Acute in vivo effects of low ethanol concentration on the capacity of urea synthesis in rats

Urea

Urea is generated by the urea cycle enzymes, which are mainly in the liver but are also ubiquitously expressed at low levels in other tissues. The metabolic process is altered in several conditions such as by diets, hormones, and diseases. Urea is then eliminated through fluids, especially urine. Blood urea nitrogen (BUN) has been utilized to evaluate renal function for decades. New roles for urea in the urinary system, circulation system, respiratory system, digestive system, nervous system, etc., were reported lately, which suggests clinical significance of urea.

Transient hyperammonemia due to L-asparaginase therapy in children with acute lymphoblastic leukemia or non-Hodgkin lymphoma

The standard treatment protocol for acute lymphoblastic leukemia (ALL) and non-Hodgkin lymphoma (NHL) in childhood includes intravenous therapy with asparaginase (Asp), which may cause hyperammonemia. In this study, all patients receiving asparaginase therapy at the Hospital for Children and Adolescents of the University of Leipzig between January 2002 and December 2007 were reviewed for the occurrence of hyperammonemia. Fifty-four patients were identified (22 girls, 32 boys; mean age 5.8 years). Blood ammonia concentrations were determined in 4 patients due to suspicious clinical signs. All showed hyperammonemia with NH(3) concentrations between 260 and 700 μmol/L. They received specific acute detoxification therapy consisting in protein restriction, administration of benzoic acid, glucose/insulin. All 4 recovered completely. All patients receiving therapeutic regimes that include asparaginase (Asp) should be monitored for the development of transient hyperammonemia.

Alcohol Acutely Down-Regulates Urea Synthesis in Normal Men

BACKGROUND

Human nitrogen balance studies suggest that alcohol up-regulates urea synthesis and promotes nitrogen catabolism, whereas animal studies conversely indicate that alcohol down-regulates urea synthesis, possibly via a redox effect. This study aimed to investigate the acute effects of alcohol exposure at a plasma concentration of about 10 mmol/liter on urea synthesis in healthy volunteers and to investigate whether methylene blue alleviates the effect of alcohol.

METHODS

Eleven males were studied three times in a randomized sequence crossover design. They received either alanine infusion to control the rate of urea synthesis (control), alanine + alcohol, or alanine + alcohol + methylene blue. The substrate independent regulation of urea synthesis was studied by means of the functional hepatic nitrogen clearance, that is, the slope of the linear relation between blood amino nitrogen concentrations and rates of urea synthesis.

RESULTS

Alcohol reduced functional hepatic nitrogen clearance to 37% and 51% during alcohol and alcohol + methylene blue infusion, respectively (p = 0.007). Accordingly, whole body nitrogen retention was higher during alcohol infusion. Glucagon, which up-regulates urea synthesis, increased during alcohol infusion. There was no change in insulin. Blood glucose was slightly lower at the end of the experiment when alcohol was infused.

CONCLUSION

Low-dose infusion of alcohol acutely down-regulated urea synthesis in healthy volunteers, transiently favoring nitrogen preservation. The effect seemed not to depend on hormonal changes. It remains to be explored how the present results can be reconciled with the reported nitrogen wasting of chronic alcoholics.

Inhibition of aldehyde dehydrogenases by methylene blue

The effect of the redox dye methylene blue on the stability of NADH and on the activity of the enzyme aldehyde dehydrogenase (ALDH; EC 1.2.1.3) was examined. NADH was measured by HPLC with fluorometric or spectrophotometric detection. The ALDH activity assays were carried out by following the formation of 3,4-dihydroxyphenylacetic acid (DOPAC) from 3,4-dihydroxyphenylacetaldehyde (DOPAL) using HPLC and electrochemical detection. Incubation of NADH solutions in the presence of methylene blue resulted in a time-dependent direct oxidation of NADH. Methylene blue inhibited the human erythrocyte and leukocyte ALDHs and the rat liver mitochondrial low-Km ALDH in a concentration-dependent manner. The inactivation was reversible by dilution, and kinetic analysis indicated that methylene blue inhibits the rat liver mitochondrial low-Km and human erythrocyte ALDHs competitively with respect to DOPAL, while no effect of the NAD+ concentration was apparent. For the rat liver low-Km ALDH, a Ki of 8.4 +/- 2.8 microM (mean +/- SD; N = 5) was calculated. The inhibition of ALDH and the resulting decrease in the redox effect on the NAD system bound to alcohol dehydrogenase (EC 1.1.1.1) could explain the protective effect of methylene blue against metabolic redox effects of ethanol.

Ethanol metabolism in isolated hepatocytes. Effects of methylene blue, cyanamide and penicillamine on the redox state of the bound coenzyme and on the substrate exchange at alcohol dehydrogenase

Ethanol metabolism in hepatocytes increases the NADH/NAD+ ratio. The mechanism was investigated by measurements of the redox state of the coenzyme bound to alcohol dehydrogenase and of ethanol-acetaldehyde exchange and concomitant hydrogen transfer between ethanol molecules. Isolated hepatocytes from fed rats were incubated with cyclohexanone and cyclohexanol or with [1,1-2H2]-and [2,2,2-2H3]ethanol, followed by gas chromatographic determination of the redox state and isotope analysis of the ethanol by gas chromatography-mass spectrometry, respectively. Cyanamide and methylene blue decreased the redox shift caused by ethanol and increased the rates of acetaldehyde reduction during the exchange. Both drugs increased the extent of hydrogen transfer between ethanol molecules during oxidoreduction. Penicillamine had no significant effect on the ethanol-induced change in redox state of the bound coenzyme although it decreased the rate of acetaldehyde reduction. The results indicate that methylene blue inhibits aldehyde dehydrogenase and that accumulation of acetaldehyde decreases the redox effects of ethanol. The redox effect appears to result primarily from rapid elimination of acetaldehyde and equilibration with the NAD system on the alcohol dehydrogenase, but is not enhanced by further decreases in acetaldehyde concentration. Thus, penicillamine could probably be used to decrease the concentration of acetaldehyde without increasing the redox effects.

Blockade of glucocorticoid receptors prevents the increase in urea synthesis after hysterectomy in rats

The postoperative increase in hepatic conversion of amino nitrogen to urea nitrogen seems to be a primary cause of post-surgical catabolism. The importance of glucocorticosteroids for the spontaneous urea nitrogen synthesis rate (UNSR) and for the maximally amino acid-stimulated capacity of urea nitrogen synthesis (CUNS) was investigated 3 and 24 h postoperatively, respectively, in hysterectomized rats. Corticosteroid effects were neutralized by glucocorticoid receptor blockade by the pharmacological analogue RU486. Hysterectomy doubled UNSR from 3.16 +/- 0.20 to 6.12 +/- 0.27 mumol (per min per 100 g body weight) after 3 h (P less than 0.01) and increased CUNS by 40% from 7.47 +/- 0.30 to 10.29 +/- 0.41 mumol (per min per 100 g body weight) after 24 h (P less than 0.01). These changes were both normalized by the receptor blockade. Hysterectomy decreased total blood alpha-amino nitrogen concentration by 25% from 3.4 +/- 0.2 to 2.6 +/- 0.2 mmol l-1 (P less than 0.05) 3 h after surgery, which was normalized by glucocorticoid receptor blockade. Hysterectomized rats lost 10 +/- 1 g the first 24 h after surgery. The blockade reduced the weight loss to 6 +/- 1 g body weight (P less than 0.05) without changing food intake. The results indicate that glucocorticoid action plays a major role in the postoperative increase in hepatic amino nitrogen conversion.