Modeling the role of osmotic forces in the cerebrovascular response to CO2.

Increases in blood osmolarity have been shown to exert a vasodilatory effect on cerebral and other vasculature, with accompanying increases in blood flow. It has also been shown that, through an influence on blood concentration of the bicarbonate ion and pH, changes in blood levels of CO2 can alter blood osmolarity sufficiently to have an impact on vessel diameter. We propose here that this phenomenon plays a previously unappreciated role in CO2-mediated vasodilation, and present a biophysical model of osmotically driven vasodilation. Our model, which is based on literature data describing CO2-dependent changes in blood osmolarity and hydraulic conductivity (Lp) of the blood-brain barrier, is used to predict the change in cerebral blood flow (CBF) associated with osmotic forces arising from a specific hypercapnic challenge. Modeled changes were then compared with actual CBF changes determined using arterial spin-labeling (ASL) MRI. For changes in the arterial partial pressure of CO2 (PaCO2) of 20 mmHg, our model predicted increases of 80% from baseline CBF with a temporal evolution that was comparable to the measured hemodynamic responses. Our modeling results suggest that osmotic forces could play a significant role in the cerebrovascular response to CO2.

Involvement of the cyclic GMP pathway in the superoxide-induced IP3 formation in vascular smooth muscle cells.

OBJECTIVE: To investigate whether cGMP or cAMP signal pathway is indirectly involved in the effect of superoxide on the IP3 formation in vascular smooth muscle cells (SMC) from rat mesenteric arteries. METHODS: Cultured smooth muscle cells from rat mesenteric arteries were prelabelled with myo-(2-(3)H) inositol for evaluation of IP3 formation. Quantitative cAMP and cGMP levels were determined using cAMP [3H] or cGMP [125I] assay systems. RESULTS: In the present study, it was found that superoxide significantly inhibited the basal level of cGMP and also suppressed the sodium nitroprusside (SNP)-induced cGMP formation in SMCs from rat mesenteric arteries. The inhibitory effect of superoxide on basal level of cGMP was similar in the absence or presence of ODQ (a guanylyl cyclase inhibitor). Moreover, the superoxide-induced increase in IP3 formation was significantly inhibited by SNP or s-nitroso- n-acetylpenicillamine but was significantly potentiated by ODQ or KT5823 (a cGMP-dependent protein kinase inhibitor). Superoxide had no effect on the basal or on the forskolin-induced cAMP production and the inhibition of adenylyl cyclase or cAMP-dependent protein kinase did not affect the superoxide-enhanced IP3 formation. CONCLUSION: The decreased cross-inhibition of IP3 pathway by cGMP may contribute to the superoxide-enhanced IP3 formation in SMCs from mesenteric arteries. The cross-talk between cGMP and IP3 pathways provides a novel mechanism for the signalling role of superoxide in vascular SMCs.

Vasorelaxant effects of the chronic treatment with melatonin on mesenteric artery and aorta of spontaneously hypertensive rats.

OBJECTIVE: To investigate the effect of a chronic treatment with melatonin on arterial pressure and a possible improvement of the vascular muscarinic and NO synthase (NOS) pathways in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. DESIGN AND METHODS: Mean arterial pressure (MAP), systolic (SBP), diastolic blood pressure (DBP), and heart rate (HR) were evaluated in conscious rats treated with 30 mg/kg per day of melatonin during 4 weeks. Changes in MAP were evaluated following an intravenous injection of the NOS inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME). Relaxant effects of acetylcholine (Ach), sodium nitroprusside (SNP), and the calcium ionophore A23187 were examined on mesenteric beds and aortic rings with or without treatment with melatonin. RESULTS: Melatonin produced a significant reduction of MAP, SBP, DBP and HR in SHR (P < 0.05). L-NAME increased the MAP of melatonin-treated SHR by the same magnitude as that of WKY rats which was significantly higher than that of non-treated SHR (P< 0.05). Melatonin treatment improved the maximal relaxation of mesenteric arteries to A23187 in SHR (P < 0.001) to the WKY level and caused a slight increment in Ach- and A23187-induced vasodilations in aorta from SHR and WKY rats (P < 0.05). CONCLUSION: The present study showed that melatonin exerted a bradycardic and an antihypertensive action in SHR. The enhancement by melatonin of the endothelium-dependent vasodilation (Ach and/or A23187) in mesenteric artery and aorta from SHR and WKY rats and the higher increase in MAP following L-NAME treatment in melatonin-treated SHR suggest the contribution of an improved vascular NOS pathway activity in the hypotensive effect of melatonin.

Effects of chronic N-acetylcysteine treatment on the actions of peroxynitrite on aortic vascular reactivity in hypertensive rats.

BACKGROUND: Peroxynitrite (ONOO-), the product of superoxide and nitric oxide, seems to be involved in vascular alterations in hypertension. OBJECTIVES: To evaluate the effects of ONOO- on endothelium-dependent and independent aortic vascular responsiveness, oxidized/reduced glutathione balance (GSSG/GSH), malondialdehyde aortic content, and the formation of 3-nitrotyrosine (3-NT), a stable marker of ONOO-, in N-acetylcysteine (NAC)-treated normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). RESULTS: In SHR only, NAC significantly reduced heart rate and systolic, but not diastolic, blood pressure. It also improved endothelium-dependent aortic relaxation in SHR, but not after exposure to ONOO-. Endothelium-dependent and independent aortic relaxations were markedly impaired by ONOO- in both strains of rat. NAC partially protected SHR against the ONOO- -induced reduction in endothelium-independent relaxation. Aortic GSSG/GSH ratio and malondialdehyde, which were higher in SHR than in WKY rats, showed a greater increase in SHR after exposure to ONOO-. NAC decreased GSSG/GSH and malondialdehyde in both strains of rat before and after exposure to ONOO-. The 3-NT concentration, which was similar in both strains of rat under basal conditions, was greater in SHR than in WKY rats after the addition of ONOO-, with a reduction only in NAC-treated SHR. CONCLUSIONS: These findings suggest an increased vulnerability of SHR aortas to the effects of ONOO- as compared with those of WKY rats. The selective improvements produced by NAC, in systolic arterial pressure, heart rate, aortic endothelial function, ONOO- -induced impairment of endothelium-independent relaxation, aortic GSSG/GSH balance, malondialdehyde content and 3-NT formation in SHR suggest that chronic administration of NAC may have a protective effect against aortic vascular dysfunction in the SHR model of hypertension.

Effect of antioxidant treatments on nitrate tolerance development in normotensive and hypertensive rats.

OBJECTIVES: To investigate the effect of chronic antioxidant treatments on the development of nitrate tolerance in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats by evaluating (i) coronary vascular reactivity, (ii) lipid peroxidation (malondialdehyde), and (iii) peroxynitrite formation (3-nitrotyrosine). METHODS: Tolerance was induced in 16-week-old male SHR and WKY, by 4 days of continuous treatment with nitroglycerin patches. Two groups were orally pre-treated (2-weeks) with antioxidants: N-acetyl-L-cysteine (NAC) or melatonin. Effects of serotonin (5-HT) and sodium nitroprusside (SNP) perfusion were tested in isolated Langendorff-perfused hearts. 3-Nitrotyrosine levels were measured in coronary sinus effluent and malondialdehyde in plasma. RESULTS: Nitrate tolerance reduced SNP-induced dilation in both strains. This alteration was differently improved by antioxidants: melatonin was effective in SHR, whereas NAC was effective in WKY. Tolerance also reduced 5-HT-mediated vasodilation in WKY, which was reversed by both antioxidants. By contrast, nitrate tolerance enhanced the vasoconstriction to 5-HT in SHR and both antioxidants prevented this response. Furthermore, tolerance was associated with higher malondialdehyde levels in both strains and with higher 3-nitrotyrosine levels in SHR. These changes were reversed by both antioxidants. CONCLUSIONS: A participation of oxidative stress was suggested during nitrate tolerance development, since antioxidants prevented the increase in lipid peroxidation and improved vascular responses to SNP and 5HT. Differential effects of antioxidants on SNP-induced vasodilation in SHR and WKY may suggest distinct mechanisms of tolerance development in hearts from hypertensive and normotensive rats. An increased peroxynitrite generation, expressed by higher 3-nitrotyrosine levels, could contribute to nitrate tolerance in the coronary circulation of SHR.

The lack of bimodality in the effects of endogenous and exogenous prostaglandins on fat cell lipolysis in rats.

The aim of this study was to investigate and clarify the role of prostaglandins (PG) on fat cell lipolysis in female rats. Incubations with adenosine deaminase (ADA) were used for the deamination of endogenous adenosine and increased basal (155%) and isoproterenol (10(-9) M) (348%) stimulation of glycerol release from adipocytes. Indomethacin and aspirin increased the effects of ADA while indomethacin further increased isoproterenol (with ADA) stimulation of lipolysis (p < or = 0.05). Exogenous PGE2 and PGI2 inhibited the isoproterenol and ADA stimulation of fat cell lipolysis (p < or = 0.05). The expected stimulatory effect of high concentrations of PGE2 and of low concentrations of PGI2 was not observed in the presence of ADA. Dose-response curves revealed that the inhibitory effects of PGs were reached at lower concentrations for PGE2 than for PGI2 (p < or = 0.05). In conclusion, this study showed that endogenous and exogenous PGs of adipose tissue only express an antilipolytic action on fat cell lipolysis. This effect appears to be highly significant when the beta-adrenergic pathway is stimulated. Our results also stress the need to control the antilipolytic effects of adenosine to study the regulation of fat cell lipolysis by PGs.

The key role of transient receptor potential melastatin-2 channels in amyloid-ß-induced neurovascular dysfunction.

Alzheimer's dementia is a devastating and incurable disease afflicting over 35 million people worldwide. Amyloid-ß (Aß), a key pathogenic factor in this disease, has potent cerebrovascular effects that contribute to brain dysfunction underlying dementia by limiting the delivery of oxygen and glucose to the working brain. However, the downstream pathways responsible for the vascular alterations remain unclear. Here we report that the cerebrovascular dysfunction induced by Aß is mediated by DNA damage caused by vascular oxidative-nitrosative stress in cerebral endothelial cells, which, in turn, activates the DNA repair enzyme poly(ADP)-ribose polymerase. The resulting increase in ADP ribose opens transient receptor potential melastatin-2 (TRPM2) channels in endothelial cells leading to intracellular Ca(2+) overload and endothelial dysfunction. The findings provide evidence for a previously unrecognized mechanism by which Aß impairs neurovascular regulation and suggest that TRPM2 channels are a potential therapeutic target to counteract cerebrovascular dysfunction in Alzheimer's dementia and related pathologies.

Chronic perfusion of angiotensin II causes cognitive dysfunctions and anxiety in mice.

High blood pressure is a major risk factor in the onset of cerebrovascular diseases and cognitive impairment. However, mechanisms by which these occur remain unclear and treatments are, therefore, ineffective to prevent cognitive decline related to cardiovascular diseases. Angiotensin II is a peptide involved in the onset and maintenance of hypertension and its effect on cognition was studied acutely but never chronically. Hence, the aim of this study is to evaluate whether chronic hypertensive levels of angiotensin II infusion alter cognitive functions in C57BL6 mice. In this study we used subcutaneous mini-pumps containing a concentration of angiotensin II (1900 ng/kg/min) that induces malignant hypertension or a saline solution for 14 and 21 days. Blood pressure was carefully monitored by a non-invasive tail-cuff method every week throughout the experiment. Spatial memory was assessed using the Morris water maze test and anxiety was measured by the elevated plus maze and the open field tests. The results indicate learning and spatial memory deficit as well as an anxious behavior induced by angiotensin II, in comparison to the vehicle group, starting at the 3rd week of perfusion. The motricity and visual acuity were equivalent in angiotensin II perfused mice compared to their respective control. These results suggest a strong relationship between angiotensin II and the development of cognitive dysfunctions and anxiety along with sustained high blood pressure.

The neurovascular dysfunction induced by angiotensin II in the mouse neocortex is sexually dimorphic.

Women are less susceptible to the cerebrovascular complications of hypertension, such as a stroke and vascular dementia. The mechanism of such protection may be related to a reduced vulnerability of women to the cerebrovascular actions of hypertension. To test this hypothesis, we used a model of hypertension based on infusion of angiotensin II (ANG II), an octapeptide that plays a key role in hypertension and produces cerebrovascular dysregulation. Cerebral blood flow (CBF) was monitored by laser-Doppler flowmetry in anesthetized (urethane-chloralose) C57BL/6J male and female mice equipped with a cranial window. ANG II administration (0.25 mug.kg(-1).min(-1) iv x 30-45 min) elevated arterial pressure equally in both sexes but attenuated the CBF increase induced by whisker stimulation or by the endothelium-dependent vasodilator acetylcholine (ACh) in male but not in female mice. The administration of ANG II for 7 days (2.74 mg.kg(-1).day(-1)), using osmotic minipumps, also attenuated these cerebrovascular responses in male, but not female, mice. The reduced susceptibility to the effect of ANG II in female mice was abolished by ovariectomy and reinstated by estrogen administration to ovariectomized mice. Administration of estrogen to male mice abolished the ANG II-induced attenuation of CBF responses. We conclude that female mice are less susceptible to the cerebrovascular dysregulation induced by ANG II, an effect related to estrogen. Such protection from the deleterious cerebrovascular effects of hypertension may play a role in the reduced vulnerability to the cerebrovascular complications of hypertension observed in women.