Effects of glucose on alanine-derived urea synthesis

Glucagon Resistance in Individuals With Obesity and Hepatic Steatosis Can Be Measured Using the GLUSENTIC Test and Index

Increased plasma levels of glucagon (hyperglucagonemia) promote diabetes development but are also observed in patients with metabolic dysfunction-associated steatotic liver disease (MASLD). This may reflect hepatic glucagon resistance toward amino acid catabolism. A clinical test for measuring glucagon resistance has not been validated. We evaluated our glucagon sensitivity (GLUSENTIC) test, which consists of 2 study days: a glucagon injection and measurements of plasma amino acids and an infusion of mixed amino acids and subsequent calculation of the GLUSENTIC index (primary outcome measure) from measurements of glucagon and amino acids. To distinguish glucagon-dependent from insulin-dependent actions on amino acid metabolism, we also studied patients with type 1 diabetes (T1D). The δ-decline in total amino acids was 49% lower in MASLD following exogenous glucagon (P = 0.01), and the calculated GLUSENTIC index was 34% lower in MASLD (P < 0.0001) but not T1D (P > 0.99). In contrast, glucagon-induced glucose increments were similar in control participants and participants with MASLD (P = 0.41). The GLUSENTIC test and index may be used to measure glucagon resistance in individuals with obesity and MASLD.

ARTICLE HIGHLIGHTS

Role of ammonia in NAFLD: An unusual suspect

Mechanistically, the symptomatology and disease progression of non-alcoholic fatty liver disease (NAFLD) remain poorly understood, which makes therapeutic progress difficult. In this review, we focus on the potential importance of decreased urea cycle activity as a pathogenic mechanism. Urea synthesis is an exclusive hepatic function and is the body's only on-demand and definitive pathway to remove toxic ammonia. The compromised urea cycle activity in NAFLD is likely caused by epigenetic damage to urea cycle enzyme genes and increased hepatocyte senescence. When the urea cycle is dysfunctional, ammonia accumulates in liver tissue and blood, as has been demonstrated in both animal models and patients with NAFLD. The problem may be augmented by parallel changes in the glutamine/glutamate system. In the liver, the accumulation of ammonia leads to inflammation, stellate cell activation and fibrogenesis, which is partially reversible. This may be an important mechanism for the transition of bland steatosis to steatohepatitis and further to cirrhosis and hepatocellular carcinoma. Systemic hyperammonaemia has widespread negative effects on other organs. Best known are the cerebral consequences that manifest as cognitive disturbances, which are prevalent in patients with NAFLD. Furthermore, high ammonia levels induce a negative muscle protein balance leading to sarcopenia, compromised immune function and increased risk of liver cancer. There is currently no rational way to reverse reduced urea cycle activity but there are promising animal and human reports of ammonia-lowering strategies correcting several of the mentioned untoward aspects of NAFLD. In conclusion, the ability of ammonia-lowering strategies to control the symptoms and prevent the progression of NAFLD should be explored in clinical trials.

Arginine-induced glucagon secretion and glucagon-induced enhancement of amino acid catabolism are not influenced by ambient glucose levels in mice

Amino acids stimulate the secretion of glucagon, and glucagon receptor signaling regulates amino acid catabolism via ureagenesis, together constituting the liver-α cell axis. Impairment of the liver-α cell axis is observed in metabolic diseases such as diabetes. It is, however, unknown whether glucose affects the liver-α cell axis. We investigated the role of glucose on the liver-α cell axis in vivo and ex vivo. The isolated perfused mouse pancreas was used to evaluate the direct effect of low (3.5 mmol/L) and high (15 mmol/L) glucose levels on amino acid (10 mmol/L arginine)-induced glucagon secretion. High glucose levels alone lowered glucagon secretion, but the amino acid-induced glucagon responses were similar in high and low glucose conditions (P = 0.38). The direct effect of glucose on glucagon and amino acid-induced ureagenesis was assessed using isolated perfused mouse livers stimulated with a mixture of amino acids (Vamin^R, 10 mmol/L) and glucagon (10 nmol/L) during high and low glucose conditions. Urea production increased robustly but was independent of glucose levels (P = 0.95). To investigate the whole body effects of glucose on the liver-α cell axis, four groups of mice received intraperitoneal injections of glucose-Vamin (2 g/kg, + 3.5 µmol/g, respectively, G/V), saline-Vamin (S/V), glucose-saline (G/S), or saline-saline (S/S). Blood glucose did not differ significantly between G/S and G/V groups. Levels of glucagon and amino acids were similar in the G/V and S/V groups (P = 0.28). Amino acids may overrule the inhibitory effect of glucose on glucagon secretion and the liver-α cell axis may operate independently of glucose in mice.NEW & NOTEWORTHY Glucagon is an essential regulator of our metabolism. Recent evidence suggests that the physiological actions of glucagon reside in amino acid catabolism in the so-called liver-α cell axis, in which amino acids stimulate glucagon secretion and glucagon enhances hepatic amino acid catabolism. Here, it is demonstrated that this feedback system is independent of glycemia possibly explaining why hyperglycemia in diabetes may not suppress α cell secretion.

Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-¹³C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM ¹⁵N-labeled NH₄Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

Effect of predialysis eating on measurement of urea reduction ratio and Kt/V

Physicians utilize the measurement of the urea reduction ratio (URR) and Kt/V as surrogates for the adequacy of hemodialysis, as well as to follow the course of patients longitudinally. These measurements are affected by the duration of a dialysis treatment, the type and size of the dialyzer membrane used during the treatment, the blood flow rate during the treatment, and the adequacy of vascular access. We, and others, have noted that eating during dialysis can be associated with decreases in URR and Kt/V. However, there have been no previous studies that have examined the effects of eating before dialysis on these variables. This study examined the effects of eating one-third of a daily diet 2 hours before dialysis as opposed to fasting for a minimum of 3 hours before dialysis on the measured URR and Kt/V as obtained routinely in our dialysis unit. Sixty seven patients gave informed consent for the study, and 42 completed the protocol. No differences were found in URR or Kt/V when dialysis was performed 2 hours after eating compared with performing dialysis after at least a 3-hour fast in the group as a whole or in subgroup analyses of men, women, patients with diabetes, patients in different age groups, or patients who dialyzed on different shifts. Unlike intradialytic food ingestion, moderate predialysis food intake does not affect the measurement of dialysis adequacy as determined by URR and Kt/V.

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 dietary supplementation with Yucca schidigera Roezl ex Ortgies and its saponin and non-saponin fractions on rat metabolism

Yucca schidigera Roezl ex Ortgies, family Lillaceae, was fractionated with butan-1-ol to yield a butanol extractable fraction (BE; saponin fraction) and a non-butanol fraction (NBE; non-saponin fraction). Four groups of eight male rats were allowed ad libitum access to diets supplemented with water (control) or 200 mg x kg(-1) total Y. schidigera (TOT) or 200 mg x kg(-1) of each of the fractions (NBE or BE). The effects of dietary supplementation with the fractions and their interactions in TOT were analyzed according to the factorial experimental design by two-way analysis of variance. All three supplementation groups displayed significantly reduced serum urea levels (P < 0.05). The TOT and NBE fractions were found to significantly increase serum insulin levels (P < 0.01) in the absence of any fluctuations in serum glucose levels. Urea cycle enzyme activities, namely, arginase (EC 3.5.3.1) and argininosuccinate lyase (EC 4.3.2.1), were significantly decreased (P < 0.05) in vivo, although no effect was observed in vitro. Both fractions displayed effects, indicating that the active constituents are present in both fractions.

Effects of hyperglycaemia and hyperinsulinaemia on plasma amino acid levels in obese subjects with normal glucose tolerance

OBJECTIVE

To investigate the effects of hyperglycaemia and hyperinsulinaemia on amino acid disposal in human obesity.

DESIGN

Four sequential experimental conditions: (1) overnight fasting; (2) hyperglycaemia with hyperinsulinaemia (2 h hyperglycaemic clamp at 11 mmol/l); (3) hyperglycaemia with basal insulin (1 h hyperglycaemic clamp during somatostatin infusion), (4) hyperglycaemia with resuming hyperinsulinaemia (1 h hyperglycaemic clamp after somatostatin discontinuation).

SUBJECTS

Seven non-obese and seven obese non-diabetic, normo-insulinaemic subjects.

MEASUREMENTS

Glucose infused to maintain steady-state hyperglycaemia. Plasma insulin, glucagon, free fatty acid and amino acid concentrations in the last 20 min of the four experimental conditions. Net rates of plasma amino acid disappearance and appearance (micromol/l per hour), calculated as the slopes of the regression of amino acid concentration on time.

RESULTS

The amount of glucose infused to maintain hyperglycaemia was reduced by nearly 50% in obese subjects. During hyperinsulinaemia, FFA suppression was lower in obese subjects. In all experimental conditions plasma amino acid levels were slightly, non-significantly higher in obese than in non-obese subjects. In both groups plasma amino acids decreased slightly with ongoing fasting, decreased remarkably during hyperglycaemia-hyperinsulinaemia, rose promptly when insulin concentration was suppressed by somatostatin infusion, and declined again after somatostatin discontinuation. Also the time-course of plasma branched-chain amino acids, which paralleled that of total amino acids, was similar in the two groups. The net rates of amino acid disappearance from plasma did not differ in obese and non-obese subjects both at fasting and during hyperglycaemia-hyperinsulinaemia. Also plasma amino acid appearance during hyperglycaemia with basal insulin was not different in the two groups.

CONCLUSION

The net traffic of amino acids to and from plasma in relation to insulin drive and prevailing glucose is not impaired in obese subjects with normal glucose tolerance, in spite of a decreased insulin sensitivity of glucose and lipid metabolism.

Effects of xylitol on urea synthesis in patients with cirrhosis of the liver

BACKGROUND

In individuals with cirrhosis the normal inhibiting effect of glucose on urea synthesis is lost, probably because of very high concentrations of glucagon. In agreement, glucose does not prevent the inducing effect of glucagon on urea synthesis in normal humans. In contrast, the sugar alcohol, xylitol, prevents the increasing effect of glucagon in normal humans. We, therefore, examined the effect of xylitol on urea synthesis in individuals with cirrhosis and hyperglucagonemia.

METHODS

Urea synthesis, calculated as urinary excretion rate corrected for accumulation in total body water and intestinal loss, was measured during infusion of alanine (2 mmol/[h x kg body wt]) and during infusion of alanine superimposed on infusion of xylitol (0.12 g/[h x kg body wt]) in 8 individuals with biopsy-proven alcoholic cirrhosis. The functional hepatic nitrogen clearance (FHNC), ie, urea synthesis expressed independent of changes in plasma amino acid concentration, was calculated as the slope of the linear relation between the urea synthesis rate and the plasma amino acid concentration.

RESULTS

All individuals had elevated basal plasma glucagon concentration (261 +/- 61 ng/L; mean +/- SEM) and a markedly increased response to alanine infusion (1037 +/- 226 ng/L). This was not changed by xylitol. Neither the basal urea synthesis rate (13.2 +/- 2.5 mmol/h) nor the alanine-stimulated urea synthesis rate (76.8 +/- 3.64 mmol/h) was changed by xylitol. FHNC during the infusion of alanine alone was 10.5 +/- 0.9 L/h and did not change during the concomitant infusion of xylitol (10.1 +/- 1.1 L/h).

CONCLUSIONS

Xylitol reduces neither urea synthesis nor FHNC. The data do not support an important role of xylitol as a nitrogen-sparing agent in cirrhosis.

Growth hormone prevents prednisolone-induced increase in functional hepatic nitrogen clearance in normal man

BACKGROUND/AIMS

Glucocorticoid treatment increases urea excretion and leads to negative nitrogen balance. This effect is presumed mainly to reflect actions on tissue protein metabolism, but has been shown in rats to involve an hepatic element in the form of upregulation of the kinetics of ureagenesis. Likewise, the anabolic action of growth hormone administration has been shown to involve an hepatic element, just as growth hormone administration has been shown to prevent the protein catabolic side effects of prednisolone. Whether glucocorticoids increase the ability of the liver to convert amino-N to urea-N in man, and whether growth hormone counteracts any possible effect of glucocorticoid has not been studied.

METHODS

We measured urea nitrogen synthesis rates and blood alpha-amino-N levels before, during, and after a 4-h constant i.v. infusion of alanine (2 mmol x kg BW(-1) x h(-1)). The urea nitrogen synthesis rate was estimated hourly as urinary excretion corrected for gut hydrolysis and accumulation in body water. The slope of the linear relationship between urea nitrogen synthesis rate and amino-N concentration represents the hepatic kinetics of conversion of amino- to urea-N, and is denoted the functional hepatic nitrogen clearance. Eight normal male subjects (aged 22-28 years; BMI 21.6-26.3 kg/m2) were randomly studied four times: i) after 4 days of s.c. saline injections, ii) after 4 days of s.c. growth hormone injections (0.1 IU x kg(-1) x day(-1)), iii) after 4 days of glucocorticoid administration (50 mg/d) and iv) after 4 days of growth hormone and glucocorticoid administration. All injections were given at 20 00 hours and 25 mg prednisolone was given morning and evening.

RESULTS

Growth hormone decreased functional hepatic nitrogen clearance (l/h) by 21% (from 38.8+/-1.8 l/h (control) to 30.5+/-2.7 l/h (4 d growth hormone) (mean+/-SE) (ANOVA; p<0.05)). Glucocorticoid increased functional hepatic nitrogen clearance by 23% (47.7+/-3.3 l/h, p<0.05), while growth hormone plus glucocorticoid offset any effect on functional hepatic nitrogen clearance (36.2+/-3.3 l/h, p=0.83).

CONCLUSIONS

Glucocorticoid administration leads to loss of nitrogen as urea, in part due to a specific hepatic mechanism, as shown by the increased functional hepatic nitrogen clearance. Growth hormone has the opposite effect, and also neutralises the glucocorticoid effect when given together with prednisolone. This adds to the understanding of the development and treatment possibilities of steroid catabolism.

Effects of xylitol on urea synthesis in normal humans: relation to glucagon

BACKGROUND

Xylitol exerts a nitrogen-sparing effect in stress catabolic states with hyperglucagonemia, but the mechanism(s) is unknown. We examined the effects of xylitol on urea synthesis during physiologic glucagon concentrations and during hyperglucagonemia.

METHODS

Urea synthesis was measured independently of blood amino acid concentration by means of functional hepatic nitrogen clearance (FHNC) (ie, the linear slope of the relation between urea synthesis rate and blood alpha-amino nitrogen concentration during infusion of alanine). FHNC was measured on four separate occasions in each of seven healthy subjects: during constant infusion of alanine alone, alanine superimposed on a constant infusion of xylitol (blood xylitol 1 mmol/L), alanine superimposed on infusion of glucagon, and alanine superimposed on infusions of xylitol and glucagon.

RESULTS

During alanine infusion alone, plasma glucagon rose to -170 ng/L, and FHNC was (mean +/- sem) 27.9 +/- 1.3 L/h. Xylitol did not affect plasma glucagon and only moderately reduced FHNC to 24.3 +/- 1.0 L/h (p < .05). Glucagon infusion increased plasma glucagon to -450 ng/L and FHNC twofold to 50.9 +/- 6.2 L/h; this increase was totally prevented by the addition of xylitol that reduced FHNC to 27.4 +/- 2.6 L/h (p < .01).

CONCLUSIONS

The results show that xylitol only inhibited FHNC minimally during spontaneous glucagon levels. In contrast, xylitol completely inhibits the increase in FHNC by glucagon. This suggests that the mechanism whereby xylitol reduces nitrogen loss in stress catabolic conditions with hyperglucagonemia involves an effect on liver metabolism. The mechanism is unknown but may be related to depletion of hepatocyte adenine nucleotides.

Kinetic of body nitrogen loss during a whole day infusion and withdrawal of glucose and insulin in injured patients

OBJECTIVE

To investigate the kinetics of body nitrogen (N) excretion during 24 h glucose infusion (relating glycemia with insulin supply) and during subsequent 24 h saline infusion in injured patients during a full blown stress reaction. To define the lag time between the start of the withdrawal of glucose and insulin infusion, and the modification in the N loss from the body, and the time span to reach the maximum effect and its size. The knowledge of these variables is mandatory to plan short term studies in critically ill patients, while assuring the stability of the metabolic condition during the study period, and also to assess the possible weaning of the effect on protein breakdown during prolonged glucose and insulin infusion.

DESIGN

24-36 h after injury, patients were fasted ( < 100 g glucose) for 24 h (basal day). Thereafter, a 24 h glucose infusion in amount corresponding to measured fasting energy production rate (EPR), clamping glycemia at normal level with insulin supply followed by 24 h saline infusion, was performed. Total N, urea and 3-methyl-histidine (3-MH) in urine were measures on 4 h samples starting from 20th h of the basal day.

SETTING

Multipurpose ICU in University Hospital.

PATIENTS

6 consecutive patients who underwent accidental and/or surgical injury, immediately admitted for respiratory assistance (FIO2 < 0.04). Excluded patients were those with abnormal nutritional status, cardiovascular compromise and organ failures.

MAIN RESULTS

Patients showed a 33% increase in measured versus predicted fasting EPR and a consistent increase in N and 3-MH urinary loss. An infusion of glucose at 5.95 +/- 0.53 mg/kg x min (97.20 +/- 0.03% of the fasting measured EPR) with 1.22 +/- 0.18 mU/kg x min insulin infusion reduced N and 3-MH loss after a time lag of 12 h. The peak decrease in body N (-36%) and 3-MH loss (-38%) was reached during the first 12 h of glucose withdrawal period. Thereafter, during the following 12 h, the effect completely vanished confirming that it is therapy-dependent and that the metabolic environment of the patients did not change during the three days study period.

CONCLUSION

24 h glucose withdrawal reduces N and 3-MH loss injured patients, the drug-like effect is maintained during the first 12 h of withdrawal and thereafter disappears. The study suggests that at least a 24 h study period is necessary when planning studies exploring energy-protein metabolism relationship in injured patients, and, again 24 h before changing protocol in a crossover study.

Somatostatin prevents the postoperative increases in plasma amino acid clearance and urea synthesis after elective cholecystectomy

The importance of glucagon on postoperative changes in hepatic amino-nitrogen conversion were investigated in six patients undergoing elective cholecystectomy for uncomplicated gall stones. Patients were given infusions of somatostatin (bolus of 6 micrograms/kg followed by continuous infusion of 6 micrograms/kg/h) from induction of anaesthesia to the end of investigation, the first postoperative day (30 hours). Controls were 16 patients undergoing the same procedures omitting the somatostatin infusion. In all patients blood concentration and plasma clearance of total alpha-amino-nitrogen, and amino acid stimulated rate of urea synthesis were measured. Elective cholecystectomy decreased blood alpha-amino-nitrogen concentration from mean (SEM) 2.9 (0.2) to 2.4 (0.1) mmol/l (p < 0.05), increased the clearance of total alpha-amino-nitrogen from 5.2 (0.3) to 6.6 (0.3) ml/s (p < 0.05), and increased the rate of amino acid stimulated urea synthesis from 27 (1) to 37 (2) mumol/s (p < 0.05) pointing to increased hepatic removal of amino-nitrogen at expense of plasma amino-nitrogen. Infusion of somatostatin prevented increase of glucagon for 24 hours after surgery, and prevented the negative changes in postoperative nitrogen homeostasis resulting from the postoperative changes in hepatic nitrogen conversion, suggesting glucagon as mediator. The exact mechanism remains in doubt, however, because of the multiple effects of somatostatin.

Elective laparoscopic cholecystectomy nearly abolishes the postoperative hepatic catabolic stress response

OBJECTIVE

Surgery results in a catabolic state of postoperative stress, where the efficiency of the liver to convert amino acids to urea is increased. This study measured the metabolic consequences of the less traumatic laparoscopic surgery in elective cholecystectomy compared with traditional open surgery technique.

SUMMARY BACKGROUND DATA

The authors previously have shown that open cholecystectomy doubles the urea synthesis measured by the means of the functional hepatic nitrogen clearance. Glucagon and cortisol increased by 50% (p < 0.05) and 75% (p < 0.05), respectively, after open cholecystectomy.

METHODS

Patients undergoing uncomplicated elective laparoscopic cholecystectomies were included. Preoperatively and on the first postoperative day, blood and urine samples were drawn every hour under basal conditions and during amino acid infusion. The urea synthesis rate was calculated from the urea excreted in urine and accumulated in total body water. Functional hepatic nitrogen clearance was quantified as the slope of the linear relation between blood amino-N concentration and the urea synthesis rate. The results were compared with an historic matched group of patients who underwent open cholecystectomies and were studied by the same protocol.

RESULTS

The laparoscopic cholecystectomy increased the functional hepatic nitrogen clearance by only 25% (from 8.7 +/- 0.9 to 11.1 +/- 1.5 mL/sec [mean +/- SEM; p < 0.05]), compared with a doubling after open cholecystectomy (from 9.4 +/- 0.9 to 17.6 +/- 3.3 mL/sec [p < 0.05]). The difference between the groups was significant (p < 0.05). Neither glucagon nor cortisol increased significantly after laparoscopic cholecystectomy.

CONCLUSIONS

The laparoscopic technique results in a much smaller postoperative hepatic catabolic stress response and probably reduced tissue loss of amino-N. This may be important for the more rapid convalescence and reduced postoperative fatigue.

Effects of insulin and glucose on urea synthesis in normal man, independent of pancreatic hormone secretion

We investigated the inhibitory effect of insulin and glucose on hepatic amino- to urea-nitrogen conversion independent of endogenous insulin and glucagon secretion. Alanine-stimulated urea synthesis kinetics, as quantified by functional hepatic nitrogen clearance, i.e. the slope of the linear relation between blood alpha-amino nitrogen concentration and urea synthesis rate, were measured four times in each of six healthy volunteers, namely during spontaneous hormone responses, and during hormonal control by somatostatin and maintenance of basal hormone levels and euglycaemia, hyperinsulinaemia (85 +/- 8 mU/l), or hyperglycaemia (8.4 +/- 0.5 mmol/l). Hormonal control and euglycaemia reduced functional hepatic nitrogen clearance (mean +/- SD) by two-thirds (from 32.9 +/- 5.2 l/h to 12.2 +/- 3.4 l/h, p < 0.01). Hyperinsulinaemia did not change this (13.2 +/- 2.8 l/h), whereas hyperglycaemia further reduced functional hepatic nitrogen clearance by 40% to 7.4 +/- 1.3 l/h (p < 0.01). The reduction by hormonal control and euglycaemia is attributable to the abolition of the glucagon response to alanine infusion, as glucagon is known to up-regulate functional hepatic nitrogen clearance. Insulin did not regulate hepatic amino- to urea-nitrogen conversion, implying that the effect of insulin on urea production is due to its effect on blood amino acid supply to the liver. In contrast, glucose in itself reduced hepatic amino nitrogen conversion, independent of the hormonal responses to glucose. This means that the hepatic component of the amino-N-sparing effect of glucose depends on hyperglycaemia but not on hyperinsulinaemia.

A rapid method for determination of hepatic amino nitrogen to urea nitrogen conversion ('the Functional Hepatic Nitrogen Clearance')

The Functional Hepatic Nitrogen Clearance (FHNC) is a measure of the functional liver mass as to conversion of amino-N to urea-N. FHNC is the slope of the linear regression of multiple samples (10-20) of urea-N synthesis rates (UNSR) on blood alpha-amino-N concentrations (alpha-AN) during infusion of amino acids. UNSR is measured as urinary urea-N excretion rate corrected for accumulation in total body water (TBW) and loss in gut. A simplified method which estimates FHNC from only two samples of UNSR and alpha-AN was developed. Urine was collected in two hourly intervals: before infusion of alanine, and from 2 to 3 h after start of alanine infusion. Blood-urea-N and alpha-amino-N was measured at the beginning and at the end of each urine sampling interval. TBW was estimated from a nomogram, and gut loss of urea was assigned a fixed value (14%). The two-sample FHNC was calculated as delta UNSR (mmol h-1)/delta mean alpha-AN (mmol l-1). Linear regression analysis of the two-sample estimates of FHNC on the 'true' multiple-sample values of FHNC in an independent population of control and cirrhotic subjects showed the two-sample estimates to be closely related with values of the multiple-sample method, the regression equation being: two-sample FHNC = -0.24 + 0.99 x multiple-sample FHNC, r2 = 0.98. A close relationship was also obtained when cirrhotic patients were considered alone: two-sample FHNC = 0.01 + 0.94 x multiple-sample FHNC, r2 = 0.98.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of urea synthesis by glucose and glucagon in normal man

The separate effects of glucose and glucagon on alanine stimulated hepatic amino-N to urea-N conversion, quantified by the Functional Hepatic Nitrogen Clearance (FHNC) (i.e. the linear slope of the relation between urea synthesis rate and blood alpha-amino-N concentration), were studied in 7 healthy subjects. FHNC was measured four times in each: during constant infusion of alanine alone; alanine superimposed on constant glucose infusion; alanine superimposed on glucose and low stepwise glucagon infusions; and alanine super-imposed on glucose and high constant glucagon infusions. Glucose halved the glucagon response to alanine. This reduction was abolished by the low stepwise glucagon infusion, aimed at re-establishing portal glucagon levels. The high glucagon infusion resulted in 3-fold elevated glucagon levels. During alanine infusion alone FHNC was (mean +/- SEM) 32.5 +/- 1.9 l/h. Glucose reduced FHNC by 43% to 18.4 +/- 0.9 l/h (p < 0.01). The low stepwise glucagon infusion only partially normalized FHNC as reduced by glucose (to 24.6 +/- 1.5 l/h, (p < 0.01 vs alanine alone)). The high glucagon infusion increased FHNC by 35% despite hyperglycaemia (to 44.1 +/- 1.5 l/h, (p < 0.01 vs alanine alone)). The results show that both glucose and glucagon are independent but opposite regulators of hepatic amino-N conversion. The physiological glucose effect is accomplished by a combination of both the effect of glucose itself and the inhibition by glucose of the glucagon response to alanine. Hyperglucagonaemia increases FHNC and overrules the inhibition by glucose. This may explain the defect nitrogen sparing by glucose and to some extent the catabolism in hyperglucagonaemic stress conditions, despite prevailing hyperglycaemia.

Effect of lanreotide, a somatostatin analogue, on urea synthesis in normal man

The aim was to investigate the effect of lanreotide (Angiopeptin) on urea synthesis. Lanreotide is a somatostatin analogue used in therapy trials of certain cancers. Cancer patients are often protein catabolic, thus the effect of lanreotide on whole body protein metabolism is of importance. We investigated the effect of lanreotide by measuring urea nitrogen synthesis rate (UNSR) and blood alpha-amino nitrogen levels before, during and after a 30 min iv infusion of 25 g of an electrolyte-free amino acid solution. 6 healthy male subjects were studied following, i) placebo (saline), ii) lanreotide 5 mug/kg, and iii) lanreotide 80 mug/kg. Lanreotide decreased urea nitrogen synthesis rate (mmol/h) during amino acid infusion significantly compared to saline, independent of dose of lanreotide (max +/- SE of urea nitrogen synthesis rate measurements in each study: 117 +/- 8 mmol/h (saline), 85 +/- 10 mmol/h (high dose) and 85 +/- 12 mmol/h (low dose)). This occurred in spite of significantly higher plasma alpha-amino nitrogen following lanreotide (peak +/- SE of alpha-amino nitrogen level in each study: 3.7 +/- 0.1 mmol/l placebo versus 4.8 +/- 0.2 mmol/l low dose and 4.7 +/- 0.4 mmol/l high dose (p < 0.01). We conclude that a single dose of lanreotide decreases whole body urea nitrogen synthesis rate thereby conserving body protein. The results indicate that long term lanreotide therapy may not lead to further protein catabolism in cancer patients.

Control of non-insulin-dependent diabetes mellitus partially normalizes the increase in hepatic efficacy for urea synthesis

The relation of urea synthesis rate to blood alanine concentration was assessed in seven healthy controls and eight patients with non-insulin-dependent diabetes mellitus (NIDDM) before (hemoglobin A1c [HbA1c] = 9.9% +/- 1.9%, mean +/- SD) and after (HbA1c = 7.9% +/- 0.8%) improvement of metabolic control. Following an overnight fast, alanine was infused at a rate of 2 mmol/(h.kg body weight [BW]). The hourly rate of urea synthesis was determined as the urinary excretion of urea corrected for accumulation of urea in total body water (TBW) and intestinal hydrolysis. The functional hepatic nitrogen clearance (FHNC) was calculated as the slope of the linear relation of urea synthesis rate to blood alanine concentration. The glucagon level was increased by twofold at the first investigation, but was not increased at the second. The insulin level was moderately increased at both investigations. In controls FHNC was 21.8 +/- 4.4 L/h, in poorly controlled patients it was increased to 36.6 +/- 4.3 L/h (P < .01), and following improvement of metabolic control it was not different from control levels at 28.6 +/- 4.3 L/h. By correlation analyses, FHNC was found only to be related to the fasting glucose value, albeit weakly (R2 = .39). In conclusion, hepatic kinetics of urea synthesis in poorly controlled NIDDM patients are changed in favor of increased conversion of alanine N to urea N at any amino acid concentration. This perturbation is partially normalized by improved metabolic control.

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.

Mathematical model to analyse urea synthesis following alanine infusion in control subjects and in patients with liver cirrhosis

A three-compartment model was used to analyse the urea response to an alanine infusion in control subjects and patients with liver cirrhosis. Discriminant analysis showed a good separation between model coefficients of the two groups. A single parameter was derived, able to quantify the liver functional capacity. The method provides a useful diagnostic tool in patients with liver disease.

Increased hepatic efficacy of urea synthesis from alanine in insulin-dependent diabetes mellitus

The relation of urea synthesis rate to blood alanine concentration was assessed in seven healthy controls and in 18 patients with insulin-dependent diabetes mellitus (HbAlc = 8.4 +/- 1.0% (mean +/- SD)). Following an overnight fast alanine was infused at 2 mmol h-1 kg-1 body weight. The hourly rate of urea synthesis was determined as the urinary excretion of urea corrected for accumulation of urea in total body water and intestinal hydrolysis. The functional hepatic nitrogen clearance, i.e. the relation of urea synthesis rate to blood alanine concentration, was calculated as the slope of linear regression of urea synthesis rates on blood alanine concentrations. Fasting glucagon concentrations were 85 +/- 26 ng l-1 in controls and 161 +/- 35 ng l-1 (P less than 0.01) in patients. The functional hepatic nitrogen clearances were 21.8 +/- 4.4 l h-1 in controls and 44.7 +/- 12.4 l h-1 (P less than 0.001) in patients. By multiple step-wise linear regression analysis the functional hepatic nitrogen clearance was found to correlate independently to fasting glucagon concentration, duration of diabetes, change in blood glucose and insulin following alanine infusion (r2 = 0.74). In a simple linear regression analysis the functional hepatic nitrogen clearance correlated strongly to fasting glucagon concentration (r2 = 0.54). In conclusion the kinetics of urea synthesis in insulin-dependent diabetes is changed in favour of increased conversion of alanine-N to urea-N at any blood amino acid concentration. The increased FHNC correlates strongly with hyperglucagonaemia.

Glucagon increases hepatic efficacy for urea synthesis

The effect of glucagon on the relation between urea synthesis and blood amino acid concentration was studied in seven healthy volunteers. Alanine was given as prime-continuous infusions and, after 1 hr for equilibration, the urea nitrogen synthesis rate was measured in two periods of about 2 hrs as urinary excretion corrected for accumulation and intestinal hydrolysis. During one of the periods, glucagon was infused to obtain a constant concentration of 200-1200 ng/l. The spontaneous urea synthesis during the alanine infusion was 86-141 mmol/hr and linearly related to the alanine concentrations of 1.33-2.99 mmol/l. The hepatic clearance of alanine-nitrogen to urea-nitrogen, assessed by the ratio between the increase in the urea synthesis rate and alanine concentration, was 23 +/- 4 l/hr (mean +/- S.D.). Glucagon increased the rate of urea synthesis by 35 +/- 11 mmol/hr (p less than 0.02) and decreased the alanine concentration by 0.22 +/- 0.06 mmol/l (p less than 0.01). Glucagon increased the hepatic nitrogen clearance to an average of 42 +/- 13 l/hr (p less than 0.01). The difference between infusion of amino-nitrogen and appearance of urea-nitrogen was +15 +/- 10 mmol/hr during alanine infusion alone and -11 +/- 25 mmol/hr during exogenous glucagon. The loss of nitrogen could be accounted for by depletion of non-alanine amino acids from the blood. Glucagon increases the efficacy of urea synthesis, which may be of importance for catabolism by changing the hepatic contribution to nitrogen homeostasis.

Regulation of urea production by glucose infusion in vivo

We have investigated the acute in vivo regulation of urea production in normal postabsorptive volunteers by administering a primed constant infusion of 15N2-urea to measure urea production during the constant intravenous infusion of equivalent molar quantities of exogenous nitrogen, given as alanine or glutamine, either with or without a simultaneous infusion of glucose at 4 mg.kg-.min-1. These responses were compared with the response to the infusion of glucose alone. Both amino acid infusions elicited significant (P less than 0.05) and identical (26%) increases in urea production over 4 h. When the glucose infusion was added to the amino acid infusions, urea production remained constant, despite the comparable increases in plasma total nonessential amino nitrogen, as were observed with the amino acid infusions alone. Glucose infused alone elicited a significant (P less than 0.05) reduction (18%) in urea production but no corresponding change in plasma total amino nitrogen. We conclude that 1) infused glucose or its hormonal response suppresses urea production by blunting the normal hepatic ureagenic response to a fixed nitrogen load, 2) this suppressive effect is not mediated via a reduction in substrate (nitrogen) supply, and 3) the inhibition of hepatic gluconeogenesis from amino acids represents one component of this suppressive effect, and direct suppression of urea cycle activity probably represents another component.

Increased capacity of urea synthesis in streptozotocin diabetes in rats

Diabetes was induced in Wistar rats by intravenous streptozotocin, 75 mg/kg. Four and 14 days after streptozotocin, fasting insulin decreased to about one-third, and fasting glucagon increased three-fold. The urea-N synthesis rate, stimulated by infusion of alanine, was measured at different amino acid concentrations 14 days after streptozotocin in 24 rats. The relationship was compatible with a barrier limited substrate inhibition kinetics. Data were examined accordingly by non-linear regression analysis. Among the estimated kinetic constants, only the 70% increase in Vmax was different from control values. In control rats the capacity of urea nitrogen synthesis, as measured within the amino acid concentration interval 7.3-11.6 mmol/l, was 10.2 +/- 1.1 mumol . (min 100 g BW)-1 (mean +/- SEM). The capacity was not different in 4 day diabetic rats, whereas it doubled in 14 day diabetic rats, 20.9 +/- 1.7 mumol (min 100 g BW)-1. The alanine elimination rate was 35% higher in the 14 day diabetic rats compared both to 4 day diabetic and control rats. The increase of urea synthesis is suggested to be due to enzyme induction by glucagon. The net nitrogen balance was negative at amino acid concentrations up to 25 mmol/l, indicating that the urea synthesis was increased at the expense of amino nitrogen.

The capacity of urea-N synthesis as a quantitative measure of the liver mass in rats

The capacity of urea-N synthesis (CUNS), the galactose elimination capacity (GEC) and the antipyrine clearance (APC) were measured in rats immediately after 30, 70 and 90% partial hepatectomy and after sham operation. CUNS was assessed during alanine infusion as urea accumulation in total body water, corrected for intestinal hydrolysis, and GEC was measured during constant galactose infusion in the same animals. APC was determined by the one-sample method in a separate group of animals, treated similarily. CUNS, GEC and APC were all correlated to the liver weight with correlation coefficients (r) above 0.8 and the correlation coefficients (r) between CUNS, GEC and APC were all above 0.7. It is concluded that CUNS is a quantitative measure of the functional liver mass in the rat.

A method for determination of the capacity of urea synthesis in the rat

The relationship between total blood alpha-amino nitrogen concentration and urea synthesis rate was investigated with alanine as nitrogen source in 24 rats. Alanine was given as prime-continuous doses for 70 min so that constant amino acid concentration was attained between 5.5 and 34 mmol/l. Urea synthesis rate was assessed as accumulation in body water, corrected for intestinal hydrolysis. There was a positive correlation between nitrogen balance and alpha-amino nitrogen concentration. Urea synthesis rate in relation to amino acid concentration suggested barrier-limited substrate inhibition kinetics and data were examined accordingly by non-linear regression analysis. The estimated kinetic constants (mean +/- standard deviation) were: Vmax: 19.2 +/- 3.3 mumol (min X 100 g BW)-1, Km: 1.74 +/- 0.5 mmol/l, Ki: 6.84 +/- 1.9 mmol/l, and the barrier: 5.4 +/- 0.13 mmol/l. Because of the substrate inhibition, saturation cannot be attained, but the maximum synthesis rate, i.e. the capacity of urea nitrogen synthesis (CUNS), can be measured within 95% of the theoretical maximum in the concentration interval 7.3-11.6 mmol/l. CUNS was 9.16 +/- 0.81 mumol (min X 100 g BW)-1 (mean +/- standard deviation). Substrate-independent regulation of urea synthesis, e.g. by changes in liver mass or hormonal concentration, can be studied by this measure.