Effects of aldose reductase inhibition on responses of the corpus cavernosum and mesenteric vascular bed of diabetic rats

Endothelium-Dependent Hyperpolarization (EDH) in Diabetes: Mechanistic Insights and Therapeutic Implications

Diabetes mellitus is one of the major risk factors for cardiovascular disease and is an important health issue worldwide. Long-term diabetes causes endothelial dysfunction, which in turn leads to diabetic vascular complications. Endothelium-derived nitric oxide is a major vasodilator in large-size vessels, and the hyperpolarization of vascular smooth muscle cells mediated by the endothelium plays a central role in agonist-mediated and flow-mediated vasodilation in resistance-size vessels. Although the mechanisms underlying diabetic vascular complications are multifactorial and complex, impairment of endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells would contribute at least partly to the initiation and progression of microvascular complications of diabetes. In this review, we present the current knowledge about the pathophysiology and underlying mechanisms of impaired EDH in diabetes in animals and humans. We also discuss potential therapeutic approaches aimed at the prevention and restoration of EDH in diabetes.

The aetiology of neuropathy in experimental diabetes

Most information on the aetiology of experimental diabetic neuropathy comes from studies on rodent models, particularly the streptozotocin-diabetic rat. The major factor that impairs small and large nerve fibre function is a decrease in nerve and ganglion perfusion. This leads to reduced conduction velocity, increased resistance to ischaemic conduction failure, blunted regenerative capacity, painful neuropathy, and autonomic nerve dysfunction. Hyperglycaemia, altered lipid metabolism and reduced insulin action combine to cause adverse metabolic effects on vasa nervorum, vascular endothelium being a notable target. The resultant reduced vasodilation and increased vasoconstriction causes endoneurial hypoxia. Oxidative stress is of primary importance, due to increased production of reactive oxygen species from a plethora of intra- and extracellular sources. Advanced glycation and carbonyl stress play a supporting role, as does essential fatty acid dysmetabolism. These mechanisms are associated with alterations in cell signalling mediated by protein kinases, nuclear factor Kappa B and poly (ADP-ribose) polymerase.

Pathogenesis of diabetic neuropathy: focus on neurovascular mechanisms

Neuropathies of the peripheral and autonomic nervous systems affect up to half of all people with diabetes, and are major risk factors for foot ulceration and amputation. The aetiology is multifactorial: metabolic changes in diabetes may directly affect neural tissue, but importantly, neurodegenerative changes are precipitated by compromised nerve vascular supply. Experiments in animal models of diabetic neuropathy suggest that similar metabolic sequelae affect neurons and vasa nervorum endothelium. These include elevated polyol pathway activity, oxidative stress, the formation of advanced glycation and lipoxidation end products, and various pro-inflammatory changes such as elevated protein kinase C, nuclear factor κB and p38 mitogen activated protein kinase signalling. These mechanisms do not work in isolation but strongly interact in a mutually facilitatory fashion. Nitrosative stress and the induction of the enzyme poly (ADP-ribose) polymerase form one important link between physiological stressors such as reactive oxygen species and the pro-inflammatory mechanisms. Recently, evidence points to endoplasmic stress and the unfolded protein response as forming another crucial link. This review focuses on the aetiopathogenesis of neurovascular changes in diabetic neuropathy, elucidated in animal studies, and on putative therapeutic targets the majority of which have yet to be tested for efficacy in clinical trials.

Aldose reductase and AGE-RAGE pathways: central roles in the pathogenesis of vascular dysfunction in aging rats

Aging is inevitably accompanied by gradual and irreversible innate endothelial dysfunction. In this study, we tested the hypothesis that accentuation of glucose metabolism via the aldose reductase (AR) pathway contributes to age-related vascular dysfunction. AR protein and activity levels were significantly increased in aged vs. young aortic homogenates from Fischer 344 rats. Immunostaining revealed that the principal site of increased AR protein was the aortic endothelium as well as smooth muscle cells. Studies revealed that endothelial-dependent relaxation (EDR) in response to acetylcholine was impaired in aged rats compared to young rats and that treatment with the AR inhibitor (ARI) zopolrestat significantly improved EDR in aged rats. Methylglyoxal (MG), a key precursor of advanced glycation endproducts (AGEs), was significantly increased in the aortas of aged rats vs. young rats. Consistent with central roles for AR in generation of MG in aging, ARI treatment significantly reduced MG levels in aged rat aorta to those in young rats. Treatment of aged rats with soluble(s) RAGE, a soluble form of the chief signal transduction receptor for AGEs, RAGE, significantly improved EDR in aged rats, thus establishing the contribution of age-related increases in AGEs to endothelial dysfunction. These findings reveal that significant increases in AR expression and activity in aged rat vasculature linked to endothelial dysfunction may be mitigated, at least in part, via ARI and that aging-linked increased flux via AR generates AGEs; species which transduce endothelial injury consequent to their interaction with RAGE. These data demonstrate for the first time that AR mediates aging-related vascular dysfunction, at least in part, via RAGE.

Evaluation of the aldose reductase inhibitor fidarestat on ischemia-reperfusion injury in rat retina

This study evaluated the effects of retinal ischemia-reperfusion (IR) injury and pre-treatment with the potent and specific aldose reductase inhibitor fidarestat on apoptosis, aldose reductase and sorbitol dehydrogenase expression, sorbitol pathway intermediate concentrations, and oxidative-nitrosative stress. Female Wistar rats were pre-treated with either vehicle (N-methyl-D-glucamine) or fidarestat, 32 mg kg(-1) d(-1) for both, in the right jugular vein, for 3 consecutive days. A group of vehicle- and fidarestat-treated rats were subjected to 45-min retinal ischemia followed by 24-h reperfusion. Ischemia was induced 30 min after the last vehicle or fidarestat administration. Retinal IR resulted in a remarkable increase in retinal cell death. The number of TUNEL-positive nuclei increased 48-fold in the IR group compared with non-ischemic controls (p<0.01), and this increase was partially prevented by fidarestat. AR expression (Western blot analysis) increased by 19% in the IR group (p<0.05), and this increase was prevented by fidarestat. Sorbitol dehydrogenase and nitrated protein expressions were similar among all experimental groups. Retinal sorbitol concentrations tended to increase in the IR group but the difference with non-ischemic controls did not achieve statistical significance (p=0.08). Retinal fructose concentrations were 2.2-fold greater in the IR group than in the non-ischemic controls (p<0.05). Fidarestat pre-treatment of rats subjected to IR reduced retinal sorbitol concentration to the levels in non-ischemic controls. Retinal fructose concentrations were reduced by 41% in fidarestat-pre-treated IR group vs. untreated ischemic controls (p=0.0517), but remained 30% higher than in the non-ischemic control group. In conclusion, IR injury to rat retina is associated with a dramatic increase in cell death, elevated AR expression and sorbitol pathway intermediate accumulation. These changes were prevented or alleviated by the AR inhibitor fidarestat. The results identify AR as an important therapeutic target for diseases involving IR injury, and provide the rationale for development of fidarestat and other AR inhibitors.

Different roles of 12/15-lipoxygenase in diabetic large and small fiber peripheral and autonomic neuropathies

Up-regulation of 12/15-lipoxygenase, which converts arachidonic acid to 12(S)- and 15(S)-hydroxyeicosatetraenoic acids, causes impaired cell signaling, oxidative-nitrosative stress, and inflammation. This study evaluated the role for 12/15-lipoxygenase in diabetic large and small fiber peripheral and autonomic neuropathies. Control and streptozotocin-diabetic wild-type and 12/15-lipoxygenase-deficient mice were maintained for 14 to 16 weeks. 12/15-lipoxygenase gene deficiency did not affect weight gain or blood glucose concentrations. Diabetic wild-type mice displayed increased sciatic nerve 12/15-lipoxygenase and 12(S)-hydroxyeicosatetraenoic acid levels. 12/15-lipoxygenase deficiency prevented or alleviated diabetes-induced thermal hypoalgesia, tactile allodynia, motor and sensory nerve conduction velocity deficits, and reduction in tibial nerve myelinated fiber diameter, but not intraepidermal nerve fiber loss. The frequencies of superior mesenteric-celiac ganglion neuritic dystrophy, the hallmark of diabetic autonomic neuropathy in mouse prevertebral sympathetic ganglia, were increased 14.8-fold and 17.2-fold in diabetic wild-type and 12/15-lipoxygenase-deficient mice, respectively. In addition, both diabetic groups displayed small (<1%) numbers of degenerating sympathetic neurons. In conclusion, whereas 12/15-lipoxygenase up-regulation provides an important contribution to functional changes characteristic for both large and small fiber peripheral diabetic neuropathies and axonal atrophy of large myelinated fibers, its role in small sensory nerve fiber degeneration and neuritic dystrophy and neuronal degeneration characteristic for diabetic autonomic neuropathy is minor. This should be considered in the selection of endpoints for future clinical trials of 12/15-lipoxygenase inhibitors.

Treatment with the xanthine oxidase inhibitor, allopurinol, improves nerve and vascular function in diabetic rats

Several putative sources of reactive oxygen species could potentially contribute to diabetic neuropathy and vasculopathy. The aim was to assess the involvement of elevated xanthine oxidase activity. After 6 weeks of streptozotocin-diabetes, groups of rats were given 2 weeks of high-dose allopurinol treatment (50 and 250 mg/kg) to gauge the effect of maximal blockade of xanthine oxidase. In the final experiments, rats were subjected to sensory testing and, under butabarbital anaesthesia, measurements were made on nerve conduction velocities and neural tissue blood flow estimated by hydrogen clearance microelectrode polarography. Further groups were used to study detailed responses of the isolated mesenteric vascular bed after 4 weeks of diabetes and allopurinol (150 mg/kg) treatment. Diabetes caused 20% and 14% reduction in motor and sensory conduction velocity, which were 78% and 81% corrected by allopurinol treatment respectively, both doses giving similar results. Diabetic rats showed tactile allodynia and thermal hyperalgesia, which were completely corrected by allopurinol, whereas mechanical hyperalgesia was only 45% ameliorated. Sciatic nerve and superior cervical ganglion blood flow was halved by diabetes and allopurinol corrected this by approximately 63%. Mesenteric endothelium-dependent vascular responses to acetylcholine, which depend upon nitric oxide and endothelium derived hyperpolarizing factor, were attenuated by diabetes. Allopurinol treatment gave approximately 50% protection for both components. Thus, xanthine oxidase is an important source of reactive oxygen species that contributes to neurovascular dysfunction in experimental diabetes. Inhibition of xanthine oxidase could be a potential therapeutic approach to diabetic neuropathy and vasculopathy.

Endothelial dysfunction in diabetic erectile dysfunction

Erectile dysfunction (ED) is highly prevalent in diabetes mellitus. Pathophysiological mechanisms underlying diabetes-associated ED are in large part due to endothelial dysfunction, which functionally refers to the inability of the endothelium to produce vasorelaxing messengers and to maintain vasodilation and vascular homeostasis. The precise mechanisms leading to endothelial dysfunction in the diabetic vasculature, including the penis, are not yet fully understood. Hyperglycemia affects endothelial nitric oxide synthase activity and nitric oxide production/bioavailability, nitric oxide-independent relaxing factors, oxidative stress, production and/or action of hormones, growth factors and/or cytokines, and generation and activity of opposing vasoconstrictors. Considering recent advances in the field of vascular biology and diabetes, the emphasis in this review is placed on the mechanisms of hyperglycemia-induced endothelial dysfunction in the pathophysiology of diabetes-associated ED.

Correction of endothelial dysfunction in diabetic female rats by tetrahydrobiopterin and chronic insulin

Diabetes-induced vascular dysfunction has mainly been studied in males. However, the mechanisms involved may not correspond to those in females. Here we analyzed the effects of tetrahydrobiopterin (BH(4)) and chronic insulin on the physiology of mesenteric arterioles of alloxan-diabetic female rats. The parameters studied were the mesenteric arteriolar reactivity (intravital microscopy), nitric oxide synthase (NOS) activity (conversion of L-arginine to L-citrulline), eNOS gene expression (RT-PCR), NO production (diaminofluorescein), reactive oxygen species (ROS) generation (intravital fluorescence microscopy) and Cu/Zn superoxide dismutase (SOD) activity (spectrophotometry) and gene expression (RT-PCR). The reduced endothelium-dependent vasodilation of diabetic females was corrected by both BH(4) and insulin. NOS activity was decreased by diabetes, but insulin did not correct it. However, NOS expression was not modified by either diabetes or insulin. Arterioles of diabetic rats exhibited lower NO production, which was fully corrected by BH(4) and only partially by insulin. ROS generation was increased in diabetic rats, and both BH(4) and insulin normalized it. Diabetes did not change SOD activity and gene expression. However, insulin increased SOD activity but not its expression. Our data suggest that, similarly to males, endothelial dysfunction in female diabetic rats involves an altered ROS/NO imbalance. In contrast to males, however, insulin does not regulate NOS in the microcirculation of diabetic females.

Inhibition of acetylcholine-induced EDHF response by elevated glucose in rat mesenteric artery

The effects of high glucose on endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxations of isolated rat mesenteric artery and the possible involvement of reactive oxygen species in these responses were investigated. After precontraction with phenylephrine (3 x 10(-8)-10(-7) M), acetylcholine (10(-8)-3 x 10(-6) M) and A 23187 (10(-8)-3 x 10(-6) M), a calcium ionophore, induced concentration-dependent relaxations in the presence of N(W)-nitro-l-arginine methyl ester (L-NAME) (10(-4) M) and indomethacin (10(-5) M). These relaxations were abolished in the presence of charybdotoxin (2 x 10(-7) M) plus apamin (10(-7) M) and were assumed to be mediated by EDHF. Effects of elevated glucose were examined by incubating the arterial rings for 6 h in Krebs-Henseleit solution containing 22.2 mM glucose. Under these conditions relaxation to acetylcholine was significantly attenuated but was unchanged when the tissues were incubated for 6 h in solution containing 11.1 mM mannitol used as hyperosmotic control. Addition of superoxide dismutase (SOD) (75 U/ml) and combination of SOD with catalase (200 U/ml) during incubation with high glucose significantly preserved the impairment of EDHF-mediated relaxations to acetylcholine. A 23187-induced endothelium-dependent relaxation was not affected by high glucose. Similarly, relaxations to pinacidil (10(-10)-10(-5) M) and to sodium nitroprusside (SNP) (10(-10)-3 x 10(-7) M) were also unchanged in the rings exposed to high glucose. These results suggest that in rat mesenteric arteries exposed to elevated glucose receptor-dependent EDHF-mediated relaxations (acetylcholine-induced) are impaired whereas receptor-independent ones (A 23187-induced) and responses to smooth muscle relaxants that exert their effects through mechanisms independent of endothelium are unaffected. Our findings lead us to propose that reactive oxygen species like superoxide ((.)O(2)(-)) and hydrogen peroxide (H(2)O(2)) do seem to play a role in the impairment of EDHF-mediated relaxations in the presence of elevated glucose.

Effect of ascorbic acid treatment on endothelium-dependent and neurogenic relaxation of corpus cavernosum from middle-aged non-insulin dependent diabetic rats

AIMS

The purpose of the present study was to investigate functional responses and nitric oxide synthase activity in the corpus cavernosum of young control, middle-aged control and middle-aged non-insulin dependent diabetic rats.

METHODS

The animal groups were treated with ascorbic acid.

RESULTS

Acetylcholine-mediated endothelium-dependent relaxation of cavernosal tissue was significantly attenuated from a maximum of 58.0 +/- 4.1% (1 mmol, n = 10) in young rats to 44.3 +/- 1.6% in aged-matched controls (P < 0.05) and to 23.3 +/- 2.8% in non-insulin-dependent diabetes mellitus rats (P < 0.01). These deficits in acetylcholine responsiveness were completely prevented by ascorbic acid treatment. Non-adrenergic non-cholinergic relaxations evoked by electrical field stimulation (0.5-64.0 Hz) in the corpus cavernosum of middle-aged control and non-insulin dependent (NID) diabetic rats are blunted and were not restored by ascorbic acid. The histochemical findings demonstrated a decrease of nicotinamide adenine dinucleotide phosphate-diaphorase staining in the cavernosal tissue obtained from middle-aged control rats and middle-aged diabetic rats.

CONCLUSION

Partial correction by ascorbic acid may suggest the importance of reactive oxygen species and a therapeutic approach in impotent NID diabetic men.

Downregulation of cGMP-dependent protein kinase-1 activity in the corpus cavernosum smooth muscle of diabetic rabbits

Increased guanosine 3',5'-cyclic monophosphate (cGMP), induced by nitric oxide release, is crucial for corpus cavernosum smooth muscle (CCSM) relaxation within the penis. This CCSM relaxation (necessary for penile erection) is impaired in men with erectile dysfunction (ED), especially those men with diabetes. One of the effector proteins for cGMP is cGMP-dependent protein kinase-1 (PKG-1). PKG-1 knockout mice exhibit detrusor overactivity (Am J Physiol Regul Integr Comp Physiol 279: R1112-R1120, 2000) and, more relevant to this study, ED (Proc Natl Acad Sci USA 97: 2349-2354, 2000), suggesting an in vivo role for PKG-1 in urogenital smooth muscle relaxation. In the current study, using normal rabbit CCSM, Western blot analysis revealed high expression of PKG-1 at levels almost equivalent to aorta (previously shown to have high PKG-1 expression) and that the two known alternatively spliced isoforms of PKG-1 (alpha and beta) are expressed in nearly equal amounts in the CCSM. However, in response to alloxan-induced diabetes, there was a decrease in expression of both PKG-1 isoforms at the mRNA and protein levels as determined by real-time RT-PCR and Western blotting, respectively, but with the PKG-1alpha isoform expression decreased to a greater extent. Moreover, diabetes was associated with significantly decreased PKG-1 activity of CCSM in vitro, correlating with decreased CCSM relaxation. Immunofluorescence microscopy revealed a diabetes-associated decrease in PKG-1 in the CCSM cells. In conclusion, our results demonstrate for the first time a significant downregulation of PKG-1 expression associated with decreased PKG-1 activity in the CCSM in response to diabetes. Furthermore, these results suggest a mechanistic basis for the decreased efficacy of phosphodiesterase V inhibitors in treating diabetic patients with ED.

Effects of proinsulin C-peptide in experimental diabetic neuropathy: vascular actions and modulation by nitric oxide synthase inhibition

Proinsulin C-peptide treatment can partially prevent nerve dysfunction in type 1 diabetic rats and patients. This could be due to a direct action on nerve fibers or via vascular mechanisms as C-peptide stimulates the nitric oxide (NO) system and NO-mediated vasodilation could potentially account for any beneficial C-peptide effects. To assess this further, we examined neurovascular function in streptozotocin-induced diabetic rats. After 6 weeks of diabetes, rats were treated for 2 weeks with C-peptide to restore circulating levels to those of nondiabetic controls. Additional diabetic groups were given C-peptide with NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA) co-treatment or scrambled C-peptide. Diabetes caused 20 and 16% reductions in sciatic motor and saphenous sensory nerve conduction velocity, which were 62 and 78% corrected, respectively, by C-peptide. L-NNA abolished C-peptide effects on nerve conduction. Sciatic blood flow and vascular conductance were 52 and 41%, respectively, reduced by diabetes (P < 0.001). C-peptide partially (57-66%) corrected these defects, an effect markedly attenuated by L-NNA co-treatment. Scrambled C-peptide was without effect on nerve conduction or perfusion. Thus, C-peptide replacement improves nerve function in experimental diabetes, and the data are compatible with the notion that this is mediated by a NO-sensitive vascular mechanism.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Calcium dobesilate potentiates endothelium-derived hyperpolarizing factor-mediated relaxation of human penile resistance arteries

1 We have evaluated the participation of endothelium-derived hyperpolarizing factor (EDHF) in the endothelium-dependent relaxation of isolated human penile resistance arteries (HPRA) and human corpus cavernosum (HCC) strips. In addition, the effect of the angioprotective agent, calcium dobesilate (DOBE), on the endothelium-dependent relaxation of these tissues was investigated. 2 Combined inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX) nearly abolished the endothelium-dependent relaxation to acetylcholine (ACh) in HCC, while 60% relaxation of HPRA was observed under these conditions. Endothelium-dependent relaxation of HPRA resistant to NOS and COX inhibition was prevented by raising the extracellular concentration of K(+) (35 mM) or by blocking Ca(2)(+)-activated K(+) channels, with apamin (APA; 100 nM) and charybdotoxin (CTX; 100 nM), suggesting the involvement of EDHF in these responses. 3 Endothelium-dependent relaxation to ACh was markedly enhanced by DOBE (10 micro M) in HPRA but not in HCC. The potentiating effects of DOBE on ACh-induced responses in HPRA, remained after NOS and COX inhibition, were reduced by inhibition of cytochrome P450 oxygenase with miconazole (0.3 mM) and were abolished by high K(+) or a combination of APA and CTX. 4 In vivo, DOBE (10 mg kg(-1) i.v.) significantly potentiated the erectile responses to cavernosal nerve stimulation in male rats. 5 EDHF plays an important role in the endothelium-dependent relaxation of HPRA but not in HCC. DOBE significantly improves endothelium-dependent relaxation of HPRA mediated by EDHF and potentiates erectile responses in vivo. Thus, EDHF becomes a new therapeutic target for the treatment of erectile dysfunction (ED) and DOBE could be considered a candidate for oral therapy for ED.

Minalrestat, an aldose reductase inhibitor, corrects the impaired microvascular reactivity in diabetes

We demonstrated that aldose reductase inhibition corrects the impaired microvascular responses to inflammatory mediators in diabetic rats. To study the mechanism involved in the restoring effect of aldose reductase inhibition, we examined the effects of minalrestat, another aldose reductase inhibitor, on the responses of mesenteric microvessels studied in vivo to permeability-increasing agents in diabetic and galactosemic rats. The diabetic group was treated from 3 days after the alloxan injection with minalrestat (10 mg/kg/day) for 30 days and the minalrestat treatment (10 mg/kg/day/7 days) of galactosemic rats started concomitantly with the induction of galactosemia. The mesenteric microvessel reactivity was studied using intravital microscopy and changes in vessel diameters were estimated after the topical application of vasoactive agents. The impaired responses to bradykinin, histamine, and platelet-activating factor of arterioles and venules observed in diabetic and galactosemic rats were completely prevented by minalrestat. Neither diabetes nor galactosemia affected responses to acetylcholine and sodium nitroprusside. Responses to these agents were not modified by aldose reductase inhibition. The restoring effect of minalrestat was reversed by inhibition of nitric oxide (NO) synthesis with N(omega)-nitro-L-arginine methyl ester, by blocking K(+) channel with tetraethylammonium but not by cyclooxygenase inhibition with diclofenac. Therefore, we concluded that NO, membrane hyperpolarization, but not cyclooxygenase products are involved in the beneficial effect of minalrestat on the microvascular reactivity in diabetes. Together, these findings led us to suggest that aldose reductase inhibition might ameliorate diabetic complications through the correction of the altered microvascular reactivity by a mechanism that involves NO and membrane hyperpolarization.

Possible involvement of facilitated polyol pathway in augmentation of intimal hyperplasia in rabbits with alloxan-induced hyperglycemia

Present experiments were designed to investigate whether the facilitated polyol pathway is involved in the augmentation of intimal hyperplasia with hyperglycemia. Twelve weeks after a single bolus intravenous injection of alloxan (100 mg/kg) or saline, rabbits underwent a unilateral endothelial denudation of the carotid artery. Intimal hyperplasia was evident 4 weeks after denudation and significantly augmented in hyperglycemic animals treated with alloxan. This effect was accompanied by the enhanced accumulation of endogenous NOS inhibitors (N(G)-monomethyl-l-arginine [l-NMMA] and asymmetric, N(G),N(G)-dimethyl-l-arginine [ADMA]) in regenerated endothelial cells, impairment of NO production and release, and enhanced accumulation of endothelin-1 (ET-1) within the vessel wall. Sorbitol levels in aortic endothelial cells and within the smooth muscle layer were significantly increased with hyperglycemia. All these changes associated with hyperglycemia were significantly reduced in animals treated with the selective aldose reductase inhibitor fidarestat (3 mg/kg/d). These findings suggest that the facilitated polyol pathway possibly plays an important role for the augmentation of intimal hyperplasia caused by the hyperglycemic state.

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.

Haemodynamics in microvascular complications in type 1 diabetes

Type 1 diabetes is commonly associated with microvascular complications. Most of the microvascular blood vessels are involved but those in the kidney, retina and large nerves exhibit the more significant pathology. Haemodynamic and metabolic factors both alone and through the activation of a common pathway contribute to the characteristic dysfunction observed in diabetic vasculopathy. The haemodynamic abnormalities in type 1 diabetes are characterized by increased systemic blood pressure and altered blood flow with subsequent activation of various vasoactive factors, which can contribute to the maintenance of the haemodynamic alterations and to the development and progression of the microvascular complications. These vasoactive factors include vasoconstrictors such as angiotensin II, and endothelin, as well as vasodilators such as nitric oxide (NO). Systemic hypertension and vasoactive factors independently and in interaction with the metabolic pathway activate intracellular second messengers, nuclear transcription factors and various growth factors which lead to the typical functional and structural alterations of diabetic microvascular complications. Therapeutic strategies involved in the management and prevention of diabetic complications currently include antihypertensive agents, particularly those that interrupt the renin-angiotensin system. Further understanding of the interactions among the vasoactive factors, the intracellular second messengers and the growth factors may help to identify novel strategies to influence the action of the vasoactive factors. These novel therapies, together with specific inhibitors of the metabolic pathway or the common pathway, may provide the possibility of preventing or even reversing the progression of diabetic microvascular complications.

Diabetes causes an early reduction in autonomic ganglion blood flow in rats

Impaired blood flow to peripheral nerve trunks makes a major contribution to the neuropathic complications of diabetes mellitus. Comparatively little attention has been paid to perfusion abnormalities for the cell bodies of origin of the autonomic and sensory nerves, although they are severely affected in diabetic neuropathy. The aim was to examine the time course of changes in superior cervical ganglion (SCG) perfusion in streptozotocin-induced diabetic rats. Ganglion blood flow, measured by hydrogen clearance microelectrode polarography, was approximately 70 ml min(-1) 100 g(-1). One week of diabetes caused a 46% perfusion deficit, which was maintained (54%) over 24 weeks. Thus, an early, profound, and long-lived reduction in ganglion perfusion may deleteriously affect neural cell body function and could contribute to autonomic neuropathy.

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.