Determination of reduced, protein-unbound, and total concentrations of N-acetyl-L-cysteine and L-cysteine in rat plasma by postcolumn ligand substitution high-performance liquid chromatography

A high-performance liquid chromatographic assay was developed for the quantitative determination of the sulfur-containing amino acids N-acetyl-L-cysteine (NAC) and L-cysteine (Cys) in rat plasma. The thiols were separated by reverse-phase ion-pair chromatography, and the column eluent was continuously mixed with an iodoplatinate-containing solution. The substitution of sulfur of the thiol compound with iodide was quantitatively determined by measuring changes in the absorption at 500 nm. The low-molecular-weight disulfides and mixed disulfide conjugates of thiols with proteins were entirely reduced to the original reduced compounds by dithiothreitol. By reducing these two types of disulfides separately during sample pretreatment, the reduced, protein-unbound, and total thiol concentrations could also be determined. Validation testing was performed, and no problems were encountered. The limit of detection was approximately 20 pmol of thiol on the column. The present method was used to measure the plasma concentrations of NAC and Cys in the rat after a bolus intravenous administration of NAC, focusing on disulfide formation. The binding of NAC to protein through mixed disulfide formation proceeds in a time-dependent and reversible manner. Moreover, this "stable" covalent binding might limit total drug elimination, while the unbound NAC is rapidly eliminated. Consequently, the analytical method described in this study is very useful for the determination of plasma NAC and Cys, including disulfide conjugates derived from them.

Amitriptyline-induced constipation in cynomolgus monkeys is beneficial for the evaluation of laxative efficacy

In an attempt to create an animal model of constipation in monkeys, amitriptyline was administered to cynomolgus monkeys at doses of 10-160 mg/kg body weight via a nasogastric tube. Normal control monkeys excreted feces frequently throughout the day. Monkeys treated with amitriptyline at doses of 10-40 mg/kg showed delays in feces excretion. The 60 mg/kg treated monkeys for the most part did not excrete feces during the 24 h after amitriptyline administration. The 80 and 120 mg/kg treated monkeys did not excrete feces until 24 h from administration of amitriptyline, and also showed prolonged crouching and lethargy. On the other hand, 160 mg/kg treated monkeys died within 24 h after administration. We therefore felt that the optimal dose for creating constipation in the monkeys was 60 mg/kg. We tested the appropriateness of this amitriptyline-induced constipated monkey model by observing the effects of a new laxative, the herbal medicine ND-10 and the commercially available laxative bisacodyl. Control monkeys (those not receiving ND-10 or bisacodyl) treated with 60 mg/kg amitriptyline did not excrete feces up to 32 h after amitriptyline administration in 2 of 3 monkeys. However, all monkeys treated with one tablet of ND-10 excreted feces. Also, in 4 monkeys administrated with bisacodyl, 3 excreted feces. In this study, we confirmed that constipation can be caused in cynomolgus monkeys by oral administration of amitriptyline. This model may also be useful for the evaluation of laxatives.

Water extract of defatted rice bran suppresses visceral fat accumulation in rats

Rice bran has been reported to inhibit pancreatic lipase activity in vitro. This action shows that administration of rice bran may result in a decrease in plasma triglyceride levels and suppress accumulation of fat in vivo. We administered water extract of defatted rice bran (WED-rice bran) to rats to determine its effects. Single administration of WED-rice bran at a dose of 1 g/kg body weight caused a decrease in plasma triglyceride levels in fat emulsion induced hypertriglyceridemic rats. Four week administration of WED-rice bran suppressed accumulation of visceral fat and body weight gain without influencing food consumption, liver function, and renal function. These results indicate that a reduction of plasma triglycerides and suppression of visceral fat accumulation may be induced by pancreatic lipase inhibition caused by administration of WED-rice bran.

Changes in hepatic nitrogen metabolism in isolated perfused liver during the development of thioacetamide-induced cirrhosis in rats

Changes in hepatic nitrogen metabolism in isolated perfused liver were studied during the induction of experimental cirrhosis by thioacetamide in female Sprague-Dawley rats. Cirrhosis of the micronodular type developed during 12-week administration of thioacetamide. Despite an increase in food consumption for 4 weeks after the end of administration, the physiological changes characteristic of cirrhosis were maintained. The rate of urea excretion per unit liver weight was significantly decreased compared with pair-fed control rats both during and after thioacetamide treatment. During 4 weeks of thioacetamide treatment, the rate of urea production in perfused liver from a combination of 0.25 mM NH4Cl and 1 mM glutamine decreased slightly, without a decrease in the maximum rate of urea production from 10 mM NH4Cl. In cirrhotic rats, the rate of urea production in perfused liver from NH4Cl and/or glutamine decreased, with a decrease in the maximum rate of urea production. The Km of ureagenesis for NH3 was unchanged in cirrhotic livers. During 4 weeks of thioacetamide treatment, glutamate dehydrogenase activity decreased, but the thioacetamide-induced cirrhotic state had no effect on glutamate dehydrogenase or glutaminase activity. Glutamine synthetase activity was decreased in rats treated with thioacetamide for 4 or 12 weeks. These results are consistent with the hypothesis that the capacity for urea production from NH3 and amino acids is decreased in the development of cirrhosis.

Characteristics of nitrogen metabolism in rats with thioacetamide-induced liver cirrhosis

Female Sprague-Dawley rats were given 0.03% thioacetamide (TAA) in their drinking water daily for 4 or 12 weeks, and were then given normal water for 4 weeks after the end of a 12-week TAA treatment to investigate amino acid metabolism. In the malnourished precirrhotic stage (stage 1) and the malnourished cirrhotic stage (stage 2), the aromatic amino acids (AAA), Glu, Asp, Orn, Arg and Cit increased, and the branched-chain amino acids (BCAA) decreased slightly. Because these changes normalized in the well-nourished cirrhotic stage (stage 3), they might have resulted from impairment of hepatocytes and malnutrition. The net uptake of BCAA into the liver increased in stage 2, but the AAA uptake did not exceed that in normal controls. Portal venous plasma AAA increased to the same level as arterial plasma AAA. These results suggest that the decrease in BCAA was partially due to liver uptake and that the increase in AAA was induced by reduction of liver uptake and overproduction in extrahepatic tissues. The liver contents of BCAA and AAA were unchanged in all stages, so were fully utilized in the impaired liver. The increases in Glu, Asp, Orn and Cit might have resulted from overproduction in the liver, because these contents of the liver increased in stage 2. In conclusion, the changes in amino acid metabolism in rats with cirrhosis induced by TAA closely resemble those seen in human liver cirrhosis.