Effect of dietary protein on the capacity of urea synthesis in rats

Perturbations of urea cycle enzymes during posthepatectomy rat liver failure

Posthepatectomy liver failure (PHLF) may occur after extended partial hepatectomy (PH). If malignancy is widespread in the liver, the size of PH and hence the size of the future liver remnant (FLR) may limit curability. We aimed to characterize differences in protein expression between different sizes of FLRs and identify proteins specific to the regenerative process of minimal-size FLR (MSFLR), with special focus on postoperative day (POD) 1 when PHLF is present. A total of 104 male Wistar rats were subjected to 30, 70, or 90% PH (MSFLR in rats), sham operation, or no operation. Blood and liver tissue were harvested at POD1, 3, and 5 (n = 8 per group). Protein expression was assessed by proteomic profiling by unsupervised two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) liquid chromatography tandem mass spectrometry (LC-MS/MS), followed by supervised selected reaction monitoring (SRM)-MS/MS. In all, 1,035 protein spots were detected, 54 of which were significantly differentially expressed between groups and identifiable. During PHLF after PH(90%) at POD1, urea cycle and related proteins showed significant perturbations, including the urea cycle flux-regulating enzyme of carbamoyl phosphate synthase-1, ornithine transcarbamylase, and arginase-1, as well as the ornithine aminotransferase and propionyl-CoA carboxylase alpha chain. Plasma-ammonia increased significantly at POD1 after PH(90%), followed by a prompt decrease. At the protein level, we found perturbations of urea cycle and related enzymes in the MSFLR during PHLF. Our results suggest that these perturbations may augment urea cycle function, which may be pivotal for increased ammonia elimination after extensive PHs and potential PHLF.NEW & NOTEWORTHY Posthepatectomy liver failure (PHLF) is associated with high mortality. In a rat model of 90% hepatectomy, PHLF is present. Our results on liver tissue proteomics suggest that the ability of the liver remnant to sufficiently eliminate ammonia may be brought about by perturbation related to urea cycle proteins and that enhancing the urea cycle capacity may play a key role in surviving PHLF.

The Food Energy/Protein Ratio Regulates the Rat Urea Cycle but Not Total Nitrogen Losses

Nitrogen balance studies have shown that a portion of the N ingested but not excreted is not accounted for. We compared several diets (standard, high-fat, high-protein, and self-selected cafeteria) to determine how diet-dependent energy sources affect nitrogen handling, i.e., the liver urea cycle. Diet components and rat homogenates were used for nitrogen, lipid, and energy analyses. Plasma urea and individual amino acids, as well as liver urea cycle enzyme activities, were determined. Despite ample differences in N intake, circulating amino acids remained practically unchanged in contrast to marked changes in plasma urea. The finding of significant correlations between circulating urea and arginine-succinate synthase and lyase activities supported their regulatory role of urea synthesis, the main N excretion pathway. The cycle operation also correlated with the food protein/energy ratio, in contraposition to total nitrogen losses and estimated balance essentially independent of dietary energy load. The different regulation mechanisms observed have potentially important nutritional consequences, hinting at nitrogen disposal mechanisms able to eliminate excess nitrogen under conditions of high availability of both energy and proteins. Their operation reduces urea synthesis to allow for a safe (albeit unknown) mechanism of N/energy excess accommodation.

Pivotal preclinical trial of the spheroid reservoir bioartificial liver

BACKGROUND & AIMS

The neuroprotective effect of the spheroid reservoir bioartificial liver (SRBAL) was evaluated in a porcine model of drug-overdose acute liver failure (ALF).

METHODS

Healthy pigs were randomized into three groups (standard therapy (ST) alone, ST+No-cell device, ST+SRBAL device) before placement of an implantable intracranial pressure (ICP) monitor and a tunneled central venous catheter. One week later, pigs received bolus infusion of the hepatotoxin D-galactosamine and were followed for up to 90h.

RESULTS

At 48h, all animals had developed encephalopathy and biochemical changes confirming ALF; extracorporeal treatment was initiated and pigs were observed up to 90h after drug infusion. Pigs treated with the SRBAL, loaded with porcine hepatocyte spheroids, had improved survival (83%, n=6) compared to ST alone (0%, n=6, p=0.003) and No-cell device therapy (17%, n=6, p=0.02). Ammonia detoxification, peak levels of serum ammonia and peak ICP, and pig survival were influenced by hepatocyte cell dose, membrane pore size and duration of SRBAL treatment. Hepatocyte spheroids remained highly functional with no decline in mean oxygen consumption from initiation to completion of treatment.

CONCLUSIONS

The SRBAL improved survival in an allogeneic model of drug-overdose ALF. Survival correlated with ammonia detoxification and ICP lowering indicating that hepatocyte spheroids prevented the cerebral manifestations of ALF (brain swelling, herniation, death). Further investigation of SRBAL therapy in a clinical setting is warranted.

Effect of lipopolysaccharide on in vivo and genetic regulation of rat urea synthesis

BACKGROUND

The acute phase response causes a negative nitrogen balance. It is unknown whether this involves regulation of hepatic urea synthesis.

METHODS

We examined the in vivo capacity of urea nitrogen synthesis (CUNS), mRNA levels of urea cycle enzyme genes and galactose elimination capacity (GEC) during moderate and severe acute phase response induced by low- and high-dose lipopolysaccharide (LPS) in rats.

RESULTS

Low-dose LPS doubled CUNS (P<0.05), decreased the mRNA level of the rate-limiting urea cycle enzyme (arginino succinate synthetase (ASS) by 26% (P<0.05) and did not change GEC. High-dose LPS did not change CUNS, decreased the mRNA level of the flux-generating enzyme carbamoyl phosphate synthetase (CPS) by 11% (P<0.05) and the rate-limiting urea cycle enzyme (ASS) by 27% (P<0.05) and almost halved GEC (P<0.05).

CONCLUSION

The moderate acute phase response up-regulated in vivo urea synthesis but had the opposite effect on gene level. The severe acute phase response decreased the functional liver mass that attenuated the increase in urea synthesis.

Urea synthesis in patients with chronic pancreatitis: relation to glucagon secretion and dietary protein intake

BACKGROUND & AIMS

Up-regulation of urea synthesis by amino acids and dietary protein intake may be impaired in patients with chronic pancreatitis (CP) due to the reduced glucagon secretion. Conversely, urea synthesis may be increased as a result of the chronic inflammation. The aims of the study were to determine urea synthesis kinetics in CP patients in relation to glucagon secretion (study I) and during an increase in protein intake (study II).

METHODS

In study I, urea synthesis rate, calculated as urinary excretion rate corrected for accumulation in total body water and intestinal loss, was measured during infusion of alanine in 7 CP patients and 5 control subjects on spontaneous protein intake. The functional hepatic nitrogen clearance (FHNC), i.e. urea synthesis expressed independent of changes in plasma amino acid concentration, was calculated as the slope of the linear relation between urea synthesis rate and plasma alpha -amino nitrogen concentration. In study II, 6 of the patients of study I had urea synthesis and FHNC determined before and after a period of 14 days of supplementation with a protein-enriched liquid (dietary sequence randomized).

RESULTS

Study I: Alanine infusion increased urea synthesis rate by a factor of 10 in the control subjects, and by a factor of 5 in the CP patients (P<0.01). FHNC was 31.9+/-2.4 l/h in the control subjects and 16.5+/-2.0 l/h (P<0.05) in the CP patients. The glucagon response to alanine infusion (AUC) was reduced by 75 % in the CP patients. The reduction in FHNC paralleled the reduced glucagon response (r(2)=0.55, P<0.01). Study II: The spontaneous protein intake was 0.75+/-0.14 g/(kg x day) and increased during the high protein period to 1.77+/-0.12 g/(kg x day). This increased alanine stimulated urea synthesis by a factor of 1.3 (P<0.05), FHNC from 13.5+/-2.6 l/h to 19.4+/-3.1 l/h (P<0.01), and the glucagon response to alanine infusion (AUC) by a factor of 1.8 (P<0.05).

CONCLUSIONS

Urea synthesis rate and FHNC are markedly reduced in CP patients. This is associated with, and probably a result of, impaired glucagon secretion, and predicts a lower than normal postprandial hepatic loss of amino nitrogen. An increase in dietary protein intake increases alanine stimulated urea synthesis and FHNC by a mechanism that involves an increase in glucagon. This indicates that the low FHNC during spontaneous protein intake included an adaptation to the low protein intake, effectuated by a further decrease in glucagon secretion.

Effects of growth hormone and insulin-like growth factor-I singly and in combination on in vivo capacity of urea synthesis, gene expression of urea cycle enzymes, and organ nitrogen contents in rats

Improvement of nitrogen balance is desirable in patients with acute or chronic illness. Both growth hormone (GH) and insulin-like growth factor-I (IGF-I) are promising anabolic agents, and their combined administration has been shown to reverse catabolism more efficiently than each of the peptides alone. This is believed to be mediated primarily through increased peripheral protein synthesis, whereas little attention has focused on a possible participation of amino acid metabolism in the liver. Four groups of rats were given: 1) placebo; 2) GH (200 micrograms/d); 3) IGF-I (300 micrograms/d); and 4) both GH and IGF-I. After 3 days, the maximum capacity of urea-nitrogen synthesis was determined by saturating infusion of alanine (n = 8 in each group), together with measurements of liver messenger RNA (mRNA) levels for urea cycle enzymes (n = 5 in each group) and N-contents of muscles, heart, and kidney. Basal plasma alpha-amino acid concentrations were similar in all groups. The capacity of urea-N synthesis [mumol/(min x 100 g body weight)] was reduced in a stepwise manner (placebo: 8.25 +/- 1.2; GH treatment: 6.52 +/- 0.8; IGF-I treatment: 5.5 +/- 0.6; and GH/IGF-I: 4.22 +/- 1.6 [P < .001 by ANOVA]), each step being lower than the former. Serum IGF-I increased stepwise from placebo (699 +/- 40 to 1,579 +/- 96 micrograms/L in the combined GH/IGF-I group), and was correlated negatively with the capacity of urea-nitrogen synthesis (P < .01). mRNA levels for urea cycle enzymes in the liver decreased after GH and IGF-I treatment, and the effect was more pronounced after the combined treatment in which the rate-limiting enzyme, argininosuccinate synthetase, was halved. Nitrogen contents of organs increased after both GH and IGF-I treatment, and even more so after the combination treatment, reaching an increase of 30% (P < .05). Data suggest that GH and IGF-I singly and, even more so in combination, additively inhibit urea synthesis. This is supposed to favor protein buildup in organs. We speculate that this inhibitory effect on the capacity of urea synthesis is caused by a decreased translation rate of the urea cycle enzymes caused by GH and IGF-I's down-regulatory effect on urea cycle enzyme gene transcription. The findings may indicate a novel mechanism of the protein anabolic action of GH and IGF-I.

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.

Effects of beta-blockade on hepatic conversion of amino acid nitrogen and on urea synthesis in cirrhosis

beta-Blockers are widely used to prevent gastrointestinal hemorrhage in cirrhosis. The metabolic effects of treatment are scarcely studied: hepatic function reportedly does not change significantly, but beta-adrenoceptors have been reported to regulate protein and amino acid metabolism. We studied hepatic nitrogen metabolism in response to constant alanine infusion in seven patients with cirrhosis before and 7 to 10 days after treatment with oral propranolol (60 to 100 mg/d). Beta-blockade was effective: it decreased heart rate by 25%, abolished orthostatic tachycardia, and reduced portal blood flow by 20%. Alanine-stimulated urea nitrogen synthesis rate (UNSR) was higher in patients with propranolol treatment, without any difference in aminonitrogen concentration. The kinetics of hepatic conversion of amino acid nitrogen into urea--ie, functional hepatic nitrogen clearance (FHNC)--increased by 30%, from (mean +/- SD) 17.0 +/- 4.1 to 22.0 +/- 6.6 L/h (P < .01). Increased urea production during alanine infusion resulted in negative nitrogen exchange even at the peak of alpha-aminonitrogen concentration. Basal insulin level was only slightly reduced during propranolol treatment, whereas the insulin response to alanine was significantly blunted. No differences in glucagon and cortisol were demonstrated. Epinephrine and norepinephrine levels were high-normal and did not vary after treatment. Increased urea production and stimulation of hepatic nitrogen clearance during beta-blockade may be mediated by relative hypoinsulinemia or by direct involvement of beta-adrenoceptors in the control of nitrogen metabolism, possibly by regulation of amino acid uptake and release in peripheral tissues.

Kinetics of hepatic amino-nitrogen conversion in ageing man

Previous studies have shown that hepatic function, quantitatively measured by dynamic liver function tests, progressively declines with ageing. Urea synthesis is a specific process taking place in the liver; a reduced urea synthesis in response to a protein-rich metal has previously been demonstrated in the elderly, but the process has never been standardized in relation to amino acid supply. We measured the hepatic conversion of alpha-amino nitrogen into urea nitrogen in response to alanine infusion in 32 subjects, with normal routine liver and renal function tests and without evidence of previous hepatic disorders, belonging to three different age-groups (< or = 55 years, 56-70, > or = 71). The functional hepatic nitrogen clearance was reduced on average by 20% in subjects aged 56-70 years, and by 30% in subjects over 70 years old in comparison to the age-group under 55 years (ANOVA: P = 0.0001), and significantly correlated with age (r = -0.684). No sex differences were observed on the effects of age on hepatic clearance. Also, liver volume, measured by ultrasonography, was reduced with advancing age, but the age-related decrease in hepatic nitrogen conversion was not primarily dependent on decreased liver volume. The measurement of functional hepatic nitrogen clearance has already been validated as a quantitative liver function test in clinical hepatology. In keeping with previous studies, the age-related decline in hepatic nitrogen conversion points to a decreased functional capacity of the ageing hepatic parenchyma.

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.

Effects of an increase in protein intake on hepatic efficacy for urea synthesis in healthy subjects and in patients with cirrhosis

The efficacy of urea synthesis as measured by functional hepatic nitrogen clearance (i.e., the relation of urea synthesis rate to blood alpha-amino nitrogen concentration) was studied before and after diet protein supplementation in six healthy subjects and five patients with stable cirrhosis (galactose elimination capacity about 60% of control). Daily protein intake was increased for 14 days by a protein-enriched liquid from (mean +/- S.D.) 1.01 +/- 0.32 g/kg body wt. to 1.62 +/- 0.31 g/kg body wt in the control subjects, and from 0.69 +/- 0.21 g/kg body wt. to 1.50 +/- 0.15 g/kg body wt. in the patients with cirrhosis. This increased the hepatic nitrogen clearance from 27 +/- 10 l/h to 39 +/- 15 l/h in the control subjects (p less than 0.05) and from 15 +/- 6 l/h to 21 +/- 7 l/h in the cirrhosis patients (p less than 0.05). There was no effect on the galactose elimination capacity in any group. Compared to the control subjects, the response in hepatic nitrogen clearance relative to the increase in protein intake was reduced by 60% in the patients. Basal glucagon was 75% higher in the patients and increased by 50% during high protein intake (p less than 0.05), but did not parallel the increase in hepatic nitrogen clearance, and it did not change in the control subjects. The study shows that an increase in protein intake selectively increases liver function with regard to disposal of amino nitrogen; the mechanism is qualitatively intact but quantitatively deficient in patients with cirrhosis of the liver, and does not seem to depend on glucagon.

Increased hepatic capacity of urea synthesis in acute and chronic uraemia in rats

We studied whole body nitrogen balance in female rats for 28 days after induction of experimental uraemia by 5 6 nephrectomy and at the same time the kinetics of hepatic urea synthesis by means of the in vivo capacity of urea synthesis. The N-balance of 5 uraemic rats kept in metabolic cages was negative on day 2, and positive but only half of control on day 28. In between, it was normal. The uraemic rats lost weight during the first week, and later only slowly regained their initial weight. The capacity for urea synthesis was determined during alanine loading in 5 unoperated controls, in uraemic rats in groups of 5 on days 2, 7, 14, 21, and 28, and correspondingly in sham operated rats. In the control rats the capacity was 8.9 +/- 0.7 micromol/(min 100 g BW) and the same in sham operated rats. In uraemic rats the capacity increased to 17.1 +/- 2.0 micromol/(min 100 g BW) (p < 0.01) 2 days after partial nephrectomy. On day 7, the capacity fell to 5.5 +/- 1.0 (not different from initial values), and thereafter again gradually increased to 16.5 +/- 1.5 micromol/(min 100g BW) on day 28. The early increase in the capacity may be related to glucagon, that nearly doubled on day 2, but not on day 28. The hepatic capacity for urea synthesis doubles biphasically: acutely and after 4 weeks of experimental uraemia. This may play a role in the reduction in N-balance, since an increase in the capacity implies larger hepatic amino-N conversion at any blood amino-acid concentration.