Effect of thyroid hormones on urea biosynthesis and related processes in rat liver

Co-Morbid Hypothyroidism and Liver Dysfunction: A Review

The liver and thyroid hormones interact at multiple levels to maintain homoeostasis. The liver requires large adequate amounts of thyroid hormones to execute its metabolic functions optimally, and deficiency of thyroid hormones may lead to liver dysfunction. Hypothyroidism has been associated with abnormal lipid metabolism, non-alcoholic fatty liver disease (NAFLD), hypothyroidism-induced myopathy, hypothyroidism-associated gallstones and occasionally, interferon-induced thyroid dysfunction. NAFLD remain an important association with hypothyroidism and further studies are needed that specifically compare the natural course of NAFLD secondary to hypothyroidism and primary NAFLD. Hepatic dysfunction associated with hypothyroidism is usually reverted by normalizing thyroid status. Large scale studies geared towards finding new and effective therapies, especially for NAFLD are needed. The clinician must be aware that there exists overlapping symptomatology between liver dysfunction and severe hypothyroidism which may make delay the diagnosis and treatment of hypothyroidism; this requires a high index of suspicion.

Transcriptomic and lipid profiling analysis reveals a functional interplay between testosterone and growth hormone in hypothyroid liver

Preclinical and clinical studies suggest that hypothyroidism might cause hepatic endocrine and metabolic disturbances with features that mimic deficiencies of testosterone and/or GH. The absence of physiological interactions between testosterone and GH can be linked to male differentiated liver diseases. Testosterone plays relevant physiological effects on somatotropic-liver axis and liver composition and the liver is a primary organ of interactions between testosterone and GH. However, testosterone exerts many effects on liver through complex and poorly understood mechanisms. Testosterone impacts liver functions by binding to the Androgen Receptor, and, indirectly, through its conversion to estradiol, and cooperation with GH. However, the role of testosterone, and its interaction with GH, in the hypothyroid liver, remains unclear. In the present work, the effects of testosterone, and how they impact on GH-regulated whole transcriptome and lipid composition in the liver, were studied in the context of adult hypothyroid-orchiectomized rats. Testosterone replacement positively modulated somatotropic-liver axis and impacted liver transcriptome involved in lipid and glucose metabolism. In addition, testosterone enhanced the effects of GH on the transcriptome linked to lipid biosynthesis, oxidation-reduction, and metabolism of unsaturated and long-chain fatty acids (FA). However, testosterone decreased the hepatic content of cholesterol esters and triacylglycerols and increased fatty acids whereas GH increased neutral lipids and decreased polar lipids. Biological network analysis of the effects of testosterone on GH-regulated transcriptome confirmed a close connection with crucial proteins involved in steroid and fatty acid metabolism. Taken together, this comprehensive analysis of gene expression and lipid profiling in hypothyroid male liver reveals a functional interplay between testosterone and pulsed GH administration.

A case of hyperammonemia occurring during treatment of metastatic renal cell carcinoma with axitinib

Introduction

Although the incidence of hyperammonemia as an adverse event of tyrosine kinase inhibitors is quite low, several cases of tyrosine kinase inhibitor associated hyperammonemia have been reported. We report a case of hyperammonemia, that occurred during combined treatment with axitinib and pembrolizumab in a metastatic renal cell carcinoma patient without hepatic disorder or liver metastases.

Case presentation

A 77-year-old Japanese woman was diagnosed with metastatic renal cell carcinoma and was treated with pembrolizumab and axitinib. Both agents were subsequently discontinued due to hyperammonemia with hypothyroidism. After recovery, the patient resumed single-agent therapy with axitinib. However, hyperammonemia and hypothyroidism occurred again, suggesting axitinib-inducible adverse event. After nephrectomy, a lower dose of axitinib was restarted and continued safely for residual metastases under prophylactic treatment with aminoleban, lactulose, and levothyroxine.

Conclusion

The rare occurrence of hyperammonemia should be considered during treatment with VEGFR- targeted tyrosine kinase inhibitor including axitinib, and supportive prophylactic medication may be useful.

Cord blood metabolomics reveals gestational metabolic disorder associated with anti-thyroid peroxidase antibodies positivity

Background

Thyroid disease is one of the common endocrine disorders affecting the pregnant women, in which thyroid autoimmunity can alter the progress and the outcome of pregnancy. Women with euthyroid status but anti-thyroid peroxidase (anti-TPO) antibodies positivity before pregnancy are prone to subclinical gestational hypothyroidism. However, the connections between anti-TPO antibodies positivity and gestational hypothyroidism remain largely unknown. The aim of the present study is to investigate the differences of fetal metabolic profile at birth according to maternal anti-TPO status.

Methods

We performed 1H-NMR metabolomics on cord blood of a nested case control cohort of 22 pregnant women with matched thyroid hormone levels and demographic data, including 11 women with euthyroid status but anti-thyroid antibodies positivity (into the anti-TPO antibodies positivity group) and 11 matched women as controls with euthyroid status and negative anti-thyroid antibodies (into the control group).

Results

Distinct metabolic profiles were observed between the anti-TPO antibody positivity group and the nested control group, from which a total of 10 metabolites with between-group altered abundances were structurally identified. Five out of the 10 metabolites were up-regulated in the anti-TPO antibodies positivity group, including D-Glucose, L-Glutamine, 3-Hydroxybutyric acid, Myo-Inositol, Creatinine. The other 5 metabolites were down-regulated in the anti-TPO antibodies positivity group, including L-Leucine, L-Lysine, L-Glutamic acid, L-Tyrosine, and L-Phenylalanine. All the 10 metabolites have been previously reported to be correlated with hypothyroidism. Metabolite set enrichment analysis and pathway analysis suggested that amino acid metabolism pathways (especially the phenylalanine metabolism) were associated with anti-TPO antibodies positivity.

Conclusion

The results of this study suggested that fetal metabolic disorder is correlated with anti-TPO antibodies positivity, representing by abundance alteration of hypothyroidism associated metabolites and the related disturbance of amino acid metabolism pathways.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-022-04564-8.

Noncoding sequence variants define a novel regulatory element in the first intron of the N-acetylglutamate synthase gene

N-acetylglutamate synthase deficiency is an autosomal recessive urea cycle disorder caused either by decreased expression of the NAGS gene or defective NAGS enzyme resulting in decreased production of N-acetylglutamate (NAG), an allosteric activator of carbamylphosphate synthetase 1 (CPS1). NAGSD is the only urea cycle disorder that can be effectively treated with a single drug, N-carbamylglutamate (NCG), a stable NAG analog, which activates CPS1 to restore ureagenesis. We describe three patients with NAGSD due to four novel noncoding sequence variants in the NAGS regulatory regions. All three patients had hyperammonemia that resolved upon treatment with NCG. Sequence variants NM_153006.2:c.427-222G>A and NM_153006.2:c.427-218A>C reside in the 547 bp-long first intron of NAGS and define a novel NAGS regulatory element that binds retinoic X receptor α. Sequence variants NC_000017.10:g.42078967A>T (NM_153006.2:c.-3065A>T) and NC_000017.10:g.42078934C>T (NM_153006.2:c.-3098C>T) reside in the NAGS enhancer, within known HNF1 and predicted glucocorticoid receptor binding sites, respectively. Reporter gene assays in HepG2 and HuH-7 cells demonstrated that all four substitutions could result in reduced expression of NAGS. These findings show that analyzing noncoding regions of NAGS and other urea cycle genes can reveal molecular causes of disease and identify novel regulators of ureagenesis.

Impaired Cardiovascular Function in Male Rats with Hypo- and Hyperthyroidism: Involvement of Imbalanced Nitric Oxide Synthase Levels

BACKGROUND AND OBJECTIVE

All three isoforms of nitric oxide (NO) synthase (NOS) are targets for thyroid hormones in the cardiovascular system. The aim of this study was to assess the effects of hypo- and hyperthyroidism on inducible (iNOS), endothelial (eNOS), and neural (nNOS) NOS levels in aorta and heart tissues of male rats.

METHODS

Rats were divided into control, hypothyroid, and hyperthyroid groups; hypo- and hyperthyroidism were induced by adding propylthiouracil (500 mg/L) and L-thyroxine (12 mg/L) to drinking water for a period of 21 days. On day 21, systolic blood pressure, heart rate, left ventricular developed pressure (LVDP), peak rate of positive and negative (±dp/dt) changes in left ventricular pressure as well as NO metabolites (NOx) and iNOS, eNOS, and nNOS protein levels in aorta and heart, were all measured.

RESULTS

Compared to controls, LVDP and ±dp/dt were lower in both hypo- and hyperthyroid rats. Compared to controls, heart rate and systolic blood pressure were lower in hypothyroid and higher in hyperthyroid rats. NOx levels in the heart of hypothyroid rats were lower (53%), whereas that in hyperthyroid rats were higher (56% and 40%) than controls. Compared to controls, hypothyroid rats had lower levels of eNOS, iNOS, and nNOS in the aorta (16%, 34%, and 15%, respectively) and lower iNOS and higher nNOS in heart tissue (27% and 46%). In hyperthyroid rats, eNOS levels were lower (54% and 30%) and iNOS were higher (63%, and 35%) in the aorta and heart while nNOS was lower in the aorta (18%).

CONCLUSION

Hypothyroidism increased while hyperthyroidism decreased the ratio of eNOS/iNOS in aorta and heart; these changes of NOS levels were associated with impaired cardiovascular function.

The interplay between thyroid and liver: implications for clinical practice

A complex relationship exists between thyroid and liver in health and disease. Liver plays an essential physiological role in thyroid hormone activation and inactivation, transport, and metabolism. Conversely, thyroid hormones affect activities of hepatocytes and hepatic metabolism. Serum liver enzyme abnormalities observed in hypothyroidism may be related to impaired lipid metabolism, hepatic steatosis or hypothyroidism-induced myopathy. Severe hypothyroidism may have biochemical and clinical features, such as hyperammonemia and ascites, mimicking those of liver failure. Liver function tests are frequently abnormal also in hyperthyroidism, due to oxidative stress, cholestasis, or enhanced osteoblastic activity. Antithyroid drug-associated hepatotoxicity is a rare event, likely related mainly to an idiosyncratic mechanism, ranging from a mild hepatocellular damage to liver failure. Propylthiouracil-induced liver damage is usually more severe than that caused by methimazole. On the other hand, thyroid abnormalities can be found in liver diseases, such as chronic hepatitis C, liver cirrhosis, hepatocellular carcinoma, and cholangiocarcinoma. In particular, autoimmune thyroid diseases are frequently found in patients with hepatitis C virus infection. These patients, especially if thyroid autoimmunity preexists, are at risk of hypothyroidism or, less frequently, thyrotoxicosis, during and after treatment with interpheron-alpha alone or in combination with ribavirin, commonly used before the introduction of new antiviral drugs. The present review summarizes both liver abnormalities related to thyroid disorders and their treatment, and thyroid abnormalities related to liver diseases and their treatment.

Refractory hepatic encephalopathy in a patient with hypothyroidism: Another element in ammonia metabolism

Hepatic encephalopathy (HE) remains a diagnosis of exclusion due to the lack of specific signs and symptoms. Refractory HE is an uncommon but serious condition that requires the search of hidden precipitating events (i.e., portosystemic shunt) and alternative diagnosis. Hypothyroidism shares clinical manifestations with HE and is usually considered within the differential diagnosis of HE. Here, we describe a patient with refractory HE who presented a large portosystemic shunt and post-ablative hypothyroidism. Her cognitive impairment, hyperammonaemia, electroencephalograph alterations, impaired neuropsychological performance, and magnetic resonance imaging and spectroscopy disturbances were highly suggestive of HE, paralleled the course of hypothyroidism and normalized after thyroid hormone replacement. There was no need for intervention over the portosystemic shunt. The case findings support that hypothyroidism may precipitate HE in cirrhotic patients by inducing hyperammonaemia and/or enhancing ammonia brain toxicity. This case led us to consider hypothyroidism not only in the differential diagnosis but also as a precipitating factor of HE.

3,5,3'-Triiodo-L-Thyronine- and 3,5-Diiodo-L-Thyronine- Affected Metabolic Pathways in Liver of LDL Receptor Deficient Mice

3,5,3′-triiodo-L-thyronine (T3) and 3,5-diiodo-L-thyronine (T2), when administered to a model of familial hypercholesterolemia, i.e., low density lipoprotein receptor (LDLr)-knockout (Ldlr^−/−) mice fed with a Western type diet (WTD), dramatically reduce circulating total and very low-density lipoprotein/LDL cholesterol with decreased liver apolipoprotein B (ApoB) production. The aim of the study was to highlight putative molecular mechanisms to manage cholesterol levels in the absence of LDLr. A comprehensive comparative profiling of changes in expression of soluble proteins in livers from Ldlr^−/− mice treated with either T3 or T2 was performed. From a total proteome of 450 liver proteins, 25 identified proteins were affected by both T2 and T3, 18 only by T3 and 9 only by T2. Using in silico analyses, an overlap was observed with 11/14 pathways common to both iodothyronines, with T2 and T3 preferentially altering sub-networks centered around hepatocyte nuclear factor 4 α (HNF4α) and peroxisome proliferator-activated receptor α (PPARα), respectively. Both T2 and T3 administration significantly reduced nuclear HNF4α protein content, while T2, but not T3, decreased the expression levels of the HNFα transcriptional coactivator PGC-1α. Lower PPARα levels were found only following T3 treatment while both T3 and T2 lowered liver X receptor α (LXRα) nuclear content. Overall, this study, although it was not meant to investigate the use of T2 and T3 as a therapeutic agent, provides novel insights into the regulation of hepatic metabolic pathways involved in T3- and T2-driven cholesterol reduction in Ldlr^−/− mice.

Proteomic approaches for the study of tissue specific effects of 3,5,3'-triiodo-L-thyronine and 3,5-diiodo-L-thyronine in conditions of altered energy metabolism

In vertebrates and, specifically, in mammals, energy homeostasis is achieved by the integration of metabolic and neuroendocrine signals linked to one another in an intricate network hierarchically responding to the tight modulating action of hormones among which thyroid hormones (THs) play a central role. At the cellular level, 3,5,3′-triiodo-L-thyronine (T3) acts mainly by binding to specific nuclear receptors (TRs) but actually it is becoming more and more evident that some T3- actions are independent of TRs and that other iodothyronines, such as 3,5-diiodo-L-thyronine (T2), affect energy metabolism and adiposity. In the postgenomic era, clinical and basic biological researches are increasingly benefiting from the recently developed new omics approaches including, among the others, proteomics. Considering the recognized value of proteins as excellent targets in physiology, the functional and simultaneous analysis of the expression level and the cellular localization of multiple proteins can actually be considered fundamental in the understanding of complex mechanisms such as those involved in thyroid control of metabolism. Here, we will discuss new leads (i.e., target proteins and metabolic pathways) emerging in applying proteomics to the actions of T3 and T2 in conditions of altered energy metabolism in animal tissues having a central role in the control of energy balance.

Hyperammonemic coma--barking up the wrong tree

Hepatic encephalopathy and myxedema coma share clinical features: coma, ascites, anemia, impaired liver functions, and a "metabolic" electroencephalogram (EEG). Hyperammonemia, a hallmark of hepatic encephalopathy, has also been described in hypothyroidism. Differentiation between the 2 conditions, recognition of their possible coexistence, and the consequent therapeutic implications are of utmost importance. We describe a case of an 82-year-old woman with a history of mild chronic liver disease who presented with hyperammonemic coma unresponsive to conventional therapy. Further investigation disclosed severe hypothyroidism. Thyroid hormone replacement resulted in gain of consciousness and normalization of hyperammonemia. In patients with an elevated ammonia level, altered mental status, and liver disease, who do not have a clear inciting event for liver disease decompensation, overwhelming evidence of hepatic decompensation, or who do not respond to appropriate therapy for hepatic encephalopathy, hypothyroidism should be considered and evaluated.

A proteomics approach to identify protein expression changes in rat liver following administration of 3,5,3'-triiodo-L-thyronine

We analyzed whole cell protein content of rat liver following T3 administration. Fourteen differentially expressed proteins were unambiguously identified and were involved in substrates and lipid metabolism, energy metabolism, detoxification of cytotoxic products, calcium homeostasis, amino acid catabolism, and the urea cycle. This study represents the first systematic identification of T3-induced changes in liver protein expression profile and provides novel information at the molecular, cellular, and tissue level of T3 action.

Elevated corticosterone and inhibition of ACTH responses to CRH and ether in the neonatal rat: effect of hypoxia from birth

Hypoxia is a common cause of neonatal morbidity and mortality. We have previously demonstrated a dramatic ACTH-independent activation of adrenal steroidogenesis in hypoxic neonatal rats, leading to increases in circulating corticosterone levels. The purpose of the present study was to determine if this ACTH-independent increase in corticosterone inhibits the ACTH response to acute stimuli. Neonatal rats were exposed to normoxia (control) or hypoxia from birth to 5 or 7 days of age. At the end of the exposure, plasma ACTH and corticosterone were measured before and after either ether vapors were administered for 3 min or CRH (10 microg/kg) was given intraperitoneally. Thyroid function, pituitary pro-opiomelanocortin (POMC) mRNA and ACTH content, and hypothalamic corticotropin-releasing hormone (CRH), neuropeptide Y (NPY), and AVP mRNA were also assessed. Hypoxia led to a significant increase in corticosterone without a large increase in ACTH, confirming previous studies. The ACTH responses to ether or CRH administration were almost completely inhibited in hypoxic pups. Hypoxia did not affect the established regulators of the neonatal hypothalamic-pituitary-adrenal axis, including pituitary POMC or ACTH content, hypothalamic CRH, NPY, or AVP mRNA (parvo- or magnocellular), or thyroid function. We conclude that hypoxia from birth to 5 or 7 days of age leads to an attenuated ACTH response to acute stimuli, most likely due to glucocorticoid negative feedback. The neural and biochemical mechanism of this effect has yet to be elucidated.

Hepatic amino nitrogen conversion and organ N-contents in hypothyroidism, with thyroxine replacement, and in hyperthyroid rats

BACKGROUND/AIMS

The role of thyroid hormones in the regulation of hepatic conversions of amino nitrogen to urea is unresolved. The present study was designed to assess ureagenesis in rats with experimentally well-established hypo- and hyperthyroidism. The possible role of propylthiuracil (PTU), used for induction of hypothyroidism, was ascertained during thyroxine replacement of PTU treated hypothyroid rats.

METHODS

Basal blood amino nitrogen concentrations (AAN), the urea nitrogen synthesis rate (UNSR) and the maximal hepatic capacity for urea nitrogen synthesis (CUNS) obtained during alanine infusion were determined together with N-contents in the soleus muscle and kidneys in experimentally hypothyroid rats (n = 19), upon thyroxine replacement (n = 14) and in experimentally hyperthyroid rats (n = 19). Hypothyroidism was induced by adding propylthiouracil (0.05%) to the drinking water for 5 weeks. Hyperthyroidism was induced by thyroxine 100 micrograms/100 g body weight.

RESULTS

During hyperthyroidism, T3 fell to less than 10%, food intake was halved, and body weight fell by 13%. Basal blood AAN fell by 25% (p < 0.01), UNSR more than doubled (p < 0.01), and CUNS rose by 45% (p < 0.05). N-contents of the soleus muscle fell by 13% and by 20% in kidneys, respectively (p < 0.05). Thyroxine replacement normalized AAN, UNSR, CUNS and reduced N-loss to 7% in the soleus muscle (NS) and kidneys (p < 0.05), respectively. During hyperthyroidism, T3 rose five-fold, food intake rose by two thirds, and body weight fell by 10%. Basal AAN rose by 20% (p < 0.05), UNSR doubled (p < 0.01), and CUNS rose by 25% (p < 0.05). N-contents of the soleus muscle decreased by 19%, whereas kidney N-contents increased by 25% (p < 0.05). Overall liver function assessed by galactose elimination capacity did not differ among groups. Both conditions increased the rate of urea synthesis; in the hypothyroid state the hepatic waste of amino-N was limited by low blood concentration of amino-N, probably due to lower proteolysis. In the hyperthyroid state hepatic amino-N loss was aggravated by higher blood concentration of amino-N, probably due to higher proteolysis. This difference may explain the markedly different dietary nitrogen economy between the two groups.

CONCLUSIONS

The findings suggest that distinct hepatic acceleration of urea synthesis may contribute to the protein loss seen in both myxedema and in thyrotoxicosis in humans.

Hepatic conversion of amino-nitrogen to urea in thyroid diseases. II. A study in hyperthyroid patients

Conflicting data have been reported on the influence of thyroid hormones on hepatic nitrogen metabolism and on liver metabolic activity. We studied the urea-nitrogen synthesis rate (UNSR) and the kinetics of the process of hepatic amino-nitrogen to urea-nitrogen conversion in response to constant alanine infusion (ie, the functional hepatic nitrogen clearance [FHNC]) in five hyperthyroid female patients before and after the achievement of a stable euthyroid status. In the same patients, galactose elimination capacity and antipyrine clearance were also measured as quantitative indices of hepatic function. The basal urea synthesis rate was nearly doubled in hyperthyroid patients (35.6 +/- 8.5 mmol.h-1 v 17.6 +/- 7.7 in euthyroid patients, P < .05) and increased linearly with increasing alpha-amino-nitrogen (alpha-AN) concentrations in both conditions. The urea synthesis rate during alanine infusion was still higher by approximately 30 mmol.h-1 in hyperthyroid subjects. The FHNC, calculated as the slope of the linear relation between the UNSR in each time interval and the corresponding average alpha-AN concentration, was not different (hyperthyroidism, 30.6 +/- 7.2 L.h-1; euthyroidism, 28.5 +/- 4.4; normal values > 25). The hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hyperthyroid patients and did not change significantly after therapy. Our data show that the hepatic nitrogen metabolism of hyperthyroid patients is characterized by an upregulation of amino-nitrogen catabolism and loss of the sparing mechanism at low plasma amino acid levels, without any change in different metabolic activities.

Hepatic conversion of amino nitrogen to urea nitrogen in hypothyroid patients and upon L-thyroxine therapy

Conflicting studies have been reported regarding the influence of thyroid hormones on hepatic nitrogen metabolism and liver metabolic activity. We studied urea N synthesis rate (UNSR), functional hepatic N clearance (FHNC), galactose elimination capacity, and antipyrine clearance in six hypothyroid female patients before and after achievement of a stable euthyroid status. In both conditions, UNSR measured at intervals in response to constant alanine infusion was linearly related to the average alpha-amino N concentrations. In the hypothyroid state, peak UNSR was decreased by 31% in comparison with values measured in euthyroidism, which were in the normal range. FHNC (ie, the slope of the linear relation between UNSR and blood alpha-amino N concentration) is a measure of the kinetics of the process of hepatic amino N to urea N conversion; it was 19.8 +/- 4.0 L.h-1 in hypothyroid patients and increased to normal values after L-thyroxine replacement (30.4 +/- 3.3 L.h-1, P < .01; normal values > 25 L.h-1). Hepatic microsomal and cytosolic activities (antipyrine clearance and galactose elimination) were normal in hypothyroid patients and did not change significantly after therapy. Our data show a specific defect in hepatic handling of amino acids in hypothyroid patients, leading to reduced alpha-amino N to urea N conversion, in the absence of any detectable impairment in different hepatic metabolic activities.

Reducing-equivalent transfer to the mitochondria during gluconeogenesis and ureogenesis in hepatocytes from rats of different thyroid status

Isolated hepatocytes from hypothyroid, euthyroid and hyperthyroid rats have been employed to investigate the relative importance of reducing-equivalent shuttles for the transfer of hydrogen between cytoplasm and mitochondria during simultaneous ureogenesis and gluconeogenesis. In cells from hypothyroid animals, a 58% depression of glucose formation and 68% reduction in ureogenesis were induced by n-butylmalonate, an inhibitor of the malate shuttle. A more reduced state of the cytoplasmic compartment and a substantial fall in the concentrations of pyruvate, aspartate, alanine and glutamate accompanied this inhibition. Preincubation of cells with n-butylmalonate yielded greater inhibitory effects than observed in the absence of preincubation. The inhibitory effects on gluconeogenesis and ureogenesis were less in cells from euthyroid rats and were very much reduced in the case of glucose synthesis and absent in the case of ureogenesis, in cells from hyperthyroid rats. It is inferred that both the malate-aspartate and alpha-glycerophosphate shuttles may function in the transfer of reducing equivalents from cytoplasm to mitochondria during ureogenesis in hepatocytes. The major inhibition by n-butylmalonate of glucose and urea synthesis in hepatocytes from hypothyroid rats is due to the diminished activity of the alpha-glycerophosphate shuttle in these cells. Moreover, it follows that the NADH arising from the cytoplasmic malate dehydrogenase-catalysed reaction is accessible to both the malate-aspartate shuttle and the alpha-glycerophosphate shuttle.

Effect of caffeine on urea biosynthesis and some related processes, ketone bodies, ATP and liver amino acids

An increase in urea synthesis has been found in rats administered large doses of caffeine. A parallel increase in urea biosynthesis was also found in hepatocytes isolated from caffeine-treated rats, which confirms a greater capacity for urea synthesis induced by caffeine. This increase appeared only after some days of caffeine treatment; during the first days there was no increase in serum urea levels or in in vitro studies of urea synthesis in isolated hepatocytes. However, no appreciable changes were found in either cytosolic or mitochondrial redox states, or in ATP levels in in vivo and in vitro studies. A parallelism was observed between the decreased amino acid levels in caffeine-treated rats and in isolated hepatocytes incubated with different concentrations of caffeine. Several possible mechanisms to explain these findings are considered in the discussion.