[The influence of N-[imino(1-piperidinyl)methyl]guanidine and N-[imino(4-morpholinyl)methyl]guanidine on citrate content, aconitase and citrate synthase activities at ischemia-reperfusion of rats brain]

The influence of some guanidine derivatives on the level of brain citrate, brain activities of aconitase and citrate synthase has been investigated in rats subjected to ischemia-reperfusion. Administration of N-[imino(1-piperidinyl)methyl]guanidine and N-[imino(4-morpholinyl)methyl]guanidine resulted in changes of specific activities of aconitase and citrate synthase towards control values. Under these conditions the citrate level considerably decreased versus rats with untreated ishemia-reperfusion. Treatment with these compounds also decreased the degree of DNA fragmentation markedly increased in rats with ischemia-reperfusion.

Effect of N-[Imino(4-morpholyl)methyl]guanidine on the oxidative status in rats with toxic hepatitis

The hepatoprotective and antioxidant properties of a synthetic biguanide N-[imino(4-morpholyl)methyl]guanidine (IMMG) were prognosticated by the method of computer prediction. Administration of IMMG was accompanied by a decrease in serum transaminase activity in rats with toxic hepatitis, which reflects inhibition of hepatocyte cytolysis. IMMG treatment was followed by a decrease in biochemiluminescence parameters reflecting the intensity of free radical oxidation. We revealed an increase in activity of aconitase, which was reduced during toxic hepatitis. The content of citrate in the liver and serum was returned to normal under these conditions. IMMG also increased activities of superoxide dismutase and catalase and total antioxidant activity in rat liver. Our results suggest that the hepatoprotective effect of IMMG is associated with its antioxidant activity.

Guanidinium derivatives bind preferentially and trigger long-distance conformational changes in an engineered T4 lysozyme

The binding of guanidinium ion has been shown to promote a large-scale translation of a tandemly duplicated helix in an engineered mutant of T4 lysozyme. The guanidinium ion acts as a surrogate for the guanidino group of an arginine side chain. Here we determine whether methyl- and ethylguanidinium provide better mimics. The results show that addition of the hydrophobic moieties to the ligand enhances the binding affinity concomitant with reduction in ligand solubility. Crystallographic analysis confirms that binding of the alternative ligands to the engineered site still drives the large-scale conformational change. Thermal analysis and NMR data show, in comparison to guanidinium, an increase in protein stability and in ligand affinity. This is presumably due to the successive increase in hydrophobicity in going from guanidinium to ethylguanidinium. A fluorescence-based optical method was developed to sense the ligand-triggered helix translation in solution. The results are a first step in the de novo design of a molecular switch that is not related to the normal function of the protein.

Advanced glycation end product precursors impair ABCA1-dependent cholesterol removal from cells

Abnormal HDL metabolism may contribute to the increased atherosclerosis associated with diabetes. The ATP-binding cassette transporter A1 (ABCA1) is an atheroprotective cell protein that mediates cholesterol transport from cells to apolipoprotein (apo) A-I, the major protein in HDL. Because formation of advanced glycation end products (AGEs) is associated with diabetic vascular complications, we examined the effects of carbonyls implicated in AGE formation on the ABCA1 pathway in cultured fibroblasts and macrophages. Treating cells with glycolaldehyde (GA) and glyoxal (GO) strongly inhibited ABCA1-dependent transport of cholesterol from cells to apoA-I, while methylglyoxal had little effect. This occurred under conditions where other lipoprotein receptors or lipid metabolic pathways were little affected, indicating that ABCA1 was uniquely sensitive to these carbonyls. GA and GO destabilized ABCA1 and nearly abolished its binding of apoA-I, indicating that these carbonyls directly modified ABCA1. Immunohistology of coronary arteries from hyperlipidemic swine revealed that inducing diabetes with streptozotocin increased atherosclerotic lesion area and dramatically reduced the fraction of macrophages that expressed detectable ABCA1. These results raise the possibility that reactive carbonyl-mediated damage to ABCA1 promotes accumulation of cholesterol in arterial macrophages and thus contribute to the increased cardiovascular disease associated with diabetes, insulin resistance, and other inflammatory conditions.

Effect of methylguanidine in a model of septic shock induced by LPS

Septic shock, a severe form of sepsis, is characterized by cardiovascular collapse following microbial invasion of the body. The progressive hypotension, hyporeactivity to vasopressor agents and vascular leak leads to circulatory failure with multiple organ dysfunction and death. Many inflammatory mediators (e.g. TNF-alpha, IL-1 and IL-6) are involved in the pathogenesis of shock and, among them, nitric oxide (NO). The overproduction of NO during septic shock has been demonstrated to contribute to circulatory failure, myocardial dysfunction, organ injury and multiple organ failure. We have previously demonstrated with in vitro and in vivo studies that methylguanidine (MG), a guanidine compound deriving from protein catabolism, significantly inhibits iNOS activity, TNF-alpha release and carrageenan-induced acute inflammation in rats. The aim of the present study was to evaluate the possible anti-inflammatory activity of MG in a model of septic shock induced by lipopolysaccharide (LPS) in mice. MG was administered intraperitoneally (i.p.) at the dose of 30 mg/kg 1 h before and at 1 and 6 h after LPS-induced shock. LPS injection (10 mg/kg in 0.9% NaCl; 0.1 ml/mouse; i.p.) in mouse developed a shock syndrome with enhanced NO release and liver, kidney and pancreatic damage 18 h later. NOx levels, evaluated as nitrite/nitrate serum levels, was significantly reduced in MG-treated rats (78.6%, p < 0.0001; n = 10). Immunohistochemistry revealed, in the lung tissue of LPS-treated group, a positive staining for nitrotyrosine and poly(adenosine diphosphate [ADP] ribose) synthase, both of which were reduced in MG-treated mice. Furthermore, enzymatic evaluation revealed a significant reduction in liver, renal and pancreatic tissue damage and MG treatment also improved significantly the survival rate. This study provides evidence that MG attenuates the degree of inflammation and tissue damage associated with endotoxic shock in mice. The mechanisms of the anti-inflammatory effect of MG is, at least in part, dependent on the inhibition of NO formation.

Effect of methylguanidine in carrageenan-induced acute inflammation in the rats

In vitro and in vivo studies have demonstrated that methylguanidine, an inhibitor of nitric oxide synthase (NOS), is also able to reduce tumour necrosis factor-alpha (TNF-alpha) release. In the present study, we evaluated the anti-inflammatory potential of methylguanidine treatment in two models of acute inflammation (carrageenan-induced paw edema and pleurisy) where oxyradical, nitric oxide (NO) and prostaglandins play a crucial role in the inflammatory processes. Our data show that methylguanidine, given intraperitoneally at the dose of 30 mg/kg, inhibits the inflammatory response reducing significantly (P<0.05) paw swelling, pleural exudates formation, mononuclear cell infiltration and histological injury. Furthermore, our data suggests that there is a significant (P<0.05) reduction in the activity and expression both of the inducible NOS (iNOS) and of cyclooxygenase-2 in lung tissue of pleurisy model. Methylguanidine is also able to reduce the appearance of nitrotyrosine and of the nuclear enzyme poly(adenosine diphosphate [ADP]-ribose) synthase immunoreactivity in the inflamed lung tissues. Treatment with aminoguanidine, the reference drug, significantly reduced all the evaluated pro-inflammatory parameters in carrageenan-treated rats. Taken together, the present results demonstrate that methylguanidine exerts potent anti-inflammatory effects that could be, in part, related to an inhibition of the expression/activity of the iNOS and cyclooxygenase-2 and, another part, may be related to a reduction of TNF-alpha release.

A selective human H(4)-receptor agonist: (-)-2-cyano-1-methyl-3-[(2R,5R)-5- [1H-imidazol-4(5)-yl]tetrahydrofuran-2-y] methylguanidine

A series of 16 compounds related to chiral 4(5)-(5-aminomethyltetrahydrofuran-2-yl)imidazoles (1) have been designed, synthesized, and examined in vitro by radioligand displacement studies and functional assays for both the human H(3)- and H(4)-receptors expressed in SK-N-MC cells. Among them, the (2S,5S)-isomer 1d of amino compounds showed approximately 300-fold higher selectivity at the H(3)-receptor than the H(4)-receptor. On the other hand, (2R,5S)- and (2R,5R)-cyanoguanidines 3b and 3c, in which the amino group of the compounds 1b and 1c was substituted by the cyanoguanidino moiety, bound to the H(4)-receptor with a pEC(50) value of 6.65 and 7.11, respectively, and had >40-fold selectivities over the H(3)-receptor. As such, 3b and 3c are the first selective H(4) receptor agonists.

Increased retinal endothelial cell monolayer permeability induced by the diabetic milieu: role of advanced non-enzymatic glycation and polyol pathway activation

BACKGROUND

Increased vascular permeability could be involved in the pathogenesis of diabetic retinopathy. The present study was aimed at assessing whether high glucose concentrations can impair retinal endothelial cell barrier function directly, irrespective of changes in other determinants of permeability, and the role of non-enzymatic glycation and polyol pathway activation in these alterations.

METHODS

Bovine retinal endothelial cells (BREC) were exposed for various periods to high glucose vs iso-osmolar mannitol and normal glucose containing media+/-agents mimicking or inhibiting advanced glycation end product (AGE) formation and polyol pathway activation. Monolayer permeability was assessed by measuring the transendothelial passage of (125)I-labeled proteins.

RESULTS

Permeability increased significantly (up to +70%) in BREC exposed to high glucose, but not to mannitol, for 1-30 days, vs normal glucose control cells. Exposure to AGE-modified bovine serum albumin (BSA) (> or = 90%) and, to a lesser extent, sorbitol (+28%) mimicked the high glucose effect. The AGE formation and nitric oxide synthase (NOS) inhibitor aminoguanidine significantly reduced (by 60%) changes induced by 30-day exposure to high glucose, whereas methylguanidine, which inhibits only NOS activity, did not affect permeability. Aldose reductase or sorbitol dehydrogenase inhibitors decreased (by approximately 40%) the enhanced leakage produced by 1-day, but not 30-day, incubation in high glucose.

CONCLUSIONS

The present results indicate that high glucose is capable of impairing retinal endothelial cell barrier function directly and that non-enzymatic glycation and polyol pathway activation may mediate these changes, with AGEs participating in the long-term alterations and increased flux through the sorbitol pathway in the short-term effect.

Effects of some guanidino compounds on human cerebral arteries

BACKGROUND AND PURPOSE

Accumulation of endogenous guanidino-substituted analogues of L-arginine in chronic renal failure might contribute to some of the vascular and neurological disorders of this pathology. We tested the hypothesis that in human cerebral arteries, some guanidino compounds may increase vascular tone, through nitric oxide (NO) synthase inhibition, and impair endothelium-dependent relaxation.

METHODS

Rings of human middle cerebral artery were obtained during autopsy of 26 patients who had died 3 to 12 hours before. The rings were suspended in organ baths for isometric recording of tension. We then studied the responses to N(G)-monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (asymmetrical dimethylarginine; ADMA), aminoguanidine (AG), and methylguanidine (MG).

RESULTS

L-NMMA (10(-6) to 3x10(-4) mol/L) and ADMA (10(-6) to 3x10(-4) mol/L) caused concentration- and endothelium-dependent contractions (median effective concentrations [EC(50)]=1.1x10(-5) and 1.6x10(-5) mol/L, respectively; E(max)=35. 5+/-7.9% and 43.9+/-5.9% of the response to 100 mmol/L KCl). AG (10(-5) to 3x10(-3) mol/L) and MG (10(-5) to 3x10(-3) mol/L) produced endothelium-independent contractions (E(max)=44.3+/-8.8% and 45.7+/-5.8% of the response to 100 mmol/L KCl, respectively). L-Arginine (10(-3) mol/L) prevented the contractions by L-NMMA and ADMA but did not change contractions induced by AG and MG. L-NMMA and ADMA inhibited endothelium-dependent relaxation induced by acetylcholine in a concentration-dependent manner; AG and MG were without effect.

CONCLUSIONS

The results suggest that the contractions induced by L-NMMA and ADMA are due to inhibition of endothelial NO synthase activity, whereas AG and MG do not affect the synthesis of NO. An increase in the plasma concentration of L-NMMA and ADMA associated with uremia is likely to represent a diminished release or effect of NO, and consequently, an increased cerebrovascular tone in uremic patients is highly conceivable.

In vitro and in vivo TNFalpha synthesis modulation by methylguanidine, an uremic catabolyte

This study was performed in order to examine whether the uraemic toxin, methylguanidine (MG), can modulate tumor necrosis factor alpha (TNF alpha) release by activated macrophages. In this study we have evaluated the ability of MG to influence TNF alpha release in vitro, in Escherichia coli lypopolysaccharide- (LPS)-stimulated J774 cells preincubated overnight with MG, and in vivo in rats treated with MG before and after LPS challenge. Parallel experiments employing N(G)-nitro-L-arginine methyl esther (L-NAME) were also carried out for comparison. The effect of LPS (6 x 10(3) u/ml) on TNF alpha release by J774, following overnight incubation with MG or L-NAME (1 mM), was examined 3 hours after LPS challenge. LPS-stimulated J774 released 287.83+/-88 u/ml TNF alpha into the culture medium. MG (1 mM) significantly inhibited TNF alpha release by 73% (P<0.05). L-NAME (1 mM) significantly inhibited TNF alpha release too by 72.88% (P<0.05). The effect of MG and L-NAME have been also studied in vivo. Serum TNF alpha levels in LPS treated rats 2 h after LPS challenge were 88.33+/-31.7 u/ml as compared to the serum TNF alpha levels of control rats (undetectable). Treatment of rats with MG (30 mg/kg, i.p.) strongly and significantly reduced TNF alpha release (98.71% inhibition; with P<0.001); in the same experimental setting L-NAME (10 mg/kg, i.p.) also significantly reduced TNF alpha serum levels (76.47% inhibition; with P<0.01). These results could indicate that immune disfunction related to uremia may be related to the inhibitory capability of uremic catabolyte, MG, on TNF alpha synthesis and release.

Uraemic guanidino compounds inhibit gamma-aminobutyric acid-evoked whole cell currents in mouse spinal cord neurones

Guanidine, creatinine (CTN), methylguanidine (MG) and guanidinosuccinic acid (GSA) are four endogenous guanidino compounds with proven neuroexcitatory actions, and putative pathophysiological significance as uraemic toxins. The effects of these uraemic guanidino compounds, were studied on whole-cell current evoked by gamma-amino butyric acid (GABA) on mouse spinal cord neurones in vitro. CTN, MG and GSA concentration dependently blocked GABA-evoked current with calculated IC50 values (+/-SE) of 9.6 +/- 0.9, 9.7 +/- 1.5 and 5.1 +/- 0.4 mM, respectively. CTN, MG and GSA were shown to block inward and outward currents to the same extent, demonstrating voltage independent block of GABA-evoked current by these compounds. Guanidine, however, evoked inward whole-cell currents, which were almost completely blocked by strychnine, indicating that the guanidine-evoked current might have been due to glycine receptor activation.

Comparison of antioxidant activities of aminoguanidine, methylguanidine and guanidine by luminol-enhanced chemiluminescence

1. The objective of this study was to investigate the ability of aminoguanidine, methylguanidine and guanidine to inhibit free radicals or metabolites generated by either stimulated human leucocytes or cell-free systems using luminol-enhanced chemiluminescence (CL). 2. Aminoguanidine (0.1 microM-10 mM), methylguanidine (10 microM-10 mM) and guanidine (10 microM-10 mM) produced concentration-dependent inhibition (96+/-0.1%, n=7, 59+/-1.3%, n=6, and 62+/-3%, n=6, P<0.05 at 10 mM, respectively) in FMLP-stimulated leucocytes CL. 3. In cell-free experiments, hydrogen peroxide (H2O2), hypochlorous acid (HOCl), hydroxyl radical and peroxynitrite-induced CL responses were initiated by hydrogen peroxide (3.5 mM), NaOCl (50 microM), FeSO4 (40 nM) and peroxynitrite (20 nM), respectively. Aminoguanidine, methylguanidine and guanidine produced concentration-dependent inhibition in H2O2-(69+/-0.7%, n=7, 26+/-1%, n=6, and 15+/-0.5%, n=6, at 1 mM, respectively) and HOCl-(84+/-0.3%, n=6, 50+/-1%, n=6, and 29+/-1%, n=7, at 1 mM, respectively) induced luminol CL. Peroxynitrite-induced CL was markedly attenuated in a concentration-dependent manner by aminoguanidine (99+/-0.1%, n=6, at 10 mM), methylguanidine (5+/-0.2%, n=6, at 10 mM) and guanidine (27+/-0.4%, n=7, at 10 mM). However, inhibition with aminoguanidine was found to be more marked than with methylguanidine and guanidine. Aminoguanidine (95+/-0.5%, n=6, at 1 mM) and methylguanidine (25+/-1%, n=6, at 1 mM), but not guanidine (2+/-1%, n=6, at 1 mM), significantly decreased ferrous iron-induced CL. 4. Collectively, these data suggest that aminoguanidine and a high concentration (> or = 0.1 mM) of methylguanidine have direct scavenging activities against H2O2, HOCl, hydroxyl radical and peroxynitrite. Guanidine, at a high concentration (> or = 0.1 mM), scavenges H2O2, HOCl and peroxynitrite, but not the hydroxyl radical. These direct scavenging properties may contribute to inhibitory effects of these compounds on human leucocyte CL.

Synthesis and pharmacological evaluation of N-(2,5-disubstituted phenyl)-N'-(3-substituted phenyl)-N'-methylguanidines as N-methyl-D-aspartate receptor ion-channel blockers

In the mammalian central nervous system, the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors may play an important role in brain diseases such as stroke, brain or spinal cord trauma, epilepsy, and certain neurodegenerative diseases. Compounds which specifically antagonize the actions of the neurotransmitter glutamate at the NMDA receptor ion-channel site offer a novel approach to treating these disorders. CERESTAT (4, aptiganel CNS 1102) is currently undergoing clinical trial for the treatment of traumatic brain injury and stroke. Previously, we reported that analogues of N-1-naphthyl-N'-(3-ethylphenyl)-N'-methylguanidine (4) bound to the NMDA receptor ion-channel site with high potency and selectivity. Recently, molecules active at both sigma receptors and NMDA receptor sites were investigated. A series of substituted diphenylguanidines 6 which are structurally related to N-1-naphthyl-N'-(3-ethylphenyl)-N'-methylguanidine was prepared. Compounds containing appropriate substitution pattern in one of the phenyl rings of diphenylguanidines displayed high affinity. For example, N-(2,5-dibromophenyl)-N'-(3-ethylphenyl)-N'- methylguanidine (27b, R2 = R5 = Br, R3 = C2H5) exhibited potency at both sigma receptors and NMDA receptor sites; 27b also showed high efficacy in vivo in a neonatal rat excitotoxicity model. Further studies indicated that substituent effects were important in this compound series, and 2,5-disubstituted phenyl was the preferred substitution pattern for high-affinity binding at NMDA receptor sites. Bromo and methylthio were the optimal substituents for the R2 and R5 positions of the 2,5-disubstituted phenyl group, respectively. N-(2-Bromo-5-(methylthio)phenyl)-N'- (3-ethylphenyl)-N'-methylguanidine (34b, R2 = Br, R5 = SMe, R3 = C2H5) was highly active at NMDA receptor sites. We found that the binding affinity of guanidines of type 6 could be further enhanced with the appropriate substitution at R3. Optimal activity in this series are afforded by 43b and 44b (R2 = Cl or Br, R5 = R3 = SCH3). Both 43b and 44b bound to NMDA receptor sites with high potency and selectivity (Ki vs [3H]MK-801: 1.87 and 1.65 nM, respectively); these compounds are active in vivo in various animal models of neuroprotection. The structure--activity relationships for these compounds at the NMDA receptor ion-channel site are discussed.

Effect of methylguanidine, guanidine and structurally related compounds on constitutive and inducible nitric oxide synthase activity

The inhibitory activity of methylguanidine, guanidine and their precursors, creatine and creatinine, on both the neuronal constitutive and lung inducible isoforms of nitric oxide synthase were examined in this study. Methylguanidine and guanidine (0.01-3 mM) significantly (P<0.01) inhibited in a concentration-dependent manner both isoforms of the enzyme. Furthermore analysis of the inhibition curves by ANOVA revealed that methylguanidine and guanidine act as non selective inhibitors of both nitric oxide synthases (P>0.4 for both methylguanidine and guanidine). In contrast, creatine and creatinine, although containing guanidine group, were totally ineffective on either enzyme even at concentration up to 3 mM. The results obtained for tested compounds also suggest a role for the lateral chain of guanidine group in the enzyme inhibition. The lack of selectivity of methylguanidine and guanidine in inhibiting both the nitric oxide synthase enzymes could account for some pathological manifestations like neurological disorders, host defense impairment and probably hypertension, that often occur in patients with uremia or chronic renal failure.

Relative contributions of advanced glycation and nitric oxide synthase inhibition to aminoguanidine-mediated renoprotection in diabetic rats

Advanced glycation end products (AGEs) have previously been shown to be increased in the diabetic kidney. Aminoguanidine, an inhibitor of advanced glycation, has been shown to attenuate the development of AGEs as well as the progression of renal disease in experimental diabetes. However, the precise mechanisms through which aminoguanidine acts remain to be elucidated since it is also able to act as an inhibitor of nitric oxide synthase (NOS). This study has therefore compared the effects of aminoguanidine with the effects of two other inhibitors of NOS, L-NAME and methylguanidine, on the development of experimental diabetic nephropathy. Diabetic rats were randomised to receive no treatment, aminoguanidine (1 g/l in drinking water), L-NAME (5 mg/l in drinking water) or methylguanidine (1 g/l in drinking water). Diabetic rats had increased levels of albuminuria and urinary nitrite/nitrate excretion when compared to control rats. Renal AGEs measured by fluorescence as well as by a carboxymethyllysine reactive radioimmunoassay, were elevated in diabetic rats. No changes in inducible NOS (iNOS) protein expression were detected in experimental diabetes nor did aminoguanidine affect iNOS expression. Aminoguanidine did not affect blood glucose or HbA1c but it did prevent increases in albuminuria, urinary nitrites/nitrates and renal AGE levels as measured by fluorescence and radioimmunoassay. L-NAME and methylguanidine did not retard the development of albuminuria, nor did they prevent increases in renal AGE levels, as assessed by fluorescence. However, these treatments did prevent increases in AGEs, as measured by radioimmunoassay. This study indicates that the renoprotective effect of aminoguanidine in experimental diabetes cannot be reproduced by L-NAME or methylguanidine. It is likely that the effect of aminoguanidine is mediated predominantly by decreased AGE formation rather than via NOS inhibition. It also raises the possibility that inhibition of fluorescent AGE formation may be more renoprotective than inhibition of the formation of carboxymethyllysine-containing AGEs.

The increase in blood pressure induced by inhibition of nitric oxide synthase in anesthetized Wistar rats is inversely related to basal blood pressure value

Nitric oxide produced by endothelial cells is a mediator involved in the regulation of vascular tone. Indeed, in vitro inhibition of basal nitric oxide release increase responses to vasoconstrictor agents, and in vivo, the parenteral or dietary administration of nitric oxide inhibitors produces an increase in blood pressure. However, the correlation of nitric oxide production and basal blood pressure in normotensive subjects is still unclear. In this study, we showed that administration, in urethane-anesthetized Wistar rat, of two inhibitors of nitric oxide synthase, such as NG-nitro-L-arginine methyl ester and methylguanidine, produced a significant increase in mean arterial blood pressure that was inversely related to basal mean arterial blood pressure at all doses tested. On the other hand, the increase in mean arterial blood pressure induced by infusion of angiotensin II did not correlate with basal mean arterial blood pressure. The pretreatment of rats with hexamethonium did not change the results observed in normal rats, ruling out an involvement of sympathetic nervous system. In conclusion, this study further confirms the presence of a tonic amount of nitric oxide, produced by endothelial cells in the bloodstream, which plays a key role in the regulation of basal blood pressure, and its reduction or inhibition may be the main cause of certain pathologic conditions characterized by high levels of blood pressure.

Discordant effects of guanidines on renal structure and function and on regional vascular dysfunction and collagen changes in diabetic rats

We examined the effects of aminoguanidine and methylguanidine on vascular dysfunction, glomerular structural changes, and indexes of early and late nonenzymatic glycation in 7-month streptozotocin-induced diabetic rats. Kidney weight, glomerular volume, fractional mesangial volume, glomerular capillary basement membrane width, and urinary albumin excretion were increased in diabetic rats. Diabetes also 1) increased vascular albumin permeation twofold in retina, sciatic nerve, aorta, skin, and kidney; 2) decreased renal collagenase-soluble collagen; 3) increased collagen-associated fluorescence in kidney and skin but not in aorta; and 4) increased glycated hemoglobin levels and aortic pentosidine levels. Aminoguanidine reduced albuminuria by 70% after 4 months, and both guanidines 1) normalized aortic pentosidine levels and renal collagenase-soluble collagen, 2) had no effect on glycated hemoglobin levels or collagen-associated fluorescence (in aorta, kidney, or skin), and 3) had little or no effect on regional albumin permeation. These discordant effects of aminoguanidine on diabetes-induced vascular changes versus parameters of nonenzymatic glycation are consistent with a multifactorial pathogenesis of diabetic complications, including roles for metabolic imbalances independent of nonenzymatic glycation. To the extent that glomerular matrix accumulation and increased regional albumin permeation in chronically diabetic rats are sequelae of nonenzymatic glycation, these findings point to an important role for early glycation reactions and products.

Effects of 1,1-dimethylguanidine administration on blood pressure, heart rate and renal sympathetic nerve activity in normotensive and spontaneously hypertensive rats

1,1-dimethylguanidine (DMG) is an endogenous nitric oxide (NO) synthesis inhibitor. This study investigates the effects of exogenous DMG administration, in anaesthetized spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). Mean blood pressure (MBP), heart rate (HR) and renal sympathetic nerve activity (SNA) were recorded in 12- to 14-week old, anaesthetized SHR and WKY. Each rat received increasing bolus injections of DMG intravenously (1.03, 2.05, 6.39, 20.45 and 51.15 mg kg-1). In separate experiments, SHR received L-arginine or D-arginine in a dose of 300 mg kg-1 followed by DMG at 6.39 mg kg-1. Thirty minutes later they received the same doses of the respective arginines followed by DMG at 20.45 mg kg-1 DMG induced dose-dependant increases in MBP in SHR and WKY. In SHR, HR increased with increasing doses of DMG (except at the near-toxic doses of 51.15 mg kg-1), whereas in WKY HR decreased with increasing doses of DMG. The net change of renal SNA ranged from -5 +/- 3 to -55 +/- 12% in SHR and from -6 +/- 8 to -66 +/- 8% in WKY. Pre-treatment with L-arginine in SHR partly inhibited the pressor effect and attenuated the inhibition of renal SNA induced by DMG, but had little effect on HR. Thus the administration of NO inhibitor DMG could alter cardiovascular function and sympathetic nerve activity, and subsequently resulted in tachycardia in SHR and bradycardia in WKY.

N-methyl-D-aspartate receptor activation by guanidinosuccinate but not by methylguanidine: behavioural and electrophysiological evidence

Levels of methylguanidine (MG) and guanidinosuccinate (GSA) are known to be highly increased in uraemic patients. In the present work, the effects of these uraemic guanidino compounds on the excitatory amino acid system were investigated in vivo and in vitro. It was found that convulsions induced by intracerebroventricular GSA injection in mice were antagonized by N-methyl-D-aspartate (NMDA) receptor blockade, whereas those induced by MG were not significantly altered. Application of GSA (between 25 and 10,000 microM) to mouse spinal cord neurones in primary dissociated cell cultures, evoked depolarizing, inward whole-cell currents in a dose-dependent fashion and with reversal potential at 0 mV; MG did not produce such effects. GSA-induced whole-cell currents were caused by NMDA receptor activation since NMDA receptor antagonists (2-amino-5-phosphonovalerate, Mg2+ and ketamine) blocked GSA-evoked whole-cell currents completely and reversibly, whereas co-application of a non-NMDA receptor antagonist (6-cyano-7-nitroquinoxaline-2,3-dione) did not affect GSA-induced current. Evoked field potentials in CA1 region of rat hippocampal slices were completely abolished by GSA, and this effect was antagonized by NMDA receptor blockade. All data were consistent with selective agonist action of GSA upon the NMDA-type glutamate receptor. In view of the results presented here, it should be examined whether NMDA receptors contribute to the neurological complications of renal failure through GSA-induced inappropriate or excessive activation of NMDA receptors.

Macula densa derived nitric oxide in regulation of glomerular capillary pressure

Nitric oxide (NO) is produced by enzymes called nitric oxide synthases (NOS). At least three different isoforms of NOS have been identified in the kidney. This study examines the effects of selective inhibition of the inducible isoform (iNOS) and the neuronal isoform (bNOS) on the glomerular capillary pressure (PGC), through studies of the tubuloglomerular feedback (TGF) mechanism in anaesthetized rats. The proximal tubular stop-flow pressure (PSF) was measured to estimate changes in PGC obtained after activation of the TGF system by varying the loop of Henle perfusion rate with artificial ultrafiltrate including vehicle, NOS inhibition or L-arginine. Infusion of nonspecific NOS inhibition (N omega-Nitro-L-arginine) increased maximal TGF responses (delta PSF) by 84% and L-arginine decreased delta PSF by 37%. Aminoguanidine, a selective iNOS-inhibitor, failed to increase delta PSF, whereas the nonspecific NOS inhibitor methylguanidine increased delta PSF by 64%. 7-Nitro indazole (7-NI), a selective bNOS inhibitor, increased delta PSF by 57% when infused intratubularly, and intraperitoneal administration of 7-NI increased delta PSF by 78%, without any change in blood pressure. Since bNOS is exclusively located in the macula densa (MD) cells, these results confirm and strengthen the obligatory role of MD-produced NO in regulation of TGF and PGC, which has been suggested earlier. iNOS, widely expressed in the kidney, does not seem to play any important role in regulation of PGC.

Inactivation of nitric oxide synthase isoforms by diaminoguanidine and NG-amino-L-arginine

Diaminoguanidine (DAG) and NG-amino-L-arginine each produced a time- and concentration-dependent inactivation of the citrulline-forming activity of all three NOS isoforms. DAG inactivates both the NADPH-oxidase and the citrulline-forming activities of GH3 pituitary nNOS while NG-amino-L-arginine inactivates only its citrulline-forming activity. The inactivation by DAG of GH3 nNOS NADPH-oxidase and citrulline forming activities is stimulated by (6R)-5,6,7,8-tetrahydrobiopterin (BH4) cofactor, follows pseudo-first-order kinetics and is not substrate saturable. DAG-induced inactivation of the citrulline-forming activity for the iNOS and eNOS isoforms displayed maximal inactivation rates of 0.37 and 0.14 min-1 and Ki values of 385 and 670 microM, respectively. At 1 mM DAG and saturating BH4, half-times of inactivation of 0.7, 8, and 2 min were observed for the nNOS, eNOS, and iNOS isoforms, respectively. NG-Amino-L-arginine-induced inactivation of the citrulline-forming activity of the nNOS, iNOS, and eNOS isoforms displayed maximal inactivation rates of 0.35, 0.26, and 0.53 min-1 and Ki values of 0.3, 3, and 2.5 microM, respectively. The inactivation of the NOS activities by both DAG and NG-amino-L-arginine in preincubations required the presence of oxygen and Ca2+, consistent with an inactivation mechanism that requires active metabolism by NOS. Methylguanidine and 1,1-dimethylguanidine exhibited a reversible inhibition pattern in contrast to all three NOS isoforms. Neither agent exhibited significant isoform selectivity.

Aminoguanidine does not inhibit aldose reductase activity in galactose-fed rats

Aminoguanidine, nucleophilic hydrazine derivative, has been shown to inhibit diamine oxidase, the formation of advanced glycation endproducts, nitric oxide synthase, and catalase. Prompted by the reports that aminoguanidine also inhibits aldose reductase (AR), we have investigated the effect of aminoguanidine, 1,3-diaminoguanidine, and methylguanidine on AR activity in vitro, and in vivo. In vitro, we have measured the inhibition of AR isolated from bovine lenses; in vivo, we have examined the effect on the galactitol levels in the red blood cells, sciatic nerve, retina, and lens of rats administered the test compounds for 11 days in the drinking water and, for the last 4 days, given access to a 20% galactose diet. Two known, structurally distinct AR inhibitors, tolrestat and compound WAY-121,509, were used as reference. In vitro, at concentrations up to 1.0 mmol/L, none of the tested guanidine derivatives had any effect on AR. As a corollary, in vivo, at doses ranging from 201 to 349 mg/kg/day, none of the guanidine derivatives affected tissular galactitol levels. We conclude that, in short-term galactose-fed rats, at the doses tested, aminoguanidine, 1,3-diaminoguanidine, and methylguanidine do not inhibit AR.

Effect of methylguanidine on rat blood pressure: role of endothelial nitric oxide synthase

1. The effect of acute i.v. administration of methylguanidine (MG) on mean arterial blood pressure (MABP) was investigated in anaesthetized male Wistar rats. 2. MG (1-30 mg kg-1 i.v.) produced an increase in MABP in a dose-dependent manner both in normal and in hexamethonium (5 mg kg-1, i.v)-treated rats. 3. L-Arginine (30 or 150 mg kg-1, i.v.), but not its enantiomer D-arginine (30 or 150 mg kg-1, i.v.), reversed the effect of MG on MABP in both normal and hexamethonium-treated rats. 4. L-Arginine (150 mg kg-1, i.v.) administered 2 min before MG (30 mg kg-1, i.v.) prevented the increase in MABP caused by MG in either normal or hexamethonium-treated rats. This effect was not observed with D-arginine (150 mg kg-1, i.v.). 5. Thus, the rise in MABP caused by MG in the anaesthetized rat is due to inhibition of endothelial NO-synthase activity. We speculate that the rise in the plasma concentration of endogenous MG associated with uraemia may contribute to the hypertension seen in patients with chronic renal failure.

Fast atom bombardment mass spectrometric determination of the molecular weight range of uremic compounds that displace phenytoin from protein binding: absence of midmolecular uremic toxins

Uremic compounds are known to displace phenytoin from protein binding, resulting in a higher concentration of the pharmacologically active free fraction of phenytoin. The true chemical identities and molecular weight range of all these compounds are still unknown. We demonstrated that indoxyl sulfate and hippuric acid, which are found at high concentrations in uremic patients, can only partially explain the elevated free phenytoin concentration. Other known uremic compounds, guanidine, methylguanidine, and guanidinosuccinic acid, do not displace phenytoin from the protein-binding sites. Uremic compounds from sera of patients on maintenance hemodialysis were removed using activated charcoal. These compounds were then backextracted from activated charcoal using methanol and analyzed by fast atom bombardment mass spectroscopy. Mass spectra of uremic sera showed no peak over m/z 450, indicating that midmolecular uremic toxins are not involved in displacing phenytoin from protein binding. We also observed additional peaks in the mass spectrum of uremic compounds when compared with the normal serum extract, indicating the presence of several endogenous compounds in uremic sera, as expected.

Inhibition of nitric oxide formation by guanidines

Aminoguanidine, N,N'-diaminoguanidine, methylguanidine, and 1,1-dimethylguanidine were compared to NG-monomethyl-L-arginine (L-NMMA) for their ability to inhibit nitric oxide (NO) formation by cytokine-inducible and vascular constitutive isoforms of NO synthase. These comparisons were performed by assessing (1) cytokine-induced production of nitrite by RINm5F cells, (2) vasoconstrictor responses of isolated rat mesenteric arteries, and (3) in vivo blood pressure responses following intravenous bolus injection into anesthetized rats. Aminoguanidine and L-NMMA were the most potent inhibitors of cytokine-induced NO formation in RINm5F cells, while the other guanidine compounds were 10 (1,1-dimethylguanidine) to 100 (methylguanidine) times less potent. L-NMMA and 1,1-dimethylguanidine were the most potent inhibitors of the vascular constitutive isoform of NO synthase in both assay systems, while aminoguanidine and N,N'-diaminoguanidine were the least potent. These results (1) confirm the selective inhibition of the inducible isoform of NO synthase by aminoguanidine, (2) indicate that N,N'-diaminoguanidine, while approximately 30 times less potent than aminoguanidine in inhibiting inducible NO synthase, has very little effect on constitutive NO synthase activity, and (3) 1,1-dimethylguanidine, like L-NMMA, is a relatively potent inhibitor of both isoforms of NO synthase.

Effect of some products of protein catabolism on the endothelium-dependent and -independent relaxation of rabbit thoracic aorta rings

We have investigated the effect that some products of protein catabolism have on endothelium-dependent and -independent relaxation of rabbit aorta rings precontracted with phenylephrine (PE). All the products tested, i.e., creatinine (CRT), guanidinosuccinic acid (GSA), urea (UR), guanidine (GND) and methylguanidine (MG), are structurally related to L-arginine (L-ARG), the substrate for nitric oxide (NO) biosynthesis which accounts for the biological properties of endothelium-derived relaxing factor (EDRF). Endothelium-derived NO (EDNO) release was induced by agents acting via a receptor- [acetylcholine (ACh)] or a nonreceptor-mediated mechanism (calcium ionophore A23187), and the endothelial-independent relaxation was induced by the NO donor glyceryl trinitrate (GTN). CRT (0.1-10 mM) did not modify the endothelium-dependent relaxation caused by ACh or A23187 but produced a small increase in the response to the endothelium-independent vasorelaxant GTN. Concentrations of GSA up to 1 mM did not affect the relaxation of rabbit aortic rings induced by either ACh or A23187, but at 10 mM, GSA enhanced the relaxation produced by these agents. UR (1-100 mM) inhibited, in a concentration-dependent manner, the relaxation induced by ACh, but not that caused by A23187 or GTN. By comparison, GND and MG (0.1-10 mM) produced a concentration-related inhibition of both ACh- and A23187-induced relaxation. The inhibition by these compounds was either completely or partially reversed by L-ARG. In contrast, the relaxation induced by GTN was inhibited only by higher concentrations (10 mM) of GND or MG. These results indicate that some products of protein catabolism can reduce EDNO formation in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Guanidine derivatives restore activity to carboxypeptidase lacking arginine-127

Arg-127 stabilizes the oxyanion of the tetrahedral intermediate formed during Zn2+ carboxypeptidase A-catalyzed hydrolysis. Mutant carboxypeptidases lacking Arg-127 exhibit substantially reduced rates of hydrolysis with the change manifest almost entirely in kcat (kcat/Km is decreased by 10(4) for R127A). Therefore, Arg-127 stabilizes the enzyme-transition state complex but not the ground state enzyme-substrate complex (Phillips, M.A., Fletterick, R., & Rutter, W.J., 1990, J. Biol. Chem. 265, 20692-20698). The addition of guandine, methylguanidine, or ethylguanidine to R127A increases the kcat for hydrolysis of Bz-gly(o)phe by 10(2) without changing the Km. Dissociation constants (Kd) for the guanidine derivatives range from 0.1 to 0.5 M. The binding affinity for the transition state analog Cbz-phe-alaP(o)ala is increased similarly by 10(2); in contrast, the binding affinity of the ground state inhibitor benzylsuccinic acid is not altered. Thus, guanidine derivatives mimic Arg-127 in stabilizing the rate-limiting transition state. Hydrolysis of Bz-gly-(o)phe by wild-type carboxypeptidase, R127K, or R127M is not substantially affected by guanidine derivatives. Additionally, primary amines do not change the activity of R127A. These observations imply that guanidine binds in the cavity vacated by Arg-127 specifically and in a productive conformation for catalysis.

Effects of methylguanidine on sodium and organic ion transport

We studied the acute effect of methylguanidine (MG), a suspected uremic toxin that accumulates in renal failure, on p-aminohippurate (PAH) and tetraethylammonium (TEA) uptake in rabbit kidney slices, on Na+,K+ ATPase activity in the microsomal fraction of rabbit kidneys, and on transepithelial active Na transport across toad skin. MG at concentrations ranging from 0.05 (similar to that reported in uremic patients) to 1.0 mM does not affect the organic anion (PAH) uptake, although it exhibits a concentration-dependent inhibition of organic cation (TEA) uptake. MG at concentrations from 0.05 to 5 mM had no effect on kidney Na+,K+ ATPase activity or on active transepithelial Na transport across toad skins when applied to the outside bathing solution; however, MG (greater than 1 mM) stimulated Na transport when applied to the inside bathing solution. These results are not consistent with the hypothesis that MG is a potential uremic toxin that causes the natriuresis and other toxic effects. However, long-term toxic effects of MG on the kidney were not assessed in the present study.

[Studies of the effects of low-molecular uremic toxins on anaerobic metabolism of human erythrocytes]

The effect of methylguanidine (MG) and guanidinosuccinic++ acid (GSA) on the anaerobic glycolysis of normal human red blood cells was studied in vitro after three-hours incubation at temperature 37 degrees C. The changes of glucose and lactic acid levels as well as intermediate metabolites and adenine nucleotides of carbohydrate metabolism were determined. Glucose and lactic acid were measured with enzymatic methods, however main phosphate compounds and nucleotide coenzymes were determined by a method of column chromatography. The studies have shown, that GSA in concentrations approximate to those appearing in the plasma patients with chronic renal failure, insignificant enhanced glucose utilization in normal erythrocytes with simultaneous significant decrease produce of lactic acid. Methylguanidine have weakly effected on this process. Subsequent studies have demonstrated that mixture MG (5.4 x 10(-5) mol/dm3) and GSA (2.8 x 10(-5) mol/dm3) induced characteristic changes in composition of main phosphate compounds of carbohydrate metabolism in human red blood cells. The statistically significant decrease of ATP in blood cells with simultaneous increase ADP and AMP were observed. The concentrations of hexose monophosphate, fructose-1,6-diphosphate, triosephosphate, and 2,3-diphosphoglycerinic acid were also higher. The inorganic phosphate concentration was over 80% higher than in control cells. The results of our studies have revealed that energetic metabolism was inhibited in red blood cells which were incubated with MG and GSA.

Inhibitory effect of methylguanidine on insulin binding to its receptor. Mechanism underlying insulin resistance in uremia

To elucidate the mechanism responsible for the decreased insulin binding to erythrocytes in uremic patients, the effects of incubation with sera obtained from uremic patients or with methylguanidine, respectively, on insulin binding were examined. Insulin binding to erythrocytes from uremic patients was lower than that from normal subjects (3.1 +/- 0.19% vs 6.6 +/- 0.33%, Mean +/- SEM, P less than .005), being due mainly to decreased binding affinity (58% of control). Incubation of erythrocytes with 1:5 diluted sera of uremic patients resulted in decreased insulin binding (65 +/- 5% of control) and this decrease was restored to the level of 78 +/- 3% of the controls after incubation with buffer for 12 h. Methylguanidine inhibited insulin binding to erythrocytes in a dose-dependent manner. Post-dialyzed serum with 100 ng/ml of methylguanidine (as seen in pre-dialyzed uremic patients) inhibited insulin binding to erythrocytes as much as pre-dialyzed serum (54.3 +/- 3% vs 47 +/- 1% of control). Incubation of IM-9 lymphocytes with 100 ng/ml of methylguanidine did not alter the insulin receptor mRNA level. These results suggest that methylguanidine inhibits insulin binding to its receptor, resulting in decreased insulin binding to erythrocytes.

[Effect of methylguanidine and guanidine succinic acid on methemoglobin reductase activity in human red cells]

Activity of methemoglobin reductase was studied in human red cells treated with methylguanidine and guanidinosuccinic acid in concentrations similar to those in plasma of patients with chronic renal failure. Enzyme activity was measured with Richterich technique following an incubation at 37 degrees C for three hours. Results have shown that methylguanidine in concentration of 5.4 x 10(-5) mol/l decreases activity of methemoglobin reductase in human red cells on average by 13.9%. Higher concentrations potentiate this effect. Similar changes in methemoglobin reductase activity were noted after introduction of guanidine-succinic acid into the mixture. This agent in concentration 5.6 x 10(-5) mol/l inhibited activity of the tested enzyme by 34.2% on average. Combined methylguanidine in concentration of 5.4 x 10(-5) mol/l and guanidine-succinic acid in concentration of 2.8 x 10(-5) mol/l inhibited methemoglobin reductase activity by 33.0% on average. It may be suggested, that methylguanidine and guanidine-succinic acid being low molecular uremic toxins may significantly decrease methemoglobin reductase activity in red cells of patients with chronic renal failure.

Guanidino compounds that are increased in cerebrospinal fluid and brain of uremic patients inhibit GABA and glycine responses on mouse neurons in cell culture

Four guanidino compounds that have been found to be markedly increased in cerebrospinal fluid and brain tissue of uremic patients, namely, guanidine, methylguanidine, creatinine, and guanidinosuccinic acid, were applied to mouse spinal cord neurons in primary dissociated cell culture to evaluate their effects on postsynaptic responses to gamma-aminobutyric acid (GABA) and glycine. Intracellular microelectrode recording techniques were used. Guanidine, methylguanidine, creatine, and guanidinosuccinic acid reversibly and in a dose-dependent manner inhibited both GABA and glycine responses. Guanidinosuccinic acid was the most potent inhibitor of the amino acid responses, followed in decreasing potency by methylguanidine, guanidine, and creatinine. Guanidinosuccinic acid inhibited responses to GABA and glycine, at concentrations similar to those found in cerebrospinal fluid and brain tissue of patients with terminal renal insufficiency. The other guanidino compounds tested exerted their effects only at concentrations higher than those found in uremic biological fluids and tissues. The inhibitory effect of guanidine and methylguanidine on responses to GABA was additive. The effect of the guanidino compounds on GABA responses was not antagonized by coapplication of the benzodiazepine-receptor antagonist CGS 9896. The results suggest that guanidine, methylguanidine, creatinine, and guanidinosuccinic acid inhibited responses to the inhibitory neurotransmitters GABA and glycine by blocking the chloride channel. The observed action of the studied guanidino compounds might contribute to the pathogenesis of the complex neurological symptomatology encountered in uremia.

[Effect of methylguanidine and guanidinosuccinic acid on pyruvate kinase activity in human red cells]

An effect of methylguanidine and guanidinosuccinic acid on pyruvate kinase activity in human red cells was determined in vitro following a 3-hour incubation at 37 degrees C. The obtained results have shown that methylguanidine in the concentration of 1.8 x 10(-5) M/l inhibits pyruvate kinase activity by 20.8%. Pyruvate kinase activity was statistically significantly inhibited on addition of methylguanidine in the concentration of 5.4 x 10(-5) M/l whereas higher concentrations have no such an effect Guanidinesuccinic acid exerted similar but weaker effect on the activity of pyruvate kinase in human red cells. Mixture of methylguanidine (5.4 x 10(-5) m/l) and guanidinesuccinic acid (2.8 x 10(-5) M/l) does not affect pyruvate kinase activity in normal human red cells under identical experimental conditions.

Effect of polyamines, methylguanidine, and guanidinosuccinic acid on calcitriol synthesis

Previous study from our laboratory has demonstrated that infusion of uremic plasma ultrafiltrate to normal rats suppressed their calcitriol synthesis. In order identify the uremic toxins responsible for the suppression of the calcitriol synthesis, we studied the effects of known uremic toxins: spermidine, spermine, methylguanidine (MG), and guanidinosuccinic acid (GSA) on calcitriol metabolism in the rats. Metabolic clearance rate (MCR) and production rate (PR) of calcitriol were measured in normal rats after they were infused for 24 hours with approximately 10 ml of normal saline containing one of the following substances: 0.8 mumoles spermidine, 0.3 mumoles spermine, 150 micrograms MG and 180 micrograms GSA. Control groups of rats were infused with 10 ml of normal saline for 24 hours. MCR of calcitriol was not altered by the infusion of each toxin; however, plasma concentration of calcitriol (controls, 105.3 +/- 6.7 pg/ml; versus GSA, 58.9 +/- 2.5 pg/ml, p less than 0.001) and PR of calcitriol (controls, 39.0 +/- 2.9 ng/kg/day, versus GSA, 22.5 +/- 1.62 ng/kg/day, p less than 0.001) were significantly suppressed by the infusion of GSA. The concentration (1.8 mg/dl) of GSA in the infusate was similar to that in the uremic plasma ultrafiltrate (2.32 +/- 1.41 mg/dl) used in the previous study, though the total amount of GSA infused to the rats was lower in the present study. GSA is, therefore, considered a uremic toxin that suppresses calcitriol synthesis.

Studies on the effect of low-molecule uremic toxins on the activity of glucose-6-phosphate dehydrogenase (E.C.1.1.1.49) and transketolase (E.C.2.2.1.1) in human red blood cells

The effect in vitro was studied of methylguanidine (MG) and guanidinosuccinic acid (GSA) on the activity of glucose-6-phosphate dehydrogenase (G-6-P DH) and transketolase (ETKA) of normal erythrocytes. The results show that MG in concentration (1.8 X 10(-5) mol/l) approximate to or higher then its concentration in plasma of patients with chronic renal failure (ch.r.f.) inhibits the activity of G-6-P DH. The changes of similar nature have been observed in case of GSA. In the same incubation conditions MG or GSA bring about the inhibition of ETKA activity in red blood cells. MG and GSA jointly introduced into incubation mixture in concentrations approximate to those appearing in plasma of patients with ch.r.f. cause the inhibition of G-6-P DH by 35.6% on the average, with no statistically significant differences in ETKA activity.

Studies on bethanidine and meobentine: direct and indirect effects of antifibrillatory drugs

The electrophysiological effects of bethanidine and meobentine were studied on isolated canine cardiac tissues and the in situ dog heart using standard techniques. The "direct" electrophysiological effects of bethanidine (in the beta-adrenergic-blocked Purkinje fiber) resemble the effects of meobentine in the normal canine Purkinje fiber; both drugs produce use-dependent decreases of the maximum rate of depolarization of phase 0 and action potential amplitude. In addition, meobentine prolongs action potential duration (100%) of Purkinje fibers. In ventricular muscle cells, the only significant effect of meobentine is a decrease in the maximum rate of depolarization. In studies of ouabain-induced tachycardias and 24-h infarct-induced ventricular arrhythmias, bethanidine tends to increase heart rate and/or exacerbate the ectopic activity (due to its sympathomimetic effects), whereas meobentine tends to reduce heart rate and restore normal sinus rhythm. Both bethanidine and meobentine increase ventricular fibrillation threshold. This increase is evident following bethanidine injection after the subsidence of the sympathomimetic effects. Finally, moderate increases of ventricular fibrillation threshold following treatment with meobentine are accompanied by partial cardiac sympathetic blockade, as indicated by reduced chronotropic responses to stellate ganglion stimulation. The antiarrhythmic and antifibrillatory effects of bethanidine and meobentine may be explained by the use-dependent effects of these drugs on phase 0 of the action potential and by their sympatholytic actions on the autonomic nervous system. Meobentine may, in addition, exert antiarrhythmic effects by decreasing automaticity in partially depolarized cells.

Effects of saxitoxin analogues and ligand competition on sodium currents of squid axons

Saxitoxin (STX) and several STX analogues from dinoflagellates (genus Protogonyaulax) block sodium conductance in squid giant axons with variable potencies. Toxins, analyzed under voltage clamp, are 21-sulfosaxitoxin, 21-sulfosaxitoxin 11 alpha-hydroxysulfate, 21-sulfosaxitoxin 11 beta-hydroxysulfate, (B1, C1, C2, respectively) and gonyautoxins 2 and 3. The potency sequence for the toxins examined is STX greater than gonyautoxin 3 greater than B1 greater than C2 greater than gonyautoxin 2 much greater than C1. Guanidine, when substituted for sodium in external seawater, reduced the potency of STX to block inward current but did not affect tetrodotoxin activity. Methylguanidine also reduced the ability of STX to block outward sodium current. Inhibitory constants for guanidine and methylguanidine were 116 and 187 mM, respectively. Competition can be explained by binding at or near the toxin binding site but not by surface potential alteration.

The effect of oxalic acid on the aggregability of human platelet rich plasma

Human platelet aggregation induced by ADP was studied after incubation with urea, methylguanidine, creatinine and oxalic acid. Oxalic acid significantly (p less than 0.05) inhibits human thrombocyte aggregation. Physiopathological implications are discussed.

A comparison of the effects of bethanidine, meobentine and quinidine on the electrical activity of rat hearts in vivo and in vitro

Glass microelectrodes were used to record transmembrane electrical activity from cells located just beneath the endocardial surface of segments of the right ventricular free wall of the rat heart during superfusion and electrical stimulation in vitro at 37 degrees C. The sulphates of bethanidine, meobentine or quinidine (4 to 20 microM) applied in vitro caused a prolongation of action potential duration and a delayed and slowed return of electrical excitability following an action potential. Intracardiac electrical stimulation of the urethane-anaesthetized rat heart in situ was used to measure ventricular refractory periods from the electrocardiogram. Intravenous administration of bethanidine, meobentine or quinidine (10 to 20 mg kg-1) caused a prolongation of ventricular refractory periods. Quinidine had a briefer duration of action than either of the other two drugs tested. Urethane-anaesthetized open-chested rats which were subjected to left coronary artery occlusion displayed ventricular tachyarrhythmias in their electrocardiogram. These arrhythmias occurred during the period of occlusion and even more prominently after release of the occlusion. Intravenous administration of bethanidine, meobentine or quinidine (1 to 20 mg kg-1) protected rats against these arrhythmias. The protective effect of quinidine was briefer than that of either of the other two drugs tested.

Structure-activity relationship between guanidine alkyl derivatives and norepinephrine release: site(s) and mechanism(s) of action

The effect of guanidine alkyl derivatives on the evoked release of [3H]norepinephrine [( 3H]NE) from spleen strips was examined. Guanidine, methyl guanidine and N,N-dimethyl guanidine all enhanced the field-stimulated release of [3H]NE 2- to 3-fold, whereas N,N'-dimethyl guanidine and propyl guanidine were without effect. The latter compound blocked the stimulatory effect of an equimolor concentration (4 mM) of guanidine. Guanidine enhanced moderately the field-stimulated release of [3H]NE from spleen strips pretreated with phenoxybenzamine. The efflux of [3H]NE from spleen slices induced by calcium ionophore A-23187 was not altered by guanidine incubation. The effect of guanidine on intracellular calcium movement was also tested by monitoring the effect of the drug on evoked secretion of ATP from human platelets. Guanidine did not modify this release. It is concluded that guanidine and its active structural derivatives augment [3H]NE release by increasing the influx of calcium through the voltage-sensitive calcium channels, but not by the mobilization of intracellular calcium pools. The biochemical basis for the action of the guanidinium cation is discussed.

Antidysrhythmic actions of meobentine sulfate

The antiarrhythmic efficacy of meobentine sulfate, a bethanidine derivative lacking inhibitory effects on adrenergic neuronal function, was assessed in three canine models. Intravenous meobentine sulfate, administered in dosages of 5.0, 10,0, and 20.0 mg/kg, produced a dose-related increase in the ventricular fibrillation threshold (VFT) under nonischemic conditions (7.6 +/- 1.8 mA vs 37.8 +/- 8.6 mA) (20 mg/kg; p less than 0.05) and during regional myocardial ischemia (5.6 +/- 1.5 mA vs 41.8 +/- 9.1 mA) (20 mg/kg; p less than 0.05). The VFT was also increased in the presence of chronic ischemic injury (6.4 +/- 1 mA to 31 +/- 10 mA) (20 mg/kg; p 0.05). In the conscious dog, 4 days after an anterior myocardial infarction, programmed electrical stimulation (PES) produced nonsustained ventricular tachycardia (VT) in five dogs. After meobentine sulfate administration, eight of nine animals had sustained VT and one animal developed ventricular fibrillation (VF). At a dose of 20 mg/kg, there was prolongation of the cycle length of the VT (169 +/- 11 msec to 237 +/- 20 msec), prolongation of the QRS duration (58 +/- 2.6 msec to 71 +/- 3.7 msec), and prolongation of the delay in epicardial activation. There was an enhanced potential after meobentine administration for programmed stimulation to produce ventricular arrhythmias with the introduction of fewer premature impulses. In the third canine model, conscious dogs with a previous anterior myocardial infarction developed VF in response to electrically induced left circumflex coronary artery injury. Meobentine (20 mg/kg) failed to prevent VF in eight of eight dogs. These results suggest that while meobentine sulfate significantly increases the electrical VFT, it does not protect the conscious canine from the induction of ventricular tachyarrhythmias in response to PES, and it does not prevent VF in a conscious canine model of sudden coronary death.(ABSTRACT TRUNCATED AT 250 WORDS)

Current-dependent block of endplate channels by guanidine derivatives

Methyl- and ethylguanidine block the endplate current in frog muscle. Both derivatives blocked inward-going endplate currents without affecting outward endplate currents. Repetitive stimulation that evoked several inward endplate currents enhanced the block, which suggests that these agents interact with open endplate channels. The relative conductance vs. potential curve exhibited a transition from a low to a high value near the reversal potential for the endplate current, both in normal and in 50% Na solution. In the latter solution, the reversal potential for endplate current was shifted by a mean value of 16 mV in the direction of hyperpolarization. The results suggest that methyl- and ethylguanidine block open endplate channels in a manner dependent on the direction of current flow rather than on the membrane potential.

[Investigations for pathogenesis of depression of lymphocyte transformation in chronic uremia (author's transl)]

In order to investigate the depressive effect of the hypothetical low molecular weight "uremic toxins" methylguanidine and phenol on lymphocyte transformation these substances were added to lymphocyte cultures with increasing concentrations. 3H-thymidine uptake and LDH-activity in the supernatant culture medium as a parameter of continuous cell destruction were analyzed after incubation for 5 days. Both substances induced in spontaneous as well as in PHA-stimulated cultures dose-dependent depression of 3H-thymidine uptake and simultaneous increase in LDH-activity. Thus the effect of both toxins may be cytotoxic depression of lymphocyte viability. Furthermore in the same arrangement-way higher molecular weight metabolites from urine of healthy and chronic uremic persons were added in increasing concentrations to lymphocyte cultures. The same result: dose-dependent depression of transformation and increase of LDH-activity was slightly diminished by urine metabolites from uremic patients as compared to healthy persons. This may be due to a shift toward non-toxic compounds. But as these metabolites are excreted in 20-30 fold larger quantities in chronic renal insufficiency, an accumulation in the serum and classification as "uremic toxins" can be excluded.