Dysfunction of soluble guanylyl cyclase in aorta and kidney of Goto-Kakizaki rats: influence of age and diabetic state

Vascular nitric oxide resistance in type 2 diabetes

Vascular nitric oxide (NO•) resistance, manifested by an impaired vasodilator function of NO• in both the macro- and microvessels, is a common state in type 2 diabetes (T2D) associated with developing cardiovascular events and death. Here, we summarize experimental and human evidence of vascular NO• resistance in T2D and discuss its underlying mechanisms. Human studies indicate a ~ 13-94% decrease in the endothelium (ET)-dependent vascular smooth muscle (VSM) relaxation and a 6-42% reduced response to NO• donors, i.e., sodium nitroprusside (SNP) and glyceryl trinitrate (GTN), in patients with T2D. A decreased vascular NO• production, NO• inactivation, and impaired responsiveness of VSM to NO• [occurred due to quenching NO• activity, desensitization of its receptor soluble guanylate cyclase (sGC), and/or impairment of its downstream pathway, cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG)] are the known mechanisms underlying the vascular NO• resistance in T2D. Hyperglycemia-induced overproduction of reactive oxygen species (ROS) and vascular insulin resistance are key players in this state. Therefore, upregulating vascular NO• availability, re-sensitizing or bypassing the non-responsive pathways to NO•, and targeting key vascular sources of ROS production may be clinically relevant pharmacological approaches to circumvent T2D-induced vascular NO• resistance.

Factors influencing the soluble guanylate cyclase heme redox state in blood vessels

Soluble guanylate cyclase (sGC) plays an important role in maintaining vascular homeostasis, as an acceptor for the biological messenger nitric oxide (NO). However, only reduced sGC (with a ferrous heme) can be activated by NO; oxidized (ferric heme) and apo (absent heme) sGC cannot. In addition, the proportions of reduced, oxidized, and apo sGC change under pathological conditions. Although diseased blood vessels often show decreased NO bioavailability in the vascular wall, a shift of sGC heme redox balance in favor of the oxidized/apo forms can also occur. Therefore, sGC is of growing interest as a drug target for various cardiovascular diseases. Notably, the balance between NO-sensitive reduced sGC and NO-insensitive oxidized/apo sGC in the body is regulated in a reversible manner by various biological molecules and proteins. Many studies have attempted to identify endogenous factors and determinants that influence this redox state. For example, various reactive nitrogen and oxygen species are capable of inducing the oxidation of sGC heme. Conversely, a heme reductase and some antioxidants reduce the ferric heme in sGC to the ferrous state. This review summarizes the factors and mechanisms identified by these studies that operate to regulate the sGC heme redox state.

Voltage dependence of the Ca2+ transient in endocardial and epicardial myocytes from the left ventricle of Goto-Kakizaki type 2 diabetic rats

Diabetes mellitus is a major global health disorder and, currently, over 450 million people have diabetes with 90% suffering from type 2 diabetes. Left untreated, diabetes may lead to cardiovascular diseases which are a leading cause of death in diabetic patients. Calcium is the trigger and regulator of cardiac muscle contraction and derangement in cellular Ca²⁺ homeostasis, which can result in heart failure and sudden cardiac death. It is of paramount importance to investigate the regional involvement of Ca²⁺ in diabetes-induced cardiomyopathy. Therefore, the aim of this study was to investigate the voltage dependence of the Ca²⁺ transients in endocardial (ENDO) and epicardial (EPI) myocytes from the left ventricle of the Goto-Kakizaki (GK) rats, an experimental model of type 2 diabetes mellitus. Simultaneous measurement of L-type Ca²⁺ currents and Ca²⁺ transients was performed by whole-cell patch clamp techniques. GK rats displayed significantly increased heart weight, heart weight/body weight ratio, and non-fasting and fasting blood glucose compared to controls (CON). Although the voltage dependence of L-type Ca²⁺ current was unaltered, the voltage dependence of the Ca²⁺ transients was reduced to similar extents in EPI-GK and ENDO-GK compared to EPI-CON and ENDO-CON myocytes. TPK L-type Ca²⁺ current and Ca²⁺ transient were unaltered. THALF decay of L-type Ca²⁺ current was unaltered; however, THALF decay of the Ca²⁺ transient was shortened in ENDO and EPI myocytes from GK compared to CON rat hearts. In conclusion, the amplitude of L-type Ca²⁺ current was unaltered; however, the voltage dependence of the Ca²⁺ transient was reduced to similar extents in EPI and ENDO myocytes from GK rats compared to their respective controls, suggesting the possibility of dysfunctional sarcoplasmic reticulum Ca²⁺ transport in the GK diabetic rat hearts.

Calcium Signaling in the Ventricular Myocardium of the Goto-Kakizaki Type 2 Diabetic Rat

The association between diabetes mellitus (DM) and high mortality linked to cardiovascular disease (CVD) is a major concern worldwide. Clinical and preclinical studies have demonstrated a variety of diastolic and systolic dysfunctions in patients with type 2 diabetes mellitus (T2DM) with the severity of abnormalities depending on the patients' age and duration of diabetes. The cellular basis of hemodynamic dysfunction in a type 2 diabetic heart is still not well understood. The aim of this review is to evaluate our current understanding of contractile dysfunction and disturbances of Ca²⁺ transport in the Goto-Kakizaki (GK) diabetic rat heart. The GK rat is a widely used nonobese, nonhypertensive genetic model of T2DM which is characterized by insulin resistance, elevated blood glucose, alterations in blood lipid profile, and cardiac dysfunction.

The Pattern of mRNA Expression Is Changed in Sinoatrial Node from Goto-Kakizaki Type 2 Diabetic Rat Heart

BACKGROUND

In vivo experiments in Goto-Kakizaki (GK) type 2 diabetic rats have demonstrated reductions in heart rate from a young age. The expression of genes encoding more than 70 proteins that are associated with the generation and conduction of electrical activity in the GK sinoatrial node (SAN) have been evaluated to further clarify the molecular basis of the low heart rate.

MATERIALS AND METHODS

Heart rate and expression of genes were evaluated with an extracellular electrode and real-time RT-PCR, respectively. Rats aged 12-13 months were employed in these experiments.

RESULTS

Isolated spontaneous heart rate was reduced in GK heart (161 ± 12 bpm) compared to controls (229 ± 11 bpm). There were many differences in expression of mRNA, and some of these differences were of particular interest. Compared to control SAN, expression of some genes were downregulated in GK-SAN: gap junction, Gja1 (Cx43), Gja5 (Cx40), Gjc1 (Cx45), and Gjd3 (Cx31.9); cell membrane transport, Trpc1 (TRPC1) and Trpc6 (TRPC6); hyperpolarization-activated cyclic nucleotide-gated channels, Hcn1 (HCN1) and Hcn4 (HCN4); calcium channels, Cacna1d (Ca_v1.3), Cacna1g (Ca_v3.1), Cacna1h (Ca_v3.2), Cacna2d1 (Ca_vα2δ1), Cacna2d3 (Ca_vα2δ3), and Cacng4 (Cav _γ 4); and potassium channels, Kcna2 (K_v1.2), Kcna4 (K_v1.4), Kcna5 (K_v1.5), Kcnb1 (K_v2.1), Kcnd3 (K_v4.3), Kcnj2 (K_ir2.1), Kcnk1 (TWIK1), Kcnk5 (K_2P5.1), Kcnk6 (TWIK2), and Kcnn2 (SK2) whilst others were upregulated in GK-SAN: Ryr2 (RYR2) and Nppb (BNP).

CONCLUSIONS

This study provides new insight into the changing expression of genes in the sinoatrial node of diabetic heart.

Chronic intermittent hypoxia accelerates coronary microcirculatory dysfunction in insulin-resistant Goto-Kakizaki rats

Chronic intermittent hypoxia (IH) induces oxidative stress and inflammation, which impair vascular endothelial function. Long-term insulin resistance also leads to endothelial dysfunction. We determined, in vivo, whether the effects of chronic IH and insulin resistance on endothelial function augment each other. Male 12-wk-old Goto-Kakizaki (GK) and Wistar control rats were subjected to normoxia or chronic IH (90-s N2, 5% O2 at nadir, 90-s air, 20 cycles/h, 8 h/day) for 4 wk. Coronary endothelial function was assessed using microangiography with synchrotron radiation. Imaging was performed at baseline, during infusion of acetylcholine (ACh, 5 μg·kg−1·min−1) and then sodium nitroprusside (SNP, 5 μg·kg−1·min−1), after blockade of both nitric oxide (NO) synthase (NOS) with Nω-nitro-l-arginine methyl ester (l-NAME, 50 mg/kg) and cyclooxygenase (COX, meclofenamate, 3 mg/kg), and during subsequent ACh. In GK rats, coronary vasodilatation in response to ACh and SNP was blunted compared with Wistar rats, and responses to ACh were abolished after blockade. In Wistar rats, IH blunted the ability of ACh or SNP to increase the number of visible vessels. In GK rats exposed to IH, neither ACh nor SNP were able to increase visible vessel number or caliber, and blockade resulted in marked vasoconstriction. Our findings indicate that IH augments the deleterious effects of insulin resistance on coronary endothelial function. They appear to increase the dependence of the coronary microcirculation on NO and/or vasodilator prostanoids, and greatly blunt the residual vasodilation in response to ACh after blockade of NOS/COX, presumably mediated by endothelium-derived hyperpolarizing factors.

Frailty and sarcopenia as the basis for the phenotypic manifestation of chronic diseases in older adults

Frailty is a functional status that precedes disability and is characterized by decreased functional reserve and increased vulnerability. In addition to disability, the frailty phenotype predicts falls, institutionalization, hospitalization and mortality. Frailty is the consequence of the interaction between the aging process and some chronic diseases and conditions that compromise functional systems and finally produce sarcopenia. Many of the clinical manifestations of frailty are explained by sarcopenia which is closely related to poor physical performance. Reduced regenerative capacity, malperfusion, oxidative stress, mitochondrial dysfunction and inflammation compose the sarcopenic skeletal muscle alterations associated to the frailty phenotype. Inflammation appears as a common determinant for chronic diseases, sarcopenia and frailty. The strategies to prevent the frailty phenotype include an adequate amount of physical activity and exercise as well as pharmacological interventions such as myostatin inhibitors and specific androgen receptor modulators. Cell response to stress pathways such as Nrf2, sirtuins and klotho could be considered as future therapeutic interventions for the management of frailty phenotype and aging-related chronic diseases.

Diabetes and ageing-induced vascular inflammation

Diabetes and the ageing process independently increase the risk for cardiovascular disease (CVD). Since incidence of diabetes increases as people get older, the diabetic older adults represent the largest population of diabetic subjects. This group of patients would potentially be threatened by the development of CVD related to both ageing and diabetes. The relationship between CVD, ageing and diabetes is explained by the negative impact of these conditions on vascular function. Functional and clinical evidence supports the role of vascular inflammation induced by the ageing process and by diabetes in vascular impairment and CVD. Inflammatory mechanisms in both aged and diabetic vasculature include pro-inflammatory cytokines, vascular hyperactivation of nuclear factor-кB, increased expression of cyclooxygenase and inducible nitric oxide synthase, imbalanced expression of pro/anti-inflammatory microRNAs, and dysfunctional stress-response systems (sirtuins, Nrf2). In contrast, there are scarce data regarding the interaction of these mechanisms when ageing and diabetes co-exist and its impact on vascular function. Older diabetic animals and humans display higher vascular impairment and CVD risk than those either aged or diabetic, suggesting that chronic low-grade inflammation in ageing creates a vascular environment favouring the mechanisms of vascular damage driven by diabetes. Further research is needed to determine the specific inflammatory mechanisms responsible for exacerbated vascular impairment in older diabetic subjects in order to design effective therapeutic interventions to minimize the impact of vascular inflammation. This would help to prevent or delay CVD and the specific clinical manifestations (cognitive decline, frailty and disability) promoted by diabetes-induced vascular impairment in the elderly.

Reduced vascular responses to soluble guanylyl cyclase but increased sensitivity to sildenafil in female rats with type 2 diabetes

Impaired nitric oxide (NO), soluble guanylyl cyclase (sGC), and cyclic guanosine monophosphate (cGMP) signaling (NO-sGC-cGMP) has been implicated in the pathogenesis of diabetic vascular dysfunction. Efforts to directly target this signaling have led to the development of sGC agonists that activate the heme group of sGC (stimulators) or preferentially activate sGC when the heme is oxidized (activators). In this study, we hypothesized that resistance arteries from female rats with spontaneous type 2 diabetes (Goto-Kakizaki rats, GK) would have reduced vasodilatory responses to heme-dependent sGC activation and increased responses to heme-independent sGC activation compared with control rats (Wistar). Endothelium-dependent and -independent relaxation was assessed in isolated segments from mesenteric resistance arteries (MA) mounted in a wire myograph. GK MA had reduced responses to acetylcholine (pEC50: 7.96 ± 0.06 vs. 7.66 ± 0.05, P < 0.05) and sodium nitroprusside (pEC50: 8.34 ± 0.05 vs. 7.77 ± 0.04, P < 0.05). There were no group differences in 8-bromoguanosine cGMP-induced relaxation and protein kinase G1 expression (P > 0.05). GK MA had attenuated responses to BAY 41-2272 (heme-dependent sGC stimulator; pEC50: 7.56 ± 0.05 vs. 6.93 ± 0.06, P < 0.05) and BAY 58-2667 (heme-independent sGC activator; pEC50: 10.82 ± 0.07 vs. 10.27 ± 0.08, P < 0.05) and increased sensitivity to sildenafil [phosphodiesterase 5 (PDE5) inhibitor; pEC50: 7.89 ± 0.14 vs. 8.25 ± 0.13, P < 0.05]. Isolated resistance arteries from female rats of reproductive age that spontaneously develop type 2 diabetes have increased sensitivity to PDE5 inhibition and reduced responsiveness to sGC activators and stimulators.

Augmented dilation to nitric oxide in uterine arteries from rats with type 2 diabetes: implications for vascular adaptations to pregnancy

Pre-existing diabetes increases the risk of maternal and fetal complications during pregnancy, which may be due to underlying maternal vascular dysfunction and impaired blood supply to the uteroplacental unit. Endothelial dysfunction and reduced vascular smooth muscle responsiveness to nitric oxide (NO) are common vascular impairments in type 2 diabetes (T2D). We hypothesized that uterine arteries from diabetic rats would have reduced vascular smooth muscle sensitivity to NO compared with nondiabetic rats due to impairment in the NO/soluble guanylate cyclase (sGC)/cGMP signaling pathway. Uterine arteries from pregnant Goto-Kakizaki (GK; model of T2D) and Wistar (nondiabetic) rats were studied in a wire myograph. GK nonpregnant uterine arteries had reduced responses to ACh and sodium nitroprusside (SNP) but increased responses to propylamine propylamine NONOate and greater sensitivity to sildenafil compared with Wistar nonpregnant arteries. In late pregnancy, Wistar rats had reduced uterine vascular smooth muscle responsiveness to SNP, but GK rats failed to show this adaptation and had reduced expression of sGC compared with the nonpregnant state. GK rats had a smaller litter size (13.9 ± 0.48 vs. 9.8 ± 0.75; P < 0.05) and a greater number of resorptions compared with Wistar controls (0.8 ± 0.76% vs. 19.9 ± 6.06%; P < 0.05). These results suggest that uterine arteries from rats with T2D show reduced sensitivity of uterine vascular smooth muscle sGC to NO. During pregnancy, the GK uterine vascular smooth muscle fails to show relaxation responses similar to those of arteries from nondiabetic rats.

Co-administration of Grape Seed Extract and Exercise Training Improves Endothelial Dysfunction of Coronary Vascular Bed of STZ-Induced Diabetic Rats

Background

One of the known complications of diabetes mellitus is vascular dysfunction. Inability of the coronary vascular response to cardiac hyperactivity might cause a higher incidence of ischemic heart disease in diabetic subjects. It has been indicated that regular exercise training and antioxidants could prevent diabetic cardiovascular problems enhanced by vascular damage.

Objectives

The aim of this study was to determine the effects of grape seed extract (as antioxidant), with and without exercise training on coronary vascular function in streptozotocin induced diabetic rats.

Materials and Methods

Fifty male Wistar rats weighing 200 – 232 grams were randomly divided into five groups of 10 rats each: sedentary control, sedentary diabetic, trained diabetic, grape seed extract (200 mg/kg) treated sedentary diabetic and, grape seed extract treated trained diabetic. Diabetes was induced by one intraperitoneal injection of streptozotocin. After eight weeks, coronary vascular responses to vasoactive agents were determined.

Results

The endothelium dependent vasorelaxation to acetylcholine was reduced significantly in diabetic animals; exercise training or grape seed extract administration partially improves this response. However, exercise training in combination with grape seed extract restores endothelial function completely. The endothelium independent vasorelaxation to sodium nitroprusside was improved by combination of exercise training and grape seed extract. On the other hand, the basal perfusion pressure and vasoconstrictive response to phenylephrine did not change significantly.

Conclusions

The data indicated that co-administration of grape seed extract and exercise training had more significant effects than exercise training or grape seed extract alone; this may constitute a convenient and inexpensive therapeutic approach to diabetic vascular complications.

The specific VPAC2 agonist Bay 55-9837 increases neuronal damage and hemorrhagic transformation after stroke in type 2 diabetic rats

VPAC2 receptor is a potential target for the treatment of type 2 diabetes and may also convey neuroprotective effects. The aim of this study was to determine the potential efficacy of the VPAC2 receptor agonist Bay 55-9837 against stroke in type-2 diabetic Goto-Kakizaki (GK) rats. GK rats were treated intravenously once daily for 7 days with 0.25 or 0.025 nmol/kg Bay 55-9837 or vehicle before inducing stroke by transient middle cerebral artery occlusion. Treatments were then continued for 7 further days. The glycemic effects of Bay 55-9837 were assessed by measuring fasting blood glucose and oral glucose tolerance. The severity of stroke was measured by assessing ischemic volume. The results show that Bay 55-9837 is not effective in lowering fasting glycemia and does not facilitate glucose disposal. The highest dose of Bay 55-9837 (0.25 nmol/kg) led to increased mortality and brain hemorrhage when compared to control. The lower dose of Bay 55-9837 (0.025 nmol/kg) did not increase mortality rate but caused a threefold increase of the ischemic lesion size with signs of brain hemorrhages as compared to control. In conclusion, Bay 55-9837 did not show antidiabetic or antistroke efficacy in the type 2 diabetic GK rat. Contrarily, Bay 55-9837 treatment led to increased mortality and worsening of the severity of stroke.

Glucagon-like peptide-2 (GLP-2) modulates the cGMP signalling pathway by regulating the expression of the soluble guanylyl cyclase receptor subunits in cultured rat astrocytes

The aim of this work was to study the effect of glucagon-like peptide-2 (GLP-2) on the cyclic guanosine monophosphate (cGMP) signalling pathway and whether insulin or epidermal growth factor (EGF) might modulate the effects of GLP-2. GLP-2 produced a dose-dependent decrease in intracellular sodium nitroprusside-induced cGMP production. However, insulin induced an increase in the levels of cGMP that was dose-dependently decreased by the addition of GLP-2. By contrast, EGF induced a decrease in cGMP production, which was further reduced by the addition of GLP-2. To assess whether variations in cGMP production might be related with changes in some component of soluble guanylyl cyclase (sGC), the expression of the α1, α2, and β1 subunits were determined by Western blot analysis. At 1 h, GLP-2 produced a decrease in the expression of both α1 and β1 in the cytosolic fraction, but at 24 h only β1was reduced. As expected, insulin induced an increase in the expression of both subunits after 1 h of incubation; this was decreased by the addition of GLP-2. Likewise, incubation with EGF for 24 h produced a decrease in the expression of both subunits that was maximal when GLP-2 was added. In addition, incubation with insulin for 1 h produced an increase in the expression of the α2 subunit, which was reduced by the addition of GLP-2. These results suggest that GLP-2 inhibits cGMP production by decreasing the cellular content of at least one subunit of the heterodimeric active form of the sGC, independently of the presence of insulin or EFG. This may open new insights into the actions of this neuropeptide.

The GK rat: a prototype for the study of non-overweight type 2 diabetes

Type 2 diabetes mellitus (T2D) arises when the endocrine pancreas fails to secrete sufficient insulin to cope with the metabolic demand because of β-cell secretory dysfunction and/or decreased β-cell mass. Defining the nature of the pancreatic islet defects present in T2D has been difficult, in part because human islets are inaccessible for direct study. This review is aimed to illustrate to what extent the Goto Kakizaki rat, one of the best characterized animal models of spontaneous T2D, has proved to be a valuable tool offering sufficient commonalities to study this aspect. A comprehensive compendium of the multiple functional GK abnormalities so far identified is proposed in this perspective, together with their time-course and interactions. A special focus is given toward the pathogenesis of defective β-cell number and function in the GK model. It is proposed that the development of T2D in the GK model results from the complex interaction of multiple events: (1) several susceptibility loci containing genes responsible for some diabetic traits; (2) gestational metabolic impairment inducing an epigenetic programming of the offspring pancreas and the major insulin target tissues; and (3) environmentally induced loss of β-cell differentiation due to chronic exposure to hyperglycemia/hyperlipidemia, inflammation, and oxidative stress.

Oxidative stress and endothelial dysfunction in hypertension

Systemic arterial hypertension is a highly prevalent cardiovascular risk factor that causes significant morbidity and mortality, and is becoming an increasingly common health problem because of the increasing longevity and prevalence of predisposing factors such as sedentary lifestyle, obesity and nutritional habits. Further complicating the impact of this disease, mild and moderate hypertension are usually asymptomatic, and their presence (and the subsequent increase in cardiovascular risk) is often unrecognized. The pathophysiology of hypertension involves a complex interaction of multiple vascular effectors including the activation of the sympathetic nervous system, of the renin-angiotensin-aldosterone system and of the inflammatory mediators. Subsequent vasoconstriction and inflammation ensue, leading to vessel wall remodeling and, finally, to the formation of atherosclerotic lesions as the hallmark of advanced disease. Oxidative stress and endothelial dysfunction are consistently observed in hypertensive subjects, but emerging evidence suggests that they also have a causal role in the molecular processes leading to hypertension. Reactive oxygen species (ROS) may directly alter vascular function or cause changes in vascular tone by several mechanisms including altered nitric oxide (NO) bioavailability or signaling. ROS-producing enzymes involved in the increased vascular oxidative stress observed during hypertension include the NADPH oxidase, xanthine oxidase, the mitochondrial respiratory chain and an uncoupled endothelial NO synthase. In the current review, we will summarize our current understanding of the molecular mechanisms in the development of hypertension with an emphasis on oxidative stress and endothelial dysfunction.

Advanced glycation end-products induce vascular dysfunction via resistance to nitric oxide and suppression of endothelial nitric oxide synthase

OBJECTIVE

A number of factors contribute to diabetes-associated vascular dysfunction. In the present study, we tested whether exposure to advanced glycation end-products (AGEs) impairs vascular reactivity independently of hyperglycemia and examined the potential mechanisms responsible for diabetes and AGE-associated vascular dysfunction.

METHODS

Vasodilator function was studied using infusion of exogenous AGEs into Sprague-Dawley rats as compared with control and streptozotocin-induced diabetic rats all followed for 16 weeks (n = 10 per group). The level of arginine metabolites and expression of endothelial nitric oxide synthase (eNOS) and downstream mediators of nitric oxide-dependent signaling were examined. To further explore these mechanisms, cultured bovine aortic endothelial cells (BAECs) were exposed to AGEs.

RESULTS

Both diabetic and animals infused with AGE-modified rat serum albumin (AGE-RSA) had significantly impaired vasodilatory response to acetylcholine. Unlike diabetes-associated endothelial dysfunction, AGE infusion was not associated with changes in plasma arginine metabolites, asymmetric dimethyl-L-arginine levels or eNOS expression. However, expression of the downstream mediator cGMP-dependent protein kinase 1 (PKG-1) was significantly reduced by both AGE exposure and diabetes. AGEs also augmented hyperglycemia-associated depletion in endothelial nitric oxide production and eNOS protein expression in vitro, and the novel AGE inhibitor, alagebrium chloride, partly restored these parameters.

CONCLUSION

We demonstrate that AGEs represent a potentially important cause of vascular dysfunction, linked to the induction of nitric oxide resistance. These findings also emphasize the deleterious and potentially additive effects of AGEs and hyperglycemia in diabetic vasculature.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Role of NAD(P)H oxidase in superoxide generation and endothelial dysfunction in Goto-Kakizaki (GK) rats as a model of nonobese NIDDM

Background

Cardiovascular disease is the leading cause of mortality in diabetics, and it has a complex etiology that operates on several levels. Endothelial dysfunction and increased generation of reactive oxygen species are believed to be an underlying cause of vascular dysfunction and coronary artery disease in diabetes. This impairment is likely the result of decreased bioavailability of nitric oxide (NO) within the vasculature. However, it is unclear whether hyperglycemia per se stimulates NADPH oxidase-derived superoxide generation in vascular tissue.

Methods and Results

This study focused on whether NADPH oxidase-derived superoxide is elevated in vasculature tissue evoking endothelial/smooth muscle dysfunction in the hyperglycemic (169±4 mg%) Goto-Kakizaki (GK) rat. By dihydroethidine fluorescence staining, we determined that aorta superoxide levels were significantly elevated in 9 month-old GK compared with age matched Wistar (GK; 195±6%, Wistar; 100±3.5%). Consistent with these findings, 10⁻⁶ mol/L acetylcholine-induced relaxation of the carotid artery was significantly reduced in GK rats compared with age matched Wistar (GK; 41±7%, Wistar; 100±5%) and measurements in the aorta showed a similar trend (p = .08). In contrast, relaxation to the NO donor SNAP was unaltered in GK compared to Wistar. Endothelial dysfunction was reversed by lowering of superoxide with apocynin, a specific Nox inhibitor.

Conclusions

The major findings from this study are that chronic hyperglycemia induces significant vascular dysfunction in both the aorta and small arteries. Hyperglycemic induced increases in NAD(P)H oxidase activity that did not come from an increase in the expression of the NAD(P)H oxidase subunits, but more likely as a result of chronic activation via intracellular signaling pathways.

Impairment of α1-adrenoceptor-mediated contractile activity in caudal arterial smooth muscle from type 2 diabetic Goto-Kakizaki rats

1. In the present study, we compared the responsiveness of de-endothelialized caudal artery smooth muscle strips, isolated from Type 2 diabetic Goto-Kakizaki (GK) and normal Wistar rats, to alpha(1)-adrenoceptor stimulation (cirazoline) and membrane depolarization (K(+)). 2. The contractile and myosin 20 kDa light chain (LC(20)) phosphorylation responses to 0.3 micromol/L cirazoline of caudal artery strips isolated from 12-week-old GK rats were significantly reduced compared with those of age-matched Wistar rats, whereas the contractile and LC(20) phosphorylation responses to 60 mmol/L K(+) were unaltered. 3. Stimulation of fura 2-AM-loaded strips from GK rats with 0.3 micromol/L cirazoline induced a significantly smaller rise in [Ca(2+)](i) (by approximately 20%) compared with that in strips from Wistar rats, whereas comparable Ca(2+) transients were evoked by K(+) in both. 4. Using quantitative polymerase chain reaction, no significant differences were detected in the mRNA expression of alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptor subtypes between GK and Wistar rats. 5. Cirazoline (1 micromol/L)- and caffeine (20 mmol/L)-induced contractions in the absence of extracellular Ca(2+) were unaltered in GK rats, suggesting that the release of Ca(2+) from the sarcoplasmic reticulum in response to cirazoline does not differ between GK and Wistar rats. 6. The results of the present study suggest that Ca(2+) entry from the extracellular space via alpha(1)-adrenoceptor-activated, Ca(2+)-permeable channels, but not via membrane depolarization and voltage-gated L-type Ca(2+) channels, is impaired in caudal artery smooth muscle of GK rats.

The soluble guanylate cyclase activator HMR1766 reverses hypoxia-induced experimental pulmonary hypertension in mice

Severe pulmonary hypertension (PH) is a disabling disease with high mortality, characterized by pulmonary vascular remodeling and right heart hypertrophy. In mice with PH induced by chronic hypoxia, we examined the acute and chronic effects of the soluble guanylate cyclase (sGC) activator HMR1766 on hemodynamics and pulmonary vascular remodeling. In isolated perfused mouse lungs from control animals, HMR1766 dose-dependently inhibited the pressor response of acute hypoxia. This dose-response curve was shifted leftward when the effects of HMR1766 were investigated in isolated lungs from chronic hypoxic animals for 21 days at 10% oxygen. Mice exposed for 21 or 35 days to chronic hypoxia developed PH, right heart hypertrophy, and pulmonary vascular remodeling. Treatment with HMR1766 (10 mg x kg(-1) x day(-1)), after full establishment of PH from day 21 to day 35, significantly reduced PH, as measured continuously by telemetry. In addition, right ventricular (RV) hypertrophy and structural remodeling of the lung vasculature were reduced. Pharmacological activation of oxidized sGC partially reverses hemodynamic and structural changes in chronic hypoxia-induced experimental PH.

Inactivation of soluble guanylate cyclase by stoichiometric S-nitrosation

Dysfunction of vascular nitric oxide (NO)/cGMP signaling is believed to contribute essentially to various cardiovascular disorders. Besides synthesis and/or bioavailability of endothelial NO, impaired function of soluble guanylate cyclase (sGC) may play a key role in vascular dysfunction. Based on the proposal that desensitization of sGC through S-nitrosation contributes to vascular NO resistance ( Proc Natl Acad Sci U S A 104: 12312-12317, 2007 ), we exposed purified sGC to dinitrosyl iron complexes (DNICs), known as potent nitrosating agents. In the presence of 2 mM GSH, DNICs stimulated cGMP formation with EC(50) values of 0.1 to 0.5 microM and with an efficacy of 70 to 80% of maximal activity measured with 10 microM 2,2-diethyl-1-nitroso-oxyhydrazine (DEA/NO). In the absence of GSH, the efficacy of DNICs was markedly reduced, and sGC stimulation was counteracted by the inhibition of both basal and DEA/NO-stimulated cGMP formation at higher DNIC concentrations. Inactivation of sGC was slowly reversed in the presence of 2 mM GSH and associated with stoichiometric S-nitrosation of the protein (2.05 +/- 0.18 mol S-nitrosothiol per mol of 143-kDa heterodimer). S-Nitrosoglutathione and sodium nitroprusside caused partial inhibition of DEA/NO-stimulated sGC that was prevented by GSH, whereas nitroglycerin (0.3 mM) had no effect. Our findings indicate that nitrosation of two cysteine residues in sGC heterodimers results in enzyme inactivation. Protection by physiologically relevant concentrations of GSH (10 microM to 3 mM) suggests that S-nitrosation of sGC may contribute to vascular dysfunction in inflammatory disorders associated with nitrosative and oxidative stress and GSH depletion.

Exercise training enhanced myocardial endothelial nitric oxide synthase (eNOS) function in diabetic Goto-Kakizaki (GK) rats

BACKGROUND

Different mechanisms of diabetic-induced NO dysfunction have been proposed and central to most of them are significant changes in eNOS function as the rate-limiting step in NO bioavailability. eNOS exists in both monomeric and dimeric conformations, with the dimeric form catalyzing the synthesis of nitric oxide, while the monomeric form catalyzes the synthesis of superoxide (O2-). Diabetic-induced shifts to decrease the dimer:monomer ratio is thought to contribute to the degradation of nitric oxide (NO) bioavailability. Exercise has long been useful in the management of diabetes. Although exercise-induced increases expression of eNOS has been reported, it is unclear if exercise may alter the functional coupling of eNOS.

METHODS

To investigate this question, Goto-Kakizaki rats (a model of type II diabetes) were randomly assigned to a 9-week running program (train) or sedentary (sed) groups.

RESULTS

Exercise training significantly (p < .05) increased plantaris muscle cytochrome oxidase, significantly improved glycosylated hemoglobin (sed: 7.33 +/- 0.56%; train: 6.1 +/- 0.18%), ad improved insulin sensitivity. Exercise increased both total eNOS expression and the dimer:monomer ratio in the left ventricle LV (sed: 11.7 +/- 3.2%; train: 41.4 +/- 4.7%). Functional analysis of eNOS indicated that exercise induced significant increases in nitric oxide (+28%) production and concomitant decreases in eNOS-dependent superoxide (-12%) production. This effect was observed in the absence of tetrahydrobiopterin (BH4), but not in the presence of exogenous BH4. Exercise training also significantly decreased NADPH-dependent O2- activity.

CONCLUSION

Exercise-induced increased eNOS dimerization resulted in an increased coupling of the enzyme to facilitate production of NO at the expense of ROS generation. This shift that could serve to decrease diabetic-related oxidative stress, which should serve to lessen diabetic-related complications.

Chromium supplementation improves glucose tolerance in diabetic Goto-Kakizaki rats

Chromium supplementation (Cr) may be useful in the management of diabetes and appears to improve some aspects of glucose handling. However, several studies have used either high doses of Cr supplementation or have placed control animals on a Cr-deficient diet. We therefore wanted to test whether Cr dosages in the ranges that more closely approximate recommended levels of supplementation in humans are efficacious in glycemic control under normal dietary conditions. Euglycemic Wistar or diabetic Goto-Kakizaki (GK) rats (a model of nonobese NIDDM) were assigned to water (control) or chromium picolinate (Cr-P) supplementation (1 or 10 mg/kg/day) groups for up to 32 weeks. Glucose tolerance was tested following an overnight fast by injecting sterile glucose (1.0 g/kg, i.p.) and then measuring blood glucose at select times to determine the sensitivity to glucose by calculation of the area under the curve. Cr-P did not significantly alter the growth of the animals. In the euglycemic Wistar rats, Cr-P supplementation did not alter the response to a glucose tolerance test. In the GK rats, Cr-P supplementation significantly improved glucose tolerance at both levels of Cr-P supplementation (1 mg/kg/day: H20; 100 +/- 11%; Cr-P 70 +/- 8%; 10 mg/kg/day: H(2)0; 100 +/- 10%; Cr-P 66 +/- 9 %). Cr-P supplementation produced a small improvement in some indices of glycemic control. There were no differences observed for the two levels of Cr-P supplementation suggested that we did not identify a threshold for Cr-P effects, and future studies may use lower doses to find a threshold effect for improving glucose tolerance in diabetics.

Nitric oxide, tetrahydrobiopterin, oxidative stress, and endothelial dysfunction in hypertension

Endothelial dysfunction in the setting of cardiovascular risk factors such as hypercholesterolemia, diabetes mellitus, chronic smoking, as well hypertension, is, at least in part, dependent of the production of reactive oxygen species (ROS) and the subsequent decrease in vascular bioavailability of nitric oxide (NO). ROS-producing enzymes involved in increased oxidative stress within vascular tissue include NADPH oxidase, xanthine oxidase, and mitochondrial superoxide producing enzymes. Superoxide produced by the NADPH oxidase may react with NO, thereby stimulating the production of the NO/superoxide reaction product peroxynitrite. Peroxynitrite in turn has been shown to uncouple eNOS, therefore switching an antiatherosclerotic NO producing enzyme to an enzyme that may accelerate the atherosclerotic process by producing superoxide. Increased oxidative stress in the vasculature, however, is not restricted to the endothelium and also occurs within the smooth muscle cell layer. Increased superoxide production has important consequences with respect to signaling by the soluble guanylate cyclase and the cGMP-dependent kinase I, which activity and expression is regulated in a redox-sensitive fashion. The present review will summarize current concepts concerning eNOS uncoupling, with special focus on the role of tetrahydrobiopterin in mediating eNOS uncoupling.

Upregulation of AT2 receptor and iNOS impairs angiotensin II-induced contraction without endothelium influence in young normotensive diabetic rats

Diabetes and insulin resistance are associated with an increased risk of hypertension and cardiovascular disease. Recent evidence demonstrates that AT2 receptors (AT2R) play an important role in the hemodynamic control of hypertension by vasodilation. The quantitative significance of AT2R in the establishment of diabetic vascular dysfunction, however, is not well defined and needs further investigation. Goto-Kakizaki (GK) rats, a polygenic model of spontaneous normotensive type 2 diabetes, were used to examine any abnormalities in cardiovascular function associated with AT2R at the early stage of the disease without endothelium influence. Using a myograph to measure the isometric force, we observed that ANG II-induced contraction was impaired in denuded GK aorta compared with control Wistar-Kyoto (WKY) aorta and exhibited a retarded AT1R antagonist response and enhanced Rho kinase signaling. When AT1R were blocked, ANG II induced a significant vasodilation of precontracted GK aorta via AT2R. The protein and mRNA of AT2R were increased in diabetic GK denuded aorta. Blocking AT2R restored the ANG II-induced contraction in the GK vasculature to control levels, demonstrating a counteractive role for AT2R in AT1R-induced contraction. Inhibition of inducible nitric oxide synthase (iNOS) by NG-monomethyl-L-arginine mimicked AT2R inhibition in denuded GK aorta, suggesting that AT2R-induced vasodilation was dependent on iNOS/NO generation. The protein and mRNA of iNOS were also increased in GK aorta. In conclusion, these results clearly demonstrate that enhanced AT2R and iNOS-induced, NO-mediated vasodilation impair ANG II-induced contraction in an endothelium-independent manner at the early stage of type 2 diabetes.

Diet-induced obesity and diabetes reduce coronary responses to nitric oxide due to reduced bioavailability in isolated mouse hearts

AIM

The aim of the present study was to examine nitric oxide (NO)-mediated coronary vascular responses in a mouse model of obesity and diabetes induced by a high-fat, high-carbohydrate diet. We hypothesized that endogenous NO bioavailability would be reduced in obese/diabetic mouse hearts due to enhanced superoxide anion production, and that coronary smooth muscle responses to exogenous NO would be reduced.

METHODS

Age-matched, male C57BL/6J mice were fed either a control diet or a high-fat, high-carbohydrate diet. After 15 weeks, the mice were anesthetized and their hearts were removed and perfused by the Langendorff method under constant flow conditions with an oxygenated buffer solution, and changes in coronary vascular resistance were quantified.

RESULTS

Mice fed the high-fat, high-carbohydrate diet became obese, hyperglycaemic and hyperinsulinaemic. Coronary vasoconstrictor responses to NO synthase inhibition by N(omega)-nitro-L-arginine methyl ester were reduced in obese/diabetic mice; normal responses were restored by pretreatment with the superoxide dismutase mimetic 2,2,6,6-tetramethyl-1-piperidinyloxy (Tempol). Coronary endothelium-independent vasodilation to the NO donor (+/-)-S-nitroso-N-acetylpenicillamine (SNAP) was reduced; however, 8-bromo-cyclic guanosine monophosphate (cGMP)-induced vasodilation was unchanged in obese/diabetic hearts.

CONCLUSIONS

These findings suggest that in a diet-induced mouse model of obesity and diabetes, NO bioavailability is reduced by increased superoxide NO scavenging leading to impaired NO-mediated vasodilation. Furthermore, the attenuation of SNAP-induced vasodilation may be due to increased reactive oxygen species scavenging of exogenous NO because normal vascular smooth muscle NO signalling is maintained as indicated by similar 8-bromo-cGMP responses in control and obese/diabetic hearts.

Insulin-induced impairment via peroxynitrite production of endothelium-dependent relaxation and sarco/endoplasmic reticulum Ca(2+)-ATPase function in aortas from diabetic rats

We designed this study to determine whether a high insulin level and a diabetic state need to exist together to cause an impairment of endothelium-dependent relaxation. In diabetic rat aortas organ-cultured with insulin [vs both control rat aortas cultured with insulin and diabetic rat aortas cultured in serum-free medium]: (1) the relaxation responses to both acetylcholine (endothelium-dependent relaxation) and Angeli's salt (nitric oxide donor) were significantly weaker, (2) acetylcholine-stimulated nitric oxide production was significantly smaller, (3) superoxide and nitric oxide production into the culture medium was greater, and (4) the levels of both nitrotyrosine and tyrosine-nitrated sarco/endoplasmic reticulum calcium ATPase (SERCA) protein were greater. The insulin-induced effects were prevented by cotreatment with either a superoxide scavenger or a peroxynitrite scavenger. After preincubation with an irreversible SERCA inhibitor, the relaxation induced by the nitric oxide donor was significantly impaired in control aortas cultured with or without insulin and in diabetic aortas cultured without insulin, but not in diabetic aortas cultured with insulin. These results suggest that the coexistence of a high insulin level and an established diabetic state may lead to an excessive generation of peroxynitrite, and that this may in turn trigger an impairment of endothelium-dependent relaxation via a decrease in SERCA function.

AK, Meurer S, Deile M, Taye A, Knorr A, Lapp H, Müller H, Turgay Y, Rothkegel C, Tersteegen A, Kemp-Harper B, Müller-Esterl W, Schmidt HH. Targeting the heme-oxidized nitric oxide receptor for selective vasodilatation of diseased blood vessels

ROS are a risk factor of several cardiovascular disorders and interfere with NO/soluble guanylyl cyclase/cyclic GMP (NO/sGC/cGMP) signaling through scavenging of NO and formation of the strong oxidant peroxynitrite. Increased oxidative stress affects the heme-containing NO receptor sGC by both decreasing its expression levels and impairing NO-induced activation, making vasodilator therapy with NO donors less effective. Here we show in vivo that oxidative stress and related vascular disease states, including human diabetes mellitus, led to an sGC that was indistinguishable from the in vitro oxidized/heme-free enzyme. This sGC variant represents what we believe to be a novel cGMP signaling entity that is unresponsive to NO and prone to degradation. Whereas high-affinity ligands for the unoccupied heme pocket of sGC such as zinc-protoporphyrin IX and the novel NO-independent sGC activator 4-[((4-carboxybutyl){2-[(4-phenethylbenzyl)oxy]phenethyl}amino) methyl [benzoic]acid (BAY 58-2667) stabilized the enzyme, only the latter activated the NO-insensitive sGC variant. Importantly, in isolated cells, in blood vessels, and in vivo, BAY 58-2667 was more effective and potentiated under pathophysiological and oxidative stress conditions. This therapeutic principle preferentially dilates diseased versus normal blood vessels and may have far-reaching implications for the currently investigated clinical use of BAY 58-2667 as a unique diagnostic tool and highly innovative vascular therapy.

Metformin delays the manifestation of diabetes and vascular dysfunction in Goto-Kakizaki rats by reduction of mitochondrial oxidative stress

BACKGROUND AND AIM

This study was undertaken to test the hypothesis that hyperglycaemia induces the generation of reactive oxygen species (ROS) by mitochondria and that the oxidative stress thereby exerted is diminished by treatment with metformin. As a parameter of mitochondrial ROS formation, the activity of mitochondrial aconitase activity was determined using Goto-Kakizaki (GK) rats as model of type 2 diabetes.

METHODS

In parallel with the development of diabetes (glucose, insulin), the generation of oxidative stress was determined in aortic tissue, heart and kidney of GK rats by measurement of lipid peroxides, oxidized proteins (carbonyl activity) and mitochondrial aconitase activity. Vascular activity was determined in aortae by measuring the endothelium-dependent vasodilatation in response to acetylcholine, and vasoconstriction in response to phenylephrine.

RESULTS

At the age of 12-14 weeks, blood glucose levels rose dramatically from 7.5 up to 16.2 mM, indicating the manifestation of an overt diabetes. In addition, the glucose tolerance was impaired. The increase in blood glucose was not accompanied by changes in plasma insulin. Whereas the lipid peroxides in plasma only showed a tendency to increase, the amount of oxidized proteins (carbonyl moieties) increased from 4.6 to 10.9 micromol/mg protein (2.4 fold). In addition, the lipid peroxides in tissue were increased. Mitochondrial aconitase activity was reduced in the aorta and kidney, but not in the heart of diabetic animals. Treatment with metformin nearly normalized the hyperglycaemia and prevented the rise in carbonyl, tissue lipid peroxides and the fall in aconitase activity. Whereas the endothelium-dependent vasodilatation was not affected by the diabetes, the reaction of aortae in response to phenylephrine was strongly enhanced, changes which were prevented by treatment with metformin.

CONCLUSIONS

These observations provide in vivo evidence that the generation of ROS plays an important role in the onset of diabetes and the development of vascular dysfunction in GK rats with type 2 diabetes.

Effects of angiotensin II receptor antagonist on impaired endothelium-dependent and endothelium-independent relaxations in type II diabetic rats

BACKGROUND

Diabetes mellitus is an important risk factor for cardiovascular diseases, and vasodilator dysfunction may contribute to vascular complications in diabetes. We previously demonstrated that the angiotensin II receptor blocker (ARB) corrected the impaired endothelium-derived hyperpolarizing factor (EDHF)-mediated arterial hyperpolarization and relaxation associated with hypertension or aging, partially independently of blood pressure.

OBJECTIVE

To test whether EDHF-mediated, as well as endothelium-independent, relaxations would be altered in arteries from type II diabetic Goto-Kakizaki rats, and whether ARB would correct these alterations.

METHODS

Goto-Kakizaki rats were treated with either the ARB candesartan or a combination of hydralazine and hydrochlorothiazide for 8 weeks, beginning at 10 weeks of age. Membrane potentials and contractile responses were recorded from the isolated mesenteric arteries.

RESULTS

The two treatments lowered blood pressure comparably. Acetylcholine-induced, EDHF-mediated hyperpolarization and relaxation in mesenteric arteries were markedly impaired in untreated Goto-Kakizaki rats compared with age-matched Wistar rats, and neither ARB nor the combination therapy improved these responses. On the other hand, relaxations to endothelium-derived nitric oxide, assessed in rings precontracted with high potassium solution, were similar among the four groups. Relaxation to the nitric oxide donor sodium nitroprusside and that to levcromakalim, an ATP-sensitive K-channel opener, were also impaired in untreated Goto-Kakizaki rats, and the response to sodium nitroprusside was partially improved in treated Goto-Kakizaki rats.

CONCLUSIONS

These findings suggest that EDHF-mediated hyperpolarization and relaxation and endothelium-independent relaxations are both impaired in arteries of type II diabetic rats, and antihypertensive treatment with or without ARB partially corrects endothelium-independent relaxations to the nitric oxide donor but not EDHF-mediated responses.

Expression of the skeletal muscle dystrophin-dystroglycan complex and syntrophin-nitric oxide synthase complex is severely affected in the type 2 diabetic Goto-Kakizaki rat

The inability of insulin to stimulate glucose metabolism in skeletal muscle fibres is a classic characteristic of type 2 diabetes. Using the non-obese Goto-Kakizaki rat as an established animal model of this type of diabetes, sucrose gradient centrifugation studies were performed and confirmed the abnormal subcellular location of the glucose transporter GLUT4. In addition, this analysis revealed an unexpected drastic reduction in the surface membrane marker beta-dystroglycan, a dystrophin-associated glycoprotein. Based on this finding, a comprehensive immunoblotting survey was conducted which showed a dramatic decrease in the Dp427 isoform of dystrophin and the alpha/beta-dystroglycan subcomplex, but not in laminin, sarcoglycans, dystrobrevin, and excitation-contraction-relaxation cycle elements. Thus, the backbone of the trans-sarcolemmal linkage between the extracellular matrix and the actin membrane cytoskeleton might be structurally impaired in diabetic fibres. Immunohistochemical studies revealed that the reduction in the dystrophin-dystroglycan complex does not induce obvious signs of muscle pathology, and is neither universal in all fibres, nor fibre-type specific. Most importantly, the expression of alpha-syntrophin and the syntrophin-associated neuronal isoform of nitric oxide synthase, nNOS, was demonstrated to be severely reduced in diabetic fibres. The loss of the dystrophin-dystroglycan complex and the syntrophin-nNOS complex in selected fibres suggests a weakening of the sarcolemma, abnormal signalling and probably a decreased cytoprotective mechanism in diabetes. Impaired anchoring of the cortical actin cytoskeleton via dystrophin might interfere with the proper recruitment of the glucose transporter to the surface membrane, following stimulation by insulin or muscle contraction. This may, at least partially, be responsible for the insulin resistance in diabetic skeletal muscles.

Vascular and renal effects of vasopeptidase inhibition and angiotensin-converting enzyme blockade in spontaneously diabetic Goto-Kakizaki rats

BACKGROUND

The renin-angiotensin system plays an important role in the pathogenesis of diabetes-induced vascular and renal complications. Vasopeptidase inhibitors simultaneously inhibit angiotensin-converting enzyme (ACE) and neutral endopeptidase.

OBJECTIVE

To compare the effectiveness of vasopeptidase inhibition and ACE inhibition in preventing hypertension, endothelial dysfunction and diabetic nephropathy in spontaneously diabetic Goto-Kakizaki (GK) rats.

METHODS

Eight-week-old GK rats received omapatrilat (40 mg/kg) or enalapril (30 mg/kg) for 12 weeks, either during a normal-sodium or high-sodium diet (7% w/w). Blood pressure, arterial functions and renal morphology were determined.

RESULTS

Blood pressure and albuminuria were increased in GK rats compared to non-diabetic Wistar controls. Endothelium-dependent vascular relaxation in response to acetylcholine (ACh) and endothelium-independent vascular relaxation in response to sodium nitroprusside (SNP) were impaired in GK rats. Experiments with N-nitro-L-arginine methyl ester (L-NAME), diclofenac, and L-NAME + diclofenac suggested that cyclooxygenase and endothelium-derived hyperpolarizing factor components of endothelium-dependent vascular relaxation were also impaired. A high-sodium diet aggravated hypertension and diabetes-induced vascular and renal complications. Omapatrilat and enalapril normalized blood pressure and albuminuria during the normal-sodium diet, and effectively ameliorated diabetes-induced renal complications also during the high-sodium diet. However, omapatrilat improved endothelium-dependent relaxation to ACh to a greater extent (85 +/- 5%) than enalapril (68 +/- 6%, P < 0.05). Diclofenac pre-incubation eliminated this difference between omapatrilat and enalapril in ACh-induced vascular relaxation, suggesting that it was mediated, at least in part, via the cyclooxygenase pathway.

CONCLUSIONS

Despite comparable blood pressure-lowering and renoprotective properties, omapatrilat may be more effective in preventing vascular dysfunction during diabetes compared to enalapril in GK rats.

Stimulation of soluble guanylate cyclase slows progression in anti-thy1-induced chronic glomerulosclerosis

BACKGROUND

A critical role of soluble guanylate cyclase and nitric oxide-dependent cyclic 3',5'-guanosine monophosphate (cGMP) production for glomerular matrix expansion has recently been documented in a rat model of acute anti-thy1 glomerulonephritis. The present study analyzes the renal activity of the nitric oxide-cGMP signaling cascade in and the effect of the specific soluble guanylate cyclase stimulator Bay 41-2272 on a progressive model of anti-thy1-induced chronic glomerulosclerosis.

METHODS

Anti-thy1 glomerulosclerosis was induced by injection of anti-thy1 antibody into uninephrectomized rats. One week after disease induction, animals were randomly assigned to chronic glomerulosclerosis, chronic glomerulosclerosis plus Bay 41-2272 (10 mg/kg body weight/day) or chronic glomerulosclerosis plus hydralazine (15 mg/kg body weight/day). In week 16, analysis included effects on systolic blood pressure, proteinuria, kidney function, glomerular and tubulointerstitial matrix protein accumulation, expression of transforming growth factor-beta1 (TGF-beta1), fibronectin and plasminogen activator inhibitor type 1 (PAI-1), macrophage infiltration, cell proliferation, basal and nitric oxide-stimulated cGMP production as well as tubulointerstitial mRNA expression of alpha 1 and beta 1 soluble guanylate cyclase.

RESULTS

The moderately elevated systolic blood pressure seen in the chronic glomerulosclerosis group was comparably decreased by both treatments. Compared to normal controls, soluble guanylate cyclase mRNA expression and nitric oxide-stimulated cGMP production were up-regulated in the tubulointerstitium of the untreated chronic glomerulosclerosis animals, while its activity was decreased in glomeruli. Bay 41-2272 treatment enhanced glomerular and tubulointerstitial nitric oxide-cGMP signaling significantly. This went along with markedly reduced glomerular and tubulointerstitial macrophage infiltration, number of proliferating cells, matrix expression and accumulation, as well as improved kidney function. In contrast, hydralazine therapy did not significantly affect renal nitric oxide-cGMP signaling, macrophage number, cell proliferation, matrix protein expression and accumulation.

CONCLUSION

Glomerular and tubulointerstitial soluble guanylate cyclase activity are discordantly altered in anti-thy1-induced chronic glomerulosclerosis. Stimulation of soluble guanylate cyclase signaling by Bay 41-2272 limits the progressive course of this model toward tubulointerstitial fibrosis and impaired renal function at least in part in a blood pressure-independent manner. The results suggest that soluble guanylate cyclase activation counteracts fibrosis and progression in chronic renal disease.

Vasopeptidase inhibition has beneficial cardiac effects in spontaneously diabetic Goto–Kakizaki rats

In this study we examined diabetes- and hypertension-induced changes in cardiac structure and function in an animal model of type 2 diabetes, the Goto-Kakizaki (GK) rat. We hypothesized that treatment with omapatrilat, a vasopeptidase inhibitor, which causes simultaneous inhibition of angiotensin converting enzyme and neutral endopeptidase, provides additional cardioprotective effects, during normal- as well as high sodium intake, compared to treatment with enalapril, a selective inhibitor of angiotensin converting enzyme. Fifty-two GK rats were randomized into 6 groups to receive either normal-sodium (NaCl 0.8%) or high-sodium (NaCl 6%) diet and enalapril, omapatrilat or vehicle for 12 weeks. The GK rats developed hypertension, cardiac hypertrophy and overexpression of cardiac natriuretic peptides and profibrotic connective tissue growth factor compared to nondiabetic Wistar rats. The high dietary sodium further increased the systolic blood pressure, and changed the mitral inflow pattern measured by echocardiography towards diastolic dysfunction. Enalapril and omapatrilat equally decreased the systolic blood pressure compared to the control group during normal- as well as high-sodium diet. Both drugs had beneficial cardioprotective effects, which were blunted by the high dietary sodium. Compared to enalapril, omapatrilat reduced the echocardiographically measured left ventricular mass during normal-sodium diet and improved the diastolic function during high-sodium diet in GK rats. Furthermore, omapatrilat reduced relative cardiac weight more effectively than enalapril during high sodium intake. Our results suggest that both the renin-angiotensin and the neutral endopeptidase system are involved in the pathogenesis of diabetic cardiomyopathy since vasopeptidase inhibition was shown to provide additional benefits in comparison with selective angiotensin converting enzyme inhibition alone.

Enhancing cGMP in experimental progressive renal fibrosis: soluble guanylate cyclase stimulation vs. phosphodiesterase inhibition

cGMP serves as the main second messenger of nitric oxide (NO). Antifibrotic effects of enhancing renal cGMP levels have recently been documented in experimental acute anti-Thy-1 glomerulonephritis. The present study compares the effects of the cGMP production-increasing soluble guanylate cyclase (sGC) stimulator BAY 41-2272 with those of the cGMP degradation-limiting phosphodiesterase inhibitor pentoxifylline (PTX) in a progressive model of renal fibrosis. At 1 wk after induction of anti-Thy-1-induced chronic glomerulosclerosis (cGS), rats were randomly assigned to groups as follows: cGS, cGS + BAY 41-2272 (10 mg x kg body wt(-1) x day(-1)), or cGS + PTX (50 mg x kg body wt(-1) x day(-1)). BAY 41-2272 and PTX reduced systolic blood pressure significantly. At 16 wk, tubulointerstitial expressions of sGC mRNA and NO-induced cGMP synthesis were increased in untreated cGS animals, whereas their glomerular activity was depressed compared with normal controls. Tubulointerstitial and glomerular cGMP production in response to NO were significantly enhanced in animals treated with BAY 41-2272, but not in those treated with PTX. BAY 41-2272 administration resulted in marked reductions of glomerular and tubulointerstitial histological matrix accumulation, expression of TGF-beta1 and fibronectin, macrophage infiltration, and cell proliferation as well as improved renal function. In contrast, only moderate and nonsignificant renoprotective changes were observed in the cGS + PTX group. In conclusion, increasing renal cGMP production through BAY 41-2272 significantly improved renal NO-cGMP signaling and limited progression in anti-Thy-1-induced chronic renal fibrosis, whereas inhibition of cGMP degradation by PTX was only moderately effective. The findings indicate that pharmacological enhancement of renal cGMP levels by sGC stimulation represents a novel and effective antifibrotic approach in progressive kidney disorders.

Dysregulation of the endothelial nitric oxide synthase–soluble guanylate cyclase pathway is normalized by insulin in the aorta of diabetic rat

Antiatherogenic effects of nitric oxide (NO) are mediated by activation of soluble guanylate cyclase (sGC) and are impaired by diabetes in animals and humans. We investigated whether uncontrolled diabetes and insulin therapy effect expression and function of the main enzymes of the endothelial nitric oxide (eNOS)-sGC signaling pathway in vivo. Expression and function of eNOS, sGC and protein kinase G (PKG) were studied by Western blot analysis and vasorelaxation to NO-donor in thoracic aortas from control (CON) and streptozotocin (SZT)-induced diabetic rats during uncontrolled diabetes (DM) and insulin treatment (INS) for 8 weeks. Protein level of eNOS was increased (+300%, P < 0.05), while sGC (-50%) and PKG (-65%) proteins were reduced (P < 0.03) in aortas of DM. Insulin treatment normalized these defects resulting in eNOS, sGC and PKG aortic protein content comparable to control. In aortic rings, diethylamine NONOate (DEA-NONOate)-induced vasorelaxation was attenuated (P< or =0.05) in DM compared to control and returned to normal in INS. Thus, experimental diabetes decreases sGC and PKG expression and their NO-dependent activation in aorta despite overexpression of eNOS. These abnormalities are normalized by insulin treatment and improved metabolic control.

Expression and activity of soluble guanylate cyclase in injury and repair of anti-thy1 glomerulonephritis

BACKGROUND

Activation of soluble guanylate cyclase and generation of cyclic 3',5'-guanosine monophosphate (cGMP) is the main signal transducing event of the L-arginine-nitric oxide pathway. The present study analyzes the expression and activity of the nitric oxide-cGMP signaling cascade in and the effect of the specific soluble guanylate cyclase stimulator Bay 41-2272 on the early injury and subsequent repair phase of acute anti-thy1 glomerulonephritis.

METHODS

Anti-thy1 glomerulonephritis was induced by OX-7 antibody injection in rats. In protocol 1 (injury), Bay 41-2272 was given starting 6 days before antibody injection. One day after disease induction, parameters of mesangial cell injury (glomerular cell number and inducible nitric oxide synthesis) were analyzed. In protocol 2 (repair), Bay 41-2272 treatment was started one day after antibody injection. On day 7, parameters of glomerular repair [glomerular matrix score, expression of transforming growth factor (TGF)-beta1, fibronectin, and plasminogen-activator-inhibitor (PAI)-1, infiltration with macrophages and fibrinogen deposition (indicating platelet localization)] were determined. In both protocols, tail bleeding time, systolic blood pressure, plasma cGMP levels, glomerular mRNA expression of endothelial nitric oxide synthase (eNOS), alpha1 and beta1 soluble guanylate cyclase, and basal and nitric oxide-stimulated glomerular cGMP production were analyzed.

RESULTS

Bay 41-2272 prolonged bleeding time, reduced blood pressure, and increased plasma cGMP levels in both protocols. In the injury experiment, disease induction increased inducible nitric oxide synthesis and reduced glomerular cell number, while expression and activity of soluble guanylate cyclase was almost completely diminished. Bay 41-2272 did not affect parameters of mesangial cell injury and glomerular soluble guanylate cyclase expression and activity. In the repair protocol, expression and activity of soluble guanylate cyclase was markedly increased by disease. Bay 41-2272 further enhanced soluble guanylate cyclase expression and activity. This went along with significant reductions in proteinuria, glomerular matrix accumulation, expression of TGF-beta1, fibronectin, and PAI-1, macrophage infiltration and fibrinogen deposition as compared to the untreated anti-thy1 animals.

CONCLUSION

Glomerular nitric oxide signaling via cGMP is markedly impaired during injury of anti-thy1 glomerulonephritis, while it is highly up-regulated during subsequent repair. Further pharmacologic soluble guanylate cyclase stimulation limits glomerular TGF-beta overexpression and matrix expansion, suggesting that the soluble guanylate cyclase enzyme represents an important antifibrotic pathway in glomerular disease.

The PI3-K/Akt pathway: roles related to alterations in vasomotor responses in diabetic models

Macro- and microvascular disease states currently represent the principal causes of morbidity and mortality in patients with type I or type II diabetes mellitus. Abnormal vasomotor responses and impaired endothelium-dependent vasodilation have been demonstrated in various beds in different animal models of diabetes and in humans with type I or type II diabetes. Several mechanisms leading to endothelial dysfunction have been reported, including changes in substrate avail ability, impaired release of NO, and increased destruction of NO. The principal mediators of diabetes-associated endothelial dysfunction are (a) increases in oxidized low density lipoprotein, endothelin-1, angiotensin II, oxidative stress, and (b) decreases in the actions of insulin or growth factors in endothelial cells. An accumulating body of evidence indicates that abnormal regulation of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway may be one of several factors contributing to vascular dysfunction in diabetes. The PI3-K pathway, which activates serine/threonine protein kinase Akt, enhances NO synthase phosphorylation and NO production. Several studies suggest that in diabetes the relative ineffectiveness of insulin and the hyperglycemia act together to reduce activity in the insulin-receptor substrates (IRS)/PI3-K/Akt pathway, resulting in impairments of both IRS/PI3-K/Akt-mediated endothelial function and NO production. This article summarizes the PI3-K/Akt pathway-mediated contraction and relaxation responses induced by various agents in the blood vessels of diabetic animals.

Nitric oxide-sensitive soluble guanylyl cyclase activity is preserved in internal mammary artery of type 2 diabetic patients

Vascular reactivity to nitric oxide (NO) is mediated by NO-sensitive soluble guanylyl cyclase (sGC). Since a diminished activity of vascular sGC has been reported in an animal model of type 2 diabetes, the sGC activity was assayed in vitro in internal mammary artery specimens obtained during bypass surgery from patients with and without type 2 diabetes. The sensitivity of sGC to NO, which is dependent on Fe(2+)-containing heme, was measured in vitro using stimulation with diethylamine NONOate (DEA/NO). In addition, the novel cyclic guanosine monophosphate-elevating compound HMR-1766 was used to test the stimulation of the oxidized heme-Fe(3+)-containing form of sGC. Basal activity of sGC and its sensitivity to stimulation by DEA/NO and HMR-1766 were not different between control and type 2 diabetic patients: maximum stimulation by DEA/NO amounted to 475 +/- 67 and 418 +/- 59 pmol. mg(-1). min(-1) in control and type 2 diabetic patients, respectively. The maximum effects of HMR-1766 were 95 +/- 18 (control subjects) and 83 +/- 11 pmol. mg(-1). min(-1) (type 2 diabetic patients). Hypertension, hyperlipidemia, drug treatment with statins, ACE inhibitors, or nitrates had no effect on sGC activity. In conclusion, the present findings do not support the hypothesis that desensitization of sGC contributes to the pathogenesis of diabetic vascular dysfunction in humans.

Differential expression of α2D-adrenoceptor and eNOS in aortas from early and later stages of diabetes in Goto-Kakizaki rats

The aim of the present study was to compare vascular dysfunction between the early (12 wk old) and later (36 wk old) stages of spontaneous diabetes in Goto-Kakizaki (GK) rats. We also evaluated the aortic expression of the α2D-adrenoceptor and endothelial nitric oxide synthase (eNOS). Vascular reactivity was assessed in thoracic aortas from age-matched control rats and 12- and 36-wk GK rats. Using RT-PCR and immunoblots, we also examined the changes in expression of the α2D-adrenoceptor and eNOS. In aortas from GK rats (vs. those from age-matched control rats): 1) the relaxation response to ACh was enhanced at 12 wk but decreased at 36 wk; 2) the relaxation response to sodium nitroprusside was decreased at both 12 and 36 wk, 3) norepinephrine (NE)-induced contractility was decreased at 12 wk but not at 36 wk, 4) the expressions of α1B- and α1D-adrenoceptors were unaffected, whereas those of α2D-adrenoceptor and eNOS mRNAs were increased at both 12 and 36 wk; and 5) NE- and ACh-stimulated NOx (nitrite and nitrate) levels were increased at 12 wk, although at 36 wk ACh-stimulated NOx was lower, whereas NE-stimulated NOx showed no change. These results clearly demonstrate that enhanced ACh-induced relaxation and impaired NE-induced contraction, due to NO overproduction via eNOS and increased α2D-adrenoceptor expression, occur in early-stage GK rats and that the impaired ACh-induced relaxation in later-stage GK rats is due to reductions in both NO production and NO responsiveness (but not in eNOS expression).

Alterations in vascular endothelial function in the aorta and mesenteric artery in type II diabetic rats

We used the partial protection exerted by suitable dosages of nicotinamide against the β-cytotoxic effect of streptozotocin (STZ) to create an experimental diabetic syndrome in adult rats that appears closer to type II diabetes mellitus than other available animal models. The dosage of 230 mg/kg of nicotinamide given intraperitoneally 15 min before STZ administration (65 mg/kg i.v.) yielded animals with hyperglycemia (187.8 ± 17.8 vs. 103.8 ± 2.8 mg/dL in controls; P < 0.001) and preservation of plasma insulin levels. This study assessed the relationship between endothelial dysfunction and agonist-induced contractile responses in such rats. In the thoracic aorta, the acetylcholine (ACh) induced relaxation was significantly reduced and the noradrenaline (NA) induced contractile response was significantly increased in diabetic rats compared with age-matched control rats. In the superior mesenteric artery, the ACh-induced relaxation was similar in magnitude between diabetic and age-matched control rats; however, the ACh-induced endothelium-derived hyperpolarizing factor (EDHF) type relaxation was significantly weaker in diabetic rats than in the controls. The phenylephrine (PE) induced contractile response was not different between the two groups. The plasma concentration of NOx (NO2– + NO3–) was significantly lower in diabetic rats than in control rats. We conclude that vasomotor activities in conduit arteries are impaired in this type II diabetes model.Key words: aorta, contraction, endothelium-derived hyperpolarizing factor, endothelium-mediated relaxation, mesenteric artery, type II diabetes.

Effects of the endothelin a receptor antagonist darusentan on blood pressure and vascular contractility in type 2 diabetic Goto-Kakizaki rats

The present study evaluated the effects of long-term treatment with the endothelin A (ET(A)) receptor antagonist darusentan (LU135252) on blood pressure (BP) and vascular target-organ damage in spontaneously type 2 diabetic Goto-Kakizaki (GK) rats. BP was monitored by radiotelemetry in untreated and darusentan-treated GK rats from 10-24 weeks of age. Relaxation of mesenteric artery segments by acetylcholine (ACh) and sodium nitroprusside (SNP) was measured to assess endothelium-dependent and -independent vasorelaxation. Aortic soluble guanylyl cyclase (sGC) activity was studied in vitro after stimulation by the nitric oxide (NO) donor diethylamine-NONOate. Untreated GKs were mildly hypertensive and showed a blunted vascular relaxation by ACh and SNP and a reduction in NO-stimulated sGC activity in comparison with Wistar control rats. Darusentan led to a small but sustained reduction in 24-h BP but did not restore the endothelium-dependent vasorelaxation nor the NO-stimulated cGMP formation in GK rats. The present findings suggest that an activated endothelin pathway may contribute to elevated BP but is not involved in vascular dysfunction in this animal model of type II diabetes.