Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat

Agmatine stimulates hepatic fatty acid oxidation: a possible mechanism for up-regulation of ureagenesis

We demonstrated previously in a liver perfusion system that agmatine increases oxygen consumption as well as the synthesis of N-acetylglutamate and urea by an undefined mechanism. In this study our aim was to identify the mechanism(s) by which agmatine up-regulates ureagenesis. We hypothesized that increased oxygen consumption and N-acetylglutamate and urea synthesis are coupled to agmatine-induced stimulation of mitochondrial fatty acid oxidation. We used 13C-labeled fatty acid as a tracer in either a liver perfusion system or isolated mitochondria to monitor fatty acid oxidation and the incorporation of 13C-labeled acetyl-CoA into ketone bodies, tricarboxylic acid cycle intermediates, amino acids, and N-acetylglutamate. With [U-13C16] palmitate in the perfusate, agmatine significantly increased the output of 13C-labeled beta-hydroxybutyrate, acetoacetate, and CO2, indicating stimulated fatty acid oxidation. The stimulation of [U-13C16]palmitate oxidation was accompanied by greater production of urea and a higher 13C enrichment in glutamate, N-acetylglutamate, and aspartate. These observations suggest that agmatine leads to increased incorporation and flux of 13C-labeled acetyl-CoA in the tricarboxylic acid cycle and to increased utilization of 13C-labeled acetyl-CoA for synthesis of N-acetylglutamate. Experiments with isolated mitochondria and 13C-labeled octanoic acid also demonstrated that agmatine increased synthesis of 13C-labeled beta-hydroxybutyrate, acetoacetate, and N-acetylglutamate. The current data document that agmatine stimulates mitochondrial beta-oxidation and suggest a coupling between the stimulation of hepatic beta-oxidation and up-regulation of ureagenesis. This action of agmatine may be mediated via a second messenger such as cAMP, and the effects on ureagenesis and fatty acid oxidation may occur simultaneously and/or independently.

Biosynthesis of agmatine in isolated mitochondria and perfused rat liver: studies with 15N-labelled arginine

An important but unresolved question is whether mammalian mitochondria metabolize arginine to agmatine by the ADC (arginine decarboxylase) reaction. 15N-labelled arginine was used as a precursor to address this question and to determine the flux through the ADC reaction in isolated mitochondria obtained from rat liver. In addition, liver perfusion system was used to examine a possible action of insulin, glucagon or cAMP on a flux through the ADC reaction. In mitochondria and liver perfusion, 15N-labelled agmatine was generated from external 15N-labelled arginine. The production of 15N-labelled agmatine was time- and dose-dependent. The time-course of [U-15N4]agmatine formation from 2 mM [U-15N4]arginine was best fitted to a one-phase exponential curve with a production rate of approx. 29 pmol x min(-1) x (mg of protein)(-1). Experiments with an increasing concentration (0- 40 mM) of [guanidino-15N2]arginine showed a Michaelis constant Km for arginine of 46 mM and a Vmax of 3.7 nmol x min(-1) x (mg of protein)(-1) for flux through the ADC reaction. Experiments with broken mitochondria showed little changes in Vmax or Km values, suggesting that mitochondrial arginine uptake had little effect on the observed Vmax or Km values. Experiments with liver perfusion demonstrated that over 95% of the effluent agmatine was derived from perfusate [guanidino-15N2]arginine regardless of the experimental condition. However, the output of 15N-labelled agmatine (nmol x min(-1) x g(-1)) increased by approx. 2-fold (P<0.05) in perfusions with cAMP. The findings of the present study provide compelling evidence that mitochondrial ADC is present in the rat liver, and suggest that cAMP may stimulate flux through this pathway.

Agmatine attenuates stress- and lipopolysaccharide-induced fever in rats

Physiological stress evokes a number of responses, including a rise in body temperature, which has been suggested to be the result of an elevation in the thermoregulatory set point. This response seems to share similar mechanisms with infectious fever. The aim of the present study was to investigate the effect of agmatine on different models of stressors [(restraint and lipopolysaccharide (LPS)] on body temperature. Rats were either restrained for 4 h or injected with LPS, both of these stressors caused an increase in body temperature. While agmatine itself had no effect on body temperature, treatment with agmatine (20, 40, 80 mg/kg intraperitoneally) dose dependently inhibited stress- and LPS-induced hyperthermia. When agmatine (80 mg/kg) was administered 30 min later than LPS (500 microg/kg) it also inhibited LPS-induced hyperthermia although the effect became significant only at later time points and lower maximal response compared to simultaneous administration. To determine if the decrease in body temperature is associated with an anti-inflammatory effect of agmatine, the nitrite/nitrate levels in plasma was measured. Agmatine treatment inhibited LPS-induced production of nitrates dose dependently. As an endogenous molecule, agmatine has the capacity to inhibit stress- and LPS-induced increases in body temperature.

Effect of agmatine on the development of morphine dependence in rats: potential role of cAMP system

Agmatine is an endogenous amine derived from arginine that potentiates morphine analgesia and blocks symptoms of naloxone-precipitated morphine withdrawal in rats. In this study, we sought to determine whether treatment with agmatine during the development of morphine dependence inhibits the withdrawal symptoms and that the effect is mediated by cAMP system. Exposure of rats to morphine for 7 days resulted in marked naloxone-induced withdrawal symptoms and agmatine treatment along with morphine significantly decreasing the withdrawal symptoms. The levels of cAMP were markedly increased in morphine-treated rat brain slices when incubated with naloxone and this increase was significantly reduced in rats treated with morphine and agmatine. The induction of tyrosine hydroxylase after morphine exposure was also reduced in locus coeruleus when agmatine was administered along with morphine. We conclude that agmatine reduces the development of dependence to morphine and that this effect is probably mediated by the inhibition of cAMP signaling pathway during chronic morphine exposure.

Effects of agmatine accumulation in human colon carcinoma cells on polyamine metabolism, DNA synthesis and the cell cycle

Putrescine, spermidine and spermine are low molecular polycations that play important roles in cell growth and cell cycle progression of normal and malignant cells. Agmatine (1-amino-4-guanidobutane), another polyamine formed through arginine decarboxylation, has been reported to act as an antiproliferative agent in several non-intestinal mammalian cell models. Using the human colon adenocarcinoma HT-29 Glc(-/+) cell line, we demonstrate that agmatine, which markedly accumulated inside the cells without being metabolised, exerted a strong cytostatic effect with an IC50 close to 2 mM. Agmatine decreased the rate of L-ornithine decarboxylation and induced a 70% down-regulation of ornithine decarboxylase (ODC) expression. Agmatine caused a marked decrease in putrescine and spermidine cell contents, an increase in the N1-acetylspermidine level without altering the spermine pool. We show that agmatine induced the accumulation of cells in the S and G2/M phases, reduced the rate of DNA synthesis and decreased cyclin A and B1 expression. We conclude that the anti-metabolic action of agmatine on HT-29 cells is mediated by a reduction in polyamine biosynthesis and induction in polyamine degradation. The decrease in intracellular polyamine contents, the reduced rate of DNA synthesis and the cell accumulation in the S phase are discussed from a causal perspective.

Mitochondrial expression of arginase II in male and female rat inner medullary collecting ducts

Microdissected rat proximal straight tubules (PST) and inner medullary collecting ducts (IMCD) highly produce urea from l-arginine, supporting the expression of the mitochondrial arginase II. However, IMCD contain a very low density of mitochondria compared with PST. Recently, arginase II has been localized by immunohistochemistry in rat PST but not IMCD. This study was designed to verify whether rat IMCD express arginase II and to identify its subcellular localization. We developed an antibody raised against arginase II that allowed the detection of a band of 38 kDa corresponding to arginase II on immunoblots. In male and female rat kidneys, Western blot analyses revealed that arginase II was highly expressed in the inner medulla (IM), the outer stripe of the outer medulla (osOM), and the deep cortex. Immunocytochemistry demonstrated that arginase II was homogeneously expressed in IMCD. Proteins of the cytosolic and mitochondrial fractions extracted from osOM and IM and analyzed by Western blot showed that 86% of arginase II was associated with mitochondria. The molecular weight of arginase II was similar in the cytosolic and mitochondrial fractions. Immunoelectron microscopy confirmed the presence of arginase II in the mitochondria of IMCD. In conclusion, arginase II is expressed in mitochondria of male and female rat IMCD.

Localization and differential expression of arginase II in the kidney of male and female mice

Arginase II (AII) has been almost exclusively studied in male mammalian kidneys. Our investigations were conducted to localize AII gene expression in the female mouse kidney, and to analyze the differential expression of AII gene at the transcriptional and translational levels in the kidneys of female and male mice. Total RNAs and soluble proteins extracted from renal zones and whole kidneys were analyzed by Northern and Western blots, respectively. Mitochondrial and cytosolic proteins were analyzed by Western blot. L-[guanidino-14C]arginine hydrolysis by AII was detected in microdissected tubules and the 14CO2 released from [14C]urea hydrolysis was quantified. The results of these experiments showed that: (1) both AII mRNA and protein were highly expressed in the deep cortex and the outer stripe of the outer medulla, (2) urea was produced mainly in the proximal straight tubules (PST), (3) the 38-kDa AII protein was more abundant in the mitochondria than the cytosol, and (4) the renal content of AII mRNA and protein was about three-fold higher in female than in male mice. In conclusion, in both genders, AII gene expression is restricted to the PST and localized into mitochondria. AII gene is differentially expressed in the kidney of female and male mice since higher levels of AII mRNA, protein and activity were observed in the kidneys of the former than those of the latter. Renal AII gene expression was gender-dependent in mice but not in rats. Finally, in the PST of females, L-arginine-derived ornithine may be a precursor for the renal production of L -glutamate and L-glutamine because high levels of AII, ornithine aminotransferase and glutamine synthetase are expressed in this nephron segment.

Crystal Structure of Agmatinase Reveals Structural Conservation and Inhibition Mechanism of the Ureohydrolase Superfamily

Agmatine is the product of arginine decarboxylation and can be hydrolyzed by agmatinase to putrescine, the precursor for biosynthesis of higher polyamines, spermidine, and spermine. Besides being an intermediate in polyamine metabolism, recent findings indicate that agmatine may play important regulatory roles in mammals. Agmatinase is a binuclear manganese metalloenzyme and belongs to the ureohydrolase superfamily that includes arginase, formiminoglutamase, and proclavaminate amidinohydrolase. Compared with a wealth of structural information available for arginases, no three-dimensional structure of agmatinase has been reported. Agmatinase from Deinococcus radiodurans, a 304-residue protein, shows approximately 33% of sequence identity to human mitochondrial agmatinase. Here we report the crystal structure of D. radiodurans agmatinase in Mn(2+)-free, Mn(2+)-bound, and Mn(2+)-inhibitor-bound forms, representing the first structure of agmatinase. It reveals the conservation as well as variation in folding, oligomerization, and the active site of the ureohydrolase superfamily. D. radiodurans agmatinase exists as a compact homohexamer of 32 symmetry. Its binuclear manganese cluster is highly similar but not identical to the clusters of arginase and proclavaminate amidinohydrolase. The structure of the inhibited complex reveals that inhibition by 1,6-diaminohexane arises from the displacement of the metal-bridging water.

Regulation of fibronectin levels by agmatine and spermine in mesangial cells under high-glucose conditions

Amines such as agmatine, putrescine, spermidine and spermine have been reported to be involved in a variety of physiological and biochemical phenomena. However, it is not known whether they are also involved in the homeostasis of intracellular fibronectin content via upregulation of protein kinase C (PKC), extracellular signal-regulated kinase (ERK), and transforming growth factor-beta1 (TGF-beta1). To determine this, we have studied the effect of multiple amines on fibronectin, TGF-beta1, ERK, and PKC levels in mesangial cells under high glucose conditions. All the amines tested (at 0.1-1 mM) affected neither the viability of mesangial cells for 42 h nor LDH release into the medium. Agmatine reduced TGF-beta1 and ERK levels but not PKC at concentrations of 0.1-1 mM. However, levels of fibronectin, TGF-beta1, ERK, and PKC were unaffected by either putrescine or spermidine. A decrease in fibronectin secretion was accompanied by decreases in TGF-beta1 and ERK. Such cumulative results lead us to hypothesize that agmatine reduces high glucose-induced fibronectin secretion via several pathways including ERK-TGF-beta1-fibronectin and spermine, via a decrease in TGF-beta1. Possible roles of enzymes involved in agmatine and polyamine biosynthesis are discussed in relation to secretion of ECM proteins.

Putrescine biosynthesis in mammalian tissues

L-ornithine decarboxylase provides de novo putrescine biosynthesis in mammals. Alternative pathways to generate putrescine that involve ADC (L-arginine decarboxylase) occur in non-mammalian organisms. It has been suggested that an ADC-mediated pathway may generate putrescine via agmatine in mammalian tissues. Published evidence for a mammalian ADC is based on (i) assays using mitochondrial extracts showing production of 14CO2 from [1-14C]arginine and (ii) cloned cDNA sequences that have been claimed to represent ADC. We have reinvestigated this evidence and were unable to find any evidence supporting a mammalian ADC. Mitochondrial extracts prepared from freshly isolated rodent liver and kidney using a metrizamide/Percoll density gradient were assayed for ADC activity using L-[U-14C]-arginine in the presence or absence of arginine metabolic pathway inhibitors. Although 14CO2 was produced in substantial amounts, no labelled agmatine or putrescine was detected. [14C]Agmatine added to liver extracts was not degraded significantly indicating that any agmatine derived from a putative ADC activity was not lost due to further metabolism. Extensive searches of current genome databases using non-mammalian ADC sequences did not identify a viable candidate ADC gene. One of the putative mammalian ADC sequences appears to be derived from bacteria and the other lacks several residues that are essential for decarboxylase activity. These results indicate that 14CO2 release from [1-14C]arginine is not adequate evidence for a mammalian ADC. Although agmatine is a known constituent of mammalian cells, it can be transported from the diet. Therefore L-ornithine decarboxylase remains the only established route for de novo putrescine biosynthesis in mammals.

Intestinal tumor and agmatine (decarboxylated arginine): low content in colon carcinoma tissue specimens and inhibitory effect on tumor cell proliferation in vitro

BACKGROUND

The polyamine system is a promising target for anticancer therapy. Ideally, an antineoplastic compound affecting this system should inhibit both ornithine decarboxylase and the polyamine transporter, and toxicity should be mild. Agmatine, decarboxylated L-arginine, appears to be such a compound.

METHODS

Adenosine triphosphate levels and the protein content of cell populations in culture were identified as surrogate markers for cell count. Agmatine content in cells and tissue specimens was measured by high-performance liquid chromatography. Antizyme levels were estimated by Western blotting.

RESULTS

Agmatine inhibited the proliferation of six human intestinal tumor cell lines in a concentration-dependent manner; this inhibition probably was attributable to an interaction between agmatine and the intracellular polyamine system. Consistent with the inverse relation between cell proliferation and agmatine concentration was the finding that agmatine content in human colon carcinoma tissue was approximately one-half as great as it was in adjacent macroscopically normal tissue.

CONCLUSIONS

The results of the current study were compatible with the hypothesis that agmatine possesses antineoplastic action against intestinal tumor cells. It is likely that this activity is attributable to agmatine's regulatory role in polyamine homeostasis.

Agmatine signaling: odds and threads

Agmatine is a metabolite of L-arginine. It is formed by the decarboxylation of L-arginine via arginine decarboxylase in bacteria, plants and mammals. It is becoming clear that it has multiple physiological functions as a potential transmitter. Agmatine binds to alpha2-adrenoceptors and to imidazoline binding sites. It blocks NMDA receptors and other ligand-gated cation channels. It also inhibits nitric oxide synthase, induces release of peptide hormones and antizyme and plays a role during cell proliferation by interacting with the generation and transport of polyamines. Although the precise function of endogenously released agmatine is presently still unclear, this review will summarize several aspects concerning the biological function of agmatine.

L-arginine as a therapeutic tool in kidney disease

Infusion of L-arginine in experimental animals increases renal plasma flow (RPF) and glomerular filtration rate (GFR). It is likely that a component of these hemodynamic changes are mediated by nitric oxide (NO) as suggested by studies with specific antagonists of L-arginine metabolism. L-arginine administration ameliorates the infiltration of the renal parenchyma by macrophages in rats with obstructive nephropathy or rats with puromycin-induced nephrotic syndrome. L-arginine administration also blunts the increase in interstitial volume, collagen IV, and alpha-smooth muscle actin. Rats with a remnant kidney given 1% L-arginine in the drinking water had a greater GFR and RPF. L-arginine administration also decreased proteinuria. Diabetic rats given L-arginine had significantly lower excretion of protein and cyclic guanosine monophosphate than diabetic rats not receiving L-arginine. Despite persistent hyperglycemia, the administration of L-arginine prevented the development of hyperfiltration and ameliorated proteinuria in diabetic rats. In the setting of ischemic acute renal failure, the administration of L-arginine had a beneficial effect on GFR and RPF, decreased O2- production, diminished up-regulation of soluble guanylate cyclase, and prevented up-regulation of inducible NO synthase (iNOS). The pharmacokinetics of L-arginine indicate that side effects are rare and mostly mild and dose dependent.

Modulation by alpha-difluoromethyl-ornithine and aminoguanidine of pain threshold, morphine analgesia and tolerance

The effects of alpha-difluoromethyl-ornithine (DFMO) and aminoguanidine, which might influence the metabolism of endogenous agmatine, on pain threshold, morphine analgesia and tolerance were investigated in mice. In the mouse acetic acid writhing test, intracerebroventricular (i.c.v.) injection of DFMO or aminoguanidine significantly elevated the pain threshold as indicated by a decrease in the number of writhings. DFMO or aminoguanidine obviously increased the analgesic effect of morphine in the mouse acetic acid writhing test and the mouse heat radiation tail-flick assay. These effects of DFMO and aminoguanidine were antagonized by idazoxan (3 mg/kg, i.p.), which is a selective antagonist of the imidazoline receptor. In the mouse heat radiation tail-flick assay, aminoguanidine significantly prolonged the tail-flick latency of animals, suggesting that the pain threshold was elevated. Furthermore, both DFMO and aminoguanidine enhanced morphine analgesia and inhibited acute morphine tolerance in the mouse heat radiation tail-flick assay. Neither DFMO nor aminoguanidine inhibited the activity of nitric oxide synthase in different brain areas in mice in vivo. These results indicate that the substances involved in the metabolism of endogenous agmatine could modulate the pain threshold, morphine analgesia and tolerance, indicating the possible role of endogenous agmatine in the pharmacological effects of morphine.

Expression of human arginine decarboxylase, the biosynthetic enzyme for agmatine

Agmatine, an amine formed by decarboxylation of L-arginine by arginine decarboxylase (ADC), has been recently discovered in mammalian brain and other tissues. While the cloning and sequencing of ADC from plant and bacteria have been reported extensively, the structure of mammalian enzyme is not known. Using homology screening approach, we have identified a human cDNA clone that exhibits ADC activity when expressed in COS-7 cells. The cDNA and deduced amino acid sequence of this human ADC clone is distinct from ADC of other forms. Human ADC is a 460-amino acid protein that shows about 48% identity to mammalian ornithine decarboxylase (ODC) but has no ODC activity. While naive COS-7 cells do not make agmatine, these cells are able to produce agmatine, as measured by HPLC, when transfected with ADC cDNA. Northern blot analysis using the cDNA probe indicated the expression of ADC message in selective human brain regions and other human tissues.

Agmatine inhibits the proliferation of rat hepatoma cells by modulation of polyamine metabolism

BACKGROUND/AIMS

Previous experiments have shown that agmatine, the product of arginine decarboxylase, is transported in competition with putrescine into quiescent rat hepatocytes, where it promotes several effects, including marked decrease of intracellular polyamines and induction of apoptosis. The primary aim of the present study was to assess the action of agmatine on transformed and proliferating hepatic rat cells.

METHODS

To assess the effect of agmatine on hepatoma cells, analysis by flow cytometry, Western blotting, reverse transcription-polymerase chain reaction, scanning and transmission electron microscopy, immunofluorescence detection of beta-actin and alpha-tubulin were performed.

RESULTS

The results showed that agmatine has antiproliferative effects on the cell lines studied (HTC, JM2, HepG2). Further experiments were performed on HTC cells. The effect was proportional to agmatine concentration (in a range between 50 and 500 microM). It was not correlated with induction of necrosis or apoptosis and was accompanied by accumulation in G(2)/M cell cycle phase and by dramatic modification of cell morphology. Spermidine reversed these effects, suggesting that the marked decrease of the polyamine pool is the main target of agmatine .

CONCLUSIONS

The results obtained show a relationship between the decrease of intracellular polyamine content, the rate of cell growth and the cytoskeleton organization.

Agmatine induces glutamate release and cell death in cultured rat cerebellar granule neurons

We investigated the effect of agmatine on cell viability of rat cerebellar granule neurons in a high-K+ (27.5 mM) medium. Exposure of cultured rat cerebellar granule neurons to agmatine (200-800 microM) resulted in a significant decrease in cell viability. Agmatine-induced neuronal death began to occur 6-12 h after addition, and gradually progressed. The agmatine neurotoxicity was attenuated by N-methyl-D-aspartate (NMDA) receptor antagonists and by enzymatic degradation of L-glutamate with glutamic pyruvic transaminase. Furthermore, a significant increase in extracellular L-glutamate concentration was detected before cell death occurred. In addition, agmatine-induced glutamate release and cell death were both blocked by pretreatment with botulinum toxin C, which is known to specifically inhibit the exocytosis. The agmatine neurotoxicity was not observed when extracellular K+ concentration was lower (10 mM). These results suggest that agmatine induces glutamate release through the exocytosis and thereby causes NMDA receptor-mediated neuronal death in conditions in which extracellular K+ concentrations are elevated.

Biogenic amines and polyamines: similar biochemistry for different physiological missions and biomedical applications

Biogenic amines are organic polycations derived from aromatic or cationic amino acids. All of them have one or more positive charges and a hydrophobic skeleton. Nature has evolved these molecules to play different physiological roles in mammals, but maintains similar patterns for their metabolic and intracellular handling. As deduced from this review, many questions still remain to be solved around their biochemistry and molecular biology, blocking our aims to control the relevant pathologies in which they are involved (cancer and immunological, neurological, and gastrointestinal diseases). Advances in this knowledge are dispersed among groups working on different biomedical areas. In these pages, we put together the most relevant information to remark how fruitful it can be to learn from Nature and to take advantage of the biochemical similarities (key protein structures and their regulation data on metabolic interplays and binding properties) to generate new hypothesis and develop different biomedical strategies based on biochemistry and molecular biology of these compounds.

Signal transduction by histamine in the cerebellum and its modulation by N-methyltransferase

Histamine has been suggested to have roles as a neurotransmitter or a neuromodulator. Direct fiber connections between the hypothalamus and the cerebellum have recently been demonstrated and it is suggested that the cerebellum is involved in the control of autonomic and emotional functions. These fibers include histaminergic fibers. The components of histaminergic signal transmission are demonstrated in the cerebellum as follows: (1) the histaminergic fibers are visualized immunohistochemically in the cerebellar cortex of rat, guinea pig and human; (2) histamine H1 receptors are visualized by autoradiographic studies in the molecular layer of mouse and guinea pig. In situ hybridization study also detects the expression of H1 receptors in the Purkinje cells. H2 receptors are expressed in the Purkinje cells and granule cells of guinea pig; and (3) the application of histamine to the slices of guinea pig or rat cerebellar cortex elicits an increase in the turnover of phosphoinositides, so H1 receptors in the cerebellum are functional. Additionally, we have recently shown in the guinea pig that Purkinje cells express one of the histamine inactivating enzymes, and that inhibition of this enzyme enhances phosphoinositide turnover by histamine. Therefore, all the components of histaminergic neurotransmission are demonstrated in the cerebellum. These data suggest that histamine is involved in the signal transmission from the hypothalamus to the cerebellum. Here we review each component of histaminergic neurotransmission in the cerebellum.

Influence of dietary arginine on sexual dimorphism of arginine metabolism in mice

We have studied the influence of dietary arginine on tissue arginine content, and arginine metabolism in CD1 mice. Dietary arginine restriction produced by feeding mice with a low arginine diet (0.06%) produced a marked decrease in arginine concentrations in the plasma, skeletal muscle and kidney of female mice (72%, 67% and 54%, respectively) while in male mice the decreases were smaller (58% in blood and 18% in the skeletal muscle). This diet abolished not only the sexual dimorphism in arginine content observed in mice fed with the diet containing 1% arginine, but also reduced renal activities of arginase and nitric oxide synthase in the female mice and ornithine decarboxylase and the decarboxylation of arginine in the male mice. Urinary putrescine excretion was dramatically reduced by arginine restriction in the male mice whereas orotic acid excretion increased about 30 fold in both sexes; urea and creatinine excretion did not change. Taken together our results indicate that dietary arginine plays a relevant role in the maintenance of the sexual dimorphism in arginine content and arginine metabolism in CD1 mice, and that this may have physiological significance because of the important effects that arginine-derived products exert on a variety of cellular processes.

Agmatine reduces hydrogen peroxide in mesangial cells under high glucose conditions

Agmatine, an amine and organic cation, reduced H(2)O(2) that was generated by hyperglycemia, and transcription factors such as NF-kappaB and AP-1 activity in the mesangial cells that were exposed to high glucose. However, spermine which shares a strong nucleophilic structure with agmatine decreased the H(2)O(2) levels and AP-1, but not the NF-kappaB activity. Possible roles for agmatine and spermine in decreasing fibronectin are discussed, and the signaling pathway for agmatine-reduced fibronectin accumulation is presented.

Arginase inhibition slows the progression of renal failure in rats with renal ablation

Exogenous arginine slows the progression of chronic renal failure (CRF) in remnant rats through a nitric oxide (NO)-dependent mechanism. We tested whether the inhibition of arginase could induce similar results through the increased availability of endogenous arginine. Three groups of remnant rats were studied for 8 wk: 1) untreated rats (REM); 2) remnant rats treated with 1% l-arginine (ARG); and 3) remnant rats administered a Mn(2+)-free diet to inhibit arginase (MNF). Normal rats (NOR) were used as controls. Liver arginase activity was depressed in MNF rats (-35% vs. REM, P < 0.01). No difference in metabolic data was detected among the groups throughout the study; blood pressure was significantly lower in MNF vs. ARG and REM rats after 6 wk (P < 0.001). The glomerular filtration rate (GFR) was greatly depressed in REM rats (-47% vs. NOR, P < 0.03) but was higher in ARG and MNF rats (+40 and +43% vs. REM, respectively, P < 0.05), with comparable changes in renal hemodynamics. Despite the better GFR, proteinuria was decreased in both ARG and MNF rats (-42%, P < 0.05, and -57%, P < 0.01, respectively, vs. REM rats). Arginine plasma levels, significantly reduced in REM rats (-41% vs. NOR, P < 0.01), were partially restored in MNF rats (+38% vs. REM), and urinary nitrite excretion, greatly depressed in REM rats (-76% vs. NOR, P < 0.01), was significantly increased in MNF rats (+209% vs. REM, P < 0.05). At the renal level, arginase activity was only slightly depressed in MNF rats (-18% vs. REM), but intrarenal concentrations of arginine were lower in this latter group (P < 0.05 vs. other groups). Beyond the hemodynamic modifications, MNF rats showed a lower glomerular sclerosis index (P < 0.05 vs. REM and ARG). Inhibition of arginase slows the progression of CRF in remnant rats similarly to arginine-treated rats; the better histological protection in MNF rats, however, suggests that additional factors are involved in these modifications.

L-arginine supplementation accelerates renal fibrosis and shortens life span in experimental lupus nephritis

BACKGROUND

Inducible, high-output nitric oxide (NO) production has been identified as a central mediator of cell injury in immune-mediated renal disease. In acute anti-thy-1 glomerulonephritis prefeeding with the NO precursor L-arginine increases mesangial cell injury and the subsequent fibrosis. The present study tested the hypothesis that L-arginine supplementation may also be detrimental in chronic, NO-mediated murine lupus nephritis.

METHODS

Groups (N = 18) of female MRL/lpr mice with lupus nephritis were fed the following diets: (1) normal protein (22% casein); (2) normal protein and 1.0% L-arginine in the drinking water; (3) low protein (6% casein); (4) low protein + 0.4%l-arginine; and (5) low protein + 1.0% L-arginine. After 40 days mouse survival, albuminuria, matrix accumulation, inflammatory cell infiltration, immunoglobulin G (IgG) deposition, expression of transforming growth factor-beta 1 (TGF-beta 1), fibronectin and plasminogen activator inhibitor-1 (PAI-1) mRNA and protein, anti-DNA antibody titer, inducible nitric oxide synthase (iNOS) mRNA expression, blood amino acid levels, blood urea nitrogen (BUN) concentrations and blood and urinary NOx (nitrite + nitrate) levels were assessed.

RESULTS

L-Arginine supplementation increased mortality significantly (P < 0.02). The death rate increased from 0% in the lowest to 50% in the highest L-arginine intake group (normal protein + 1.0% L-arginine). L-Arginine administration increased albuminuria, renal matrix accumulation, TGF-beta 1, fibronectin, PAI-1, blood L-arginine, L-citrulline, BUN and blood and urine NOx levels, while protein restriction reduced these parameters. Renal cell infiltration and iNOS mRNA expression were decreased in the low protein group only. Anti-ds DNA-IgG and renal IgG deposition were comparable in all groups

CONCLUSIONS

Increasing L-arginine intake increases the severity of renal fibrosis and the likelihood of death in MRL/lpr mice. The results appear to be at least in part mediated through enhanced cytotoxic NO generation via iNOS. The data suggest that L-arginine restriction should be considered in human immune-mediated renal diseases.

Agmatine is efficiently transported by non-neuronal monoamine transporters extraneuronal monoamine transporter (EMT) and organic cation transporter 2 (OCT2)

Agmatine has received considerable attention recently. Available evidence suggests that agmatine functions as a neurotransmitter and inhibits, via induction of antizyme, cellular proliferation. Because of its positive charge, agmatine will not appreciably cross cellular membranes by simple diffusion. Indeed, all physiological models require a channel or transporter protein in the plasma membrane to effect inactivation or nonexocytotic release of agmatine. However, a transport mechanism for agmatine has not been identified on a molecular level so far. In the present study, the non-neuronal monoamine transporters, organic cation transporter (OCT) 1, OCT2, and extraneuronal monoamine transporter (EMT) (gene symbols SLC22A1-A3), both from human and rat, were examined, stably expressed in 293 cells, for [(3)H]agmatine transport. Our results indicate that OCT2 and EMT, but not OCT1, efficiently translocate agmatine. The structural homolog putrescine was not accepted as substrate. Uptake of agmatine via EMT and OCT2 was saturable, with K(m) values of 1 to 2 mM. The affinity of OCT1 was 10-fold lower. Carrier-mediated efflux of agmatine was documented in a trans-stimulation experiment. Finally, uptake of agmatine increased dramatically with increasing pH. Thus, only the singly charged species of agmatine is accepted as substrate. In conclusion, both EMT and OCT2 must be considered for the control of agmatine levels in rat and human.

Agmatine modulates the in vivo biosynthesis and interconversion of polyamines and cell proliferation

Agmatine has recently gained wide interest as a bioactive arginine metabolite with a multitude of physiological functions. This study evaluates the in vivo role of agmatine in the modulation of metabolism and intracellular level of polyamines. Here, we report that agmatine, administered to mice, differentially affects the renal and liver activity of the two key enzymes regulating polyamine biosynthesis and interconversion/degradation. Thus, agmatine exerts a negative regulation of ODC activity and protein content, and positive regulation of SSAT activity, having no effect on ODC and SSAT transcript level. Agmatine modulation of ODC and SSAT activities is noticeably augmented by the inhibitor of its catabolism, aminoguanidine. Antizyme and eIF4E protein content appears to be affected by agmatine only insignificantly and apparently do not contribute to agmatine-induced down-regulation of ODC content. The homeostasis of spermidine and spermine is preserved after agmatine injection, while the putrescine level decreases. Furthermore, when tested in a mouse kidney injury model, agmatine, partially but significantly, reduces [3H] thymidine incorporation into DNA. This is consistent with suppressed renal tubule epithelial cell proliferation. The findings provide in vivo evidence of a substantial role of agmatine as a modulator of polyamine biosynthesis and degradation and suggest its suppressive effect on cell proliferation.

Agmatine inhibits arginine vasopressin-stimulated urea transport in the rat inner medullary collecting duct

BACKGROUND

Agmatine, a putative endogenous ligand for imidazoline receptors, induces numerous biological effects. The agonist clonidine binds to alpha-2 (alpha2) adrenoceptors and imidazoline receptors, and inhibits arginine vasopressin (AVP)-stimulated urea permeability (Pu) in the rat inner medullary collecting duct (IMCD). Dexmedetomidine, a selective alpha2 agonist, does not inhibit AVP-stimulated Pu. This study was conducted to determine if agmatine affects Pu in the rat IMCD and to investigate the possibility of an imidazoline-mediated mechanism.

METHODS

The isolated-perfused tubule technique was used to measure Pu in IMCDs from Wistar rats. AVP at 220 pmol/L or 8-chlorophenylthio cyclic adenosine monophosphate (8CPT cAMP) was used to stimulate Pu. Agmatine and other agents were added to the bath.

RESULTS

Agmatine at 1 micromol/L inhibited AVP-stimulated Pu by 50%. Agmatine-induced inhibition could not be separated completely from inhibition produced by the non-imidazoline, catecholamine epinephrine. Of three antagonists selective for alpha2 adrenoceptors (rauwolscine, yohimbine, and RX821002), only rauwolscine reversed inhibition, whereas each of the three imidazoline-selective antagonists tested (atipamezole, idazoxan, and BU239) produced a significant reversal. Agmatine did not affect basal Pu or inhibit 8CPTcAMP-stimulated Pu.

CONCLUSION

Our results indicate that agmatine inhibits AVP stimulated Pu by a cAMP-dependent mechanism. Imidazoline receptors are probably not involved. The possibility exists of an unknown agmatine-selective receptor modulating urea transport in the rat IMCD.

Effect of chronic morphine treatment on the biosynthesis of agmatine in rat brain and other tissues

Agmatine is an endogenous amine derived from the decarboxylation of arginine by arginine decarboxylase (ADC), and metabolized to putrescine by agmatinase. Exogenously administered agmatine has several biological actions including its ability to potentiate morphine analgesia and block symptoms of morphine tolerance/withdrawal in rats. To investigate the role of endogenous agmatine in this action, we sought to determine whether chronic exposure to morphine and induction of withdrawal modulate the synthesis of agmatine in rat brain and other tissues. Exposure of rats to morphine for three days significantly decreases the activity of ADC and the levels of agmatine in rat liver, kidney, brain, aorta and intestine with no changes in agmatinase activity. The precipitation of withdrawal syndrome by injecting naloxone further decreases ADC activity and agmatine levels in these tissues. We conclude that endogenous agmatine may play an important role in regulating morphine tolerance/dependence and withdrawal symptoms.

Exposure of rat isolated stomach and rats in vivo to [(14)C]agmatine: accumulation in the stomach wall and distribution in various tissues

The aims of the present study were: (i) to investigate the accumulation of radioactivity in the stomach wall after luminal exposure of the rat isolated stomach to[(14)C]agmatine and (ii) to determine the distribution of radioactivity in various tissues after oral administration of this radiolabelled polyamine to rats in vivo. In isolated rat stomach, [(14)C]agmatine was accumulated in part by an energy-dependent uptake process that could be inhibited by phentolamine. These findings correspond to properties of the recently identified specific agmatine transporter in human glioma cells, suggesting that in rat stomach [(14)C]agmatine is taken up by such a carrier. In in vivo experiments, rats received 0.5 microCi [(14)C]agmatine adsorbed to 5 g rat standard chow after a fasting period of 24 h. After oral ingestion of [(14)C]agmatine, radioactivity was recovered in all organs investigated as well as in blood and urine. Radioactivity also seemed to be secreted into the pancreaticobiliary fluid, as it was recovered in the luminal content of distal ileum and sigmoid colon. Accumulation of radioactivity in organs and distal gut luminal content was dose-dependently decreased by simultaneous administration of putrescine. In conclusion, the present data are compatible with the view that agmatine can be absorbed in rat at least from the stomach and probably also from the gut by means of an energy-dependent agmatine transport mechanism. Agmatine itself and/or its degradation products, which also have the potential to be pharmacologically active, are unevenly distributed between the organs. Putative secretion of radioactivity into the pancreaticobiliary fluid suggests the potential for an enterohepatic circulation of agmatine. In view of the high intraluminal concentration of agmatine in the stomach and distal gut and the operation of an agmatine transporter, it is rather likely that agmatine in the chyme of the gut represents an important source for agmatine detected in the tissues of the organism.

Evidence for endogenous agmatine in hypothalamo-neurohypophysial tract and its modulation on vasopressin release and Ca2+ channels

Agmatine, decarboxylated from arginine by arginine decarboxylase, is particularly prominent in the hypothalamus. The present study utilized the rat hypothalamo-neurohypophysial system to determine expression and "pre-synaptic" modulation of agmatine in the central nervous system (CNS). Under confocal-laser scanning, agmatine-like immunoreactivity (Agm-LI) was found enriched in arginine-vasopressin (AVP)-containing magnocellular neurons of the supraoptic nuclei (SON) and paraventricular nuclei (PVN). In addition, using electron microscopy, Agm-LI was found closely associated with large neurosecretory-like vesicles in neurohypophysial nerve terminals of posterior pituitary gland. Radioimmunoassay revealed that 10 and 30 microM agmatine concentration-dependently inhibited the depolarization-evoked AVP release from isolated neurohypophysial terminals. Using perforated patch-clamp, effects of agmatine on whole-terminal voltage-gated ion currents in the isolated neurohypophysial nerve terminals were examined. While it did not significantly affect either tetrodotoxin (TTX)-sensitive Na(+) or sustained Ca(2+)-activated K(+) channel currents, agmatine (1-40 microM) inhibited Ca(2+) channel currents in approximately 53% of the total nerve terminals investigated. The onset of inhibitory effect was immediate, and the inhibition was reversible and concentration-dependent with an IC(50)=4.6 microM. In the remaining (approximately 47%) neurohypophysial nerve terminals, only a higher (120 microM) concentration of agmatine could moderately inhibit Ca(2+) channel currents. The results suggest that: (1) endogenous agmatine is co-expressed in AVP-containing, hypothalamic magnocellular neurons of the SON/PVN and in neurohypophysial nerve terminals of posterior pituitary gland; (2) agmatine may serve as a physiological neuromodulator by regulating the voltage-gated Ca(2+) channel and, as a result, the release of AVP from neurohypophysial nerve terminals.

Cloning and characterization of human agmatinase

Arginine decarboxylase (ADC) and agmatinase are part of an operon in Escherichia coli, which constitutes the primary pathway of polyamine synthesis from arginine. This pathway is also known to exist in plants, but until recently, neither agmatine nor ADC, the enzyme that synthesizes it, nor agmatinase the enzyme that is responsible for conversion of agmatine to putrescine, were known to exist in man or other mammals. We describe here the cloning of the agmatinase gene and the tissue distribution of its transcription product. Human agmatinase contains 352 amino acid residues and has a calculated molecular weight of 37,688 kDa. It has 56% similarity to E. coli agmatinase and 42% similarity to human arginases I and II and shares highly conserved substrate-binding domains with these well-characterized enzymes.

Inducible nitric oxide synthase in renal transplantation

The importance of the endothelial isoform of nitric oxide synthase (eNOS) has been well established. Endothelium-derived nitric oxide has been shown to be essential for vascular homeostasis and modulation of eNOS has thus become a target in prevention of cardiovascular disease. The role of the inducible form of nitric oxide synthase (iNOS) in vascular biology, however, is less clear. Classically, iNOS has been regarded as an enzyme that produces nmolar amounts of the nitric oxide radical, thereby leading to cellular damage. More recent data, however, have shown that the iNOS can be a superoxide, peroxynitrite as well as a nitric oxide-producing enzyme, while the biological effects of iNOS probably depend upon the sort of radical species released by the enzyme as well as the anti-oxidant capacity of the cellular microenvironment of the enzyme. This brief review discusses these aspects in relation to renal transplantation.

Expression of diamine oxidase (histaminase) in guinea-pig tissues

The expression of mRNA for diamine oxidase (histaminase) and the enzyme activity in guinea-pig tissues were investigated. Reverse transcription-polymerase chain reaction analysis revealed that the message corresponding to the long form present in humans and rats was expressed abundantly in the small intestine and liver. Small but detectable amounts of diamine oxidase mRNA were observed in the kidney, stomach, cerebellum, thalamus+hypothalamus, and cerebral cortex. Northern blot analysis showed that the message (2.8 kb in size) was observed abundantly in the liver and small intestine and was detectable in the kidney and stomach but not in the brain or lung. In situ hybridization showed that diamine oxidase mRNA was localized throughout the liver and epithelial cells of the small intestine. Diamine oxidase activity was detected at various levels in different tissues of the guinea-pig at the following relative abundance: liver>small intestine>lung, kidney>stomach. Histamine dose-dependently induced the contraction of sections of the guinea-pig small intestine, and the pretreatment of the tissue section with aminoguanidine (100 microM), a diamine oxidase inhibitor, but not with S-[4-(N,N-dimethylamino)butyl]isothiourea (100 microM), an inhibitor of histamine N-methyltransferase, shifted the dose-response curve of histamine-induced contraction to lower concentrations. These results suggest that diamine oxidase has a crucial role in the degradation of histamine in the guinea-pig small intestine and probably in the liver.

Cloning of human agmatinase. An alternate path for polyamine synthesis induced in liver by hepatitis B virus

Agmatinase, which hydrolyzes agmatine to putrescine and urea, not only represents a potentially important mechanism for regulating the biological effects of agmatine in mammalian cells but also represents an alternative to ornithine decarboxylase for polyamine biosynthesis. We have isolated a full-length cDNA encoding human agmatinase whose function was confirmed by complementation in yeast. The single-copy human agmatinase gene located on chromosome 1 encodes a 352-residue protein with a putative mitochondrial targeting sequence at the NH(3)-terminus. Human agmatinase has about 30% identity to bacterial agmatinases and <20% identity to mammalian arginases. Residues required for binding of Mn(2+) at the active site in bacterial agmatinase and other members of the arginase superfamily are fully conserved in human agmatinase. Agmatinase mRNA is most abundant in human liver and kidney but also is expressed in several other tissues, including skeletal muscle and brain. Its expression in human liver is induced during hepatitis B virus infection, suggesting that agmatinase may play a role in the pathophysiology of this disease.

Agmatine induces apoptosis in rat hepatocyte cultures

BACKGROUND/AIMS

Agmatine, the compound formed by decarboxylation of arginine, is believed to be an endogenous neurotransmitter through interaction with the imidazoline receptors. However, it also appears to regulate rat hepatocyte polyamines by modifying both their synthesis and their catabolism. As the decrease in polyamine content has been correlated with apoptosis, we examined the possibility that agmatine has an effect on this phenomenon.

METHODS

Apoptotic cells were detected by visualizing nuclear shrinkage/fragmentation in hepatocytes cultured at 21 and 5% oxygen tension. Caspase-3 activity, cleavage of PARP, release of cytochrome c and mitochondrial swelling were therefore measured in the two conditions and in the presence or not of agmatine.

RESULTS

In rat hepatocytes agmatine promoted apoptosis, procaspase 3 processing and increase of caspase-3 like activity. This occurred through mitochondria swelling and release of cytochrome c. Cyclosporin A and catalase blocked the swelling.

CONCLUSIONS

Our experiments show that agmatine, besides all the known biological effects, has also part, at least in hepatocytes, in the modulation of programmed cell death.

Polyamine metabolism revisited

The natural polyamines putrescine, spermidine and spermine play an essential role in cell growth and differentiation. Cellular polyamine depletion results in inhibition of growth, whereas its accumulation appears to be toxic. Intracellular levels of polyamines are regulated by a multitude of mechanisms affecting their synthesis, degradation, uptake and excretion. The three key enzymes in the regulation of polyamine metabolism have short half-lives and are inducible. Ornithine and S-adenosylmethionine decarboxylases regulate polyamine biosynthesis whereas spermidine/spermine acetyltransferase regulates polyamine interconvertion and degradation.

Suppression of inducible nitric oxide generation by agmatine aldehyde: beneficial effects in sepsis

The induction of inducible nitric oxide synthase (iNOS) serves an important immuno-protective function in inflammatory states, but ungoverned nitric oxide (NO) generation can contribute to a number of pathologic consequences. Delineation of the mechanisms that can downregulate iNOS-generated NO in inflammation could have therapeutic relevance. Here we show that agmatine, a metabolite of arginine, inhibits iNOS mediated nitric oxide generation in cytokine stimulated cell culture preparations. This effect was not cell type specific. Increased diamine oxidase (DAO) and decreased aldehyde dehydrogenase (AldDH) activities are also representative of inflammatory settings. Increasing the conversion of agmatine to an aldehyde form by addition of purified DAO or suppression of aldehyde breakdown by inhibition of AldDH activity increases the inhibitory effects of agmatine in an additive fashion. Inhibitors of DAO, but not monoamine oxidase (MAO), decreased the inhibitory effects of agmatine, as did the addition of AldDH or reacting aldehydes with phenylhydrazine. We examined rats given lipopolysaccharide (LPS) to evaluate the potential effects of agmatine in vivo. Endotoxic rats administered agmatine prevented the decreases in blood pressure and renal function normally associated with sepsis. Agmatine treatment also increased the survival of LPS treated mice. Our data demonstrate the capacity of agmatine aldehyde to suppress iNOS mediated NO generation, and indicate a protective function of agmatine in a model of endotoxic shock. How agmatine may aid in coordinating the early NO phase and the later repair phase responses in models of inflammation is discussed.

Polyamine transport system mediates agmatine transport in mammalian cells

Agmatine is a biogenic amine with the capacity to regulate a number of nonreceptor-mediated functions in mammalian cells, including intracellular polyamine content and nitric oxide generation. We observed avid incorporation of agmatine into several mammalian cell lines and herein characterize agmatine transport in mammalian cells. In transformed NIH/3T3 cells, agmatine uptake is energy dependent with a saturable component indicative of carrier-mediated transport. Transport displays an apparent Michaelis-Menten constant of 2.5 microM and a maximal velocity of 280 pmol x min(-1) x mg(-1) protein and requires a membrane potential across the plasma membrane for uptake. Competition with polyamines, but not cationic molecules that utilize the y+ system transporter, suppresses agmatine uptake. Altering polyamine transporter activity results in parallel changes in polyamine and agmatine uptake. Furthermore, agmatine uptake is abrogated in a polyamine transport-deficient human carcinoma cell line. These lines of evidence demonstrate that agmatine utilizes, and is dependent on, the polyamine transporter for cellular uptake. The fact that this transport system is associated with proliferation could be of consequence to the antiproliferative effects of agmatine.

Transport and metabolism of agmatine in rat hepatocyte cultures

Rat hepatocytes in culture take up [14C]-agmatine by both a high-affinity transport system [KM = 0.03 mM; Vmax = 30 pmol x min x (mg protein)-1] and a low-affinity system. The high-affinity system also transports putrescine, but not cationic amino acids such as arginine, and the polyamines spermidine and spermine. The rate of agmatine uptake is increased in cells deprived of polyamines with difluoromethylornithine. Of the agmatine taken up, 10% is transformed into polyamines and 50% is transformed into 4-guanidinobutyrate, as demonstrated by HPLC and MS. Inhibition by aminoguanidine and pargyline shows that this is due to diamine oxidase and an aldehyde dehydrogenase. 14C-4-aminobutyrate is also accumulated in the presence of an inhibitor of 4-aminobutyrate transaminase.

L-arginine supplementation improves function and reduces inflammation in renal allografts

Recovery from ischemia/reperfusion and immune-mediated injury in the renal transplant is associated with reduced renal hemodynamics and increased leukocyte infiltration. In diverse models of renal failure, L-arginine supplementation improved hemodynamics and reduced inflammation. However in a proinflammatory environment, L-arginine can worsen renal injury. This study investigated the therapeutic potential of L-arginine supplementation in allogeneic renal transplantation: Brown Norway rat kidneys were transplanted into Lewis rat recipients, with one native kidney remaining. Recipients received low-dose cyclosporin A (2.5 mg/kg per d subcutaneously) to obtain moderate vascular and interstitial rejection, with or without 1% L-arginine in drinking water for 7 d posttransplantation. Transplantation increased renal vasoconstriction (from 16.9 +/- 1.33 to 35.1 +/- 8.6 units; P: < 0.01), thereby reducing GFR (from 0.96 +/- 0.09 to 0.48 +/- 0.10 ml/min; P: < 0.05). Treatment with L-arginine restored renal graft function to levels found in normal donors (renal vascular resistance, 15.7 +/- 1.69 units; GFR, 0.80 +/- 0.06 ml/min). L-arginine significantly reduced vascular occlusion because of less inflammation, endothelial disruption, and thrombosis. L-arginine also decreased tubulitis, interstitial injury, and macrophage infiltration. These protective effects suggest that L-arginine might be useful as additive therapy to conventional immune suppression.

Ornithine decarboxylase, kidney size, and the tubular hypothesis of glomerular hyperfiltration in experimental diabetes

In early diabetes, the kidney grows and the glomerular filtration rate (GFR) increases. This growth is linked to ornithine decarboxylase (ODC). The study of hyperfiltration has focused on microvascular abnormalities, but hyperfiltration may actually result from a prior increase in capacity for proximal reabsorption which reduces the signal for tubuloglomerular feedback (TGF). Experiments were performed in Wistar rats after 1 week of streptozotocin diabetes. Kidney weight, ODC activity, and GFR were correlated in diabetic and control rats given difluoromethylornithine (DFMO; Marion Merrell Dow, Cincinnati, Ohio, USA) to inhibit ODC. We assessed proximal reabsorption by micropuncture, using TGF as a tool for manipulating single-nephron GFR (SNGFR), then plotting proximal reabsorption versus SNGFR. ODC activity was elevated 15-fold in diabetic kidneys and normalized by DFMO, which also attenuated hyperfiltration and hypertrophy. Micropuncture data revealed an overall increase in proximal reabsorption in diabetic rats too great to be accounted for by glomerulotubular balance. DFMO prevented the overall increase in proximal reabsorption. These data confirm that ODC is required for the full effect of diabetes on kidney size and proximal reabsorption in early streptozotocin diabetes and are consistent with the hypothesis that diabetic hyperfiltration results from normal physiologic actions of TGF operating in a larger kidney, independent of any primary malfunction of the glomerular microvasculature.

Agmatine uptake by cultured hamster kidney cells

Agmatine, the product of arginine decarboxylation, has been recently found in a wide variety of animal tissues. In spite of the emergent interest on agmatine in animals, the mechanism of agmatine uptake in mammalian cells has been scarcely studied. An analysis of radiolabeled agmatine uptake was carried out by using a classical, kinetic approach with BHK-21 hamster kidney cells in culture. A high affinity, temperature- and energy-dependent agmatine transport system in BHK-21 kidney cells is here kinetically characterized which seems to be a "general" transporter shared by di- and triamines and different to a highly specific carrier for the tetraamine spermine.

The effect of agmatine administration on ischemic-reperfused isolated rat heart

BACKGROUND

The natural polyamine Agmatine (Ag) plays a significant role in protection of nerve cell ischemic injury. A previous report indicated that Ag given intraperitoneally to rats enhanced the recovery of the heart from ischemic injury. Based on this initial observation, a larger investigation was undertaken to explore a dose-response effect and possible mechanisms underlying the protective effects.

METHODS

Using the modified Langendorff model, 36 isolated hearts were divided into five groups: group 1, hearts receiving 100 microM/L Ag pre-ischemia (n=7); group 2, hearts receiving 100 microM/L Ag pre- and post-ischemia, (n=7); group 3, hearts receiving 250 microM/L Ag pre-ischemia (n=7); group 4, hearts receiving 250 microM/L Ag pre- and postischemia (n=7); and group 5, hearts receiving Krebs-Hensleit solution served as control (n=8). The study design included 20 minutes of perfusion, 30 minutes of global ischemia, and 30 minutes of reperfusion.

RESULTS

After ischemia, group 2 developed higher left ventricular pressure P(max) (P<0.01), improved first-derivative of the rise (dP/dt max; P<0.02), and fall (dP/dt min; P<0.04) in left ventricular pressure, and the area calculated under the left-ventricle developed pressure curve (pressure-time integral; P<0.015), but coronary flow was not significantly increased (P=0.06) compared to the control group. Group 1 had improved diastolic recovery: dP/dt min (P<0.05) and coronary flow (P<0.03), compared with the control group. Group 3 had improved P(max) (P<0.01), dP/dt min (P<0.01), and coronary flow (P<0.02); group 4 had no improvement in all hemodynamic parameters.

CONCLUSION

Low doses of Ag given pre- and post-ischemia, and high doses given only pre-ischemia have favorable, protective effects on the hemodynamic recovery of isolated rat heart undergoing global ischemia and reperfusion.

Nitric oxide and substance dependence

The free-radical gas nitric oxide (NO) plays an important role in a diverse range of physiological processes. It is synthesized from the precursor L-arginine by the enzyme NO synthase (NOS), which transforms L-arginine into NO and citrulline. This synthetic pathway exists in the central nervous system (CNS), and NO appears to be a messenger molecule in the CNS, fulfilling most of the criteria of a neurotransmitter. Recent studies indicate that NO may play an important role in dependence on drugs of abuse. The purpose of this review is to address the role of NO in dependence on substances such as opioids, ethanol, psychostimulants and nicotine. Inhibitors of NOS modulate withdrawal from opioids and ethanol, diminishing many signs of withdrawal. In addition, NOS inhibitors suppress signs of withdrawal from nicotine. These data suggest that NO may be involved in the expression of withdrawal signs, and they leave open the possibility that NO may mediate the development of many of these signs. Although preliminary, data to date suggest that glutamate neurotransmission may be related to these beneficial effects of NOS inhibitors on signs of withdrawal. Emerging data further suggest that NO may have a general role in the dependence potential of various classes of drugs of abuse. Thus, modulation of NO systems may be a potential therapeutic target for treatment of substance abuse.

Agmatine inhibits cell proliferation and improves renal function in anti-thy-1 glomerulonephritis

Changes in the expression of alternate arginine metabolic pathways have been implicated in the pathogenesis of experimental glomerulonephritis. Agmatine, decarboxylated arginine, has been shown in vitro to suppress both inducible nitric oxide synthase and the rate-limiting enzyme of polyamine biosynthesis, ornithine decarboxylase (ODC). This study was undertaken to determine whether agmatine administration could reduce tissue injury by decreasing nitric oxide, and reduce cell proliferation, by diminishing ODC activity, in experimental mesangial proliferative glomerulonephritis (Thy-1 nephritis). Agmatine treatment (50 mg/kg per d intraperitoneally) in Thy-1 nephritis rats prevented a reduction in GFR at day 1. Agmatine treatment decreased nitric oxide production in Thy-1 nephritis rats by 23% and 41% at days 1 and 4, respectively. Agmatine treatment also reduced ODC activity and glomerular (3)H-thymidine incorporation on days 1, 4, and 7. Histologic evaluation revealed a decline in mesangial cell proliferation and extracellular matrix accumulation associated with agmatine treatment administered before or 24 h after Thy-1 antibody, and this was confirmed by a reduction in the number of cells expressing proliferating cell nuclear antigen on days 4 and 7. These studies provide the first in vivo evidence that agmatine administration can reduce cellular proliferation in Thy-1 nephritis and attenuate the initial reduction in renal function associated with this model.

Agmatine suppresses nitric oxide production in microglia

We investigated the effect of agmatine, an arginine metabolite synthesized in the brain, in cultured microglia obtained from neonatal rat cerebral cortex. Agmatine (1-300 microM) did not affect viability of cultured microglia. Activation of microglia by lipopolysaccharide (LPS, 1 microg/ml) caused the expression of inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) assessed as the accumulation of nitrite in the culture supernatants. Agmatine had no effect on the expression of iNOS, but significantly suppressed the LPS-induced NO production in a concentration-dependent manner. Agmatine was also effective in suppressing the production of NO induced by a combination of interferon-gamma (500 U/ml) and amyloid beta protein (10 microM). In co-cultures of rat cortical neurons and microglia, LPS caused significant loss of neuron viability. The LPS neurotoxicity was not observed in the absence of microglia, and was completely blocked by the NOS inhibitor diphenyleneiodoium chloride. The neuronal death induced by microglia-derived NO was significantly attenuated by the presence of agmatine. These results suggest that agmatine works to protect neurons by inhibiting the production of NO in microglia.

Bioactive products of arginine in sepsis: tissue and plasma composition after LPS and iNOS blockade

Blockade or gene deletion of inducible nitric oxide synthase (iNOS) fails to fully abrogate all the sequelae leading to the high morbidity of septicemia. An increase in substrate uptake may be necessary for the increased production of nitric oxide (NO), but arginine is also a precursor for other bioactive products. Herein, we demonstrate an increase in alternate arginine products via arginine and ornithine decarboxylase in rats given lipopolysaccharide (LPS). The expression of iNOS mRNA in renal tissue was evident 60 but not 30 min post-LPS, yet a rapid decrease in blood pressure was obtained within 30 min that was completely inhibited by selective iNOS blockade. Plasma levels of arginine and ornithine decreased by at least 30% within 60 min of LPS administration, an effect not inhibited by the iNOS blocker L-N(6)(1-iminoethyl)lysine (L-NIL). Significant increases in plasma nitrates and citrulline occurred only 3-4 h post-LPS, an effect blocked by L-NIL pretreatment. The intracellular composition of organs harvested 6 h post-LPS reflected tissue-specific profiles of arginine and related metabolites. Tissue arginine concentration, normally an order of magnitude higher than in plasma, did not decrease after LPS. Pretreatment with L-NIL had a significant impact on the disposition of tissue arginine that was organ specific. These data demonstrate changes in arginine metabolism before and after de novo iNOS activity. Selective blockade of iNOS did not prevent uptake and can deregulate the production of other bioactive arginine metabolites.

Characterization of arginine decarboxylase in rat brain and liver: distinction from ornithine decarboxylase

We compared the properties of mammalian arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) in rat liver and brain. Mammalian ADC is thermally unstable and associated with mitochondrial membranes. ADC decarboxylates both arginine (Km = 0.75 mM) and ornithine (Km = 0.25 mM), a reaction not inhibited by the specific ODC inhibitor, difluoromethylomithine. ADC activity is inhibited by Ca2+, Co2+, and polyamines, is present in many organs being highest in aorta and lowest in testis, and is not recognized by a specific monoclonal antibody to ODC. In contrast, ODC is thermally stable, cytosolic, and mitochondrial and is expressed at low levels in most organs except testis. Although ADC and ODC are expressed in cultured rat C6 glioma cells, the patterns of expression during growth and confluence are very different. We conclude that mammalian ADC differs from ADC isoforms expressed in plants, bacteria, or Caenorhabditis elegans and is distinct from ODC. ADC serves to synthesize agmatine in proximity to mitochondria, an organelle also harboring agmatine's degradative enzyme, agmatinase, and a class of imidazoline receptor (I2) to which agmatine binds with high affinity.

Is agmatine a novel neurotransmitter in brain?

Recent evidence suggests that agmatine, which is an intermediate in polyamine biosynthesis, might be an important neurotransmitter in mammals. Agmatine is synthesized in the brain, stored in synaptic vesicles in regionally selective neurons, accumulated by uptake, released by depolarization, and inactivated by agmatinase. Agmatine binds to alpha2-adrenoceptors and imidazoline binding sites, and blocks NMDA receptor channels and other ligand-gated cationic channels. Furthermore, agmatine inhibits nitric oxide synthase, and induces the release of some peptide hormones. As a result of its ability to inhibit both hyperalgesia and tolerance to, and withdrawal from, morphine, and its neuroprotective activity, agmatine has potential as a treatment of chronic pain, addictive states and brain injury.