In vivo whole body and organ arginine metabolism during endotoxemia (sepsis) is dependent on mouse strain and gender

Arginine metabolism involves various organs such as the kidney, the intestines, and the liver, which act together in an interorgan axis. Major pathways for arginine production are protein breakdown and de novo arginine production from citrulline; disposal of arginine is mainly used for protein synthesis or used by the enzymes arginase and nitric oxide synthase (NOS). To assess in vivo organ arginine metabolism under normal conditions and during endotoxemia we used a mouse model, and analyzed for gender and strain differences. Male and female inbred FVB and C57BL6/J mice were anesthetized and catheterized to study whole body, gut, liver, renal and muscle metabolism, using a stable isotope infusion protocol. Animals were treated with saline or lipopolysaccharide. Plasma arginine levels tended to be higher in female mice, although levels were not significantly different from male mice (P = 0.09). Although not all significantly different, whole body arginine production and arginine clearance tended to be higher in C57BL6/J mice (P < 0.1), while citrulline (P = 0.05), NO (P = 0.08), and de novo arginine (P < 0.01) production were higher in FVB mice. During endotoxemia, NO production increased in general (P < 0.05), while whole body arginine clearance increased in FVB mice, but decreased in C57BL6/J mice (P < 0.01). At the organ level, portal-drained viscera (PDV) arginine metabolism was higher in FVB than in C57BL6/J mice (P < 0.05). During endotoxemia, liver arginine metabolism decreased in general (P < 0.05), while strain differences existed for PDV, muscle, and renal arginine metabolism. In conclusion, stable isotope techniques in multicatheterized mice allow measurements of arginine metabolism on whole body and organ level. Strain and gender differences are present in arginine metabolism under physiological conditions and during endotoxemia.

Reduced arginine availability and nitric oxide production

The precursor for nitric oxide (NO) synthesis is the amino acid arginine. Reduced arginine availability may limit NO production. Arginine availability for NO synthesis may be regulated by de novo arginine production from citrulline, arginine transport across the cell membrane, and arginine breakdown by arginase.

Differential effects of selective and non-selective NOS inhibition on renal arginine and protein metabolism during endotoxemia in rats

BACKGROUND AND AIMS

The kidney is the main endogenous producer of circulating arginine. Renal arginine disposal is directed to protein synthesis, urea production and nitric oxide synthesis. The administration of nitric oxide synthase inhibitors during sepsis may be beneficial or detrimental depending on the specificity of the inhibitor. We aimed to measure the effects of two NOS inhibitors, with different specificity, on renal arginine and protein turnover in a rat model of sepsis.

METHODS

Rats were subject to double hit endotoxemia and either L-NAME (non-specific), SMT (iNOS specific) or saline. Under anesthesia, vessels supplying and draining the kidney were catheterized. Systemic and intra-renal arginine and protein metabolism were measured using a primed continuous infusion of L-[2,3-(3)H]arginine and L-[2,6-(3)H]phenylalanine.

RESULTS

Non-specific NOS reduced systemic protein and arginine turnover, whereas selective iNOS inhibition did not. In the kidney, blood flow was reduced by L-NAME, but not by SMT. In conjunction with this, non-selective NOS inhibition increased renal protein breakdown, whereas selective iNOS inhibition increased renal arginine production.

CONCLUSIONS

This study shows that non-selective NOS inhibition using L-NAME is detrimental for systemic and renal protein metabolism. Selective NOS inhibition stimulates renal arginine synthesis, without changing circulating arginine levels.

Overexpression of arginase alters circulating and tissue amino acids and guanidino compounds and affects neuromotor behavior in mice

Arginine is an intermediate of the ornithine cycle and serves as a precursor for the synthesis of nitric oxide, creatine, agmatine and proteins. It is considered to be a conditionally essential amino acid because endogenous synthesis only barely meets daily requirements. In rapidly growing suckling neonates, endogenous arginine biosynthesis is crucial to compensate for the insufficient supply of arginine via the milk. Evidence is accumulating that the intestine rather than the kidney plays a major role in arginine synthesis in this period. Accordingly, ectopic expression of hepatic arginase in murine enterocytes by genetic modification induces a selective arginine deficiency. The ensuing phenotype, whose severity correlates with the level of transgene expression in the enterocytes, could be reversed with arginine supplementation. We analyzed the effect of arginine deficiency on guanidine metabolism and neuromotor behavior. Arginine-deficient transgenic mice continued to suffer from an arginine deficiency after the arginine biosynthetic enzymes had disappeared from the enterocytes. Postweaning catch-up growth in arginine-deficient mice was characterized by increased levels of all measured amino acids except arginine. Furthermore, plasma total amino acid concentration, including arginine, was significantly lower in adult male than in adult female transgenic mice. Decreases in the concentration of plasma and tissue arginine led to significant decreases in most metabolites of arginine. However, the accumulation of the toxic guanidino compounds, guanidinosuccinic acid and methylguanidine, corresponded inversely with circulating arginine concentration, possibly reflecting a higher oxidative stress under hypoargininemic conditions. In addition, hypoargininemia was associated with disturbed neuromotor behavior, although brain levels of toxic guanidino compounds and ammonia were normal.

Metabolic flux measurements across portal drained viscera, liver, kidney and hindquarter in mice

A method was developed to measure metabolic fluxes across either portally-drained viscera (PDV) and liver or kidney and hindquarter (HQ) in anesthetized mice. The method includes a primed-constant infusion of ketamine-medetomidine anaesthesia to stabilize the mice for the surgical procedures. For measurement of metabolic fluxes across PDV and liver, blood sampling catheters were inserted in the carotid artery, portal vein and hepatic vein and infusion catheters in the jugular vein and mesenteric vein. For measurement of metabolic flux across kidney and HQ, blood sampling catheters were inserted in the carotid artery, renal vein and caval vein and infusion catheters in the jugular vein and abdominal aorta. 14C-PAH was infused to enable plasma flow measurement using an indicator dilution method. In addition, we developed a blood sampling procedure without waste of blood. We measured plasma flow and metabolic fluxes across PDV, liver, kidney and HQ. Mean plasma flow in post-absorptive mice was: PDV: 0.9+/-0.2, liver: 1.2+/-0.3, kidney: 1.0+/-0.1, HQ: 1.1+/-0.3 ml/10 g body weight (b.w.)/min. Significant glutamine release by the HQ and uptake of glutamine by the kidney and PDV was observed. In PDV, citrulline is produced from glutamine and is in turn used by the kidney for the production of arginine. In conclusion, the described model enables measurement of metabolic fluxes across PDV, liver, kidney and HQ in mice. The availability of such a small animal model allows the potential for measuring metabolic parameters in transgenic and knockout mice, and therefore may lead to an important refinement in metabolic research.

Renal amino acid metabolism during endotoxemia in the rat

BACKGROUND

The kidney has an important function in the exchange of nitrogenous metabolites. Glutamine is the most important substrate for renal ammoniagenesis and thus plays a crucial role in acid-base homeostasis. Furthermore, the kidney is the main endogenous source for de novo arginine production from citrulline, which in turn is derived from intestinal glutamine metabolism. Sepsis is a condition in which glutamine availability is reduced, whereas the need for arginine biosynthesis may be increased. Limited bioavailability of glutamine may affect arginine synthesis, which may have consequences for nitric oxide (NO) synthesis. Therefore, we studied renal glutamine and arginine metabolism in a rat model of endotoxemia and related this to NO metabolism.

MATERIALS AND METHODS

Rats were subject to double hit endotoxemia, and control rats received 0.9% NaCl. Renal blood flow was measured using para-aminohippuric acid. Concentrations of plasma amino acids and nitrate were measured in the aorta and renal vein to calculate net renal uptake or release of amino acids and address NO production.

RESULTS

The arterial concentrations of glutamine and ammonia were not changed in endotoxemic rats. Although renal glutamine uptake was reduced, total renal ammonia production was not changed during endotoxemia. The arterial concentration of citrulline and renal citrulline uptake was not altered in endotoxin-treated rats, but renal arginine production was increased. However, no effect was observed on nitric oxide production.

CONCLUSIONS

Although the kidney has very important functions in the excretion of waste products and in interorgan metabolism, this study suggests that the kidney has a limited role in glutamine, arginine, and NO metabolism during late endotoxemia in rats.

Tracer methodology in whole body and organ balance metabolic studies: plasma sampling is required. A study in post-absorptive rats using isotopically labeled arginine, phenylalanine, valine and leucine

BACKGROUND AND AIMS

Radioactive and stable amino acid isotopes are frequently used in metabolic research. Blood cells contain amino acid transporters, which may influence tracer distribution in blood. The aim of this study was to determine whether plasma or whole blood specific activity or enrichment of amino acid tracers should be used in the calculation of whole body and organ production rates.

METHODS

Seven male Wistar rats were infused with L-[2,3-(3)H]-Arginine, L-[2, 6-(3)H]-Phenylalanine, L-[3,4-(3)H]-Valine, and [L-[4,5-(3)H]-Leucine. Whole body and portal drained visceral, hepatic and renal production rates of arginine, phenylalanine, valine and leucine were determined in plasma and in whole blood.

RESULTS

Amino acid tracers that equilibrate well between plasma and blood cells (for instance phenylalanine, valine and leucine) yield similar whole body production rates when whole blood or plasma is sampled. Also, organ production rates measured using these amino acid tracers are consistent. However, a discrepancy exists between the whole body production rate and the sum of PDV, hepatic and renal production rates. When tracers are used that do not equilibrate well between plasma and blood cells (for instance arginine) the use of whole blood specific activity in the calculations yield overestimations of whole body and organ production rates.

CONCLUSION

From our data we recommend plasma sampling and strongly advise against whole blood sampling in metabolic organ balance studies in which amino acid tracers are used.

Increased lactulose/rhamnose ratio during fluid load is caused by increased urinary lactulose excretion

Noninvasive assessment of intestinal permeability in vivo is based on the measurement of urinary excretion of orally administered sugar probes. It is expressed as a ratio, usually lactulose/rhamnose or 3-O-methyl-D-glucose (3-OMG)/rhamnose. In both endotoxemic and control rats that were receiving fluid, we observed an increase in the recovery of lactulose and 3-OMG but not rhamnose in both groups, suggesting an enhancement of intestinal permeability. In the measurement of intestinal permeability, all pre- and postmucosal factors are considered equal for all sugars. We hypothesized that postmucosal factors and not changes in intestinal permeability caused the increased urinary lactulose and 3-OMG recoveries observed during fluid loading. Therefore, the effects of fluid loading on urinary excretion of the sugar probes were studied in healthy rats receiving the sugars intravenously. After intravenous injection, fluid loading increased urinary lactulose recovery threefold but not that of 3-OMG and rhamnose. In conclusion, fluid loading increases the lactulose/rhamnose ratio independent of changes in intestinal permeability. The 3-OMG/rhamnose ratio is not influenced by fluid loading.