Hypertension in L-NAME-treated diabetic rats depends on an intact sympathetic nervous system

Knockout of Macula Densa Neuronal Nitric Oxide Synthase Increases Blood Pressure in db/db Mice

[Figure: see text].

Blood Pressure Changes After Exposures Increasing Angiotensin-Converting Enzyme Activity and After Its Normalization with Dihydroquercetin in Male Wistar Rats

Changes in BP and HR were assessed after exposures increasing activity of angiotensin-converting enzyme: ionizing radiation, NO synthase inhibitor (L-NAME), and dexamethasone. Effects of dihydroquercetin and angiotensin-converting enzyme inhibitor enalapril on activity of this enzyme, BP, and HR were also evaluated under these exposures. Wistar male rats were subjected to X-ray irradiation in a dose of 2.5 Gy. Angiotensin-converting enzyme activity in the aorta sections was determined by Hip-His-Leu hydrolysis. BP and HR were recorded using a non-invasive tail-cuff method and PowerLab 8/35 software. BP and HR were not altered with the increase in activity of angiotensin-converting enzyme after irradiation. In case of prolonged (7 days) treatment with NO synthase inhibitor and dexamethasone, the increase in enzyme activity was accompanied by elevation of BP and, in the case of NO synthase inhibitor, HR reduction. Dihydroquercetin normalized the enzyme activity and lowered BP, but not to the normal level. Enalapril normalized BP, increased by NO synthase inhibitor solution intake; at the same time, the angiotensinconverting enzyme activity decreased more than 2-fold in comparison with the normal.

Blood pressure regulation in stress: focus on nitric oxide-dependent mechanisms

Stress is considered a risk factor associated with the development of various civilization diseases including cardiovascular diseases, malignant tumors and mental disorders. Research investigating mechanisms involved in stress-induced hypertension have attracted much attention of physicians and researchers, however, there are still ambiguous results concerning a causal relationship between stress and long-term elevation of blood pressure (BP). Several studies have observed that mechanisms involved in the development of stress-induced hypertension include increased activity of sympathetic nervous system (SNS), glucocorticoid (GC) overload and altered endothelial function including decreased nitric oxide (NO) bioavailability. Nitric oxide is well known neurotransmitter, neuromodulator and vasodilator involved in regulation of neuroendocrine mechanisms and cardiovascular responses to stressors. Thus NO plays a crucial role in the regulation of the stress systems and thereby in the BP regulation in stress. Elevated NO synthesis, especially in the initial phase of stress, may be considered a stress-limiting mechanism, facilitating the recovery from stress to the resting levels via attenuation of both GC release and SNS activity as well as by increased NO-dependent vasorelaxation. On the other hand, reduced levels of NO were observed in the later phases of stress and in subjects with genetic predisposition to hypertension, irrespectively, in which reduced NO bioavailability may account for disruption of NO-mediated BP regulatory mechanisms and accentuated SNS and GC effects. This review summarizes current knowledge on the role of stress in development of hypertension with a special focus on the interactions among NO and other biological systems affecting blood pressure and vascular function.

Vascular endothelial growth factor protects post-ganglionic sympathetic neurones from the detrimental effects of hydrogen peroxide by increasing catalase

AIM

Vascular production of hydrogen peroxide (H(2)O(2)) is implicated in the development and progression of vascular disease. Hydrogen peroxide also promotes neuronal degeneration, which suggests that vascular H(2)O(2) would promote degeneration of perivascular sympathetic nerves. Vascular cells also produce vascular endothelial growth factor (VEGF), which could protect perivascular nerves from the detrimental effects of H(2)O(2) . The aim of this study was to test these hypotheses.

METHODS

The effects of H(2)O(2) and VEGF on neuronal survival and noradrenaline uptake were studied in cultures of rat post-ganglionic sympathetic neurones. Western analyses of catalase and growth associated protein 43 were performed and reactive oxygen species (ROS) were measured using the fluorescent indicator 5-(and-6)-chloromethyl-2'7'-dichlorodihydrofluorescein diacetate, acetyl ester.

RESULTS

Hydrogen peroxide (30 μm) decreased the survival of post-ganglionic sympathetic neurones (57.8 ± 4.8% of control) and decreased noradrenaline uptake into the neurones (14 ± 6% of control). Hyperglycaemia, which is known to increase H(2)O(2), also decreased survival (31.4 ± 12% of control) and noradrenaline uptake (42 ± 18.4% of control). VEGF reduced the effects of H(2)O(2) (94.3 ± 12% of control) and hyperglycaemia (83.5 ± 23.6% of control) on survival. VEGF increased catalase, a primary determinant of intracellular concentrations of H(2)O(2) , and decreased H(2)O(2) -induced increases in ROS.

CONCLUSION

These results indicate that VEGF protects post-ganglionic sympathetic neurones from the detrimental effects of H(2)O(2). Our data suggest that an increase in catalase is the mechanisms underlying this neuroprotection.

Vascular-dependent effects of elevated glucose on postganglionic sympathetic neurons

Perivascular sympathetic nerves are important determinants of vascular function that are likely to contribute to vascular complications associated with hyperglycemia and diabetes. The present study tested the hypothesis that glucose modulates perivascular sympathetic nerves by studying the effects of 7 days of hyperglycemia on norepinephrine (NE) synthesis [tyrosine hydroxylase (TH)], release, and uptake. Direct and vascular-dependent effects were studied in vitro in neuronal and neurovascular cultures. Effects were also studied in vivo in rats made hyperglycemic (blood glucose >296 mg/dl) with streptozotocin (50 mg/kg). In neuronal cultures, TH and NE uptake measured in neurons grown in high glucose (HG; 25 mM) were less than that in neurons grown in low glucose (LG; 5 mM) (P < 0.05; n = 4 and 6, respectively). In neurovascular cultures, elevated glucose did not affect TH or NE uptake, but it increased NE release. Release from neurovascular cultures grown in HG (1.8 ± 0.2%; n = 5) was greater than that from cultures grown in LG (0.37 ± 0.28%; n = 5; P < 0.05; unpaired t-test). In vivo, elevated glucose did not affect TH or NE uptake, but it increased NE release. Release in hyperglycemic animals (9.4 + 1.1%; n = 6) was greater than that in control animals (5.39 + 1.1%; n = 6; P < 0.05; unpaired t-test). These data identify a novel vascular-dependent effect of elevated glucose on postganglionic sympathetic neurons that is likely to affect the function of perivascular sympathetic nerves and thereby affect vascular function.

Antioxidants but not doxycycline treatments restore depressed beta-adrenergic responses of the heart in diabetic rats

Reactive oxygen species (ROS) play important roles in the development of diabetic cardiomyopathy. Matrix metalloproteinases (MMPs) can get activated by ROS and contribute to loss of myocardial contractile function in oxidative stress injury. Previously we have shown that either a MMP-2 inhibitor doxycycline or an antioxidant selenium treatment in vivo prevented diabetes-induced cardiac dysfunction significantly. In addition, there is an evidence for impaired cardiac responsiveness to beta-adrenoceptor (beta AR) stimulation in experimental animals with diabetes. The exact nature of linkage between the functional depression in cardiac responses to catecholamines and the variations in uncoupling of beta AR in diabetes has not been clearly defined. Therefore, we aimed to evaluate the effect of in vivo administration of doxycycline on beta AR responses of isolated hearts from diabetic rats and compare these data with two well-known antioxidants; sodium selenate and (n-3) fatty acid-treated diabetic rats. We examined the changes in the basal cardiac function in response to the beta AR stimulation, adenylate cyclase activity, and beta AR affinity to its agonist, isoproterenol. These results showed that antioxidant treatment of diabetic rats could protect the hearts against diabetes-induced depression in beta AR responses, significantly while doxycycline did not have any significant beneficial action on these parameters. As a summary, present data, in part, demonstrate that antioxidants and MMP inhibitors could both regulate MMP function but may also utilize different mechanisms of action in cardiomyocytes, particularly related with beta AR signaling pathway.

Endothelial dysfunction and arterial pressure regulation during early diabetes in mice: roles for nitric oxide and endothelium-derived hyperpolarizing factor

We determined whether nitric oxide (NO) counters the development of hypertension at the onset of diabetes in mice, whether this is dependent on endothelial NO synthase (eNOS), and whether non-NO endothelium-dependent vasodilator mechanisms are altered in diabetes in mice. Male mice were instrumented for chronic measurement of mean arterial pressure (MAP). In wild-type mice, MAP was greater after 5 wk of N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 mg x kg(-1) x day(-1) in drinking water; 97 +/- 3 mmHg) than after vehicle treatment (88 +/- 3 mmHg). MAP was also elevated in eNOS null mice (113 +/- 4 mmHg). Seven days after streptozotocin treatment (200 mg/kg iv) MAP was further increased in L-NAME-treated mice (108 +/- 5 mmHg) but not in vehicle-treated mice (88 +/- 3 mmHg) nor eNOS null mice (104 +/- 3 mmHg). In wild-type mice, maximal vasorelaxation of mesenteric arteries to acetylcholine was not altered by chronic L-NAME or induction of diabetes but was reduced by 42 +/- 6% in L-NAME-treated diabetic mice. Furthermore, the relative roles of NO and endothelium-derived hyperpolarizing factor (EDHF) in acetylcholine-induced vasorelaxation were altered; the EDHF component was enhanced by L-NAME and blunted by diabetes. These data suggest that NO protects against the development of hypertension during early-stage diabetes in mice, even in the absence of eNOS. Furthermore, in mesenteric arteries, diabetes is associated with reduced EDHF function, with an apparent compensatory increase in NO function. Thus, prior inhibition of NOS results in endothelial dysfunction in early diabetes, since the diabetes-induced reduction in EDHF function cannot be compensated by increases in NO production.

LACK OF BLOOD PRESSURE SALT-SENSITIVITY SUPPORTS A PREGLOMERULAR SITE OF ACTION OF NITRIC OXIDE IN TYPE I DIABETIC RATS

1. The relationship between sodium intake and blood pressure is affected differently by changes in angiotensin (Ang) II and preglomerular resistance, and this study measured that relationship to evaluate the link between nitric oxide and blood pressure early in diabetes. 2. Rats were chronically instrumented, placed on high-sodium (HS = 12 mEq/d) or low-sodium (LS = 0.07 mEq/d) intake diets and assigned to either vehicle- (V) or Nomega-nitro-L-arginine methyl ester- (L-NAME; L) treated groups. Mean arterial pressure (MAP) was measured 18 h/day for a 6-day control and 14-day streptozotocin diabetic period in each animal. 3. The MAP of the control period averaged 95 +/- 1 and 94 +/- 1 mmHg in the LSV and HSV rats and 116 +/- 2 and 124 +/- 1 mmHg in the LSL and HSL rats, respectively (LSL vs HSL was significant at P < 0.05). Diabetes increased MAP only in the LSL and HSL rats to 141 +/- 2 mmHg and 152 +/- 2, respectively, similar to our previous reports, and those respective 25 and 28 mmHg increases were a parallel shift in the pressure natriuresis relationship. However, the apparent difference between the LSL and HSL groups when compared was a parallel of the control MAP difference. Plasma renin activity (PRA) in the control period averaged 1.5 +/- 0.5 and 8.1 +/- 1.8 ng AI/mL per h in the HSV and LSV rats, and 0.8 +/- 0.2 and 2.8 +/- 0.5 ng AI/mL per h in the HSL and LSL rats, respectively, and increased similarly by 4.6-fold in the HSL and 4.8-fold in the LSL rats during diabetes. Glomerular filtration rate (GFR) increased in the vehicle but not the L-NAME-treated groups, consistent with our previous reports. 4. Thus, the hypertension caused by the onset of diabetes in L-NAME-treated rats was not salt-sensitive. The normal modulation of PRA by salt intake and the failure of GFR to increase are consistent with our hypothesis that nitric oxide may protect against hypertension early in diabetes by preventing preglomerular vasoconstriction by AngII.

Mechanisms for Renal Blood Flow Control Early in Diabetes as Revealed by Chronic Flow Measurement and Transfer Function Analysis

The purpose of this study was to establish the roles of the myogenic response and the TGF mechanism in renal blood flow (RBF) control at the very earliest stages of diabetes. Mean arterial pressure (MAP) and RBF were measured continuously, 18 h/d, in uninephrectomized control and diabetic rats, and transfer function analysis was used to determine the dynamic autoregulatory efficiency of the renal vasculature. During the control period, MAP averaged 91 +/- 0.5 and 89 +/- 0.4 mmHg, and RBF averaged 8.0 +/- 0.1 and 7.8 +/- 0.1 ml/min in the control and diabetic groups, respectively. Induction of diabetes with streptozotocin caused a marked and progressive increase in RBF in the diabetic rats, averaging 10 +/- 6% above control on day 1 of diabetes and 22 +/- 3 and 34 +/- 1% above control by the end of diabetes weeks 1 and 2. MAP increased approximately 9 mmHg during the 2 wk in the diabetic rats, and renal vascular resistance decreased. Transfer function analysis revealed significant increases in gain to positive values over the frequency ranges of both the TGF and myogenic mechanisms, beginning on day 1 of diabetes and continuing through day 14. These very rapid increases in RBF and transfer function gain suggest that autoregulation is impaired at the very onset of hyperglycemia in streptozotocin-induced type 1 diabetes and may play an important role in the increase in RBF and GFR in diabetes. Together with previous reports of decreases in chronically measured cardiac output and hindquarter blood flow, this suggests that there may be differential effects of diabetes on RBF versus nonrenal BF control.

Dose-dependent autonomic dysfunction in chronic L-NAME-hypertensive diabetic rats

This study investigated the effects of varying doses of L-NAME on arterial pressure (AP), baroreflex control, and heart rate (HR)/AP variability in the STZ-diabetic rat. Fifty-two male Wistar rats were injected with 50 mg/kg IV STZ (diabetes, D, n = 24) or citrate (controls, C, n = 28) 30 days before recordings. After 16 days, they received 14 days of oral L-NAME, 10 (H10) or 30 (H30) mg/kg, or water. Catheters were implanted into the femoral artery and vein (PE-10) for measurements in conscious rats; recorded data were analyzed on a beat-to-beat basis. Mean AP was higher in CH30 versus C and in DH10 and DH30 versus D rats. Reflex tachycardia was blunted in CH30 and DH30 rats (b = -1.81, -1.41, -0.48 in C, CH10, and CH30, respectively, P < 0.05 and b = -1.45, -1.19, -0.28 in D, DH10, and DH30, respectively, P < 0.05). Although HR and AP variability were reduced in CH30 and DH30 rats versus C and D rats, the DH30 rat had more accentuated dysfunction. All doses of L-NAME produced similar AP responses in experimental versus control groups, independent of the disease state (diabetes). Thus, autonomic dysfunction is more related to the L-NAME dose used and to the association of diabetes and hypertension than to AP values.

Causes and consequences of increased sympathetic activity in renal disease

Much evidence indicates increased sympathetic nervous activity (SNA) in renal disease. Renal ischemia is probably a primary event leading to increased SNA. Increased SNA often occurs in association with hypertension. However, the deleterious effect of increased SNA on the diseased kidney is not only caused by hypertension. Another characteristic of renal disease is unbalanced nitric oxide (NO) and angiotensin (Ang) activity. Increased SNA in renal disease may be sustained because a state of NO-Ang II unbalance is also present in the hypothalamus. Very few studies have directly compared the efficacy of adrenergic blockade with other renoprotective measures. Third-generation beta-blockers seem to have more protective effects than traditional beta-blockers, possibly via stimulation of NO release. Although it has been extensively documented that muscle SNA is increased in chronic renal failure, data on renal SNA and cardiac SNA are not available for these patients before end-stage renal disease. It is also unknown whether additional treatment with third-generation beta-blockers can delay the progression of renal injury and prevent cardiac injury in chronic renal failure more efficiently than conventional treatment with angiotensin-converting enzyme inhibitors only.

Nitric Oxide May Prevent Hypertension Early in Diabetes by Counteracting Renal Actions of Superoxide

The dependence of blood pressure on a balance between superoxide and nitric oxide may be amplified in diabetes. We have shown that the first occurrence of sustained hyperglycemia in type I diabetes causes hypertension when induced in rats that have had nitric oxide synthesis blocked chronically (L-NAME, 10 μg/kg per minute IV). This study used tempol (18 μmol/kg per hour IV) to test the hypothesis that superoxide mediates that hypertensive response. Induction of diabetes in untreated rats had no significant effect on mean arterial pressure (MAP, measured 18 h/d), and glomerular filtration rate (GFR) increased significantly during the 2 weeks of diabetes. Chronic infusion of L-NAME in a separate group of rats increased baseline MAP from ≈90 mm Hg to a stable level of ≈120 mm Hg after 6 days of infusion, and induction of diabetes (streptozotocin, 40 mg/kg IV) in those rats caused a rapid, progressive increase in MAP that averaged 156±5 mm Hg by day 14 of diabetes that was associated with a decrease in GFR and 4-fold increase in isoprostane excretion. Tempol infusion was begun on day 2 of diabetes in a subgroup of those rats, and the progressive hypertensive response was prevented, with MAP averaging 134±10 mm Hg by day 14. In addition, the normal renal hyperfiltration response was restored by tempol and the increase in isoprostane did not occur. Thus, the hypertension and decrease in GFR caused by onset of diabetes in rats without a functioning nitric oxide system was prevented by chronic administration of the superoxide dismutase mimetic tempol.

Oral arginine reduces systemic blood pressure in type 2 diabetes: its potential role in nitric oxide generation

OBJECTIVES

Arginine is converted in the endothelial cells to nitric oxide (NO) and citrulline. NO is a potent vasodilator in humans, but diabetics may have a reduced generation of NO which results in endothelial dysfunction. The aim of this study was to evaluate the effects of oral arginine on nitric oxide production, counter-regulatory hormones and blood pressure in mildly hypertensive type 2 diabetic patients.

METHODS

A prospective, crossover clinical trial was performed over a three-day stay in the General Clinical Research Center. Six patients with type 2 diabetes mellitus and mild hypertension consented and were given orally three grams of arginine per hour for 10 hours on either day 2 or day 3. On both days 2 and 3, blood pressure was monitored between 5 AM and 4 PM and mean pressure determined.

RESULTS

Oral arginine increased plasma citrulline from 31.3 +/- 6.0 to 41.5 +/- 6.0 micro mol/L (mean +/- SEM; p < 0.05) which may reflect an increased conversion of arginine into NO and citrulline. Arginine reduced systolic BP from 135 +/- 7 to 123 +/- 8 mmHg; p < 0.05. Diastolic BP fell from 86.9 +/- 1.7 to 80.7 +/- 2.4 mmHg; p < 0.05). The reduction in BP was noted to occur two hours after starting oral arginine, and BP returned to normal within one hour of stopping the arginine. The oral arginine had no effect on C-peptide, insulin or other hormone concentrations.

CONCLUSIONS

These data suggest that oral arginine may increase endothelial nitric oxide synthase (NOS) to increase vascular NO and temporally reduce blood pressure in mildly hypertensive type 2 diabetic patients.