated with thyroid hormone disrupting doses of PB, PCN, or PTU for 3, 7, 14, 21, 28, 45, or 90 days. In this study, PTU treatment increased apoptosis and TGF-beta1 immunoreactive thyroid-follicular cells. PTU treatment of rats produced both a large increase number of TGF-beta1-positive cells (detected by immunohistochemistry), and apoptotic thyroid-follicular cells (detected by morphology). In PB- and PCN-treated rats, a moderate increase in apoptosis coincided with similar increases in TGF-beta1 immunoreactive thyroid-follicular cells. In summary, PB and PCN increase apoptosis and the percentage of TGF-beta1 positive thyroid-follicular cells. Thus, treatment with UDP-GT-inducing chemicals may increase the expression of TGF-beta1 and apoptosis in the thyroid to compensate for the thyroid hypertrophy and hyperplasia.     

A study on cerium-induced liver injury in rats after pretreatment with spironolactone, phenobarbital, pregnenolone-16 alpha-carbonitrile and nafenopin

A number of drugs known to induce liver microsomal enzymes were studied with respect to their effect on the elevated liver triglyceride (TG) concentration and serum glutamic oxaloacetic acid transaminase (GOT) and glutamic pyruvic acid transaminase (GPT) activities induced by an i.v. injection of cerium in rats. Pretreatment with pregnenolone - 16 alpha - carbonitrile (PCN) and 2-methyl-2-[p-(1,2,3,4-tetrahydronaphth-1-yl)phenoxy]-propionic acid (nafenopin) decreased the liver TG concentration significantly. Spironolactone and phenobarbital pretreatment also decreased the TG level in comparison to controls, but the difference was not statistically significant. Both GOT- and GPT-activities in serum increased on the first day after cerium injection and reached a maximum after 3 days. All the drugs significantly reduced the elevated levels. After 7 days the activity was almost normal in all the surviving animals.     

Effect of pregnenolone-16 alpha-carbonitrile and dexamethasone on acetaminophen-induced hepatotoxicity in mice.

Recently, we demonstrated that a microsomal enzyme inducer with a steroidal structure, pregnenolone-16 alpha-carbonitrile (PCN), markedly decreased the hepatotoxicity of acetaminophen (AA) in hamsters. Therefore, it was of interest to determine if PCN, as well as another steroid microsomal enzyme inducer, dexamethasone (DEX), would decrease the toxicity of AA in mice, another species sensitive to AA hepatotoxicity. Mice were pretreated with PCN or DEX (100 and 75 mg/kg, ip, for 4 days, respectively) and were given AA (300-500 mg/kg, ip). Twenty-four hours after AA administration, liver injury was assessed by measuring serum activities of sorbitol dehydrogenase and alanine aminotransferase and by histopathological examination. Neither PCN nor DEX protected markedly against AA hepatotoxicity in mice; PCN tended to decrease AA-induced hepatotoxicity, whereas DEX was found to enhance AA-induced hepatotoxicity and it produced some hepatotoxicity itself. DEX decreased the glutathione concentration (36%) in liver and increased the biliary excretion of AA-GSH, which reflects the activation of AA, whereas PCN produced neither effect. Thus, whereas PCN has been shown to markedly decrease the hepatotoxicity of AA in hamsters, apparently by decreasing the isoform of P450 responsible for activating AA to N-acetyl-p-benzoquinoneimine, this does not occur in mice after induction with either PCN or DEX. In contrast, DEX enhances AA hepatotoxicity apparently by decreasing liver GSH levels and increasing the activation of AA to a cytotoxic metabolite.     

Effect of phenobarbital or pregnenolone-16alpha-carbonitrile (PCN) pretreatment on acute carbon tetrachloride hepatotoxicity in rats

In rats, CCl₄ hepatotoxicity (reflected by augmented serum glutamic-pyruvic transaminase activity and hepatic triglyceride content) was diminished by pregnenolone-16α-carbonitrile (PCN) and markedly increased by phenobarbital. PCN, like penobarbital, caused smooth-surfaced endoplasmic reticulum proliferation in hepatocytes and enhanced hexobarbital oxidation, ethylmorphine N-demethylation, cytochrome P-450 and NADPH-cytochrome c reductase activity. Contrary to earlier views, it is concluded that the increase in these parameters is not a prerequisite for augmented CCl₄ toxicity. Perhaps the cytochrome P-450 induced by the steroid and barbiturate has different catalytic properties, which are responsible for variations in the response to CCl₄.     

Protective effect of pregnenolone-16 alpha-carbonitrile on acetaminophen-induced hepatotoxicity in hamsters

Overdosage of acetaminophen (AA) is known to produce acute liver toxicity in both humans and laboratory animals. Hamsters are especially sensitive to the hepatotoxic effect of AA. In the present study, hamsters pretreated with pregnenolone-16 alpha-carbonitrile (PCN; 75 mg/kg, ip, daily for 4 days) were given a single dose of AA (350-1200 mg/kg, ip) and liver function was determined 24 hr later. Serum activities of alanine aminotransferase (ALT) and sorbitol dehydrogenase (SDH) as well as histopathology were used as indices of hepatotoxicity. PCN pretreatment decreased AA-induced mortality. PCN dramatically decreased ALT (93-97%) and SDH (63-98%) activities relative to control values from hamsters treated with AA alone, and remarkably decreased hepatic centrilobular necrosis produced by AA. To investigate the mechanism of this protective effect, the biliary and urinary excretion of AA metabolites were measured for 1 hr after administration of AA (150 mg/kg, iv) in bile-duct-cannulated hamsters. PCN pretreatment resulted in increased urinary and biliary excretion of AA-glucuronide and decreased biliary excretion of AA-glutathione. Microsomes from PCN-pretreated hamsters produced less benzoquinoneimine intermediate than controls, as determined by the formation of AA-glutathione. In addition, hepatic UDP-glucuronic acid and UDP-glucuronosyltransferase were significantly increased in PCN-pretreated hamsters. In conclusion, PCN pretreatment protected against AA-induced hepatotoxicity. The mechanism of this protection appears to be due to decreased formation of the reactive metabolite by the cytochrome P450 pathway, and an increased detoxication by enhanced glucuronidation of AA.     

Effect of pregnenolone-16alpha-carbonitrile (PCN) on drug response in man.

Pregnenolone-16alpha-carbonitrile (PCN), which alters drug responses and induces hepatic microsomal drugs-metabolizing enzymes in experimental animals, is well-tolerated in man. There were no marked changes in the concentrations of blood sugar and urea; marked changes in the concentrations of blood sugar and urea; bromsulphophthalein (BSP) clearance; serum alkaline phosphatase activity; hematocrit, erythrocyte and leukocyte counts; as well as in ecg patterns. When the steroid was given by mouth for 4 days to patients treated with electroshock for psychiatric disorders, it decreased suxamethonium-induced skeletal muscle fasciculation and shortened thiopentone as well as propanidid anaesthesia. Breathing was resumed more rapidly than in control subjects. The protective effect of PCN is probably mediated through induction of activation or hepatic drug-metabolizing enzymes, or both.     

Onset of enzyme induction with pregnenolone-16alpha-carbonitrile in male and female rats.

Male and female rats treated with a single 20-mg/kg ip dose of pregnenolone-16alpha-carbonitrile (PCN) produced parallel increases in hepatic cytochrome P-450 and aniline hydroxylase activity. However, the onset of increase in aniline hydroxylase activity and cytochrome P-450 content was slower in male than female animals. The maximal levels achieved in male rats were 89% of the values observed in the female animals. Furthermore, the hepatic aniline hydroxylase activity and P-450 content declined more rapidly in male than in female rats.     

Phenobarbital, beta-naphthoflavone, clofibrate, and pregnenolone-16alpha-carbonitrile do not affect hepatic thyroid hormone UDP-glucuronosyl transferase activity, and thyroid gland function in mice

The effects of representative liver enzyme inducers such as clofibrate (CLO), phenobarbital (PB), pregnenolone-16alpha-carbonitrile (PCN), and beta-naphthoflavone (NF) on hepatic microsomal thyroxin (T4)- UDP-glucuronosyl transferase (UGT) and triiodothyronine (T3)- UGT activities and thyroid function were evaluated in OF-1 male mice after a 14-day po administration. CLO, PB, and PCN induced histological liver hypertrophy, increases in liver weights, in microsomal protein and cytochrome P450 contents as well as increases in specific UGT activities. Despite this, no significant changes in T4-UGT and T3-UGT activities occurred after treatment by any of these compounds. Furthermore, no significant changes in serum T4 and T3 levels were observed and thyroid histology was not affected. NF treatment induced microvacuolation of hepatocytes but did not affect any of the other tested parameters. The results show that, in contrast to the widely described effects in rats, liver enzyme inducers do not affect hepatic thyroid hormone metabolism and thyroid function in mice, suggesting that this species should be less sensitive to thyroid tumor promotion by hepatic microsomal enzyme inducers than rats.     

Nuclear receptor, pregname X receptor, is required for induction of UDP-glucuronosyltranferases in mouse liver by pregnenolone-16 alpha-carbonitrile.

The aim of this study was to determine the role of pregnane X receptor (PXR) in the induction of UDP-glucuronosyltransferases (UGTs) by pregnenolone-16 alpha-carbonitrile (PCN). Four- to six-month-old male wild-type and PXR-null mice received control or PCN-treated (1500 ppm) diet for 21 days. On day 22, livers were taken to prepare microsomes and total RNA to determine UGT activity and mRNA levels, respectively. In wild-type mice, PCN treatment significantly increased UGT activities toward bilirubin, 1-naphthol, chloramphenicol, thyroxine, and triiodothyronine. On control diet, the UGT activities toward the above substrates (except for 1-naphthol) in the PXR-null mice were significantly higher than those of wild-type mice. However, UGT activities in PXR-null mice were not increased by PCN. In agreement with the above findings, mRNA levels of mouse Ugt1a1 and Ugt1a9, which are involved in the glucuronidation of bilirubin and phenolic compounds, were increased about 100% in wild-type mice following PCN treatment, whereas the expression of Ugt1a2, 1a6, and 2b5 was not affected. In contrast, PCN treatment had no effect on the mRNA levels of these UGTs in PXR-null mice. Taken together, these results indicate that PCN treatment induces glucuronidation in mouse liver, and that PXR regulates constitutive and PCN-inducible expression of some UGTs.     

Effect of pregnenolone-16 alpha-carbonitrile on bilirubin-and sulfobromophthalein-binding to hepatic Y and Z proteins in the rat

Pregnenolone-16 alpha-carbonitrile (PCN), administered twice daily p.o. for 3 days at a dose level of 20 mumol/100 g body weight, significantly enhances in vivo binding of 14C-bilirubin and sulfobromophthalein (BSP) to hepatic Y and Z proteins in female Charles River CD rats. 14C-bilirubin-binding to Y protein showed a 61% increase, while binding of the same moiety to the Z protein fraction was augmented by 59%. BSP-binding in vivo demonstrated rises of 114 and 71% in relation to Y and Z proteins, respectively. These data correlate well with previous investigations in which PCN was found to have a beneficial influence on experimentally induced hyperbilirubinemias and, furthermore, there is an indication that phenobarbital, another potent microsomal enzyme inducer, acts via a similar mechanism.     

Postnatal expression and induction by pregnenolone-16alpha-carbonitrile of the organic anion-transporting polypeptide 2 in rat liver.

Newborn rats are more sensitive to the toxic effects of cardiac glycosides than are adult rats. This is associated with a decreased ability to remove cardiac glycosides from blood into the liver. Pregnenolone-16alpha-carbonitrile (PCN), a prototypical rodent CYP3A inducer and pregnane-X-receptor (PXR) ligand, stimulates the hepatic clearance of cardiac glycosides in newborn rats, which results in decreased toxicity of the cardiac glycosides. The mechanism responsible for this phenomenon is not clear; however, if elucidated, it would help in understanding and preventing potential drug-drug interactions. The recently cloned rat organic anion-transporting polypeptide 2 (oatp2) (Slc21a5) is a sinusoidal hepatic uptake transporter, with very high affinities for cardiac glycosides, and thus it was hypothesized that rat oatp2 increases during postnatal development and is inducible by PCN. In the present study, livers were removed from Sprague-Dawley rats from postnatal days (pnd) 0 to 45, in 5-day increments; as well as from pnd 10 to 90, in 10-day increments, after PCN (75 mg/kg i.p., for 4 days) or corn oil (vehicle for PCN) treatment. The protein and mRNA levels of rat oatp2 were determined by Western blot analysis and branched DNA signal amplification technique, respectively. Expression of rat oatp2 protein and mRNA increased gradually during postnatal development. PCN treatment increased liver to body weight ratio in both genders, and dramatically accelerated the maturation of hepatic oatp2 protein and mRNA levels. In summary, rat oatp2 undergoes age-dependent and chemical regulation during postnatal development, and is a potential target for drug-drug and age-drug interactions.     

Sex-specific modulation of hepatic covalent DNA modifications (I-compounds) by the cytochrome P450 inducer, pregnenolone-16 alpha-carbonitrile.

I-compounds are recently discovered, age-dependent covalent DNA modifications, which are detectable by 32P-postlabeling assay for DNA adducts. The effects of the catatoxic antiglucocorticoid, pregnenolone-16 alpha-carbonitrile (PCN), on hepatic and renal I-compound levels have been studied in male and female Sprague-Dawley rats together with the levels of microsomal cytochrome P450 and rates of ethylmorphine N-demethylation. PCN (50 mg/kg ip) was dissolved in corn oil and administered to rats once daily for 4 days, and animals were killed at 1 day or 8 days after the last treatment. Hepatic and renal I-compounds were analyzed by 32P-postlabeling in control and PCN-treated animals at both time points. Microsomal cytochrome P450 and ethylmorphine N-demethylase activities were also determined. Total levels of liver nonpolar and polar I-compounds were reduced in female rats by 37 and 51%, respectively, compared to controls, at 1 day. Ten out of sixteen individual I-compounds were also markedly reduced in female rat liver DNA as a result of PCN administration. In contrast to females, total levels of liver I-compounds were not significantly altered in males by PCN at 1 day; however, two individual I-compounds were lowered. I-compound levels recovered 8 days after termination of PCN treatment in both males and females. Total levels of renal I-compounds were not affected by PCN treatment in either males or females. [3H]Methylthymidine incorporation studies showed an increase in mean DNA synthesis rate at 1 day in liver of both males and females, but this was significant in males only. Marked induction of hepatic microsomal cytochrome P450 (2.2-fold) and ethylmorphine N-demethylase (4.0-fold) activity was observed in female rats treated with PCN at 1 day as compared to controls. The extent of induction of these enzymes was much higher in females than males. At 8 days the levels of cytochrome P450 and ethylmorphine N-demethylase activity had returned to uninduced values. The results are consistent with a pivotal role for PCN-inducible cytochrome P450 in the metabolism of I-compounds.     

Unusual characteristics of the dose-dependent uptake of propylthiouracil by thyroid gland in vivo. Effects of thyrotropin, iodide or phenobarbital pretreatment.

Dose dependency of propylthiouracil (PTU) uptake by the thyroid gland was investigated in intact, adult male rats after a single intraperitoneal dose of PTU. PTU pharmacokinetics in rat serum, liver and lung were independent of the size of this single dose of PTU. However, in the thyroid gland, PTU concentrations corrected for dose were disproportionately high at low doses. At low PTU doses, a transport mechanism contributes substantially to thyroidal PTU uptake, but at high PTU doses this transport system becomes saturated. Thus, at serum PTU concentrations of 5 μg/ml or above, thyroid to serum (T/S) PTU ratios are independent of PTU dose and serum concentration. Alteration in the functional status of the thyroid changed the relationship between serum and thyroidal PTU concentrations. Thyroxine administration prior to an intraperitoneal PTU dose of 5 mg/kg reduced ratios of T/S PTU concentrations. However, thyrotropic hormone (TSH) preadministration had little effect on early thyroidal PTU concentration, but appeared to exert an effect at later times. Chronic administration of either potassium iodide or phenobarbital prior to PTU increased T/S PTU ratios, again suggesting an effect of TSH on PTU metabolism in the thyroid.     

Reversal of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced depression of ouabain biliary excretion by pregnenolone-16 alpha-carbonitrile and spironolactone in isolated perfused rat livers.

Perfusate disappearance and biliary excretion of ouabain were depressed in isolated perfused livers of adult male rats 10 days after treatment with a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 25 μg/kg). When TCDD-treated rats were given pregnenolone-16α-carbonitrile (PCN, 75 mg/kg/day) or spironolactone (S, 75 mg/kg/day) on Days 6–9, the depression in ouabain perfusate disappearance and biliary excretion on Day 10 was reversed, but the degree of reversal afforded by PCN was greater than S. Liver surface membrane (LSM) Mg²⁺ ATPase and Na⁺, K⁺ ATPase activity were also depressed 10 days after TCDD treatment and when TCDD-treated rats were given S on Days 6–9 the depression in both ATPase activities was completely reversed but treatment with PCN was less effective. Liver excretory function for ouabain and LSM Na⁺, K⁺, or Mg²⁺ ATPase activity in control and TCDD-treated rats were altered independently by PCN and S treatment. Adrenalectomy on Day 6 after TCDD treatment did not reverse the depressant effects of TCDD on ouabain disposition but did prevent the increase in liver weight observed on Day 10. In view of the dissociation between hepatic excretory function for ouabain and LSM ATPase activities it is suggested that LSM Mg²⁺ ATPase and Na⁺, K⁺ ATPase are not directly involved in hepatic ouabain transport. We postulate that a LSM ouabain carrier unit, separate and distinct from these ATPases, is responsible for hepatic uptake and biliary excretion of ouabain.     

p53-independent induction of rat hepatic Mdm2 following administration of phenobarbital and pregnenolone 16alpha-carbonitrile.

Murine double minute 2 (Mdm2) negatively regulates p53 by mediating its ubiquitination and proteosomal degradation, and Mdm2 is recognized as a proto-oncogene. In the present study, hepatic gene expression patterns induced by phenobarbital (PB; 100 mg/kg) and pregnenolone 16alpha-carbonitrile (PCN, 100 mg/kg) were evaluated in male and female Sprague-Dawley rats using Affymetrix Rat Genome U34A gene arrays. In addition to changes in the hepatic expression of well-characterized drug-metabolizing enzymes, an increase in Mdm2 mRNA was observed with both compounds after single or repeat dosing (5 days). However, gene array analyses did not reveal changes in other p53-dependent genes, suggesting that induction of Mdm2 occurred in a p53-independent manner. Real-time polymerase chain reaction confirmed the microarray results, as PB increased Mdm2 mRNA approximately twofold after single or repeat doses in male and female rats. PCN treatment increased Mdm2 mRNA levels up to 5- and 12-fold in male and female rats, respectively, after 5 days of dosing. Hepatic Mdm2 protein levels were increased, and immunohistochemical evaluation of rat liver demonstrated nuclear localization of Mdm2, suggesting an interaction with p53. Consequently, p53 protein levels were also decreased by approximately 35 and 50% after 5 days of PB and PCN treatment, respectively. In direct contrast to rats, PB and PCN (100 mg/kg) did not induce Mdm2 mRNA in mouse liver after 5 days of dosing. Finally, although Mdm2 in mice and humans is reported to migrate electrophoretically as two proteins with molecular weights of 76 and 90 kDa, rat Mdm2 protein was detected primarily as a 120-kDa species. Follow-up experiments indicated that rat hepatic Mdm2 was subject to posttranslational modification with small ubiquitin-modifying (SUMO) proteins. Although the molecular mechanisms controlling Mdm2 induction by PB and PCN in rats have not yet been determined, these results suggest that early effects on cell cycle regulation, response to DNA damage or cell transformation may contribute to liver tumor development.     

Influence of combined treatment with pregnenolone-16α-carbonitrile and spironolactone or phenobarbital on drug metabolism in rats

In female rats, pretreatment with pregnenolone-16α-carbonitrile (PCN), spironolactone or phenobarbital resulted in shorter zoxazolamine and hexobarbital sleeping times and more rapid plasma clearance of zoxazolamine and bishydroxycoumarin. All of these effects were slightly enhanced by simultaneous pretreatment with PCN and spironolactone and greatly increased by concomitant administration of PCN and phenobarbital. However, the observed increases in zoxazolamine and hexobarbital metabolism rates by submitochondrial liver fractions were not further augmented by pretreatment with both of the steroids, while the rates in animals treated with PCN and phenobarbital were only slightly greater than in those which received one of these inducers. It is suggested that PCN and spironolactone share a common receptor and induction mechanism, while the action of phenobarbital is slightly different.     

Induction of rat organic anion transporting polypeptide 2 by pregnenolone-16alpha-carbonitrile is via interaction with pregnane X receptor

The rat organic anion transporting polypeptide 2 (oatp2; Slc21a5) is a liver transporter that mediates the uptake of a variety of structurally diverse compounds, and has a high affinity for cardiac glycosides. Treatment of rats with pregnenolone-16alpha-carbonitrile (PCN), a ligand for the rodent pregnane X receptor (PXR), significantly enhances the rat oatp2 gene expression. To understand the molecular mechanism of oatp2 induction by PCN, rat oatp2 gene was cloned. The rat oatp2 gene consists of 16 exons; alternative splicing of the second noncoding exon gives rise to the two published rat oatp2 cDNAs. Approximately 8700 base pairs (bp) of the 5'-flanking region of the rat oatp2 gene were linked to the luciferase reporter gene and used in transient transfection assays in H4IIE cells. Treatment of PCN induced the expression of the reporter gene in a dose-dependent manner. Four potential PXR response elements (PXREs) were identified in the 5'-flanking region of the rat oatp2 gene. One element (DR3-1) is located approximately -5000 bp with three more (DR3-2, DR3-3, and DR3-4) clustered at about -8000 bp. Results from electrophoretic mobility shift assays showed that the PXR-retinoid X receptor alpha heterodimer binds to the DR3-2 with the highest affinity, to the DR3-4 and DR3-1 with a lower affinity, and weakly or not at all to the DR3-3. Furthermore, a series of partial deletions of the 5'-flanking region illustrated that both the proximal and distal clusters of PXREs are required for maximal induction of rat oatp2 by PCN. In conclusion, these data elucidate the molecular mechanism by which PCN treatment induces rat oatp2 gene expression. In addition, this study identifies rat oatp2 as a direct PXR-targeted gene and further supports the hypothesis that activation of PXR affects a network of genes that is involved in either metabolism or transport of drugs, steroids, and bile acids.     

P-glycoprotein and an unstirred water layer barring digoxin absorption in the vascularly perfused rat small intestine preparation: induction studies with pregnenolone-16alpha-carbonitrile.

Digoxin, a substrate of P-glycoprotein (Pgp) and cytochrome P450 3a (Cyp3a), was used to illustrate the inductive effects of pregnenolone-16alpha-carbonitrile (PCN), a ligand of the pregnane X receptor, on the absorption and disposition of [3H]digoxin in the vascularly perfused rat small intestine preparation. Although increased Cyp3a protein was observed with Western blotting analysis after PCN treatment, metabolism of digoxin to the digoxigenin bis-digitoxoside metabolite in the rat small intestine remained insignificant (<4% dose). PCN pretreatment significantly decreased blood perfusate [3H]digoxin concentrations for both systemic and intraluminal administrations of [3H]digoxin due to increased Pgp levels. The apical secretion by Pgp increased at 90 min with PCN treatment, from 11.2 +/- 5.1% of dose to 20.1 +/- 8.6% of dose after systemic administration of [3H]digoxin; this increase was, however, statistically insignificant (P = 0.13) because of the high variability among preparations. When the composite data for the control and PCN-treated preparations were fit to published physiologically based pharmacokinetic models: the traditional model and the segregated flow model, suboptimal parameters were obtained. The data were further fit to expanded models with a bilayer membrane compartment housing the Pgp adjacent to the apical membrane, or an unstirred water layer (UWL) external to the apical membrane. The models with the UWL yielded improved fits and reasonable parameters associated with digoxin absorption, suggesting that the UWL posed as a barrier for digoxin absorption. Similar results were obtained with the segmental models (the segmental traditional model and the segmental segregated flow model) using the UWL, when heterogeneous distributions of Pgp in the duodenum, jejunum, and ileum were considered.