Contraction and relaxation of aortas from diabetic rats: effects of chronic anti-oxidant and aminoguanidine treatments

LQM10, a guanylhydrazone derivative, reduces nociceptive and inflammatory responses in mice

In the present study, we examined the antinociceptive and anti-inflammatory activities of a guanylhydrazone derivative, (E)-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-guanylhydrazone hydrochloride (LQM10), in mice. The antinociceptive effect was determined by assessing behavioural responses in different pain models, while anti-inflammatory activity was examined in carrageenan-induced pleurisy. Intraperitoneal LQM10 administration reduced the acetic acid-induced nociceptive behaviour, a phenomenon that was unaltered by pretreatment with yohimbine, atropine, naloxone or glibenclamide. In the formalin assay, LQM10 reduced nociceptive behaviour only in the second phase, indicating an inhibitory effect on inflammatory pain. LQM10 did not alter the pain latency in the hot plate assay and did not impact the locomotor activity of mice in the rotarod assay. In the carrageenan-induced pleurisy assay, LQM10 treatment inhibited critical events involved in inflammatory responses, namely, leucocyte recruitment, plasma leakage and increased inflammatory mediators (tumour necrosis factor Like Properties of Chalchones and Flavonoid Derivatives [TNF]-α and interleukin [IL]-1β) in the pleural exudate. Overall, these results indicate that LQM10 exhibits antinociceptive effects associated with peripheral mechanisms and anti-inflammatory activity mediated via a reduction in leucocyte migration and proinflammatory mediators, rendering this compound a promising candidate for treating pain and inflammatory process.

Pathophysiology of diabetic erectile dysfunction: potential contribution of vasa nervorum and advanced glycation endproducts

Erectile dysfunction (ED) due to diabetes mellitus remains difficult to treat medically despite advances in pharmacotherapeutic approaches in the field. This unmet need has resulted in a recent re-focus on the pathophysiology, in order to understand the cellular and molecular mechanisms leading to ED in diabetes. Diabetes-induced ED is often resistant to PDE5 inhibitor treatment, thus there is a need to discover targets that may lead to novel approaches for a successful treatment. The aim of this brief review is to update the reader in some of the latest development on that front, with a particular focus on the role of impaired neuronal blood flow and the formation of advanced glycation endproducts.

Oxidative stress is not associated with vascular dysfunction in a model of alloxan-induced diabetic rats

OBJECTIVES

To verify if an experimental model of alloxan-diabetic rats promotes oxidative stress, reduces nitric oxide bioavailability and causes vascular dysfunction, and to evaluate the effect of N-acetylcysteine (NAC) on these parameters.

METHODS

Alloxan-diabetic rats were treated or not with NAC for four weeks. Plasmatic levels of malondialdehyde (MDA) and nitrite/nitrate (NOx), the endothelial and inducible nitric oxide synthase (eNOS and iNOS) immunostaining and the vascular reactivity of aorta were compared among diabetic (D), treated diabetic (TD) and control (C) rats.

RESULTS

MDA levels increased in D and TD. NOx levels did not differ among groups. Endothelial eNOS immunostaining reduced and adventitial iNOS increased in D and TD. The responsiveness of rings to acetylcholine, sodium nitroprusside, and phenylephrine did not differ among groups.

CONCLUSIONS

NAC had no effect on the evaluated parameters and this experimental model did not promote vascular dysfunction despite the development of oxidative stress.

Structural modifications in the arterial wall during physiological aging and as a result of diabetes mellitus in a mouse model: are the changes comparable?

AIM

Vascular accelerated aging represents the major cause of morbidity and mortality in subjects with diabetes mellitus. In the present study, our aim was to compare premature functional and morphological changes in the arterial wall resulting from streptozotocin (STZ)-induced diabetes mellitus in mice over a short-term period with those that develop during physiological aging. The effect of aminoguanidine (AG) on the prevention of these alterations in the diabetic group was also analyzed.

METHODS

The vascular relaxation response to acetylcholine (ACh) in the mouse was tested in isolated segments of phenylephrine (Phe)-precontracted aorta at 2, 4 and 8 weeks (wk) of STZ-induced diabetes and compare to 12- and 84-wk-old mice. Aortic structural changes were investigated, and receptor for AGE (RAGE) aortic expression was quantified by western blot.

RESULTS

Compared to the 12-wk control group (76 ± 5%), significant endothelium-dependant relaxation (EDR) impairment was found in the group of 12-wk-old mice, which underwent a 4-wk diabetes-inducing STZ treatment (12wk-4WD) (52 ± 4%; P < 0.01) and was yet more apparent in the group of 16-wk-old mice, which underwent an 8-wk diabetes-inducing STZ treatment (16wk-8WD) (34 ± 4%; P < 0.001). The alteration in EDR was relatively comparable between the diabetic 12wk-4WD group and the 84-wk-old group (52.7 ± 4 vs. 48 ± 4%). Intima/media aortic thickening and aortic structural changes were significantly increased in the diabetic 12wk-4WD group and were even more apparent in the 84-wk group compared to the 12-wk controls. AG treatment in the 12wk-4WD+AG diabetic group significantly improved EDR, decreased RAGE expression and showed an aging preventive effect on the structural changes of the arterial wall.

CONCLUSION

Our study compared EDR linked to physiological aging with that observed in the case of STZ-induced diabetes over a short-term period, and demonstrated the beneficial effect of AG.

Resorcylidene aminoguanidine induces antithrombotic action that is not dependent on its antiglycation activity

There is good evidence supporting the notion that aminoguanidine(AG)-derived compounds prevent glycation/glycooxidation-dependent processes and therefore inhibit late diabetic complications. The aim of the present work was to analyse the antithrombotic action and antiglycation activity of beta-resorcylidene aminoguanidine (RAG) in comparison with another commonly used aminoguanidine (AG)-derived compound, pyridoxal aminoguanidine (PAG). In vitro RAG and PAG prevented exhaustive glycation and glycooxidation of BSA to a similar extent. However, merely RAG showed almost complete binding to sepharose-immobilized heparin, while PAG and other AG derivatives had much poorer affinities. In the model of in vivo thrombosis in Wistar rats with extracorporeal circulation RAG (i.v. 30 mg/kg), but not PAG, produced sustained (2 h) antithrombotic effect, which was abrogated by indomethacin (5 mg/kg) and rofecoxib (1 mg/kg). The 60-day treatment of streptozotocin-diabetic animals with RAG (p.o. 4 mg/kg) significantly decreased plasma concentration of a thromboxane B(2) and reduced whole blood platelet aggregability triggered by ADP or collagen. In conclusion, although RAG and PAG displayed similar antiglycation and antioxidation activities in vitro, only RAG showed antithrombotic activity in vivo that involved activation of COX-2/PGI(2) pathway. Our results indicate that designing novel RAG derivatives with optimal antithrombotic and antiglycation activities may prove useful to treat diabetic complications.

Alteration of aortic function from streptozotocin-diabetic rats with Kilham’s virus is associated with inducible nitric oxide synthase

Kilham's rat virus (KRV) is a parvovirus commonly known to affect laboratory rats. Qualitative immunohistochemical analysis revealed that aorta isolated from KRV-infected streptozotocin (STZ)-induced diabetic adult rats expressed markedly greater levels of inducible nitric oxide synthase (iNOS) than aorta from KRV-infected controls. In contrast with the prevailing literature, nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was not blunted by STZ-diabetes, but was comparable to relaxations of aorta from controls. However, with increasing ex vivo duration, a decreased response to acetylcholine was observed in the STZ-diabetic aorta. In addition, whereas contraction responses to phenylephrine were not significantly altered over time in control tissue, aorta from STZ-diabetic rats developed increased tensions. The data suggest that increased iNOS-derived nitric oxide masks expected acetylcholine-mediated relaxation deficits as a result of KRV-infection, and that the deficit is unmasked by iNOS turnover ex vivo.

Inhibitors of Advanced Glycation End Product Formation and Neurovascular Dysfunction in Experimental Diabetes

Advanced glycation and lipoxidation end products (AGEs/ALEs) have been implicated in the pathogenesis of the major microvascular complications of diabetes mellitus: nephropathy, neuropathy, and retinopathy. This article reviews the evidence regarding the peripheral nerve and its vascular supply. Most investigations done to assess the role of AGEs/ALEs in animal models of diabetic neuropathy have used aminoguanidine as a prototypic inhibitor. Preventive or intervention experiments have shown treatment benefits for motor and sensory nerve conduction velocity, autonomic nitrergic neurotransmission, nerve morphometry, and nerve blood flow. The latter depends on improvements in nitric oxide-mediated endothelium-dependent vasodilation and is responsible for conduction velocity improvements. A mechanistic interpretation of aminoguanidine's action in terms of AGE/ALE inhibition is made problematic by the relative lack of specificity. However, other unrelated compounds, such as pyridoxamine and pyridoxamine analogues, have recently been shown to have beneficial effects similar to aminoguanidine, as well as to improve pain-related measures of thermal hyperalgesia and tactile allodynia. These data also stress the importance of redox metal ion-catalyzed AGE/ALE formation. A further approach is to decrease substrate availability by reducing the elevated levels of hexose and triose phosphates found in diabetes. Benfotiamine is a transketolase activator that directs these substrates to the pentose phosphate pathway, thus reducing tissue AGEs. A similar spectrum of improvements in nerve and vascular function were noted when using benfotiamine in diabetic rats. Taken together, the data provide strong support for an important role for AGEs/ALEs in the etiology of diabetic neuropathy.

Correction of nitrergic neurovascular dysfunction in diabetic mouse corpus cavernosum by p38 mitogen-activated protein kinase inhibition

Increased p38 mitogen-activated protein kinase (MAPK) in response to stress stimuli, including hyperglycemia, contributes to diabetic somatic neuropathy. However, effects on autonomic nerve and vascular function have not been determined. The aim of this study was to investigate the effects of the p38 MAPK inhibitor, LY2161793, on penile neurovascular function in streptozotocin-induced diabetic mice. Diabetes duration was 6 weeks and intervention LY2161793 treatment was given for the final 2 weeks. In vitro measurements on phenylephrine-precontracted corpus cavernosum revealed a 32% reduction in maximum nitrergic nerve-mediated relaxation with diabetes that was 74% corrected by LY2161793 treatment. Maximum nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was 42% attenuated by diabetes and 88% restored by LY2161793. Moreover, treatment partially corrected a diabetic deficit in endothelium-independent relaxation to a nitric oxide donor. Thus, p38 MAPK inhibition corrects nitric oxide-dependent indices of diabetic erectile autonomic neuropathy and vasculopathy, a therapeutic approach potentially worthy of consideration for clinical trials.

Blood pressure fall and increased relaxation of aortic smooth muscle in diabetic rats

This study was designed to identify changes in endothelium-independent relaxation that could contribute to the depressed vascular reactivity and fall in blood pressure (BP) detected in rats after 5 weeks of streptozotocin (STZ)-induced (i.e. type 1) diabetes. Aortic rings were contracted by simultaneous activation of voltage-operated channels (KCl=80 mM) and alpha-adrenergic receptors (phenylephrine 1 microM) and then relaxed by simultaneous exposure to Ca2+-free PSS and 10 microM phentolamine. Additional relaxations were performed under conditions in which the plasma membrane Na-Ca exchanger (PMNaCa) or Ca-pump (PMCA), or the sarcoplasmic reticulum (SR) Ca-pump (SERCA) were blocked, to identify which mechanism(s) could modulate this process. The STZ-diabetic rats had a moderate but significant decrease of BP, and their aortic rings exhibited accelerated relaxation following a biexponential model, with a significantly decreased slow component. In control rats only the inhibition of the PMNaCa could slow down the fast component, while the slow component was insensitive to any blocking maneuver. In contrast, the diabetic animals' slow component was sensitive to the inhibition of both the PMNaCa and the SERCA. The SERCA-sensitive 45Ca2+ uptake by the SR was augmented in the aortas of STZ-treated animals. This hyperactivity of the SERCA, associated with augmented activity of the PMNaCa, at least partly induced by an increase of the plasma membrane Na+/K+-ATPase activity, could explain the decrease in BP and the accelerated aortic relaxation observed in the diabetic rats.

Early and intermediate Amadori glycosylation adducts, oxidative stress, and endothelial dysfunction in the streptozotocin-induced diabetic rats vasculature

AIMS/HYPOTHESIS

In a model of streptozotocin-induced Type 1 diabetes mellitus in rats of 9 weeks duration, we analysed time associations between the development of hyperglycaemia, early and intermediate glycosylation Amadori adducts, or AGE compared with enhancement of oxidative stress and endothelial dysfunction.

METHODS

Endothelial function was tested at several stages of streptozotocin-induced diabetes and after treatment with insulin, resulting in different concentrations of blood glucose, glycosylated haemoglobin (an Amadori adduct), and AGE. Other animals were studied antagonising the formation of AGE with aminoguanidine.

RESULTS

Relaxation in response to acetylcholine (1 nmol/l to 10 micro mol/l) was tested in isolated segments from aorta or mesenteric microvessels. Impairment of endothelium-dependent relaxations occurred after 2 weeks of untreated diabetes. Preincubation of vessels affected with 100 U/ml superoxide dismutase improved the relaxations to acetylcholine, along the time-course of the endothelial impairment. This indicates the participation of reactive oxygen species on diabetic endothelial dysfunction. The impairment of endothelium-dependent relaxations was recovered after 3 more weeks of insulin treatment. Aminoguanidine treatment did not modify this pattern of development. The time course of the rise and disappearance of endothelial dysfunction showed a higher correlation with glycosylated haemoglobin concentrations than with blood glucose or serum AGE.

CONCLUSION/INTERPRETATION

Enhancement of early and intermediate Amadori adducts of protein glycosylation was the factor showing a better relation with the development of endothelium impairment. These results are consistent with a role for these products in the development of diabetic vasculopathy.

Effects of diabetes and evening primrose oil treatment on responses of aorta, corpus cavernosum and mesenteric vasculature in rats

Diabetes causes endothelial dysfunction, with deleterious effects on nitric oxide (NO) mediated vasodilatation. However, in many vessels other local vasodilators such as endothelium-derived hyperpolarizing factor (EDHF), prostacyclin, epoxides or endocannabinoids are also important. Several of these factors may be derived from omega-6 essential fatty acids via arachidonate metabolism. Diabetes inhibits this pathway, a defect that may be bypassed by diets enriched with omega-6 gamma-linolenic acid-containing oils such as evening primrose oil (EPO). The aim was to examine the effects of preventive EPO treatment on endothelium-dependent and neurally mediated vasorelaxation. Diabetes was induced by streptozotocin in rats; duration was 8 weeks. Vascular responses were examined in vitro on thoracic aorta, corpus cavernosum and perfused mesenteric bed preparations. Diabetes caused 25% and 35% deficits, respectively, in aorta and corpus cavernosum NO-mediated endothelium-dependent relaxation to acetylcholine that were largely unaffected by EPO treatment. Moreover, a 44% reduction in maximum corpus cavernosum vasorelaxation to nitrergic nerve stimulation was not prevented by EPO. However, for the mesenteric vascular bed, a 29% diminution of responses to acetylcholine, mediated by both NO and EDHF, was 84% attenuated by EPO treatment. When the EDHF component was isolated during NO synthase inhibition, a 76% diabetic deficit was noted. This was completely prevented by EPO treatment, which also caused supernormal EDHF responses in nondiabetic rats. EPO treatment prevented the development of deficits in endothelium-dependent relaxation in diabetic rats. Effects were particularly marked on the resistance vessel EDHF system, which may have potential therapeutic relevance for diabetic microvascular complications.

Transition metals and polyol pathway in the development of diabetic neuropathy in rats

BACKGROUND

The transition metal-catalyzed reaction is a major source of oxygen free radicals, which play an important role in vascular dysfunction leading to ischemia in diabetic tissues. The inhibition of polyol pathway hyperactivity has been reported to ameliorate neurovascular abnormalities in diabetic rats and has been proposed to improve the oxygen free radical scavenging capacity. The present study was conducted to compare the effect of a transition metal chelating agent, trientine (TRI), on diabetic neuropathy with that of an aldose reductase inhibitor, NZ-314 (NZ).

METHODS

Diabetic rats were divided into three groups: (1). untreated, (2). TRI-treated, and (3). NZ-treated. TRI (20 mg/kg) or NZ (100 mg/kg) was administered by gavage or chow containing NZ, respectively, for 8 weeks. Motor nerve conduction velocity (MNCV), coefficient of variation of the R - R interval on electrocardiogram (CVr-r), sciatic nerve blood flow (SNBF), platelet aggregation activities, and serum concentrations of malondialdehyde were measured.

RESULTS

Untreated diabetic rats showed delayed MNCV, decreased CV(R-R), and reduced SNBF compared to normal rats. TRI or NZ completely prevented these deficits. Platelet hyperaggregation activities in diabetic rats were prevented by NZ, but not by TRI. Increased concentrations of malondialdehyde in diabetic rats were partially but significantly ameliorated by either TRI or NZ.

CONCLUSIONS

These observations suggest that increased free radical formation through the transition metal-catalyzed reaction plays an important role in the development of diabetic neuropathy and that the preventive effect of an aldose reductase inhibitor on diabetic neuropathy may also be mediated by decreasing oxygen free radicals.

Effects of protein kinase Cbeta inhibition on neurovascular dysfunction in diabetic rats: interaction with oxidative stress and essential fatty acid dysmetabolism

BACKGROUND

Elevated protein kinase C (PKC) activity is thought to play a substantial role in the aetiology of diabetic microvascular complications, the PKCbeta isoform being identified as particularly important. Neuropathy has a vascular component; therefore, one aim was to assess whether the PKCbeta inhibitor, LY333531, could correct nerve conduction velocity (NCV) and perfusion deficits in diabetic rats. Neurovascular dysfunction also depends on oxidant stress and impaired omega-6 essential fatty acid metabolism; correctable by antioxidant and gamma-linolenic acid (GLA) treatments, respectively. A second aim was to assess whether there were interactions between these mechanisms and PKCbeta-mediated effects.

METHODS

Diabetes was induced by streptozotocin; duration was 8 weeks. NCV was monitored and blood flow was assessed by hydrogen clearance microelectrode polarography.

RESULTS

Diabetes caused 19.7% and 13.9% reductions in sciatic motor and saphenous sensory NCV, respectively. Two weeks of LY333531 treatment dose-dependently corrected these deficits. A dose of 10 mg kg(-1) day(-1) gave non-diabetic NCV values and also completely corrected a 50% diabetic reduction in sciatic endoneurial blood flow. Low-dose (0.25 mg kg(-1) day(-1)) LY333531 had modest effects ( approximately 20% correction) on NCV and sciatic perfusion. However, when combined with equi-effective doses of the antioxidants vitamin E or alpha-lipoic acid, or GLA, motor and sensory NCV and sciatic nerve perfusion were in the non-diabetic range. The joint effect was equivalent to that of the 10 mg kg(-1) day(-1) LY333531 dose, demonstrating synergism between PKCbeta, oxidative stress and essential fatty acid mechanisms.

CONCLUSIONS

LY333531, alone or combined with antioxidants or GLA, could form the basis for therapeutic intervention in neuropathy, which requires assessment in clinical trials.

Vascular protective actions of a nitric oxide aspirin analog in both in vitro and in vivo models of diabetes mellitus

BACKGROUND

Defective endothelium-dependent relaxation is observed in experimental and human diabetes mellitus. The nature of this defect is not fully understood but may involve decreased nitric oxide (NO) bioactivity due to enhanced production of reactive oxygen species (ROS). In this paper, we examine the benefits and actions of a novel NO-donating, antioxidant called 2-acetoxybenzoic acid 2-(2-nitrooxymethyl) phenyl ester, and denoted as NCX4016, on NO-mediated endothelium-dependent relaxation in normal arteries exposed to acute elevations in glucose or in arteries derived from chronic diabetic animals.

MATERIAL AND METHODS

Intrinsic free radical scavenging by NO-NSAIDs in solution were evaluated using electron paramagnetic resonance (EPR) spectroscopy and spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In acute studies, normal rat aortas were exposed in tissue culture for 18 h to 5.5 mM or 40 mM in the presence or absence of NCX4016, a NO-donating NSAID unrelated to aspirin (NCX2216) or aspirin. Vascular reactivity of thoracic aortic rings to endothelium-dependent relaxation to acetylcholine in vitro was determined. For chronic hyperglycemia, diabetes was induced in rats by intravenous injection with streptozotocin. Vascular reactivity of thoracic aortic rings to endothelium-dependent relaxation to acetylcholine in vitro was determined after 8 wks in untreated animals or animals chronically-treated with NCX4016. Antioxidant efficacy in vivo was determined by measurement of plasma isoprostanes and by nuclear binding activity of NF-kappaB in nuclear fractions of aortae.

RESULTS

Incubation with NCX4016 and NCX2216 produced a concentration-dependent inhibition of DMPO-OH formation indicating scavenging of hydroxyl radicals (HO(*)). In contrast, little efficacy to scavenge superoxide anion radicals was noted. Acute incubation of normal arteries with elevated glucose concentration caused inhibition of normal relaxation to acetylcholine. This impairment was prevented by co-incubation with NCX4106 but not by mannitol, the parent compound (aspirin) or by NCX2216. In addition, chronic treatment with NCX4016 prevented the development of defective endothelium-dependent relaxation to acetylcholine. This protection did not occur as a result to any changes in blood glucose concentration or hemoglobin glycation. Treatment with NCX4016 did decrease the elevation in plasma isoprostanes and normalized the diabetes-induced increase in NF-kappaB binding activity in nuclear fractions derived from aortic tissue.

CONCLUSIONS

Collectively, these studies suggest that antioxidant interventions using NO-donating NSAIDs may provide an important novel therapeutic strategy to protect the diabetic endothelium.

Corpus cavernosum dysfunction in diabetic rats: effects of combined alpha-lipoic acid and gamma-linolenic acid treatment

BACKGROUND

The effects of streptozotocin-induced diabetes on nitric oxide (NO)-mediated relaxation of rat corpus cavernosum smooth muscle to neurogenic and endothelial stimulation was examined. The aim was to assess the effects of treatment with low doses of the antioxidant, alpha-lipoic acid, and the omega-6 essential fatty acid, gamma-linolenic acid, either separately or in combination.

METHODS

Treatment was preventive from diabetes induction or corrective over 4 weeks after 4 weeks of untreated diabetes. Corpus cavernosum responses were examined in vitro.

RESULTS

Neither diabetes nor treatment affected contractile responses to transmural electrical field stimulation of noradrenergic nerves. Stimulation of phenylephrine precontracted cavernosa in the presence of guanethidine and atropine caused relaxation via the nitrergic innervation. Maximum relaxation responses were 40% and 46% decreased after 4 and 8 weeks of diabetes, respectively. alpha-Lipoic acid, gamma-linolenic acid combination treatment fully prevented this deficit, and partially (52%) corrected the effect of 4 weeks of untreated diabetes. Neither alpha-lipoic acid nor gamma-linolenic components alone had significant effects, which suggests that there were synergistic interactions between the drugs. Both 4 and 8 weeks of untreated diabetes reduced maximum endothelium-dependent relaxation of phenylephrine precontracted cavernosa to acetylcholine by approximately 40%. While alpha-lipoic acid or gamma-linolenic acid were ineffective, joint treatment fully prevented and corrected this diabetic endothelial deficit. Neither diabetes nor treatment affected endothelium-independent relaxation to the NO donor, sodium nitroprusside.

CONCLUSION

The data show that alpha-lipoic acid and gamma-linolenic acid interact synergistically to improve NO-mediated neurogenic and endothelium-dependent relaxation of corpus cavernosum in experimental diabetes.

Neurovascular interactions between aldose reductase and angiotensin-converting enzyme inhibition in diabetic rats

Increased polyol pathway flux has been linked to nerve complications in diabetic rats, which are attenuated by aldose reductase inhibitors, defective nitric oxide-mediated vasodilation being a particular target. Diabetes also elevates the endothelial angiotensin system, increasing vasa nervorum vasoconstriction. The aim was to assess whether promotion of vasodilation by treatment with the aldose reductase inhibitor, ZD5522 (3',5'-dimethyl-4'-nitromethylsulphonyl-2-(2-tolyl)acetanilide), coupled with reduced vasoconstriction using the angiotensin-converting enzyme inhibitor, lisinopril, interacted positively to improve neurovascular function. After 8 weeks of streptozotocin-induced diabetes, sciatic nerve blood flow and motor conduction velocity were 51% and 21% reduced, respectively. Two weeks of lisinopril treatment dose-dependently corrected the conduction deficit (ED(50) approximately 0.9 mg kg(-1)). Low-dose lisinopril (0.3 mg kg(-1)) or ZD5522 (0.25 mg kg(-1)) had modest corrective (10-20%) effects on nerve conduction and perfusion. However, when combined, blood flow and conduction velocity reached the nondiabetic range. The ZD5522 dose used gave a approximately 45% nerve sorbitol reduction but had no significant effect on fructose content; lisinopril co-treatment did not alter ZD5522 action on polyols. Thus, there was a marked neurovascular synergistic interaction between angiotensin-converting enzyme and aldose reductase inhibition in diabetic rats. This points to a potential therapeutic benefit, which requires evaluation in clinical trials.

Nitric oxide-mediated corpus cavernosal smooth muscle relaxation is impaired in ageing and diabetes

OBJECTIVE

To examine nitric-oxide (NO)-mediated relaxation in cavernosal smooth muscle in a rat model of diabetes, as previous experiments showed that HbA1c (an isoform of glycosylated haemoglobin and a marker of long-term diabetic control) impaired NO-mediated relaxation of normal corpus cavernosal tissue through the generation of superoxide anions.

MATERIALS AND METHODS

Eight weeks after the induction of diabetes, male Wistar rats were killed and cavernosal tissue obtained. Strips were contracted with 1 micromol/L noradrenaline before applying acetylcholine or electrical field stimulation (EFS) or sodium nitroprusside (SNP). Relaxation responses were repeated in the presence of L-arginine (100 micromol/L), indomethacin (10 micromol/L) or superoxide dismutase (SOD, 120 IU/mL). Young and age-matched control animals were examined in the same way.

RESULTS

Eight weeks of uncontrolled diabetes caused a significant impairment in mean relaxation responses to acetylcholine (P < 0.05) and to EFS (P < 0.05), but not to SNP, compared with young and age-matched controls, respectively. L-arginine, indomethacin and SOD had no significant effect on this impairment. Ageing caused a lesser but significant impairment in EFS-mediated cavernosal smooth muscle relaxation (P < 0.05).

CONCLUSION

Diabetes impairs endothelial and neuronal NO-mediated cavernosal smooth muscle relaxation in rats in vitro. This effect is not mediated by an alteration in the intracellular action of NO, the availability of NO, superoxide anion inactivation of NO or the generation of constrictor prostanoids. It is possible that cholesterol or advanced glycation end products are responsible for the effect of diabetes on penile smooth function.

Correction of glycosylated oxyhemoglobin-induced impairment of endothelium-dependent vasodilatation by gliclazide

We have investigated whether gliclazide, a second-generation sulfonylurea hypoglycemic agent, interferes with the impairment of endothelium-dependent nitric-oxide-mediated relaxation produced by 14%-glycosylated human oxyhemoglobin (GHHb). For comparative purposes, other agents, like glibenclamide, aminoguanidine, ascorbic acid or superoxide dismutase (SOD), were also tested. GHHb (10 nM) caused a reduction in endothelium-dependent relaxation induced by acetylcholine (1 nM to 10 microM) in both isolated aortic segments and mesenteric microvessels from normoglycemic nondiabetic rats. Preincubation of the vessels with gliclazide (100 nM to 10 microM) prevented the impairment of endothelial relaxation, the threshold concentration of gliclazide being 300 nM. In addition, 10 microM gliclazide also prevented the reduction by 10 nM GHHb of the relaxation induced by exogenous nitric oxide (NO, 10 nM to 100 microM). Determination of superoxide anion release measured by the reduction in ferricytochrome c indicated that GHHb produced significant amounts of these free radicals that were concentration-dependently inhibited by gliclazide. The impairment of endothelium-mediated responses was also prevented by 100 U/ml SOD or 10 microM ascorbic acid, but not by 10 microM glibenclamide or 100 microM aminoguanidine. We conclude that gliclazide can reduce the impairment of nitric-oxide-mediated endothelium-dependent relaxation produced by GHHb. This reduction is likely related to the antioxidant properties of the drug, a mechanism suggested by these studies which demonstrate the inactivation of superoxide anions produced by the glycosylated protein by gliclazide.

Prevention of endothelial dysfunction in streptozotocin-induced diabetic rats by gliclazide treatment

The aim of the present work was to analyze whether the oral hypoglycemic drug gliclazide affects diabetic endothelial dysfunction in streptozotocin-induced diabetic rats. Gliclazide was compared with glibenclamide, ascorbic acid, and aminoguanidine. An insulin-dependent model of diabetes was selected to exclude insulin-releasing effects of the drugs. Both in isolated aortic segments and mesenteric microvessels, endothelium-dependent relaxation evoked by acetylcholine (ACh, 1 nM to 10 microM) was significantly reduced in vessels from diabetic animals. This impairment was reversed when the segments were previously incubated with 100 U/ml superoxide dismutase. When streptozotocin-induced diabetic rats were orally treated from the time of diabetes induction with gliclazide (10 mg/kg) or ascorbic acid (250 mg/kg), ACh-induced endothelium-dependent relaxation was well preserved both in aortic segments and mesenteric microvessels. In addition, the impaired vasodilatation to exogenous nitric oxide (NO) in aortic segments was also improved in gliclazide-treated diabetic rats. On the other hand, oral treatment with glibenclamide (1 and 10 mg/kg) or aminoguanidine (250 mg/kg) did not produce significant improvements in diabetic endothelial dysfunction. We conclude that gliclazide reverses the endothelial dysfunction associated with diabetes. This effect appears to be due not to the metabolic actions of the drug but rather to its antioxidant properties, as it can be mimicked by other antioxidants. We propose that the mechanism involved is the inactivation of reactive oxygen species, which are increased in diabetes probably as a result of increased early protein glycosylation products, such as glycosylated hemoglobin (HbA(1c)). These effects of gliclazide are not shared by other oral hypoglycemic agent such as glibenclamide, or by blockade of advanced glycosylation end product (AGE) generation with aminoguanidine.

Aminoguanidine suppresses basal macromolecular extravasation during diabetes mellitus

The goal of this study was to determine the effect of aminoguanidine on basal macromolecular efflux from the microcirculation of the hamster cheek pouch during diabetes mellitus. We used intravital fluorescent microscopy and fluorescein isothiocyanate dextrans (FITC-dextran; mw = 70 and 20K) to examine basal macromolecular extravasation in nondiabetic hamsters, nondiabetic hamsters treated with a topical application of aminoguanidine (0.5 mM), diabetic hamsters (6-10 weeks after injection of streptozotocin), and diabetic hamsters treated with a topical application of aminoguanidine (0.5 mM). Increases in macromolecular efflux were quantitated by calculating the clearance (ml/s x 10(-6)) of FITC-dextran-70K and -20K. In nondiabetic hamsters, the clearance of FITC-dextran-70K and -20K remained relatively constant during the experimental period, although the clearance of FITC-dextran-70K was less than that for FITC-dextran-20K. Topical application of aminoguanidine did not alter basal permeability characteristics in nondiabetic hamsters. In diabetic hamsters, clearance of FITC-dextran-70K and -20K also remained relatively constant during the experimental period. However, the magnitude of clearance of FITC-dextran-70K and and -20K was significantly greater in diabetic compared to nondiabetic hamsters (P < 0.05). Topical application of aminoguanidine restored basal permeability characteristics of diabetic hamsters to that observed in nondiabetic hamsters. These findings suggest that acute treatment of the microcirculation in vivo with aminoguanidine ameliorates basal increases in extravasation of macromolecules during diabetes mellitus. We suggest that aminoguanidine suppresses basal macromolecular efflux in diabetic hamsters via inhibition of nitric oxide synthase. Thus, it appears that the use of aminoguanidine may be an important therapeutic approach for the treatment of diabetes-related vascular dysfunction.

Effects of antioxidants on nerve and vascular dysfunction in experimental diabetes

Reactive oxygen species (ROS) are elevated by metabolic changes in diabetes, including autoxidation and increased advanced glycation. Endogenous protection by the glutathione redox cycle is also compromised by the competing NADPH requirement of elevated polyol pathway flux. Antioxidant treatment strategies prevent or reverse nerve conduction velocity (NCV) deficits in diabetic rats. These include lipophilic scavengers such as butylated hydroxytoluene, probucol and vitamin E, more hydrophilic agents like alpha-lipoic acid and acetyl cysteine, and transition metal chelators that inhibit autoxidation. In the long-term, elevated ROS cause cumulative damage to neurons and Schwann cells, however, they also have a deleterious effect on nerve blood flow in the short term. This causes endoneurial hypoxia, which is responsible for early NCV deficits. Antioxidant treatment corrects the blood flow deficit and promotes normal endoneurial oxygenation. ROS cause antioxidant-preventable vascular endothelium abnormalities, neutralizing nitric oxide mediated vasodilation and increasing reactivity to vasoconstrictors. Unsaturated fatty acids are a major target for ROS and essential fatty acid metabolism is impaired by diabetes. Gamma-linolenic acid stimulates vasodilator prostanoid production, and there are marked synergistic interactions between gamma-linolenic acid and antioxidants. This has encouraged the development of novel drugs such as ascorbyl-gamma-linolenic acid and gamma-linolenic acid-lipoic acid with enhanced therapeutic potential.

Effects of chelator treatment on aorta and corpus cavernosum from diabetic rats

Transition-metal catalyzed reactions contribute to oxidative stress, which has been implicated in the pathogenesis of diabetic complications. The aim was to evaluate the effects of treatment with the transition metal chelator trientine on endothelium-dependent relaxation of aorta and corpus cavernosum from streptozotocin-induced diabetes of 8 weeks duration in rats. Effects on cavernosum autonomic innervation were also examined. Diabetes caused a 30.1 +/- 3.8% reduction in maximum aorta endothelium-dependent relaxation to acetylcholine (ACh), which was markedly attenuated (72.7 +/- 10.6%) by trientine treatment. Reversal treatment (4 weeks untreated diabetes, 4 weeks trientine) did not effect endothelium-dependent relaxation compared with aortas from rats with 4 weeks of diabetes, however, there was a 22.5 +/- 6.2% improvement compared with 8 weeks of diabetes. Eight weeks of diabetes caused a 41.5 +/- 6.6% reduction in corpus cavernosum endothelium-dependent maximum relaxation to ACh that was 70.1 +/- 16.9% prevented by trientine. Cavernosum nonadrenergic, noncholinergic (NANC) nerve stimulation caused frequency-dependent relaxation to a maximum of 40.9 +/- 2.4%, which was reduced by diabetes to 24.2 +/- 2.1%. Trientine partially prevented this deficit, maximum relaxation being 31.9 +/- 2.3%. Thus, metal chelator treatment has beneficial effects on aorta and cavernosum endothelium-dependent relaxation and on cavernosum NANC innervation.

Correction of neurovascular deficits in diabetic rats by beta2-adrenoceptor agonist and alpha1-adrenoceptor antagonist treatment: interactions with the nitric oxide system

The aims were to test whether 2 weeks treatment with the beta2-adrenoceptor agonist, salbutamol, or the alpha1-adrenoceptor antagonist, doxazosin, could correct nerve blood flow and conduction velocity deficits in 8 week streptozotocin-diabetic rats and to examine neurovascular mechanisms using co-treatment with the nitric oxide synthase inhibitor, NG-nitro-L-arginine. Sciatic motor conduction velocity, 20.3% reduced by diabetes, was corrected by 88.2 and 88.5% for salbutamol and doxazosin, respectively. A 47.6% diabetic deficit in sciatic nutritive endoneurial blood, was substantially reversed by salbutamol (117.0%) and doxazosin (61.0%) treatment. The effects of alpha1-adrenoceptor blockade and beta2-adrenoceptor stimulation on nerve blood flow and conduction velocity were almost completely (76.7-91.7%) attenuated by NG-nitro-L-arginine co-treatment. Thus, the data stress the importance of vasa nervorum alpha1 and beta2 adrenoceptors and the permissive role of nitric oxide in nerve blood flow control mechanisms. They also indicate that beta2-adrenoceptor agonists may be suitable for clinical trials of diabetic neuropathy.

Diabetes-induced endothelial dysfunction is prevented by long-term treatment with the modified iron chelator, hydroxyethyl starch conjugated-deferoxamine

Oxygen radicals are believed to play a role in vascular complications of diabetes mellitus. In this study, we evaluated whether long-term treatment with an iron chelator and inhibitor of metal-catalyzed hydroxyl radicals (.OH) could prevent diabetes-induced defects in endothelium-dependent relaxation. Diabetes was induced in Sprague-Dawley rats by injection of streptozotocin. At 48 h after streptozotocin, a subgroup of diabetic rats received daily injections of 50 mg/kg hydroxyethyl starch conjugated-deferoxamine (HES-DFO) for a total of 8 weeks. Long-term treatment with HES-DFO did not modify serum insulin or blood glucose taken at the end of the study; however, a modest reduction in glycosylated hemoglobin was present. In precontracted aortic rings suspended in tissue baths, endothelium-dependent relaxation to acetylcholine was impaired in diabetic rings compared with control rings in the presence or absence of indomethacin. Endothelium-independent relaxation to nitroglycerin was unaltered. Long-term treatment with HES-DFO had no effect on relaxation to nitroglycerin but completely prevented the impaired relaxation to acetylcholine in diabetic rings in either the presence or absence of indomethacin. These data suggest that iron-catalyzed .OH formation contributes to the development of diabetes-associated endothelial dysfunction.

Aminoguanidine prolongs survival in azotemic-induced diabetic rats

Toxic effects of hyperglycemia-induced advanced glycosylated end products (AGEs) may explain some vasculopathic complications of diabetes. Aminoguanidine, a known inhibitor of AGE formation, was administered by gavage to Sprague-Dawley streptozotocin-induced diabetic rats made azotemic by surgical reduction of renal mass. All rats became hyperglycemic. Renal ablation caused renal insufficiency, as evidenced by markedly reduced endogenous creatinine clearances at days 7 and 14. Aminoguanidine-treated rats had significantly (P < 0.04) superior survival to that of untreated azotemic diabetic rats. We infer from the extended life in a rat model of uremia in diabetic nephropathy that aminoguanidine may prove beneficial in human diabetes.

Neurovascular effects of L-carnitine treatment in diabetic rats

The aims were to ascertain whether L-carnitine could prevent nerve blood flow and conduction deficits in 1-month diabetic rats and to examine potential neurovascular mechanisms using co-treatment with the nitric oxide synthase inhibitor, NG-nitro-L-arginine. A 19.8% diabetic deficit in sciatic motor conduction velocity was 57.4% attenuated by L-carnitine treatment. Similarly, a 47.7% reduction in sciatic nutritive (capillary) endoneurial blood flow was 48.6% blocked by L-carnitine. Joint treatment with NG-nitro-L-arginine completely abolished the effects on nerve conduction and nutritive flow. However, L-carnitine treatment did not alter a 50.8% diabetic deficit in total sciatic endoneurial flow, which was further depressed (61%) by NG-nitro-L-arginine co-treatment. Thus, the effect of L-carnitine on nerve conduction in diabetic rats depends on changes in the endoneurial perfusion pattern by an action that may involve the nitric oxide system of vasa nervorum.

Nerve function and regeneration in diabetic and galactosaemic rats: antioxidant and metal chelator effects

Immature rats were made diabetic with streptozotocin or were fed a 40% galactose diet to stimulate the polyol pathway. Separate diabetic and galactosaemic groups were treated with butylated hydroxytoluene or trientine. After 4 weeks the sciatic nerve was freeze-lesioned. Two weeks later, the degree of myelinated fibre regeneration was assessed electrophysiologically and nerve conduction velocity was measured in the contralateral leg. Similar sciatic motor and saphenous sensory nerve conduction velocity deficits of approximately 18% and 19%, respectively, compared to age-matched control rats were found in both models. They were partially prevented by treatment (approximately 68% for butylated hydroxytoluene and 63% for trientine). There were 12% and 10% deficits in nerve regeneration distance with diabetes and galactosaemia respectively, which were markedly attenuated (approximately 80%) by both treatments. The data emphasise the importance of elevated, free radical activity for the aetiology of neural/neurovascular deficits in experimental diabetes and galactosaemia.

Rapid reversal by aminoguanidine of the neurovascular effects of diabetes in rats: modulation by nitric oxide synthase inhibition

Aminoguanidine treatment prevents the development of nerve conduction velocity (NCV) deficits and some renal and retinal complications in diabetic rats. Pharmacological actions include inhibition of the formation of advanced glycosylation end products (AGEs) and nitric oxide (NO) synthase. The aims of the study were to determine the extent to which diabetic NCV and nerve blood flow deficits could be corrected by aminoguanidine in an intervention study, to assess the time course of drug action, and to examine the effects of cotreatment with the NO synthase inhibitor, NG-nitro-L-arginine (NOLA). A 19.3% +/- 0.9% reduction in sciatic motor NCV after 4 weeks of untreated diabetes was corrected 86.6% +/- 3.7% by aminoguanidine treatment for a further 4 weeks. Time-course studies showed that 50% of the maximal effect was attained within 6 days. Sciatic endoneurial capillary blood flow, reduced approximately 45% by diabetes, was corrected 85.6% +/- 12.1% by aminoguanidine treatment. The NCV and blood flow effects of aminoguanidine were completely blocked by cotreatment with NOLA. Thus, the data support a neurovascular mechanism for aminoguanidine involving improved NO action. The rapidity of aminoguanide's effect is consistent with inhibition of free radical production by autoxidative glycosylation or glycoxidation.