Endothelial function in the isolated perfused mesentery and aortae of rats with streptozotocin-induced diabetes: effect of treatment with the aldose reductase inhibitor, ponalrestat

Rutin ameliorates nitrergic and endothelial dysfunction on vessels and corpora cavernosa of diabetic animals

Endothelial dysfunction is an early complication of diabetes and it is related to both micro- and macroangiopathies. In addition, >70% of diabetic patients develop autonomic neuropathies. Increased oxidative stress has a major role in the development of both nitrergic and endothelial dysfunction. The aim of this work is to evaluate whether rutin, a potent antioxidant, could ameliorate nitrergic and/or endothelial dysfunction in diabetic animals. Primary and secondary treatment protocols with rutin were investigated on rat aortic rings and the mesenteric arteriolar bed, and on rabbit aortic rings and corpora cavernosa (RbCC) from both euglycemic and alloxan-diabetic animals. Acetylcholine endothelium-dependent and sodium nitroprusside endothelium-independent relaxations were compared in tissues from euglycemic or diabetic animals. Electrical field stimulation (EFS)-induced relaxation was performed only in the RbCC. Endothelial-dependent relaxations were blunted by 40% in vessels and neuronal relaxation was blunted by 50% in RbCC taken from diabetic animals when compared to euglycemic animals. Pre-treatment with rutin restored both neuronal and endothelial dependent relaxations in diabetic animals towards the values achieved in control euglycemic tissues. Rutin was able to ameliorate both endothelial dysfunction and nitrergic neuropathy in animal experimental models. Rutin could be a lead compound in the primary or secondary preventive ancillary treatment of endothelial and nitrergic dysfunction in the course of diabetes.

Pharmacological management of vascular endothelial dysfunction in diabetes: TCM and western medicine compared based on biomarkers and biochemical parameters

Diabetes, a worldwide health concern while burdening significant populace of countries with time due to a hefty increase in both incidence and prevalence rates. Hyperglycemia has been buttressed both in clinical and experimental studies to modulate widespread molecular actions that effect macro and microvascular dysfunctions. Endothelial dysfunction, activation, inflammation, and endothelial barrier leakage are key factors contributing to vascular complications in diabetes, plus the development of diabetes-induced cardiovascular diseases. The recent increase in molecular, transcriptional, and clinical studies has brought a new scope to the understanding of molecular mechanisms and the therapeutic targets for endothelial dysfunction in diabetes. In this review, an attempt made to discuss up to date critical and emerging molecular signaling pathways involved in the pathophysiology of endothelial dysfunction and viable pharmacological management targets. Importantly, we exploit some Traditional Chinese Medicines (TCM)/TCM isolated bioactive compounds modulating effects on endothelial dysfunction in diabetes. Finally, clinical studies data on biomarkers and biochemical parameters involved in the assessment of the efficacy of treatment in vascular endothelial dysfunction in diabetes was compared between clinically used western hypoglycemic drugs and TCM formulas.

The Vascular Consequences of Metabolic Syndrome: Rodent Models, Endothelial Dysfunction, and Current Therapies

Metabolic syndrome is characterized by visceral obesity, dyslipidemia, hyperglycemia and hypertension, and affects over one billion people. Independently, the components of metabolic syndrome each have the potential to affect the endothelium to cause vascular dysfunction and disrupt vascular homeostasis. Rodent models of metabolic syndrome have significantly advanced our understanding of this multifactorial condition. In this mini-review we compare the currently available rodent models of metabolic syndrome and consider their limitations. We also discuss the numerous mechanisms by which metabolic abnormalities cause endothelial dysfunction and highlight some common pathophysiologies including reduced nitric oxide production, increased reactive oxygen species and increased production of vasoconstrictors. Additionally, we explore some of the current therapeutics for the comorbidities of metabolic syndrome and consider how these benefit the vasculature.

Identification of activators of ERK5 transcriptional activity by high-throughput screening and the role of endothelial ERK5 in vasoprotective effects induced by statins and antimalarial agents

Because ERK5 inhibits endothelial inflammation and dysfunction, activating ERK5 might be a novel approach to protecting vascular endothelial cells (ECs) against various pathological conditions of the blood vessel. We have identified small molecules that protect ECs via ERK5 activation and determined their contribution to preventing cardiac allograft rejection. Using high-throughput screening, we identified certain statins and antimalarial agents including chloroquine, hydroxychloroquine, and quinacrine as strong ERK5 activators. Pitavastatin enhanced ERK5 transcriptional activity and Kruppel-like factor-2 expression in cultured human and bovine ECs, but these effects were abolished by the depletion of ERK5. Chloroquine and hydroxychloroquine upregulated ERK5 kinase activity and inhibited VCAM-1 expression in an ERK5-dependent but MAPK/ERK kinase 5- and Kruppel-like factor 2/4-independent manner. Leukocyte rolling and vascular reactivity were used to evaluate endothelial function in vivo, and we found that EC-specific ERK5 knockout (ERK5-EKO) mice exhibited increased leukocyte rolling and impaired vascular reactivity, which could not be corrected by pitavastatin. The role of endothelial ERK5 in acute cardiac allograft rejection was also examined by heterotopic grafting of the heart obtained from either wild-type or ERK5-EKO mice into allomismatched recipient mice. A robust increase in both inflammatory gene expression and CD45-positive cell infiltration into the graft was observed. These tissue rejection responses were inhibited by pitavastatin in wild-type but not ERK5-EKO hearts. Our study has identified statins and antimalarial drugs as strong ERK5 activators and shown that ERK5 activation is preventive of endothelial inflammation and dysfunction and acute allograft rejection.

Retinopathy in a novel model of metabolic syndrome and type 2 diabetes: new insight on the inflammatory paradigm

The pathogenesis of diabetic retinopathy (DR) in metabolic syndrome (MetS) and type 2 diabetes (T2D) is not well studied, partly because an appropriate model has not been developed. Recently, we introduced a novel model of spontaneous T2D and MetS that replicates the relevant features of the human disease. In the current study, we investigated the retinal vascular changes in these animals. Experimental DR in streptozotocin (STZ)-injected rodents is described as an inflammatory disease, in which intercellular adhesion molecule 1 (ICAM-1) plays a key role. In comparison, advanced diabetes (HbA1c>10%) in the Nile grass rat (NGR) was associated with lower ICAM-1 protein expression when compared with that in normal or moderately diabetic animals. Vascular cell adhesion molecule 1 (VCAM-1) expression, however, was unaffected by the disease state. As opposed to the STZ-induced model of DR, in diabetic NGRs, most leukocytes accumulated in the retinal arteries. Consistent with the ICAM-1 reduction, leukocyte accumulation was significantly reduced in advanced disease. Similarly, leukocyte adhesions were significantly lower, with elevated plasma triglycerides (>200 mg/dl), and cholesterol (>240 mg/dl). However, these adhesions were significantly higher in animals with higher plasma insulin (>5 μIU/ml) and leptin (>20 ng/ml), suggesting a role for these hormones in diabetic retinal leukostasis. Diabetic NGRs showed substantial retinal endothelial injury, primarily in the microvessels, including vascular tortuosity, obliterated acellular capillaries, and pericyte ghosts. The NGR provides a convenient and realistic model for investigation of retinal changes in MetS/T2D with convincing advantages over the commonly used STZ-induced T1D.

Caveolin-1 is a critical determinant of autophagy, metabolic switching, and oxidative stress in vascular endothelium

Caveolin-1 is a scaffolding/regulatory protein that interacts with diverse signaling molecules. Caveolin-1^null mice have marked metabolic abnormalities, yet the underlying molecular mechanisms are incompletely understood. We found the redox stress plasma biomarker plasma 8-isoprostane was elevated in caveolin-1^null mice, and discovered that siRNA-mediated caveolin-1 knockdown in endothelial cells promoted significant increases in intracellular H₂O₂. Mitochondrial ROS production was increased in endothelial cells after caveolin-1 knockdown; 2-deoxy-D-glucose attenuated this increase, implicating caveolin-1 in control of glycolytic pathways. We performed unbiased metabolomic characterizations of endothelial cell lysates following caveolin-1 knockdown, and discovered strikingly increased levels (up to 30-fold) of cellular dipeptides, consistent with autophagy activation. Metabolomic analyses revealed that caveolin-1 knockdown led to a decrease in glycolytic intermediates, accompanied by an increase in fatty acids, suggesting a metabolic switch. Taken together, these results establish that caveolin-1 plays a central role in regulation of oxidative stress, metabolic switching, and autophagy in the endothelium, and may represent a critical target in cardiovascular diseases.

A crucial role for p90RSK-mediated reduction of ERK5 transcriptional activity in endothelial dysfunction and atherosclerosis

BACKGROUND

Diabetes mellitus is a major risk factor for cardiovascular mortality by increasing endothelial cell (EC) dysfunction and subsequently accelerating atherosclerosis. Extracellular-signal regulated kinase 5 (ERK5) is activated by steady laminar flow and regulates EC function by increasing endothelial nitric oxide synthase expression and inhibiting EC inflammation. However, the role and regulatory mechanisms of ERK5 in EC dysfunction and atherosclerosis are poorly understood. Here, we report the critical role of the p90 ribosomal S6 kinase (p90RSK)/ERK5 complex in EC dysfunction in diabetes mellitus and atherosclerosis.

METHODS AND RESULTS

Inducible EC-specific ERK5 knockout (ERK5-EKO) mice showed increased leukocyte rolling and impaired vessel reactivity. To examine the role of endothelial ERK5 in atherosclerosis, we used inducible ERK5-EKO-LDLR(-/-) mice and observed increased plaque formation. When activated, p90RSK associated with ERK5, and this association inhibited ERK5 transcriptional activity and upregulated vascular cell adhesion molecule 1 expression. In addition, p90RSK directly phosphorylated ERK5 S496 and reduced endothelial nitric oxide synthase expression. p90RSK activity was increased in diabetic mouse vessels, and fluoromethyl ketone-methoxyethylamine, a specific p90RSK inhibitor, ameliorated EC-leukocyte recruitment and diminished vascular reactivity in diabetic mice. Interestingly, in ERK5-EKO mice, increased leukocyte rolling and impaired vessel reactivity were resistant to the beneficial effects of fluoromethyl ketone-methoxyethylamine, suggesting a critical role for endothelial ERK5 in mediating the salutary effects of fluoromethyl ketone-methoxyethylamine on endothelial dysfunction. Fluoromethyl ketone-methoxyethylamine also inhibited atherosclerosis formation in ApoE(-/-) mice.

CONCLUSIONS

Our study highlights the importance of the p90RSK/ERK5 module as a critical mediator of EC dysfunction in diabetes mellitus and atherosclerosis formation, thus revealing a potential new target for therapeutic intervention.

Dynamic synchrotron imaging of diabetic rat coronary microcirculation in vivo

OBJECTIVE

In diabetes, long-term micro- and macrovascular damage often underlies the functional decline in the cardiovascular system. However, it remains unclear whether early-stage diabetes is associated with in vivo functional impairment in the coronary microvasculature. Synchrotron imaging allows us to detect and quantify regional differences in resistance microvessel caliber in vivo, even under conditions of high heart rate.

METHODS AND RESULTS

Synchrotron cine-angiograms of the coronary vasculature were recorded using anesthetized Sprague-Dawley rats 3 weeks after treatment with vehicle or streptozotocin (diabetic). In the early diabetic state, in the presence of nitric oxide and prostacyclin, vessel diameters were smaller (P<0.01) and endothelium-dependent vessel recruitment was already depressed (P<0.05). Endothelium-dependent and -independent vasodilatory responses in individual coronary vessels were not different in vivo. Inhibition of NO and PGI(2) production in diabetes uncovered early localized impairment in dilation. Diabetic animals displayed focal stenoses and segmental constrictions during nitric oxide synthase/cyclooxygenase blockade, which persisted during acetylcholine infusion (P<0.05), and a strong trend toward loss of visible microvessels.

CONCLUSIONS

Synchrotron imaging provides a novel method to investigate coronary microvascular function in vivo at all levels of the arterial tree. Furthermore, we have shown that early-stage diabetes is associated with localized coronary microvascular endothelial dysfunction.

Vascular dysfunction in streptozotocin-induced experimental diabetes strictly depends on insulin deficiency

OBJECTIVE

In previous studies we and others have shown that streptozotocin (STZ)-induced diabetes in rats is associated with vascular oxidative stress and dysfunction. In the present study, we sought to determine whether vascular dysfunction and oxidative stress strictly depend on insulin deficiency.

METHODS

The effects of insulin (2.5 U/day s.c., 2 weeks) therapy on vascular disorders in STZ-induced (60 mg/kg i.v., 8 weeks) diabetes mellitus (type I) were studied in Wistar rats. The contribution of NADPH oxidase to overall oxidative stress was investigated by in vivo (30 mg/kg/day s.c., 4 days) and in vitro treatment with apocynin.

RESULTS

Insulin therapy completely normalized blood glucose, body weight, vascular dysfunction and oxidative stress as well as increased cardiac reactive oxygen and nitrogen species formation in diabetic rats, although diabetes was already established for 6 weeks before insulin therapy was started for the last 2 weeks of the total treatment interval. Apocynin normalized cardiac NADPH oxidase activity, and L-NAME effects suggest a role for uncoupled endothelial nitric oxide synthase in diabetic vascular complications.

CONCLUSIONS

Our findings indicate that STZ-induced diabetes is a model of insulin-dependent diabetes (type 1) and that cardiovascular complications are probably not associated with systemic toxic side effects of STZ.

Changes in caveolin-1 expression and vasoreactivity in the aorta and corpus cavernosum of fructose and streptozotocin-induced diabetic rats

Hyperglycemia is a common defining feature in the development of endothelial dysfunction which plays a key role in the pathogenesis of both type 1 and type 2 diabetes. Caveolin-1 is the main structural component of caveolae which might be involved in the pathophysiology of macrovascular complications of diabetes. In this study we aimed to observe the effect of caveolin-1 on functional responses of aorta and corpus cavernosum in the streptozotocin and fructose-induced diabetes groups. Type 1 diabetes was induced by intraperitoneal administration of streptozotocin (60 mg/kg),. Type 2 diabetes by adding fructose in the rat's drinking water (10% (w/v)) for 8 weeks. For insulin treatment; rats were treated with insulin (6 U/kg) for 8 weeks. In Type I and Type II diabetic groups the contractile responses of corpus cavernosum strips to phenylephrine (EC(50):1.82 x 10(-5)M;1.47 x 10(-5)M, respectively)and relaxation responses to acetylcholine (EC(50):7.5 x 10(-5)M;4.48 x 10(-5)M, respectively)were significantly impaired. Contractile responses of aorticstrips to phenylephrine in diabetic groups were markedly decreased (EC(50):3.7.10(-7)M;2.61.10(-7)M respectively) and dose-dependent relaxation responses to acetylcholine were also attenuated (EC(50):3.23.10(-6)M; 2.0.10(-6)M respectively). Treatment with insulin improved the functional responses in the aorta and corpus cavernosum. Protein expression of caveolin-1 was increased in the aorta and corpus cavernosum of the diabetic groups, but this increase seen in the streptozotocin group was more significant than the fructose group. Our findings indicate that an attenuation of the functional responses in both diabetes groups were probably associated with an enhanced expression of caveolin-1, and therefore a decrease in the eNOS activity with a concomitant decrease in NO synthesis.

Effects of streptozotocin-induced diabetes on the pharmacology of rat conduit and resistance intrapulmonary arteries

Background

Poor control of blood glucose in diabetes is known to promote vascular dysfunction and hypertension. Diabetes was recently shown to be linked to an increased prevalence of pulmonary hypertension. The aim of this study was to determine how the pharmacological reactivity of intrapulmonary arteries is altered in a rat model of diabetes.

Methods

Diabetes was induced in rats by the β-cell toxin, streptozotocin (STZ, 60 mg/kg), and isolated conduit and resistance intrapulmonary arteries studied 3–4 months later. Isometric tension responses to the vasoconstrictors phenylephrine, serotonin and PGF_2α, and the vasodilators carbachol and glyceryl trinitrate, were compared in STZ-treated rats and age-matched controls.

Results

STZ-induced diabetes significantly blunted the maximum response of conduit, but not resistance pulmonary arteries to phenylephrine and serotonin, without a change in pEC₅₀. Agonist responses were differentially reduced, with serotonin (46% smaller) affected more than phenylephrine (32% smaller) and responses to PGF_2α unaltered. Vasoconstriction caused by K⁺-induced depolarisation remained normal in diabetic rats. Endothelium-dependent dilation to carbachol and endothelium-independent dilation to glyceryl trinitrate were also unaffected.

Conclusion

The small resistance pulmonary arteries are relatively resistant to STZ-induced diabetes. The impaired constrictor responsiveness of conduit vessels was agonist dependent, suggesting possible loss of receptor expression or function. The observed effects cannot account for pulmonary hypertension in diabetes, rather the impaired reactivity to vasoconstrictors would counteract the development of pulmonary hypertensive disease.

Effect of chronic and selective endothelin receptor antagonism on microvascular function in type 2 diabetes

Vascular dysfunction, which presents either as an increased response to vasoconstrictors or an impaired relaxation to dilator agents, results in worsened cardiovascular outcomes in diabetes. We have established that the mesenteric circulation in Type 2 diabetes is hyperreactive to the potent vasoconstrictor endothelin-1 (ET-1) and displays increased nitric oxide-dependent vasodilation. The current study examined the individual and/or the relative roles of the ET receptors governing vascular function in the Goto-Kakizaki rat, a mildly hyperglycemic, normotensive, and nonobese model of Type 2 diabetes. Diabetic and control rats received an antagonist to either the ET type A (ETA; atrasentan; 5 mg x kg(-1) x day(-1)) or type B (ET(B); A-192621; 15 or 30 mg x kg(-1) x day(-1)) receptors for 4 wk. Third-order mesenteric arteries were isolated, and vascular function was assessed with a wire myograph. Maximum response to ET-1 was increased in diabetes and attenuated by ETA antagonism. ETB blockade with 15 mg/kg A-192621 augmented vasoconstriction in controls, whereas it had no further effect on ET-1 hyperreactivity in diabetes. The higher dose of A-192621 showed an ETA-like effect and decreased vasoconstriction in diabetes. Maximum relaxation to acetylcholine (ACh) was similar across groups and treatments. ETB antagonism at either dose had no effect on vasorelaxation in control rats, whereas in diabetes the dose-response curve to ACh was shifted to the right, indicating a decreased relaxation at 15 mg/kg A-192621. These results suggest that ETA receptor blockade attenuates vascular dysfunction and that ETB receptor antagonism exhibits differential effects depending on the dose of the antagonists and the disease state.

Contribution of nitric oxide produced by inducible nitric oxide synthase to vascular responses of mesenteric arterioles in streptozotocin-diabetic rats

1. The functional changes in mesenteric arterioles of streptozotocin-induced diabetes were investigated by intravital microscopy. The mesentery was exteriorized from anesthetized rats, spread in a chamber, and superfused with Tyrode solution. All drugs tested were applied to the superfusing Tyrode solution. 2. Compared with age-matched controls, the diabetic rats showed enhanced vascular sensitivity to phenylephrine, an alpha(1)-adrenoceptor agonist. The preincubation of the mesentery with N(G)-nitro-l-arginine (l-NNA), a nitric oxide synthase (NOS) inhibitor, shifted the phenylephrine-concentration-response curves to the left in both the diabetic and control rats. Even in the presence of l-NNA, the sensitivity to phenylephrine was higher in the diabetic rats than in the control. 3. Acetylcholine relaxed the mesenteric arterioles in both groups, but to a significantly greater extent in the control than in the diabetic rats. However, the l-NNA-induced constriction of arterioles did not differ significantly between the groups. In contrast, the amplitude of the constrictions of mesenteric arterioles induced by S-ethylisothiourea, an inducible NOS (iNOS) inhibitor, was significantly greater in the diabetic rats than in the control. 4. Immunostaining of the mesentery with a specific antibody for iNOS revealed iNOS in the microvessels of only the diabetic rats. 5. These results suggest that constrictor responses to alpha(1)-adrenoceptor stimulation are sensitized in the mesenteric arterioles of STZ-diabetic rats, and that iNOS expressed in the arteriolar smooth muscle plays a role in suppressing the basal tone and the reactivity of the arterioles in STZ-diabetic rats.

Vascular responses to alpha-adrenergic stimulation and depolarization are enhanced in insulin-resistant and diabetic Psammomys obesus

Since vascular complications often accompany diabetes, we examined the influence of the endothelial lining on vascular reactivity in Psammomys obesus, a desert gerbil that acquires insulin resistance and diabetes when exposed to a laboratory diet. Vasoconstriction to phenylephrine and depolarizing KCl, as well as carbachol endothelium-dependent relaxation, were assessed in rings of thoracic aortae obtained from three groups: (i) group A, normoglycemic-normoinsulinemic; (ii) group B, normoglycemic-hyperinsulinemic, and (iii) group C, hyperglycemic-hyperinsulinemic animals. As expected, marked hypertriglyceridemia and hypercholesterolemia characterized groups B and C, which developed enhanced contractile responsiveness to phenylephrine and KCl compared with controls (group A). Furthermore, both experimental groups displayed a significant decrease in endothelium-dependent relaxation to carbachol. Altered lipid profiles are considered to play some role in the observed modification of aortic reactivity. Overall, our data indicate that vascular contractile responsiveness is enhanced early in the development of insulin resistance and diabetes in the female P. obesus.

Effect of 5-aminoimidazole-4-carboxamide riboside (AICA-r) on isolated thoracic aorta responses in streptozotocin-diabetic rats

Diabetes mellitus alters the vascular responsiveness to several vasoconstrictors and vasodilators. 5-amino-4-imidazole-carboxamide riboside (AICA-r), a nucleoside corresponding to AICA-ribotide and an intermediate of the de novo pathway of purine biosynthesis, was recently proposed as a new insulinotropic tool in non-insulin-dependent diabetes mellitus. The aim of the present study was to define whether AICA-r affects altered vascular responsiveness to vasoconstrictors and vasodilators in the thoracic aorta of neonatal streptozotocin (STZ)-diabetic rats. The results of this study indicate that a 1-month treatment with AICA-r significantly increases the body weight in diabetic rats; significantly decreases the blood glucose level of diabetic rats (from 302+/-47 to 135+/-11 mg/dL, p<0.001); does not significantly affect the fast, slow, and total components of responses to noradrenaline in all the experimental groups; reverses the increased Emax values of noradrenaline in diabetic rats to near-control values; reverses the completely abolished responses of acetylcholine (pD2 and percent relaxation) in diabetic rats to control values; and reverses the decreased pD2 values of sodium nitroprussiate in diabetic rats to control values. In conclusion, AICA-r treatment in neonatal STZ-diabetic rats improved increased blood glucose levels, accelerated weight gain, reversed endothelial dysfunction, and normalized vascular responses.

Recovery of microvascular responses during streptozotocin-induced diabetes

Microvascular reactivity of cannulated and pressurised rat cremaster arterioles was studied during the progress of diabetes using mechanical (intraluminal pressure) and chemical (acetylcholine, sodium nitroprusside) stimulation. Microvessels were studied in controls and at 2, 4 and 8 weeks following induction of diabetes by streptozotocin. Mechanical responses were stable at the test pressure (70 mmHg) used for pharmacological investigations during the period of diabetes. Acetylcholine application could induce maximal dilatation in control vessels and in vessels exposed to 8 weeks of diabetes. However, acetylcholine administration failed to generate maximal dilatation at 2 and 4 weeks of diabetes. During the period of diabetes, loss of nitric oxide (NO) pathway effectiveness was revealed by diminished response to sodium nitroprusside and by reduced capacity of Nomega-nitro-L-arginine methyl ester (L-NAME) to decrease resting diameter and acetylcholine-evoked dilatation. L-NAME and indomethacin application revealed a significant non-NO, non-prostaglandin contribution to the acetylcholine response at 4 and 8 weeks of diabetes. Recovery of responsiveness to acetylcholine and stabilisation of resting vessel diameter during diabetes may, in part, be due to increasing effectiveness of non-NO, non-prostaglandin pathways.

Radiotelemetric monitoring of blood pressure and mesenteric arterial bed responsiveness in rats with streptozotocin-induced diabetes

We investigated the changes in arterial blood pressure (BP) and of mesenteric arterial bed (MAB) responsiveness that accompany streptozotocin (STZ)-induced diabetes. BP was recorded by radiotelemetry in conscious animals before and during a 4-week period following induction of the diabetic state with STZ. At the end of this period, the MAB was isolated and perfused under constant flow conditions: perfusion pressure (PP, mmHg) was taken as an index of arteriolar tone. BP was lower (P < 0.05) in STZ-treated diabetic rats (82.9 ± 5.0 mmHg) than in vehicle-treated rats (108.9 ± 6.3 mmHg). Basal perfusion pressure of the MAB was lower in STZ-treated rats than in control rats and inhibition of nitric oxide (NO) synthesis with NG-nitro-L-arginine-methyl-ester and NG-nitro-L-arginine (100 µM each) failed to change this relationship. Increases in PP of MAB to phenylephrine (Phe), norepinephrine (NE), and potassium chloride (KCl) were reduced in STZ-treated rats compared with control rats. Inhibition of NO synthesis reduced responses to Phe, NE, and KCL in both STZ and control rats. The reduced responsiveness of STZ rats to Phe, NE, and KCl persisted after inhibition of NO synthesis. Acetylcholine (ACh) evoked relaxation of the MAB in a dose-dependent fashion. Maximal responses to ACh, but not sodium nitroprusside, were lower in STZ rats than in vehicle treated rats. Inhibition of NO synthesis reduced responses to ACh in both STZ and control rats. The reduced responsiveness of STZ rats to ACh persisted after inhibition of NO synthesis. The data demonstrate that STZ-induced diabetes is associated with a fall in blood pressure when pressure is recorded with radiotelemetry. The fall in blood pressure may be related to a non-specific decrease in responsiveness to vasoconstrictor stimuli mediated at least in part by NO-independent mechanisms. A decrease in responsiveness to endothelial dependent vasodilator mechanisms appeared insufficient to restore responsiveness to vasoconstrictor stimuli.Key words: radiotelemetry, blood pressure, mesenteric arterial bed reactivity, acetylcholine, norepinephrine, STZ-diabetic rat.

Endothelial dysfunction in diabetes

Endothelial dysfunction plays a key role in the pathogenesis of diabetic vascular disease. The endothelium controls the tone of the underlying vascular smooth muscle through the production of vasodilator mediators. The endothelium-derived relaxing factors (EDRF) comprise nitric oxide (NO), prostacyclin, and a still elusive endothelium-derived hyperpolarizing factor (EDHF). Impaired endothelium-dependent vasodilation has been demonstrated in various vascular beds of different animal models of diabetes and in humans with type 1 and 2 diabetes. Several mechanisms of endothelial dysfunction have been reported, including impaired signal transduction or substrate availibility, impaired release of EDRF, increased destruction of EDRF, enhanced release of endothelium-derived constricting factors and decreased sensitivity of the vascular smooth muscle to EDRF. The principal mediators of hyperglycaemia-induced endothelial dysfunction may be activation of protein kinase C, increased activity of the polyol pathway, non-enzymatic glycation and oxidative stress. Correction of these pathways, as well as administration of ACE inhibitors and folate, has been shown to improve endothelium-dependent vasodilation in diabetes. Since the mechanisms of endothelial dysfunction appear to differ according to the diabetic model and the vascular bed under study, it is important to select clinically relevant models for future research of endothelial dysfunction.

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

We examined the effects of 2 months of streptozotocin-induced diabetes mellitus in rats on relaxation and contraction of corpus cavernosum and the mesenteric vascular bed in vitro. A further diabetic group was treated from diabetes induction with 10 mg/kg/day of the aldose reductase inhibitor, WAY121509. For corpus cavernosum, maximal acetylcholine-induced relaxation was 35.5% reduced (p < 0.001) by diabetes, and this deficit was completely prevented by WAY121509 treatment. Neither diabetes nor treatment affected contractile responses to field stimulation of noradrenergic nerves; however, nonadrenergic noncholinergic nerve relaxation responses were 32.9% decreased by diabetes and WAY 121509 attenuated this by 84% (p < 0.001). For the mesenteric vascular bed, diabetes depressed maximal endothelium-dependent vasodilation to acetylcholine by 25.2% (p < 0.001), and this was partially (50.6%; p < 0.01) prevented by WAY121509. Nitric oxide synthase blockade revealed endothelium-derived hyperpolarising factor-mediated vasodilation to acetylcholine that was 73.5% (p < 0.001) depressed by diabetes; WAY121509 provided partial (43.4%; p < 0.001) protection. Neither diabetes nor treatment affected endothelium-independent vasorelaxation to the nitric oxide donor, sodium nitroprusside, in corpus cavernosum or mesenteric vessels. Thus the data show protective effects of WAY121509 on nitric oxide-mediated cavernosal vasorelaxation responses and on mesenteric endothelium-derived hyperpolarising factor responses. Together these findings could account for the beneficial effects of aldose reductase inhibition on diabetic complications in experimental models.

The effect of long-term streptozotocin-induced diabetes on contractile and relaxation responses of coronary arteries: selective attenuation of CGRP-induced relaxations

1. This study investigates the effect of partially metabolic controlled long-term (34 weeks) streptozotocin (STZ)-induced diabetes on relaxation and contractile responses of isolated coronary arteries to seven different vasoactive agents. 2. The average fasting and non-fasting blood glucose concentrations (mM) were significantly elevated in STZ-induced diabetic rats (P<0.0001; 10.4+/-0.4 and 16. 6+/-1.1, n=15) compared to those (4.3+/-0.03 and 4.7+/-0.18, n=11) in age-matched controls. The level of glycated haemoglobin (HbA(1)) was also significantly (P<0.0001) increased in STZ-induced diabetic rats. In STZ-induced diabetic rats, the HbA(1) levels were significantly correlated with the non-fasting blood glucose concentrations (r=0.76; P=0.003; n=13). In both groups, there was no significant correlation between the HbA(1) levels and maximal responses or sensitivities to the vasoactive agents. 3. The maximal relaxation induced by rat-alphacalcitonin gene-related peptide (rat-alphaCGRP) was significantly attenuated in the coronary arteries of STZ-induced diabetic rats (P<0.05; 40+/-7%, n=15) compared to that in age-matched controls (63+/-3%, n=11). However, there was no significant difference in the sensitivity to rat-alphaCGRP between the two groups. 4. There was no significant difference in either maximal response or sensitivity to any of the six other vasoactive agents between STZ- induced diabetic rats (n=15) and age-matched controls (n=11). 5. Our results show that partially metabolic controlled long-term (34 weeks) STZ-induced diabetes causes a selective depression of rat-alphaCGRP-induced relaxation in the intramural coronary arteries of Wistar rats.

The effect of sorbinil, an aldose reductase inhibitor, on aortic function in control and streptozotocin-induced diabetic rats

1. The present study investigates the effect of treatment of 14-day streptozotocin-diabetic rats with the aldose reductase inhibitor, sorbinil, on changes ex vivo in aortic vasoconstriction and vasodilation. 2. Maximum contractile responses and aortic sensitivity to phenylephrine were significantly enhanced in aortae from 14-day diabetic rats, in accordance with our previous data. 3. Endothelium-dependent relaxations to carbachol were, in contrast, depressed, although endothelium-independent relaxations to forskolin and sodium nitroprusside were unaltered. 4. Sorbinil treatment of diabetic animals failed to prevent any of these diabetes-induced alterations in aortic function, and indeed exacerbated some of these alterations. In addition, sorbinil treatment caused altered aortic responses in control animals, which sometimes mirrored those found in diabetic animals. 5. It can be concluded that sorbinil may have actions in addition to, and independent of, polyol pathway inhibition. Thus, sorbinil may not be an effective tool for the investigation of aldose reductase inhibition within the vascular system of the rat.

Constrictor responses of resistance arterioles during diabetes mellitus

While diabetes mellitus appears to alter nitric oxide synthase-dependent vasodilatation, the effect of diabetes on constrictor responses of resistance arterioles is not clear. Our goal was to examine the effect of diabetes on constrictor responses of cheek pouch arterioles. In vivo diameter of arterioles ( approximately 50 microm) was measured in response to norepinephrine, the thromboxane analogue (U-46619) and endothelin-1 in nondiabetic and diabetic hamsters (4-6 weeks post streptozotocin). Norepinephrine (1.0 and 10 nM) and U-46619 (0.1 and 1.0 nM) produced similar dose-related vasoconstriction in nondiabetic and diabetic hamsters (P > 0.05). In contrast, vasoconstriction to endothelin-1 (0.1 and 1.0 pM) was greater in diabetic than nondiabetic hamsters (P < 0.05). Next, we examined the role of nitric oxide in basal vascular tone and whether enhanced vasoconstriction in diabetic hamsters to endothelin-1 might be related to an alteration in the modulatory role of nitric oxide. N(G)-monomethyl-L-arginine (L-NMMA) (1.0, 10 and 100 microM) produced dose-related vasoconstriction in nondiabetic, but not diabetic hamsters. Further, L-NMMA did not alter vasoconstriction in response to endothelin-1 in nondiabetic and diabetic hamsters. These findings suggest that diabetes alters constriction of cheek pouch resistance arterioles to endothelin-1 which appears to be independent of the synthesis/release of nitric oxide. In addition, based upon findings using L-NMMA, it appears that there is a reduced influence of nitric oxide on basal diameter of resistance arterioles during diabetes.

Effect of insulin treatment on smooth muscle contractility and endothelium-dependent relaxation in rat aortae from established STZ-induced diabetes

1. This study involved the chronic administration of low or high insulin to rats with established streptozotocin (STZ)-induced diabetes. We studied the effect of such treatment on smooth muscle contractility and endothelium-dependent relaxation using aortic strips. 2. Aortae from diabetic rats, but not those from high-insulin-treated diabetic rats, showed an impaired endothelium-dependent in response to acetylcholine (ACh) by comparison with untreated controls. 3. Isotonic high K+-induced contractility was impaired in diabetic aortae. This impairment was prevented by high-insulin treatment. 4. Noradrenaline (NA)-induced contractility was enhanced in aortae from high-insulin-treated diabetic rats, but not in those from untreated diabetic or low-insulin treated diabetic rats. 5. In the combined presence of the nitric oxide inhibitor N(G)-nitro-L-arginine and the cyclo-oxygenase inhibitor indomethacin, NA-induced contractility was significantly greater in aortae from high-insulin-treated diabetic rats than in those from controls or untreated diabetic rats. 6. An increased expression of the mRNA for the alpha1D and alpha1B adrenergic receptors was found in aortae from high-insulin-treated diabetic rats. 7. These results demonstrate that in rats with established STZ-induced diabetes, high-insulin treatment prevents the development of an impaired endothelium-dependent relaxation in the aorta, and that such treatment enhances NA-induced contractility. This enhancement may be related to an upregulation in the expression of the mRNA for the alpha1B or alpha1D adrenergic receptor that is secondary to the hyperinsulinaemia.

Different effects of the beta-adrenoceptor antagonists celiprolol and metoprolol on vascular structure and function in long-term type I diabetic rats

An intriguing problem of diabetes mellitus is the development of generalized angiopathy and concomitant hypertension. However, there is still a controversy whether beta-adrenoceptor antagonists can be used as antihypertensive agents in diabetes. Four groups of rats were investigated: nondiabetic controls, diabetes mellitus, diabetes + celiprolol (250 mg/kg body weight/day), diabetes + metoprolol (125 mg/kg body weight/day) after 6 months. Diabetes was induced by i.v. streptozotocin injection. We examined vascular structure and function histologically and by an in vitro microvideoangiometry of isolated perfused mesenterium. Additionally, we investigated the effects of hyperglycemia and celiprolol on NO release in cultivated aortic endothelial cells and the effect of celiprolol on transendothelial paracellular permeability. Diabetes resulted in endothelial dysfunction, characterized by a reduced response to acetylcholine and L-N(G)-nitro-arginine and an unchanged response to sodium nitroprusside (SNP). These effects were significantly antagonized by celiprolol but were not influenced by metoprolol treatment. This was supported by the finding of typical vascular changes associated with diabetes like media thickening, reduced cardiac capillary/muscle fiber ratio, and glomerulosclerosis, which were significantly reduced by celiprolol but not influenced by metoprolol treatment. Ketonuria improved after celiprolol treatment, whereas blood glucose, lipids, and body weight were not different between the diabetic groups. In cultured cells, celiprolol did not induce direct NO release but reversed the impairment of stimulated NO release caused by hyperglycemia. Furthermore, celiprolol reduced endothelial paracellular permeability. We conclude that celiprolol can exert antiangiopathic effects in diabetic rats and that both beta-adrenoceptor antagonists did not aggravate diabetic angiopathy and metabolic derangement.

Accelerated intimal hyperplasia and increased endogenous inhibitors for NO synthesis in rabbits with alloxan-induced hyperglycaemia

1. We examined whether endogenous inhibitors of NO synthesis are involved in the augmentation of intimal hyperplasia in rabbits with hyperglycaemia induced by alloxan. 2. Four weeks after the endothelial denudation of carotid artery which had been performed 12 weeks after alloxan, the intimal hyperplasia was greatly augmented with hyperglycaemia. The degree of hyperplasia was assessed using three different parameters of histopathological findings as well as changes in luminal area and intima: media ratio. 3. There were positive and significant correlations between intima:media ratio, plasma glucose, and concentrations of N(G)-monomethyl-L-arginine (L-NMMA) and N(G), N(G)-dimethyl-L-arginine (ADMA) in endothelial cells, that is, the intima:media ratio became greater as plasma glucose and endothelial L-NMMA and ADMA were increased. Furthermore, endothelial L-NMMA and ADMA were increased in proportion to the increase in plasma glucose. 4. In contrast, there were inverse and significant correlations between cyclic GMP production by carotid artery strips with endothelium and plasma glucose, between cyclic GMP production and endothelial L-NMMA and ADMA, and between the intima:media ratio and cyclic GMP production. 5. Exogenously applied L-NMMA and ADMA inhibited cyclic GMP production in a concentration-dependent manner. IC50 values were determined to be 12.1 microM for the former and 26.2 microM for the latter. The cyclic GMP production was abolished after the deliberate removal of endothelium from the artery strips. 6. These results suggest that the augmentation of intimal hyperplasia with hyperglycaemia is closely related to increased accumulation of L-NMMA and ADMA with hyperglycaemia, which would result in an accelerated reduction in NO production/release by endothelial cells.

Attenuation of diabetes-associated mesenteric vascular hypertrophy with perindopril: morphological and molecular biological studies

Vascular disease is now the major cause of morbidity and mortality in the diabetic population. Our group explored the vascular changes associated with experimental diabetes and examined whether these changes can be ameliorated by angiotensin-converting enzyme (ACE) inhibition. The ACE inhibitor perindopril (PE) was administered to streptozotocin-induced diabetic rats for 24 weeks. At death, mesenteric vessels were perfused in vivo followed by assessment of the vascular architecture by quantitative histomorphometry. In a subgroup of animals, RNA was extracted from the mesenteric vasculature for assessment of gene expression of the prosclerotic cytokine, transforming growth factor beta 1 (TGFbeta1), and the matrix protein, type IV collagen. Diabetes was associated with smooth muscle hypertrophy and extracellular matrix (ECM) accumulation. ECM accumulation, particularly collagen deposition, was observed in the medial and adventitial layers. ACE inhibition prevented mesenteric vascular hypertrophy after 24 weeks of diabetes. In addition, overexpression of TGFbeta1 in the vessels of diabetic animals was prevented by PE treatment. Similarly, type IV collagen mRNA levels were increased in diabetic vessels, and this overexpression was also prevented by PE therapy. In summary, ACE inhibition attenuates many of the vascular changes observed in experimental diabetes and may have important clinical implications as a vasoprotective agent in human diabetes.

Vascular responsiveness in alloxan-induced diabetes-susceptible (ALS) and resistant (ALR) mice

Changes in reactivity of vascular smooth muscles of male alloxan-induced diabetes-susceptible (ALS) and resistant (ALR) mice aorta were investigated at 2 weeks, 1, 2 and 4 month(s) after the injection of alloxan (45 mg/kg, i.v.). The glucose levels in blood and urine of all the alloxan-treated ALS mice were markedly elevated while those in alloxan-treated ALR and non-treated ALS and ALR mice were not altered. The magnitude of high K+ (65.4 mM)-induced contractions were not affected by the treatment of alloxan. Norepinephrine-induced contractions in vascular smooth muscles of ALS mice in a diabetic state for 2 or 4 months were significantly potentiated. The contractile sensitivity to prostaglandin F2 alpha (PGF2 alpha) was increased in the 4-month-diabetic state. Responsiveness to 5-HT did not vary in the diabetic mouse. Vasorelaxation induced by nitroprusside was attenuated in 2 weeks, 2 or 4 month-diabetic ALS mice. Similarly the inhibitory effects of levcromakalim were attenuated at 2 and 4 months. The influences of diabetes on the inhibitory effects of forskolin or verapamil were very small or not detected. The effects of the vasomodulators used in this study on the vascular smooth muscles of alloxan-treated ALR mice did not differ from those of untreated ALR mice. The results from using ALS and ALR mice suggest that the vasoreactivities to some vasomodulators are changed especially in the long-term diabetic state and that when diabetes was not induced the dose of alloxan does not have any effect on vascular smooth muscle.

Treatment with dimethylthiourea prevents impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that the synthesis/release of hydroxyl radical accounts for impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin, 50-60 mg/kg ip) in response to nitric oxide synthase-dependent agonists (acetylcholine and substance P) and a nitric oxide synthase-independent agonist (nitroglycerin). Reactivity of the basilar artery was measured in untreated nondiabetic and diabetic rats and in nondiabetic and diabetic rats treated with a daily intraperitoneal injection of dimethylthiourea (DMTU; 50 mg/kg). Injection of DMTU was started 48 h after injection of streptozotocin and was continued throughout the diabetic period (3-4 wk). Topical application of acetylcholine (0.1, 1.0, and 10 microM) and substance P (0.1 and 1.0 microM) produced similar dilatation of the basilar artery in untreated and DMTU-treated nondiabetic rats. In untreated diabetic rats, the magnitude of vasodilation produced by acetylcholine and substance P was significantly less than in untreated nondiabetic rats. However, in DMTU-treated diabetic rats, dilatation of the basilar artery in response to acetylcholine and substance P was similar to that observed in nondiabetic rats. Dilatation of the basilar artery in response to nitroglycerin was similar in untreated and DMTU-treated nondiabetic and diabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related to the synthesis/release of hydroxyl radical.

Oxidative stress and lipids in diabetes: a role in endothelium vasodilator dysfunction?

Endothelial dysfunction is a key feature of diabetes mellitus and is thought to be the major cause of vascular complications associated with the disease. The vascular endothelium demonstrates impaired synthesis of vasodilators and increased release of procoagulants and vasoconstrictors, defects which theoretically could explain the increased incidence of atherosclerosis and hypertension found within this patient group. The pathways mediating endothelial cell layer dysfunction are unknown, although many candidates have been proposed. This review concentrates on the hypothesis that increased oxidative stress combined with abnormal plasma lipid composition leads to reduced synthesis of endothelial vasodilators and hence endothelial dysfunction. Free radical generation is undoubtedly raised in diabetes but the evidence for decreased antioxidant status is debatable. The role of antioxidant and lipid-lowering therapy is considered, but few studies have directly investigated the effect of treatment on vascular function. Concern arises from individual studies of vitamin E in diabetic animals which have proved deleterious. Current literature implies that a combination therapy of vitamin E and vitamin C may be beneficial, but this needs to be investigated further in both animal and human diabetes.

Superoxide dismutase partially restores impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of superoxide dismutase restores nitric oxide synthase-mediated dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin; 50-60 mg/kg i.p.) in response to nitric oxide synthase-dependent agonists (acetylcholine and bradykinin) and a nitric oxide synthase-independent agonist (nitroglycerin) before and during application of superoxide dismutase. Topical application of acetylcholine (1.0 and 10 microM) and bradykinin (1.0 and 10 microM) produced dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. However, the magnitude of vasodilation produced by acetylcholine and bradykinin was significantly less in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. Treatment with superoxide dismutase (150 U/ml) did not alter baseline diameter of the basilar artery in nondiabetic and diabetic rats. However, topical application of superoxide dismutase partially restored nitric oxide synthase-dependent dilatation of the basilar artery in diabetic rats towards that observed in nondiabetic rats. Superoxide dismutase did not alter dilatation of the basilar artery in nondiabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related, in part, to enhanced release of oxygen-derived free radicals.

The aldose reductase inhibitor sorbinil does not prevent the impairment in nitric oxide-mediated neurotransmission in anococcygeus muscle from diabetic rats

This study investigated whether increased polyol pathway activity could contribute to alterations in nitrergic neurotransmission in anococcygeus muscles from 8-week diabetic rats. In the presence of guanethidine (10-30 microM) and clonidine (0.01-0.05 microM), relaxations obtained to nitrergic nerve stimulation (0.5-5 Hz, 10-s train), to sodium nitroprusside (5-500 nM) and to nitric oxide (0.1-3 microM) were significantly reduced in muscles from diabetic rats compared to responses from control rats. Treatment of diabetic rats with the aldose reductase inhibitor sorbinil (42 mg/kg per day via feed for 8 weeks) did not affect impaired reactivity to nitrergic nerve stimulation, sodium nitroprusside or nitric oxide. The results suggest increased polyol pathway activity does not contribute to the alterations in nitrergic neurotransmission in anococcygeus muscles from diabetic rats.

Microvascular responses in the skeletal muscle of the diabetic rat

We examined microvascular responses to acetylcholine (ACh), adenosine (ADO), nitroprusside (SNP), bradykinin (BK), histamine (HIS), and serotonin (5-HT) in control and diabetic rats. Agonists were applied topically to neurovascularly intact and environmentally controlled cremaster muscles of diabetic or control rats 3 weeks after streptozotocin or vehicle injection. Precapillary arteriolar diameters and leakage of fluorescein isothiocyanate conjugated to bovine serum albumin (FITC-BSA) in postcapillary venules were measured by using intravital microscopy techniques. All agents dilated the arterioles, and BK, HIS, and 5-HT caused concentration-dependent leakage of FITC-BSA. Of the vasodilators tested, only ACh-induced dilation was attenuated in diabetic animals. BK and HIS caused a concentration-dependent leakage of FITC-BSA that was similar in diabetic and control groups, but FITC-BSA leakage to 5-HT was significantly attenuated in diabetic animals. To determine whether the attenuated responses were attributable to an alteration of nitric oxide, the same dilational and leakage-promoting agents were tested in normoglycemic animals in the presence of N'-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthase. Only ACh-induced dilation in control animals was attenuated in the presence of L-NAME; all other responses were normal. Thus, although the attenuated ACh-induced arteriolar dilation in striated muscle of diabetic rats may be linked to an impairment of nitric oxide function or release, the reduced leakage to serotonin is not linked to nitric oxide. The impaired leakage response to 5-HT is specific for that agonist and is not an indication of a generalized decrease in vascular permeability in these diabetic animals.

Impairment of endothelium-dependent relaxation by increasing percentages of glycosylated human hemoglobin. Possible mechanisms involved

High levels of glycosylated human hemoglobin impair nitric oxide-mediated responses. However, the percentage of glycosylation for which this effect is observed and the mechanisms involved are unknown. We tested endothelium-dependent relaxations caused by acetylcholine in rat aortic segments either in control conditions or after preincubation with increasing percentages of glycosylated human hemoglobin. Human hemoglobin (1 and 10 nmol/L) inhibited endothelium-dependent relaxations only when glycosylated at 9% or higher. We evaluated the effect of 14% glycosylated human hemoglobin on acetylcholine-evoked responses in vessels preincubated with scavengers of superoxide anions, hydroxyl radical, or hydrogen peroxide (superoxide dismutase, deferoxamine, and catalase, respectively); with inhibitors of xanthine oxidase, cyclooxygenase, or thromboxane synthase (allopurinol, indomethacin, and dazoxiben, respectively); with blockers of thromboxane A2/prostaglandin H2 or endothelin receptors (SQ 30741 and BQ-123); and with the precursor of nitric oxide synthesis L-arginine. Superoxide dismutase abolished the effect of glycosylated hemoglobin, and the other substances did not have any effect. Glycosylated hemoglobin at 14% did not modify either the vasoconstrictions induced by the blocker of nitric oxide synthase NG-nitro-L-arginine methyl ester or the relaxations evoked in deendothelialized vessels by sodium nitroprusside and 8-bromo-cGMP. However, it inhibited the vasodilations evoked by exogenous nitric oxide. Superoxide dismutase abolished this latter effect. We conclude that the threshold for glycosylated human hemoglobin (Hb A1) to inhibit endothelium-dependent relaxation is 9%. This effect is due to interference with endothelial nitric oxide by means of superoxide anion production.

The effect of insulin treatment and of islet transplantation on the resistance artery function in the STZ-induced diabetic rat

1. This study was designed to investigate the influence of insulin treatment and islet transplantation on the smooth muscle contractility and endothelium-dependent and independent relaxation of resistance arteries in the chemically induced streptozotocin (STZ) diabetic rat after 6-8 weeks, and 12-14 weeks of diabetes, compared to non-diabetic age-matched controls. 2. The morphology, and contractile responses to high potassium physiological salt solution (KPSS), KPSS containing 10(-5) M noradrenaline (NAK), and concentration-response curves to noradrenaline (NA) of mesenteric resistance arteries were recorded, along with the endothelium-dependent relaxation responses to acetylcholine (ACh) and bradykinin (BK), and endothelium-independent relaxation to sodium nitroprusside (SNP). Concentration-response curves were then repeated in the presence of a nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG). 3. Insulin-treated diabetic rats in the 12 week study demonstrated enhanced vascular contractility to KPSS, NAK and NA, compared to age-matched non-diabetic controls. 4. Incubation with L-NOARG resulted in both a significant increase in maximum contractile response, and sensitivity (pD2) to NA in the untreated diabetic group (6 weeks). A significant shift in sensitivity was also seen in the insulin-treated diabetic group. In the 12 week study, incubation with L-NOARG resulted in an increased maximum contractile response and sensitivity to NA in the insulin-treated diabetics. An increase in sensitivity was also observed in the untreated diabetic group. 5. Endothelium-dependent relaxation to ACh was significantly augmented in the untreated diabetics (6-weeks), compared to the control group. In the 12-week study, relaxation to both ACh and BK was not significantly different in any of the experimental groups when compared to the sham-operated non-diabetic controls. 6. Incubation with L-NOARG resulted in a significant attenuation of the maximum relaxation response to ACh and BK in all of the experimental groups, in the 6- and the 12-week study. 7. There was no significant difference in the maximum relaxation response or sensitivity to sodium nitroprusside between the diabetic groups and their age-matched controls in either the 6-week or the 12-week study. 8. The results of this study suggest an enhanced release of nitric oxide in the early stages of diabetes, which is more evident in the untreated diabetic rats than the insulin treated, and appears to normalize as the duration of diabetes progresses. This study also shows that the alteration in vascular reactivity of the resistance arteries can be restored to within normal limits by the transplantation of islets of Langerhans, and that islet transplantation is an effective strategy in the correction of the metabolic abnormalities associated with insulin-dependent diabetes.

Effects of streptozotocin-induced diabetes on vascular reactivity in genetically hyperinsulinaemic obese Zucker rats

Although the fa/fa Zucker rat shows many of the features of type II diabetes, the absence of consistent cardiovascular complications in this model may be due to the absence of significant hyperglycaemia. We studied the consequences of streptozotocin (STZ)-induced insulin deficiency and hyperglycaemia on vascular reactivity in the fa/fa Zucker rat. Hyperinsulinaemic obese Zucker rats were rendered diabetic by injection of STZ (50-60 mg/kg intraperitoneally, i.p.), and vascular tissue was removed for study 10-12 weeks later. In isolated aorta, there was no difference in the phenylephrine (PE) concentration-response relation between lean and obese control animals, but the concentration-response curve was shifted to the left in diabetic animals, (pD2 7.56 +/- 0.04 in STZ diabetic animals, n = 8; 7.4 +/- 0.04 in obese control, n = 9, p < 0.05). The maximum response was also enhanced in both aorta and perfused mesentery of STZ-treated animals. In contrast, the potency of serotonin (5-HT) in inducing contractions of isolated aorta were enhanced in tissues from obese as compared with lean animals (pD2 6.63 +/- 0.06, n = 9; 6.17 +/- 0.07, n = 7 respectively; p < 0.01) and was attenuated in animals with STZ-induced diabetes (pD2 6.31 +/- 0.09, n = 8, p = 0.05). The differential effects of hyperglycaemia on PE-and 5-HT-induced vasoconstriction suggest that the long-lasting modulation of vasoconstrictor responses induced by increases in blood glucose level may be specific for some agonists.

Enhanced contractile responses of arteries from streptozotocin diabetic rats to sodium fluoride

1. Previous studies from this laboratory have demonstrated that alpha 1-adrenoceptor-mediated increases in tension and phosphoinositide metabolism are enhanced in the aorta and mesenteric arteries from diabetic rats. The purpose of the present investigation was to determine whether contractile responses to sodium fluoride (NaF), which directly stimulates GTP-binding proteins (G-proteins), are also enhanced in diabetic arteries. 2. NaF (1-20 mM) in the presence of 10 microM aluminium chloride produced slowly developing, concentration-dependent contractions in mesenteric arteries from three month streptozotocin-diabetic (60 mg kg-1, i.v.) male Wistar rats and age-matched control rats. The maximum contractile response but not the sensitivity to NaF was significantly greater in mesenteric arteries from diabetic than from control rats, as was the response to noradrenaline (NA). Maximum contractile responses of aorta and caudal artery from diabetic rats to NaF were also significantly enhanced. 3. Removal of the endothelium and denervation with 6-hydroxydopamine did not significantly alter the maximum contractile response of mesenteric arteries from either control or diabetic rats to NaF. Similarly, NaF had no effect on cyclic AMP levels in aorta, and no difference in cyclic AMP levels, either basally or in the presence of NaF, was detected between control and diabetic rat aorta. 4. Contractile responses of mesenteric arteries from both control and diabetic rats to NaF were diminished in calcium-free Krebs solution, but the NaF response remained significantly elevated in mesenteric arteries from diabetic rats compared to control. 5. Ryanodine (30 microM) which depletes intracellular calcium stores, nifedipine (3 microM) which blocks dihydropyridine-sensitive calcium channels and calphostin C (0.5 microM) which selectively inhibits protein kinase C, all significantly inhibited maximum contractile responses of mesenteric arteries from control and diabetic rats to NaF. There were no significant differences between control and diabetic arteries in the relative magnitude of the inhibition produce by the three antagonist. 6. These data suggest that there may be increased activation of the same signalling processes that mediate NA-stimulated vasoconstriction, perhaps contraction-associated G-proteins or the effectors coupled to these G-proteins, in response to NaF in mesenteric arteries from diabetic rats. This may also be responsible for the enhanced contractile responses of these arteries to alpha 1-adrenoceptor stimulation.

L-Arginine does not restore dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of L-arginine, a substrate for the synthesis of nitric oxide, restores endothelium-dependent dilatation of the basilar artery during diabetes mellitus. We measured the diameter, of the basilar artery in vivo in nondiabetic and diabetic (streptozotocin; 50-60 mg/kg i.p.) rats in response to endothelium-dependent agonists (acetylcholine and bradykinin) and an endothelium-independent agonist (nitroglycerin) before and during application of L-arginine. Topical application of acetylcholine (1.0 and 10 muM) and bradykinin (1.0 and 10 microM) produced dilatation in nondiabetic rats of the basilar artery which was impaired in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dilatation of the basilar artery in nondiabetic and diabetic rats. Topical application of L-arginine (0.1 and 3 mM) did not enhance dilatation of the basilar artery in response to acetylcholine and bradykinin in diabetic rats. Thus, impairment of dilatation of the basilar artery in diabetic rats in response to acetylcholine and bradykinin appears to be related to a mechanism unrelated to the availability of L-arginine for nitric oxide synthase.

Preservation of endothelium-dependent relaxation in cholesterol-fed and streptozotocin-induced diabetic mice by the chronic administration of cholestyramine

1. Experiments were designed to investigate the effects of the low density lipoprotein (LDL)-lowering drugs cholestyramine on serum LDL levels and endothelium-dependent relaxation to acetylcholine (ACh) in cholesterol-fed or streptozotocin (STZ)-induced diabetic mice. 2. In aortic rings from control mice, ACh or A23187 caused concentration-dependent relaxation. The relaxations caused by ACh or A23187 were significantly attenuated in aortic rings from cholesterol-fed and STZ-diabetic mice. The attenuated vasodilatation in both cholesterol-fed and diabetic mice was returned to normal by chronic administration of cholestyramine. The endothelium-independent relaxations of aortic rings induced by sodium nitroprusside (SNP) were not significantly different between control, cholesterol-fed and STZ-induced diabetic mice. 3. The increased LDL levels in cholesterol-fed and diabetic mice were returned to normal by the chronic administration of cholestyramine. Chronic administration of cholestyramine had no effects on serum glucose levels. 4. These results suggest that attenuated endothelium-dependent vasodilatations in both cholesterol-fed and STZ-diabetic mice are improved by the chronic administration of cholestyramine, and these effects are, at least in part, due to lowering serum LDL levels.

Fosinopril improves regulation of vascular tone in mesenteric bed of diabetic rats

Because diabetes mellitus leads to vascular dysfunction, we examined the microvascular endothelial and smooth muscle function in long-term diabetes and a possible influence of fosinopril treatment (10 mg/kg). We investigated isolated perfused mesenteric beds of diabetic rats (4 groups: control, control + fosinopril, diabetes, diabetes + fosinopril; diabetes of 6-month duration, induced by streptozotocin, STC) were investigated using computer-assisted microvideoangiometry. Vascular diameter of four different vascular regions [classified as conductive (G1, 303 +/- 6.5 mu m and G2, 239 +/- 6.3 mu m) and resistance (G3, 192 +/- 4.5 mu m and G4, 124 +/- 2.6 mu m) vessel generations; resting conditions, control group] were increased in diabetes by approximately 20%. However, the endothelium-dependent relaxation in response to 1 mu M acetylcholine (ACh) was reduced from 38-44% to 20-25% (diabetes mellitus) with maximal impairment in G4 vessels. This could be significantly antagonized by fosinopril treatment. Similarly, vasodilation in response to 1 mu M glyceroltrinitrate (GTN) was reduced from 50-58 to 20-30%, but was partially prevented by fosinopril (32-38%), whereas potassium chloride (KCl)-induced vasoconstriction did not show differences between the groups. Inhibition of nitric oxide (NO) synthesis by 3 mu M L-NG-nitro arginine (L-NNA) resulted in a slight vasoconstriction of all vessels (12-25%), with maximum response in G3/G4. This was not altered by disease or treatment. We conclude that (a) long-term diabetes leads to endothelial and smooth muscle dysfunction with reduced capability of vasodilation and either an impairment of NO release or a reduced smooth muscle responsiveness to and (b) a predominant impairment of NO-dependent regulation in small resistance vessels, and (c) that fosinopril treatment can at least partially prevent this vascular dysfunction.

Attenuated 5-HT2 receptor-mediated responses in hindquarters of diabetic rats

Vasoconstrictor responses to 5-hydroxytryptamine (5-HT), alpha-methyl-5-HT, endothelin-1, arachidonic acid and the thromboxane A2-mimetic U46619 ((15S)-hydroxy-11 alpha, 9 alpha-(epoxymethano)prosta-5Z,13E-dienoic acid) were obtained in blood-perfused hindquarters of 6-week streptozotocin-diabetic rats. When compared to responses obtained in hindquarters of control rats, responses to 5-HT, alpha-methyl-5-HT, and arachidonic acid were attenuated in hindquarters of diabetic rats. However, responses to endothelin-1 or U46619 were not significantly different between controls and diabetics. These results suggest that 5-HT2, but not endothelin ETA receptor-mediated responses are reduced in hindquarters of diabetic rats. The results utilising arachidonic acid and U46619 suggest that there may also be a defect in the cyclo-oxygenase cascade during diabetes.

Impaired endothelium-dependent relaxation in isolated resistance arteries of spontaneously diabetic rats

1. Previous studies have shown that endothelium-dependent relaxation in the aorta of spontaneously diabetic bio bred rats (BB) is impaired. 2. We have investigated noradrenaline (NA) contractility, endothelium-dependent acetylcholine (ACh) and bradykinin (BK) relaxation, and endothelium-independent sodium nitroprusside (SNP) relaxation in mesenteric resistance arteries of recent onset BB rats and established insulin treated BB rats, compared to their age-matched non diabetic controls. 3. There was no significant difference in the maximum contractile response or sensitivity to noradrenaline in either of the diabetic groups compared to their age-matched controls. 4. Incubation with the nitric oxide synthetase inhibitor NG-nitro-L-arginine (L-NOARG) resulted in a significant increase in maximum contractile response to noradrenaline in the recent onset age-matched control group (P < 0.05). Analysis of the whole dose-response curve (using ANOVA for repeated measures with paired t test) showed a significant left-ward shift following the addition of L-NOARG (P < 0.001). A similar but less marked shift (P < 0.01) was evident in vessels from recent onset diabetics. An overall shift in both sensitivity and maximum response was also evident in the age-matched non diabetic controls of the insulin-treated group (P < 0.05). However, by contrast, there was no significant change in sensitivity in the insulin-treated diabetic rats. 5. ACh-induced endothelium-dependent relaxation was significantly impaired in the recent onset diabetic rats compared to their age-matched controls (47 +/- 11% versus 92 +/- 2%, P < 0.05, n = 6), and in the insulin treated diabetic rats (34 +/- 5% versus 75 +/- 6%, P < 0.05, n = 6). The relaxation responses to BK also were significantly impaired in the diabetic rats compared to their age-matched controls (recent onset: 20 +/- 3% versus 72 +/- 7%, P < 0.05, n = 6; insulin treated: 12 +/- 9% versus 68 +/- 7%, P < 0.05, n = 7). 6. Incubation with either the nitric oxide synthetase substrate, U-arginine, or the free radical scavenging enzyme superoxide dismutase (150 mu ml-1) failed to improve the attenuated response of acetylcholine-induced relaxation in the diabetic vessels. 7. Endothelium-dependent relaxation mediated by ACh and BK was significantly attenuated in both the diabetic and control vessels after incubation with L-NOARG. 8. Pretreatment with a cyclo-oxygenase inhibitor, indomethacin, significantly enhanced the relaxation to ACh in both the recent onset and insulin treated diabetic rats (42 +/- 10%, n = 7 versus 64 +/- 7%, n = 7, P < 0.05, and 40 +/- 5%, n = 7 versus 65 +/- 9%, n = 6, P < 0.05). 9. Following endothelium removal, there was a marked impairment in endothelium-dependent relaxation responses to ACh and BK in both the diabetic and control vessels. 10. Incubation with the thromboxane A2 receptor antagonist SQ29548, did not significantly improve the ACh endothelium-dependent relaxation response in the diabetic vessels. 11. Endothelium-independent relaxation to sodium nitroprusside was significantly impaired in the first group of diabetic vessels studied; however, subsequent studies showed no impairment of the sodium nitroprusside response in the diabetic vessels. 12. In conclusion, the ability of the endothelium to regulate vascular contractility is reduced in recent onset diabetic vessels, and significantly impaired in established insulin treated diabetics. Relaxation to the endothelium-dependent vasodilators ACh and BK was impaired in both the recent onset and the established insulin treated diabetics, and the ACh response was significantly improved following pretreatment with indomethacin, suggesting a role for a cyclo-oxygenase-derived vasoconstrictor. Preliminary studies with a thromboxane A2, receptor antagonist, SQ29548 did not significantly improve the impaired relaxation to ACh, indicating that the vasoconstrictor prostanoid is not thromboxane A2.

Effects of streptozotocin-diabetes on sympathetic nerve, endothelial and smooth muscle function in the rat mesenteric arterial bed

Mesenteric arterial function was assessed in constantly perfused preparations isolated from rats 12 weeks after treatment with streptozotocin (65 mg kg-1, i.p.) to induce diabetes. Frequency-dependent vasoconstrictor responses to electrical field stimulation of sympathetic nerves (4-32 Hz, 0.1 ms, 90V, 30 s) were severely attenuated in preparations from streptozotocin-diabetic rats, although dose-dependent vasoconstrictions to the sympathetic cotransmitters noradrenaline and ATP, as well as to potassium chloride, were not significantly changed. Dose-dependent relaxations to the endothelium-dependent vasodilators acetylcholine and ATP were significantly impaired in preparations from streptozotocin-diabetic rats, although endothelium-independent vasodilatation to sodium nitroprusside was unimpaired. These results suggest 12 weeks after induction of streptozotocin-diabetes in rats there is pre-junctional impairment of sympathetic neurotransmission and impaired endothelial function of the mesenteric arteries. This is in contrast to our previous findings that at 8 weeks after induction of streptozotocin-diabetes sympathetic nerve and endothelial function is normal, although sensory-motor vasodilatation is severely attenuated. It is suggested that selective changes occur in mesenteric arterial function after streptozotocin treatment depending on the duration of diabetes; sensory-motor nerves are affected first, followed by sympathetic nerves and the endothelium, while the smooth muscle is relatively resistant to change.

Responses of the skin microcirculation to acetylcholine and sodium nitroprusside in patients with NIDDM

The mechanisms involved in the pathogenesis of microangiopathy occurring in non-insulin-dependent diabetes mellitus (NIDDM) are unclear. In the present study, blood flow responses to the vasodilators acetylcholine (which acts via the endothelium) and sodium nitroprusside (a smooth muscle relaxant) were evaluated in this patient group. In 14 male patients with NIDDM, treated with either diet alone (n = 6) or diet plus insulin, (mean age 59 years) and 14 age-pair-matched control subjects, forearm skin perfusion following multiple doses of iontophoretically applied 1% acetylcholine and 0.01% sodium nitroprusside was recorded by laser Doppler perfusion imaging. Basal skin blood flow was not significantly different in the diabetic group compared with the control group. The following results are expressed as drug-minus-vehicle response. Acetylcholine significantly increased forearm skin perfusion (p < 0.001, analysis of variance) in all subjects, but the vasodilatation was attenuated in the patient group compared with control subjects (0.86 +/- 0.09 vs 1.36 +/- 0.14 arbitrary units of volts (V) respectively, at the fifth measurement point, mean +/- SEM, p < 0.01). Skin perfusion significantly increased following sodium nitroprusside (p < 0.001) but was lower in patients than control subjects (0.12 +/- 0.05 vs 0.45 +/- 0.11 V, respectively, at the fifth measurement point, p < 0.01). These data suggest that endothelial and/or smooth muscle function may be impaired in the skin microcirculation of patients with NIDDM.

Diabetic vascular hypertrophy and albuminuria: effect of angiotensin converting enzyme inhibition

The role of angiotensin-converting enzyme (ACE) inhibition with ramipril on mesenteric vascular hypertrophy and urinary albumin excretion was explored in a normotensive model of experimental diabetes. Serial measurements of albuminuria were performed in Sprague-Dawley control, diabetic rats, and diabetic rats treated with ramipril. Over 24 weeks, urinary albumin excretion showed a continuous rise in the untreated diabetic rats. Ramipril prevented the increase in albuminuria over the whole study period. After 6 months, animals were perfused with glutaraldehyde and sacrificed for measurement of mesenteric vessel wall/lumen ratio and kidney weight. Diabetes was associated with increased mesenteric wall/lumen ratio and kidney weight. ACE inhibition, despite no effect on glycemic control, attenuated mesenteric vascular hypertrophy but did not decrease kidney weight. In addition to the well-described renoprotective effects of ACE inhibition in diabetes, this class of agents may have a favorable effect on diabetic vascular disease.

Vascular changes in the diabetic kidney: effects of ACE inhibition

The effects of angiotensin-converting enzyme (ACE) inhibition with perindopril on renal vascular structure were studied in control and streptozotocin diabetic male Sprague-Dawley rats after 3 weeks. After kidneys were perfusion-fixed at systolic blood pressure, morphometric analysis of vascular structural changes in the media of the renal vasculature at the cortico-medullary junction was performed. Vascular hypertrophy was present in the diabetic vessels, as assessed by an increase in medial cross-sectional area for a given lumen size. This relative increase in medial area was prevented by perindopril treatment, consistent with an antitrophic effect on diabetic kidney vessels by ACE inhibition. The diabetic kidney had an increased proportion of small vessels less than 50 microns diameter at the cortico-medullary junction, perhaps representing diabetes induced angiogenesis. This subpopulation of vessels was reduced in number after perindopril treatment. Our data support a role for increased activity of angiotensin converting enzyme as a mechanism for vascular hypertrophy, which may be involved in the pathogenesis of diabetic vasculopathy and nephropathy.