Vascular reactivity in diabetic rats: effect of melatonin

Effect of sulfasalazine on endothelium-dependent vascular response by the activation of Nrf2 signalling pathway

Background: Diabetes mellitus leads to endothelial dysfunction and accumulation of oxygen radicals. Sulfasalazine-induced Nrf2 activation reduces oxidative stress in vessels. Thus, in the present study, we investigated the effects of sulfasalazine on endothelial dysfunction induced by high glucose. We also ascribed the underlying mechanism involved in glucose-induced endothelial dysfunction.

Methods: For this experiment we used 80 Wistar Albino rats thoracic aorta to calculate the dose response curve of noradrenaline and acetylcholine. Vessels were incubated in normal and high glucose for 2 h. To investigate glucose and sulfasalazine effects the vessels of the high glucose group were pre-treated with sulfasalazine (300 mM), JNK inhibitor (SP600125), and ERK inhibitor (U0126) for 30 min. The dose response curve was calculated through organ bath. The eNOS, TAS, TOS, and HO-1 levels were estimated by commercially available ELISA kits.

Results: In the high glucose group, the E_max for contraction was significantly higher (p < 0.001), and E_max for relaxation was lower than that of control. These functional changes were parallel with the low levels of eNOS (p < 0.05). High glucose vessel treated with sulfasalazine showed low E_max value for contraction (p < 0.001) however, the E_max for relaxation was significantly high (p < 0.001) when compared to high glucose group. In the JNK group, E_max for contraction and relaxation was inhibited (p < 0.001) compared to sulfasalazine treated vessels. HO—1 enzyme levels were significantly low (p < 0.01) with sulfasalazine but higher with ERK inhibitor (p < 0.05).

Conclusion: High glucose induced endothelial dysfunction and sulfasalazine reduced damage in high glucose vessels by activating eNOS, antioxidant effect through HO-1 enzymes and particularly inducing Nrf2 via the ERK and JNK pathways.

The DPP-4 inhibitor linagliptin and the GLP-1 receptor agonist exendin-4 improve endothelium-dependent relaxation of rat mesenteric arteries in the presence of high glucose

The aim of the study was to investigate the effects of the DPP-4 inhibitors and GLP-1R agonist, exendin-4 on the mechanism(s) of endothelium-dependent relaxation in rat mesenteric arteries exposed to high glucose concentration (40 mM). Organ bath techniques were employed to investigate vascular endothelial function in rat mesenteric arteries in the presence of normal (11 mM) or high (40 mM) glucose concentrations. Pharmacological tools (1μM TRAM-34, 1μM apamin, 100 nM Ibtx, 100 μM l-NNA, 10 μM ODQ) were used to distinguish between NO and EDHF-mediated relaxation. Superoxide anion levels were assessed by L-012 and lucigenin enhanced-chemiluminescence techniques. Incubation of mesenteric rings with high glucose for 2 h caused a significant increase in superoxide anion generation and a significant impairment of endothelium-dependent relaxation. Exendin-4 and DPP-4 inhibitor linagliptin, but not sitagliptin or vildagliptin, significantly reduced vascular superoxide and improved endothelium-dependent relaxation in the presence of high glucose. The beneficial actions of exendin-4, but not linagliptin, were attenuated by the GLP-1R antagonist exendin fragment (9-39). Further experiments demonstrated that the presence of high glucose impaired the contribution of both nitric oxide and endothelium-dependent hyperpolarisation to relaxation and that linagliptin improved both mechanisms involved in endothelium-dependent relaxation. These findings demonstrate that high glucose impaired endothelium-dependent relaxation can be improved by exendin-4 and linagliptin, likely due to their antioxidant activity and independently of any glucose lowering effect.

Melatonin: buffering the immune system

Melatonin modulates a wide range of physiological functions with pleiotropic effects on the immune system. Despite the large number of reports implicating melatonin as an immunomodulatory compound, it still remains unclear how melatonin regulates immunity. While some authors argue that melatonin is an immunostimulant, many studies have also described anti-inflammatory properties. The data reviewed in this paper support the idea of melatonin as an immune buffer, acting as a stimulant under basal or immunosuppressive conditions or as an anti-inflammatory compound in the presence of exacerbated immune responses, such as acute inflammation. The clinical relevance of the multiple functions of melatonin under different immune conditions, such as infection, autoimmunity, vaccination and immunosenescence, is also reviewed.

Interleukin-2 Improves Vascular Functions in Streptozotocin-induced Diabetic Rats

To investigate changes in vascular functions in streptozotocin (STZ)-induced diabetic rats and the effect of interleukin-2 (IL-2), male Sprague-Dawley rats were randomly divided into a normal group, a diabetic control group, and diabetic groups treated with a low dose (5000 U/kg/d) or a high dose of IL-2 (50000 U/kg/d) for five weeks. Rats were injected with STZ (60 mg/kg, i.p.) to induce diabetes. The contractions in response to KCl, phenylephrine (PE), and CaCl₂, and the vasorelaxant effect of acetylcholine (ACh) on rings from thoracic aortae preconstricted with PE were determined using organ bath technique. In the diabetic group, the contractile responses to PE were significantly impaired. Treatment with IL-2 improved this response, but the high dose was less effective. However, IL-2 attenuated the contractile response to KCl in the diabetic groups in a dose-dependent manner. The high concentration of IL-2 decreased the contractile response to CaCl₂. ACh caused a dose-dependent relaxation of aortic rings that was impaired in the diabetic group. The vascular responses in IL-2 treated groups were preserved. The results indicate that IL-2 can improve vascular function in diabetic rat.

Metabolic disorders and adipose tissue insulin responsiveness in neonatally STZ-induced diabetic rats are improved by long-term melatonin treatment

Diabetes mellitus is a product of low insulin sensibility and pancreatic β-cell insufficiency. Rats with streptozotocin-induced diabetes during the neonatal period by the fifth day of age develop the classic diabetic picture of hyperglycemia, hypoinsulinemia, polyuria, and polydipsia aggravated by insulin resistance in adulthood. In this study, we investigated whether the effect of long-term treatment with melatonin can improve insulin resistance and other metabolic disorders in these animals. At the fourth week of age, diabetic animals started an 8-wk treatment with melatonin (1 mg/kg body weight) in the drinking water at night. Animals were then killing, and the sc, epididymal (EP), and retroperitoneal (RP) fat pads were excised, weighed, and processed for adipocyte isolation for morphometric analysis as well as for measuring glucose uptake, oxidation, and incorporation of glucose into lipids. Blood samples were collected for biochemical assays. Melatonin treatment reduced hyperglycemia, polydipsia, and polyphagia as well as improved insulin resistance as demonstrated by constant glucose disappearance rate and homeostasis model of assessment-insulin resistance. However, melatonin treatment was unable to recover body weight deficiency, fat mass, and adipocyte size of diabetic animals. Adiponectin and fructosamine levels were completely recovered by melatonin, whereas neither plasma insulin level nor insulin secretion capacity was improved in diabetic animals. Furthermore, melatonin caused a marked delay in the sexual development, leaving genital structures smaller than those of nontreated diabetic animals. Melatonin treatment improved the responsiveness of adipocytes to insulin in diabetic animals measured by tests of glucose uptake (sc, EP, and RP), glucose oxidation, and incorporation of glucose into lipids (EP and RP), an effect that seems partially related to an increased expression of insulin receptor substrate 1, acetyl-coenzyme A carboxylase and fatty acid synthase. In conclusion, melatonin treatment was capable of ameliorating the metabolic abnormalities in this particular diabetes model, including insulin resistance and promoting a better long-term glycemic control.

Melatonin in septic shock: some recent concepts

Melatonin is a versatile molecule, synthesized not only in the pineal gland, but also in many other organs. Melatonin plays an important physiologic role in sleep and circadian rhythm regulation, immunoregulation, antioxidant and mitochondrial-protective functions, reproductive control, and regulation of mood. Melatonin has also been reported as effective in combating various bacterial and viral infections. Melatonin is an effective anti-inflammatory agent in various animal models of inflammation and sepsis, and its anti-inflammatory action has been attributed to inhibition of nitric oxide synthase with consequent reduction of peroxynitrite formation, to the stimulation of various antioxidant enzymes thus contributing to enhance the antioxidant defense, and to protective effects on mitochondrial function and in preventing apoptosis. In a number of animal models of septic shock, as well as in patients with septic disease, melatonin reportedly exerts beneficial effects to arrest cellular damage and multiorgan failure. The significance of these actions in septic shock and its potential usefulness in the treatment of multiorgan failure are discussed.

Glycoxidative stress and cardiovascular complications in experimentally-induced diabetes: effects of antioxidant treatment

Diabetes mellitus (DM) is a common metabolic disease, representing a serious risk factor for the development of cardiovascular complications, such as coronary heart disease, peripheral arterial disease and hypertension. Oxidative stress (OS), a feature of DM, is defined as an increase in the steady-state levels of reactive oxygen species (ROS) and may occur as a result of increased free radical generation and/or decreased anti-oxidant defense mechanisms. Increasing evidence indicates that hyperglycemia is the initiating cause of the tissue damage in DM, either through repeated acute changes in cellular glucose metabolism, or through long-term accumulation of glycated biomolecules and advanced glycation end products (AGEs). AGEs are formed by the Maillard process, a non-enzymatic reaction between ketone group of the glucose molecule or aldehydes and the amino groups of proteins that contributes to the aging of proteins and to the pathological complications of DM. In the presence of uncontrolled hyperglycemia, the increased formation of AGEs and lipid peroxidation products exacerbate intracellular OS and results in a loss of molecular integrity, disruption in cellular signaling and homeostasis, followed by inflammation and tissue injury such as endothelium dysfunction, arterial stiffening and microvascular complications. In addition to increased AGE production, there is also evidence of multiple pathways elevating ROS generation in DM, including; enhanced glucose auto-oxidation, increased mitochondrial superoxide production, protein kinase C-dependent activation of NADPH oxidase, uncoupled endothelial nitric oxide synthase (eNOS) activity, increased substrate flux through the polyol pathway and stimulation of eicosanoid metabolism. It is, therefore, not surprising that the correction of these variables can result in amelioration of diabetic cardiovascular abnormalities. A linking element between these phenomena is cellular redox imbalance due to glycoxidative stress (GOS). Thus, recent interest has focused on strategies to prevent, reverse or retard GOS in order to modify the natural history of diabetic cardiovascular abnormalities. This review will discuss the links between GOS and diabetes-induced cardiovascular disorders and the effect of antioxidant therapy on altering the development of cardiovascular complications in diabetic animal models.

Increased melatonin synthesis in pineal glands of rats in streptozotocin induced type 1 diabetes

It is well-documented that melatonin influences insulin secretion. The effects are mediated by specific, high-affinity, pertussis-toxin-sensitive, G protein-coupled membrane receptors (MT(1) as well MT(2)), which are present in both the pancreatic tissue and islets of rats and humans, as well as in rat insulinoma cells (INS1). Via the Gi-protein-adenylatecyclase-3',5'-cyclic adenosine monophosphate (cAMP) and, possibly, the guanylatecyclase-cGMP pathways, melatonin decreases insulin secretion, whereas, by activating the Gq-protein-phospholipase C-IP(3) pathway, it has the opposite effect. For further analysis of the interactions between melatonin and insulin, diabetic rats were investigated with respect to melatonin synthesis in the pineal gland and plasma insulin levels. In this context, recent investigations have proven that type 2 diabetic rats and humans display decreased melatonin levels, whereas type 1 diabetic IDDM rats or those with diabetes induced by streptozotocin (STZ) of the present study show increased plasma melatonin levels and elevated AA-NAT-mRNA. Furthermore, the mRNA of pineal insulin receptors and beta1-adrenoceptors, including the clock genes Per1 and Bmal1 and the clock-controlled output gene Dbp, increases in both young and middle-aged STZ rats. The results therefore indicate that the decreased insulin levels in STZ-induced type 1 diabetes are associated with higher melatonin plasma levels. In good agreement with earlier investigations, it was shown that the elevated insulin levels observed in type 2 diabetes, are associated with decreased melatonin levels. The results thus prove that a melatonin-insulin antagonism exists. Astonishingly, notwithstanding the drastic metabolic disturbances in STZ-diabetic rats, the diurnal rhythms of the parameters investigated are maintained.

Effect of melatonin on vascular responses in aortic rings of aging rats

In old animals a marked reduction in endothelium-dependent relaxation occurs. Since there is evidence that the endothelial dysfunction associated with aging may be partly related to the local formation of reactive oxygen species, the purpose of this study was to examine the effect of the natural antioxidant melatonin (10(-5)mol/l) on in vitro contractility of aged aortic rings under conditions of increased oxidative stress (40 m mol/l glucose concentration in medium). Experiments were carried out in 18-20 months old, Wistar male rats, using adult (6-7 months old) animals as controls. A higher plasma lipid peroxidation was found in aged rats as compared to the younger ones. In a first experiment, dose-response curves for acetylcholine-induced relaxation of aortic rings were conducted. Analyzed as a main factor in a factorial ANOVA, age decreased and melatonin augmented the relaxing response to acetylcholine. melatonin's restoring effect on aortic ring relaxation was found in aged aortic rings only and was more pronounced in the presence of a high glucose medium. In a second experiment, the effect of melatonin on the contractility response to phenylephrine of intact or endothelium-denuded aortic rings obtained from aged or control rats was examined in normal or high glucose medium. A main factor analysis in the factorial ANOVA indicated that age and operation augmented, and melatonin decreased, aortic ring contractility response to phenylephrine. Melatonin's restoring effect on aortic contractility was seen in aged aortic rings. The effect of age or a high glucose medium on phenylephrine-induced contractility was more pronounced in the absence of an intact endothelium. Aging did not affect the relaxant response of intact or endothelium-denuded rings to sodium nitroprusside. The results support the improvement by melatonin of vascular response in aging rats, presumably via its antioxidant activity.

Comparative effects of melatonin and vitamin E in restoring aortic relaxation in pancreatectomized rats

In a previous study we reported the efficacy of melatonin to restore the decreased relaxation response to acetylcholine (ACh) or to sodium nitroprusside (SNP) in aortic rings of rats turned hyperglycemic by subtotal pancreatectomy. The effect was amplified by pre-incubation in a high (44 mmol/l) glucose solution, a situation that resulted in oxidative stress. We hereby compare the effect of another antioxidant, vitamin E, with that of melatonin on ACh response in intact aortic rings or on SNP response in endothelium-denuded aortic rings obtained from pancreatectomized or sham-operated rats. Dose-response curves to ACh or SNP were performed in the presence or absence of melatonin or vitamin E (10-5 mol/1) in 10 or 44 mmol/1 glucose medium. Melatonin was more effective than vitamin E in restoring ACh- or SNP-induced relaxation of aortic rings in a high glucose medium. The differences between the two antioxidants may rely on the ability of melatonin to diffuse readily into intracellular compartments.

Interleukin-2 protects against endothelial dysfunction induced by high glucose levels in rats

AIMS

Interleukin-2 (IL-2) can modulate cardiovascular functions, but the effect of IL-2 on vascular endothelial function in diabetes is not known. We hypothesized that IL-2 may attenuate endothelial dysfunction induced by high glucose or diabetes. So the aim of this study was to investigate the effect of IL-2 on endothelium-response of aortas incubated with high glucose or from diabetic rats and its underlying mechanism.

METHODS

Acetylcholine (ACh)-induced endothelium-dependent relaxation (EDR), sodium nitroprusside (SNP)-induced endothelium-independent relaxation (EIR), superoxide dismutase (SOD) and nitric oxide synthase (NOS) were measured in aortas isolated from non-diabetic rats and exposed to a high glucose concentration and from streptozotocin-induced diabetic rats.

RESULTS

Incubation of aortic rings with high glucose (44 mM) for 4 h resulted in a significant inhibition of EDR, but had no effects on EIR. Co-incubation with IL-2 for 40 min prevented the inhibition of EDR caused by high glucose in a concentration-dependent manner. Similarly, high glucose decreased SOD and NOS activity in aortic tissue. IL-2 (1000 U/ml) significantly attenuated the decrease of SOD and NOS activity caused by high glucose. In addition, EDR declined along with the decrease of serum NO level in aortas from STZ-induced diabetic rats. Injection of IL-2 (5000 and 50,000 U kg(-1) d(-1), s.c.) for 5 weeks prevented the inhibition of EDR and the decrease of serum NO levels caused by diabetes.

CONCLUSIONS

IL-2 significantly ameliorated the endothelial dysfunction induced by hyperglycemia, in which the activation of the NO pathway and SOD may be involved.

Increased RhoA translocation in aorta of diabetic rats

AIM

To analyze RhoA expression and activation in the aorta of diabetic rats.

METHODS

Male SD rats (n=70) were divided into 2 groups: the diabetic group and the control group. Diabetes was induced by intravenous injection of streptozotocin (55 mg/kg). The Rats were studied 3 weeks after the induction of diabetes. Western blotting was used to measure the expression and activation of Rho.

RESULTS

Heart rate was measured 24 h/d; it decreased by 58+/-13 beats/min in the diabetic rats. Isometric tension showed that the contraction of diabetic aorta was significantly reduced compared with that of control aorta when stimulated by KCl and serotonin. The relaxation of the diabetic aorta was reduced when stimulated by acetylcholine. An enhanced RhoA translocation in the aortic tissues of diabetic rats was determined by a 90% increase in membrane-bound RhoA, indicating that the activation of RhoA is markedly increased in the diabetic aorta.

CONCLUSION

Our data suggest that upregulated RhoA could be involved in the vascular dysfunction of diabetic rats.

Vascular Reactivity to Various Vasoconstrictor Agents and Endothelium-Dependent Relaxations of Rat Thoracic Aorta in the Long-Term Period of Pinealectomy

In this study, the effects of reduced melatonin concentrations in the long-term period of pinealectomy on mean arterial blood pressure (BP) and vascular responses in isolated rat thoracic aorta were investigated. Rats were pinealectomized (Px) two months before the beginning of the studies. Rings of endothelium-intact and -denuded rat arteries were mounted in isolated tissue baths for the measurements of isometric contractile force. No significant difference was determined between the arterial BP of Px (88.1 +/- 1.9 mmHg) and control (83.8 +/- 1.2 mmHg) rats. All arteries isolated from control and Px rats showed essentially identical contractions in response to phenylephrine, serotonin, calcium, clonidine, vasopressin, and angiotensin-II. Only endothelin-1 (ET-1)-induced contractions in the endothelium-denuded vessels isolated from Px rats were found to be increased to some extent. Pinealectomy did not affect acetylcholine or sodium nitroprusside-induced relaxation in the rat aorta either. These data suggest that reduced melatonin levels two months after pinealectomy did not modify either the vascular reactivity to various vasoconstrictor agents except the partially increased contractile responses to ET-1 in the endothelium-denuded thoracic aortas of Px rats or the endothelium-dependent and -independent relaxations in rat thoracic aorta. Restoration of the increased vascular responses to some vasoconstrictor agents, which were reported previously, may be the reason of why the hypertension is temporary following pinealectomy.

Melatonin restores endothelium-dependent relaxation in aortic rings of pancreatectomized rats

In rats turned hyperglycemic by a subtotal pancreatectomy, a decreased relaxation response of aortic rings to acetylcholine (ACh) was found; this effect was amplified by preincubation in a high glucose medium (44 mmol/L). The relaxation response to ACh did not occur in endothelium-denuded rings or after the aortic rings were exposed to l-nitro-arginine methyl ester [L-NAME, a nitric oxide (NO) synthase inhibitor]. Incubation with the NO donor sodium nitroprusside (SNP) restored the impaired relaxation response seen in endothelium-denuded or L-NAME-treated aortic rings. Pancreatectomy decreased the vasorelaxation of aortic rings caused by SNP. Only in pancreatectomized rats, incubation in a high glucose medium impaired the relaxation effect of SNP. To assess whether melatonin preincubation reversed the impaired relaxation response to ACh (intact endothelium aortic rings) or to SNP (endothelium-denuded or L-NAME-treated rings) in hyperglycemic rats, cumulative dose-response curves were performed in the presence of 10(-5) mol/L melatonin. Melatonin preincubation did not modify ACh-induced relaxation of aortic rings in a normal glucose concentration but was highly effective in preventing the impairment of relaxation caused by a high glucose solution. Melatonin was also effective in restoring the impaired SNP-induced vasorelaxation seen in endothelium-denuded or L-NAME-treated aortic rings from hyperglycemic rats. The results further support the improvement by melatonin of the endothelial-mediated relaxation in blood vessels of diabetic rats.

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

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

D-4F Induces Heme Oxygenase-1 and Extracellular Superoxide Dismutase, Decreases Endothelial Cell Sloughing, and Improves Vascular Reactivity in Rat Model of Diabetes

Background— Apolipoprotein A1 mimetic peptide, synthesized from D-amino acid (D-4F), enhances the ability of HDL to protect LDL against oxidation in atherosclerotic animals.

Methods and Results— We investigated the mechanisms by which D-4F provides antioxidant effects in a diabetic model. Sprague-Dawley rats developed diabetes with administration of streptozotocin (STZ). We examined the effects of daily D-4F (100 μg/100 g of body weight, intraperitoneal injection) on superoxide (O 2 − ), extracellular superoxide dismutase (EC-SOD), vascular heme oxygenase (HO-1 and HO-2) levels, and circulating endothelial cells in diabetic rats. In response to D-4F, both the quantity and activity of HO-1 were increased. O 2 − levels were elevated in diabetic rats (74.8±8×10 3 cpm/10 mg protein) compared with controls (38.1±8×10 3 cpm/10 mg protein; P <0.01). D-4F decreased O 2 − levels to 13.23±1×10 3 ( P <0.05 compared with untreated diabetics). The average number of circulating endothelial cells was higher in diabetics (50±6 cells/mL) than in controls (5±1 cells/mL) and was significantly decreased in diabetics treated with D-4F (20±3 cells/mL; P <0.005). D-4F also decreased endothelial cell fragmentation in diabetic rats. The impaired relaxation typical of blood vessels in diabetic rats was prevented by administration of D-4F (85.0±2.0% relaxation). Western blot analysis showed decreased EC-SOD in the diabetic rats, whereas D-4F restored the EC-SOD level.

Conclusions— We conclude that an increase in circulating endothelial cell sloughing, superoxide anion, and vasoconstriction in diabetic rats can be prevented by administration of D-4F, which is associated with an increase in 2 antioxidant proteins, HO-1 and EC-SOD.

Melatonin treatment protects against diabetes-induced functional and biochemical changes in rat aorta and corpus cavernosum

Enhanced oxidative stress due to diabetes is accepted to lead to endothelial dysfunction, and this is known to play a key role in the pathogenesis of diabetic vascular diseases and complications. This study was designed to determine the possible protective effect of melatonin and/or insulin treatment on the functional and biochemical changes caused by hyperglycemia in aorta and corpus cavernosum of diabetic rats. Wistar albino male rats were rendered diabetic by injecting streptozotocin (60 mg/kg, intraperitoneally (i.p.)). Melatonin (10 mg/kg, i.p.) and/or insulin (6 U/kg, subcutaneously (s.c.)) were administered for 8 weeks. In the diabetic group, the contractile responses of aortic strips to phenylephrine were significantly impaired (EC(50) 5.5 x 10(-7) M in diabetic and EC(50) 1.47 x 10(-7) M in the control group, P<0.001). Treatment with melatonin (EC(50) 4.6 x 10(-7) M) or insulin+melatonin (EC(50) 1.68 x 10(-7) M, P<0.001) improved the contractile responses. Acetylcholine caused a dose-dependent relaxation response (EC(50) 1.58 x 10(-7) M) which was impaired in the diabetic group (EC(50) 26 x 10(-7) M, P<0.001). There was less impairment in melatonin-, insulin- and insulin+melatonin-treated groups (EC(50) 11.61 x 10(-7), 7.3 x 10(-7) and 1.41 x 10(-7) M, respectively, P<0.01). Contractile responses to phenylephrine were also impaired in the corpus cavernosum strips (EC(50) 2.06 x 10(-5) M in diabetic and 0.94 x 10(-5) M in the control group, P<0.001). In the melatonin- (EC(50) 1.59 x 10(-5) M) and insulin+melatonin-treated (EC(50) 1.53 x 10(-5) M, P<0.5) groups contractile responses were improved. In the diabetic group, the relaxation responses of corpus cavernosum strips to acetylcholine were impaired (EC(50) 24.12 x 10(-5) M, P<0.001), and treatment with melatonin (EC(50) 0.68 x 10(-5) M), insulin (EC(50) 0.53 x 10(-5) M) or insulin+melatonin (0.98 x 10(-5) M, P<0.001) restored the responses to acetylcholine. In diabetic tissues, malondialdehyde levels were increased while glutathione levels were decreased, demonstrating oxidative damage. This was also prevented by treatment with melatonin or the melatonin and insulin combination. The diabetic state enhances the generation of free radicals, and both melatonin and insulin treatments reduced this oxidative stress; however, treatment with the combination was the most efficient in preventing diabetes-induced damage. Thus, our results suggested that giving diabetic patients adjuvant therapy with melatonin may have some benefit in controlling diabetic complications.

Effect of melatonin on vascular reactivity in pancreatectomized rats

The present study was undertaken to assess whether the improvement of contractile performance of aortic rings by melatonin described in streptozotocin diabetic rats also occurs in another model of type I diabetes, the pancreatectomized rats. Adult male Wistar rats submitted to a subtotal pancreatectomy and exhibiting altered levels of fasting glucose and an abnormal tolerance glucose test, were used. Sham-operated laparotomized rats were employed as controls. Dose-response curves for acetylcholine-induced, endothelium-related relaxation of aortic rings (after previous exposure to phenylephrine) and for phenylephrine-induced vasoconstriction were conducted. This protocol was repeated with rings pre-incubated in a high glucose solution (44 mmol/l). Pancreatectomy decreased significantly acetylcholine-induced relaxation of aortic rings, but not phenylephrine-induced vasoconstriction, the effect being amplified by preincubation in high glucose solution. The deleterious effect of a high glucose medium was more pronounced in pancreatectomized rats. Melatonin (10(-5) M) did not modify acetylcholine-induced relaxation in normal glucose concentration but was effective to prevent the impairment of relaxation brought about by exposure to high glucose solution. The contractile response to phenylephrine of aortic rings obtained from pancreatectomized rats was not affected by melatonin. The results further support the improvement by melatonin of endothelial-mediated relaxation in blood vessels of diabetic rats.