Hepatic handling of a synthetic gamma-labeled bile acid (75SeHCAT)

Time of flight dual photon emission computed tomography

Time-of-flight dual photon emission computed tomography (TOF-DuPECT) is an imaging system that can obtain radionuclide distributions using time information recorded from two cascade-decay photons. The potential decay locations in the image space, a hyperbolic response curve, can be determined via time-difference-of-arrival (TDOA) estimations from two instantaneous coincidence photons. In this feasibility study, Monte Carlo simulations were performed to generate list-mode coincidence data. A full-ring positron emission tomography-like detection system geometry was built in the simulation environment. A contrast phantom and a Jaszczak-like phantom filled with Selenium-75 (Se-75) were used to evaluate the image quality. A TOF-DuPECT system with varying coincidence time resolution (CTR) was then evaluated. We used the stochastic origin ensemble (SOE) algorithm to reconstruct images from the recorded list-mode data. The results indicate that the SOE method can be successfully employed for the TOF-DuPECT system and can achieve acceptable image quality when the CTR is less than 100 ps. Therefore, the TOF-DuPECT imaging system is feasible. With the improvement of the detector with time, future implementations and applications of TOF-DuPECT are promising. Further quantitative imaging techniques such as attenuation and scatter corrections for the TOF-DuPECT system will be developed in future.

Recycling rate of bile acids in the enterohepatic recirculation as a major determinant of whole body 75SeHCAT retention

Measurement of the whole body retention of orally administered (75)SeHCAT is used to investigate patients with unexplained diarrhoea. Retention values of <15 % at 7 days post-administration are taken to indicate bile acid malabsorption (BAM). Whilst idiopathic BAM is frequently diagnosed with (75)SeHCAT, functional and morphological studies of the terminal ileum rarely show any abnormality, so the disorder may be more appropriately termed bile acid diarrhoea (BAD). In addition to malabsorption, excess bile acid may reach the colon, where the events leading to diarrhoea take place, as a result firstly of increased bile acid synthesis and secondly of an increased recycling rate of bile acids. Increased recycling has been largely ignored as a cause of BAD, but, as shown in this study, can readily result in excess bile acids reaching the colon even when ileal absorption efficiency is normal (i.e. 95-97 %). There needs to be a re-evaluation of the causes of BAD in patients without a history of previous intestinal resection or evidence of ileal pathology, such as Crohn's disease.

Measurement of gallbladder motor functions: an overview

Control of gallbladder motor functions involve a constant interplay between several stimulatory and inhibitory hormones and neurotransmitters. Gallbladder response to a stimulus is complicated involving rapid alternation of emptying and refilling during the postprandial period. Conventional methodology is not capable of evaluating both emptying and refilling in a quantitative manner, and hence previous studies have yielded a large variation in results in health and conflicting results in gallstone patients. There is therefore, a need for improved methodology. Postprandial refilling and turnover of bile are important parameters that need to be assessed when addressing gallbladder motor function and its role in the pathogenesis of cholesterol gallstone disease.

Review article: measurement of gall-bladder motor function in health and disease

Normal gall-bladder (GB) motor function plays an important role in the enterohepatic circulation of bile acids in health, and abnormal GB motor function has serious clinical implications. The old concept that the GB empties gradually during meals and fills between meals is still believed by many. This paper describes the techniques used in the measurement of GB motor function and discusses the information provided by these techniques in health and disease.

Absence of correlation between hepatic function and characteristics of migrating motor complexes in the gastrointestinal tract

BACKGROUND

Cyclic changes in gallbladder filling and emptying during the migrating motor complex (MMC) cycle have been demonstrated by scintigraphy. However, a possible cyclic change in the hepatic function and handling of the pharmacologic agents used for scintigraphy during the MMC cycle could have an influence on these results. The aim of the present study was to investigate the hepatic handling of cholic acid and mebrofenin in relation to the MMCs of the gastrointestinal tract.

METHODS

The plasma disappearance rate of 14C-cholic acid and the hepatic uptake and excretion of 99mTc-mebrofenin were examined during phase I and phase II of the MMC in six healthy male volunteers.

RESULTS

The plasma disappearance rate of 14C-cholic acid showed a biexponential course with an initial rapid and late slow phase after a bolus injection. There were no significant differences between the initial or late plasma disappearance rate of 14C-cholic acid during phase I as compared with phase II. The results of the time-activity curves from the 99mTc-mebrofenin scintigraphy showed an exponential rapid increase in radioactivity followed by an almost linear slow decrease after a bolus injection. There was no significant difference between phase I and phase II in any of the variables studied.

CONCLUSION

The lack of a relationship between hepatic handling of cholic acid and mebrofenin and MMC excludes this as a possible source of error in the investigations of the dynamic function of the enterohepatic circulation and especially gallbladder motility by the use of either cholic acids or iminodiacetic acid derivatives as investigative agents.

Kinetics of hepatic bile acid handling in cholestatic liver disease: effect of ursodeoxycholic acid

BACKGROUND/AIMS

Ursodeoxycholic acid (UDCA) is clinically beneficial in chronic cholestatic liver disease, but the underlying mechanisms are unclear. It has been suggested that intrahepatic retention of endogenous hydrophobic bile acids contributes to cholestasis and that the hydrophilic bile acid UDCA reduces this retention; the aim of our study was to test these hypotheses.

METHODS

Twelve patients with primary biliary cirrhosis (PBC) and 5 with primary sclerosing cholangitis (PSC) were studied before and during UDCA (10 mg.kg-1.day-1) and compared with 11 healthy controls. Following intravenous 75Se labeled homocholic acid taurine (75SeHCAT) in the fasting state, abdominal gamma camera imaging was performed for 90 minutes. Initial hepatic uptake, transit time, net, and absolute excretory rates for 75SeHCAT were measured.

RESULTS

Mean initial hepatic uptake was not different between patients and controls (17.2% and 19.9% dose/minute, not significant). However, net and absolute excretory rates were significantly reduced in patients (1.4% vs. 3.7% dose/minute, P < 0.0001; and 2.35% vs. 3.96% dose/minute, P < 0.02, respectively), and hepatic transit time was prolonged (18.7 minutes vs. 11.6 minutes, P < 0.002). UDCA improved net and absolute hepatic excretory rates and transit time (1.43% to 1.96% dose/minute, P < 0.001; 2.35% to 3.15% dose/minute, P < 0.005 and 18.7 to 14.7 minutes, P < 0.001, respectively). However, UDCA did not alter initial hepatic uptake.

CONCLUSIONS

In PBC and PSC, there is a defect in hepatic bile acid excretion but not in uptake, implying bile acid retention. This retention is reduced by UDCA.

Bile acid malabsorption in patients with chronic diarrhoea

The presence of bile acid malabsorption was studied in 24 patients with chronic diarrhoea without established cause despite extensive investigations. Bile acid absorption was evaluated with the 75Se-homocholic acid taurine (SeHCAT) test. A therapeutic trial of cholestyramine was performed in 11 patients. Fourteen of the patients (58%) showed evidence of bile acid malabsorption. Of the 11 patients who were treated with cholestyramine, 3 had no improvement of their diarrhoea and also had a normal SeHCAT test result. Of the other eight patients, who also had pathologic SeHCAT test result, five improved on treatment, whereas three had no change of their diarrhoea. Seven of the 24 patients had a previous history of cholecystectomy. Four of them showed bile acid malabsorption; three of these were treated with cholestyramine and responded favourably. The results suggest that bile acid malabsorption may be common in chronic diarrhoea patients but may not always be the primary cause of diarrhoea.

Serum 7 alpha-hydroxy-4-cholesten-3-one concentrations in the evaluation of bile acid malabsorption in patients with diarrhoea: correlation to SeHCAT test

The synthesis of bile acids is regulated by a homeostatic mechanism in which bile acids returning to the liver from the intestine inhibit their own synthesis. Serum concentrations of the bile acid intermediate 7 alpha-hydroxy-4-cholesten-3-one reflect the rate of bile acid synthesis whereas bile acid malabsorption can be determined by the SeHCAT test. This study was done to evaluate the correlation between the two tests in humans. Twenty eight patients with chronic diarrhoea were included in the study. Fasting serum was collected for the determination of 7 alpha-hydroxy-4-cholesten-3-one, and on the same day the gamma emitting bile acid analogue SeHCAT was given orally and its fractional catabolic rate assessed by repeated external counting over the upper abdomen during the next seven days. There was a highly significant positive correlation between the two tests (Rs = 0.80, p < 0.001). The results show a close relation between intestinal loss and hepatic synthesis of bile acids and imply that analysis of 7 alpha-hydroxy-4-cholesten-3-one in serum should now be evaluated as a possible convenient method for assessing bile acid malabsorption in patients with diarrhoea.

Effect of ursodeoxycholic acid treatment on ileal absorption of bile acids in man as determined by the SeHCAT test

The effects of urodeoxycholic acid on ileal absorption of bile acids and on serum bile acid and lipoprotein concentrations were studied. Eight healthy subjects were investigated. The gamma emitting bile acid analogue, SeHCAT, was given orally and its fractional catabolic rate and seven day retention were assessed by repeated external counting over the upper abdomen during the next seven days. Ursodeoxycholic acid was then given orally at a dose of 15 mg/kg/day for three weeks and the study was repeated during treatment. The fractional catabolic rate increased by 64% (mean (SD), 0.333 (0.159) v 0.203 (0.061)/day; p less than 0.05) and seven day retention decreased by 44% (15(10) v 27(10)%, p less than 0.001), indicating bile acid malabsorption. Total serum cholesterol fell from 5.79 (1.22) to 5.50 (1.18) mmol/l (p = 0.05), while serum ursodeoxycholic acid increased 22 fold (7.87 (2.67) v 0.34 (0.24) mumol/l, p less than 0.001). Five of the subjects continued taking 30 mg/kg/day of ursodeoxycholic acid for one week and showed an increase in fractional catabolic rate of 81% (0.300 (0.091) v 0.166 (0.037)/day; p less than 0.05) and a fall in seven day retention of 50% (16 (12) v 32 (8)%, p less than 0.01). There were significant reductions in total cholesterol (5.36 (1.71) v 6.08 (1.47) mmol/l; p less than 0.05) and low density lipoprotein cholesterol (3.70 (1.33) v 4.58 (1.16) mmol/l; p less than 0.05). The results support the concept tht ursodeoxycholic acid treatment interferes with the absorption of endogenous bile acids, and emphasise the beneficial effects of this treatment of lipoprotein concentrations in man.

[Increased bile acid retention in biliodigestive Roux-Y anastomosis in animal experiment]

In an experimental study on 65 rats the influence of different Roux-Y biliary anastomoses on the enterohepatic bile acid circulation was investigated. Long (10 cm) and short (3 cm) isoperistaltic and long (10 cm) anisoperistaltic Roux-Y loops were studied and compared to sham-operated controls. 75-SeHCAT a y-labeled synthetic bile acid was used for assessment of enterohepatic circulation. Hepatic excretion and enteric bile flow was investigated by 99m-Tc-HIDA. Intestinal transit time was evaluated by 51-Cr-EDTA an inert marker of intestinal contents. Animals with biliary anastomoses showed an increased retention of the synthetic bile acid 75-SeHCAT compared to the controls. Highest retention values were found in animals with anisoperistaltic loops (t1/2 = 98.6 h). Isoperistaltic loops showed uniform values independently of the length of the Roux-limb (t1/2 = 59.8 h resp. 59.3 h). Differences to the controls (t1/2 = 42.6 h) were highly significant (p less than 0.001). In the 99m-Tc-HIDA hepatic excretion into the connected small bowel was normal in all groups. But in rats with biliary anastomoses a marked stasis of the 99m-Tc-HIDA in the Roux-Y loop was apparent. Bowel transit time tested by 51-Cr-EDTA showed comparable results in all groups without significant differences. The results demonstrate an increased bile acid retention in Roux-Y biliary anastomoses. An alteration of the enterohepatic circulation due to stasis in the Roux-Y limb seems to be the underlying mechanism.

Bile acid retention after Roux-en-Y biliary reconstruction demonstrated by tauro-23-(75Se)selena-25-homocholic acid. An experimental animal study

In this study of 65 laboratory rats the influence of biliary Roux-en-Y anastomosis on the enterohepatic circulation was investigated. Three different types of Roux-en-Y limbs were studied and compared with sham-operated controls. To collect information about the influence of the length and motility of the Roux limb, 10-cm and 3-cm isoperistaltic Roux segments and 10-cm anisoperistaltic Roux limbs were investigated. 75SeHCAT, a tau-labelled synthetic bile acid, was used for assessment of enterohepatic circulation. Intestinal transit time was evaluated by measuring the decrease of whole-body counts of 51Cr-labelled ethylenediamintetraacetic acid (EDTA), an inert marker of intraluminal contents. 99mTc-labelled dimethyl-iminodiacetic acid (HIDA) was used to detect possible alterations of bile flow. 75SeHCAT results showed marked retention of bile acids in all Roux-en-Y animals compared with controls. Results of short and long Roux limbs did not differ significantly from each other (t1/2 10 cm, 59.8 h; 3 cm, 59.3 h; control, 42.6 h; p less than or equal to 0.001). The anisoperistaltic Roux limbs showed prolonged retention even when compared with isoperistaltic limbs (t1/2 98.6 h; p less than or equal to 0.001). 51Cr-EDTA results did not show significant differences between all groups. 99mTc-HIDA showed undelayed excretion in all animals but increase of radioactivity at the site of the Roux-en-Y enteroanastomosis, indicating marked stasis in the biliodigestive Roux limb. The results demonstrate the impact of biliary Roux-en-Y anastomosis on bile acid retention due to stasis in the defunctioning jejunal segment.