Protective effects of imidapril on He-Ne laser-induced thrombosis in cerebral blood vessels of stroke-prone spontaneously hypertensive rats

Inhibitors of angiotensin converting enzyme (ACE) have been developed recently for therapeutic purposes in hypertension and ischemic cardiovascular diseases. Ogiku et al. reported that one such inhibitor, imidapril, significantly prolonged survival in stroke-prone spontaneously hypertensive rats (SHRSP). The present study was designed to investigate the effect of imidapril on cerebral blood vessels in SHRSP to clarify role of the ACE inhibitor in mechanisms of cerebral thrombosis and stroke. Imidapril was administered orally at 1.0 and 5.0 mg/kg/day for 3 weeks from the age of 7 weeks, and was shown to prevent the usual increase in blood pressure seen in these animals. It also delayed He-Ne laser-induced cerebral thrombosis and increased significantly the plasma concentration of nitric oxide metabolites (NO2/NO3). To confirm the association between nitric oxide (NO) and these effects of imidapril, an inhibitor of nitric oxide synthase, N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) was dissolved in drinking water and administered to the animals for 3 weeks. Four of six rats died from stroke when L-NAME was given alone. When imidapril (5.0 mg/kg/day) was administered with L-NAME, however, the animals showed no signs or symptoms of stroke. In these instances, therefore, the concurrent administration of L-NAME with imidapril reversed significantly the effects of imidapril. Intravenous injection of imidaprilat (100 microg/kg), an active metabolite of imidapril, also decreased blood pressure significantly and increased the plasma levels of NO2/NO3 after 5 min. Moreover, imidaprilat enlarged arteriolar diameters and caused an increase in red cell velocity and mean blood flow in pial arterioles after 15 min. The results strongly suggested that imidapril protects cerebral vessels in SHRSP by elevating the release of NO, thereby improving the cerebral circulation and reducing the tendency to thrombosis and stroke.     

Experimental pneumococcal meningitis: Cerebrovascular alterations,brain edema,and meningeal inflammation are linked to the production of nitric oxide

We investigated whether treatment with the nitric oxide synthase inhibitor N-nitro-L-arginine (L-NA) and the free radical scavenger superoxide dismutase influences cerebral blood flow changes, brain edema, and cerebrospinal fluid pleocytosis in early experimental pneumococcal meningitis. Compared to untreated infected rats, superoxide dismutase given 3 hours after infection significantly attenuated the increase of brain water content, intracranial pressure, and cerebrospinal fluid white blood cell count, but did not modulate the increase in regional cerebral blood flow. N-Nitro-L-arginine treatment (5 mg/kg intravenously, followed by 5 mg/kg/hour) reversed the increase in regional cerebral blood flow; prevented an increase in brain water content, intracranial pressure, and cerebrospinal fluid nitrite concentrations; and reduced cerebrospinal fluid white blood cell count. With a closed cranial window preparation, N-nitro-L-arginine prevented pneumococci-induced dilatation of pial arterioles. When the effective dose was increased twofold, the effects of N-nitro-Lvarginine became more pronounced but resulted in the death of 4 of 5 rats, probably due to hemodynamic side effects. In primary cultures of rat cerebral endothelial cells, nitrite concentrations increased after pneumococcal stimulation, which could be prevented by NvnitrovLvarginine and cycloheximide. These data suggest that (a) nitric oxide accounts for regional cerebral blood flow changes and pial arteriolar dilatation in the early phase of experimental pneumococcal meningitis; (b) both superoxide radical and nitric oxide are involved as mediators of brain edema and meningeal inflammation; and (c) cerebral endothelial cells can be stimulated by pneumococci to release nitric oxide presumably via the inducible nitric oxide synthase.     

Modulation by nitric oxide of cerebral neutrophil accumulation after transient focal ischemia in rats.

A beneficial role of nitric oxide (NO) after cerebral ischemia has been previously attributed to its vascular effects. Recent data indicate a regulatory role for NO in initial leukocyte-endothelial interactions in the cerebral microcirculation under basal and ischemic conditions. In this study, the authors tested the hypothesis that endogenous NO production during and/or after transient focal cerebral ischemia can also be neuroprotective by limiting the process of neutrophil infiltration and its deleterious consequences. Male Sprague-Dawley rats were subjected to 2 hours occlusion of the left middle cerebral artery and the left common carotid artery. The effect of NG-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg, intraperitoneally), an NO synthase inhibitor, was examined at 48 hours after ischemia on both infarct size and myeloperoxidase activity, an index of neutrophil infiltration. L-NAME given 5 minutes after the onset of ischemia increased the cortical infarct volume by 34% and increased cortical myeloperoxidase activity by 60%, whereas administration of L-NAME at 1, 7, and 22 hours of reperfusion had no effect. Such exacerbations of infarction and myeloperoxidase activity produced when L-NAME was given 5 minutes after the onset of ischemia were not observed in rats rendered neutropenic by vinblastine. These results suggest that after transient focal ischemia, early NO production exerts a neuroprotective effect by modulating neutrophil infiltration.     

Is intracellular brain pH a dependent factor in NOS inhibition during focal cerebral ischemia?

The interaction between nitric oxide (NO.) and focal cerebral ischemia is multifaceted. Experiments have shown that inhibition of nitric oxide synthase (NOS) either ameliorates or exacerbates focal cerebral ischemia. Recent in vitro experiments have shown that NOS activity is pH-dependent. Previous work from this laboratory has demonstrated that N(G)-nitro-L-arginine-methyl-ester (L-NAME) mitigated cerebral ischemia independent from regional cerebral blood flow (rCBF) changes during moderate focal cerebral ischemia. This study examined the effects of L-NAME inhibition on brain pH(i), rCBF, and NADH redox state during 3 h of severe focal cerebral ischemia. Fifteen fasted rabbits under 1.5% halothane were equally divided into three groups: ischemic controls and two drug groups receiving either 1.0 or 10 mg/kg L-NAME intravenously 30 min prior to ischemia. In the ischemic controls, brain pH(i) declined from 6.95+/-0.04 to 6.60+/-0.05, rCBF declined from 48+/-7 to 10+/-3 ml/100 g/min, and NADH fluorescence increased by 149+/-15% 3 h after onset of ischemia (p<0.01 for all three parameters). L-NAME at either dose did not significantly alter these values. Infarct volume was not significantly different between both the L-NAME treated groups and the ischemic control group. This data suggests that during severe focal cerebral ischemia, NO. mechanisms of injury have a less important punitive role. One possible explanation is that the severity of acidosis secondary to anaerobic metabolism during severe focal cerebral ischemia attenuates NOS activity.     

Quantitative measurement of blood flow velocity in feline pial arteries during hemorrhagic hypotension and hypercapnia

Due to methodologic difficulties, few investigations have been made on the blood flow velocity in the cerebral microcirculation. Using a newly developed video camera method, we simultaneously measured the blood flow velocity and diameter of pial arteries during hemorrhagic hypotension, after blood pressure recovery, and during CO2 inhalation in cats. When the mean arterial blood pressure was lowered from 129.7 +/- 6.6 to 71.5 +/- 4.1 mm Hg, the blood flow velocity inevitably decreased from 36.6 +/- 5.3 to 27.0 +/- 3.9 mm/sec (p less than 0.001). The calculated blood flow rate [pi X (diameter/2)2 X flow velocity] was preserved in cases with concomitant vasodilation. Conversely, the blood flow velocity increased from 25.3 +/- 5.1 to 31.0 +/- 5.4 mm/sec (p less than 0.001) after mean arterial blood pressure recovery from 67.1 +/- 3.7 to 129.8 +/- 5.8 mm Hg. The blood flow rate was again preserved in vessels with a vasoconstrictive response. Each pial artery apparently dilated or constricted in proportion to the decrease or increase in flow velocity during blood pressure changes, maintaining a constant cerebral blood flow. This indicated the importance of the pial arteries in the mechanisms of cerebral blood flow autoregulation. During 5% CO2 inhalation, the blood flow velocity increased markedly from 25.4 +/- 4.6 to 37.2 +/- 10.0 mm/sec (p less than 0.05), while the pial artery diameter (85.0 +/- 13.7 microns) increased by 9.6 +/- 1.5% (p less than 0.01). The increased flow velocity might be attributable to preferential dilatation of small arterioles or intraparenchymal vessels during hypercapnia.(ABSTRACT TRUNCATED AT 250 WORDS)     

Eicosanoid synthesis elicited by norepinephrine in piglet parietal cortex

We investigated effects of exogenous norepinephrine and isoproterenol on pial arterial diameter and cerebral eicosanoid synthesis in anesthetized newborn pigs. Norepinephrine in artificial cerebrospinal fluid (CSF) constricted pial arteries from 203 +/- 27 micron (X +/- S.E.M.) to 164 +/- 18 micron (20 +/- 2%) (n = 21 vessels from 16 animals) at 10(-4) M. In the same animals, norepinephrine caused the concentration in CSF of 6-keto-prostaglandin F1 alpha to increase from 768 +/- 91 to 1544 +/- 151 pg/ml, thromboxane B2 to increase from 188 +/- 37 to 269 +/- 38 pg/ml, and prostaglandin E2 to increase from 2067 +/- 448 to 6575 +/- 751 pg/ml. Topical application of prostaglandin E2 in CSF to the cortical surface demonstrated that concentrations as low as 10,000 pg/ml were able to dilate pial arteries substantially. Blockade of cyclo-oxygenase activity by indomethacin (5-10 mg/kg, i.v.) potentiated pial arterial constriction to norepinephrine. Topical isoproterenol dilated pial arteries, but isoproterenol did not affect levels of measured eicosanoids in CSF. We conclude that norepinephrine elicits release of prostanoids from the cortical surface, and that these substances limit cerebrovascular constriction to norepinephrine.     

Effect of caffeine on cerebral blood flow response to somatosensory stimulation.

Despite caffeine's wide consumption and well-documented psychoactive effects, little is known regarding the effects of caffeine on neurovascular coupling. In the present study, we evaluated the effects of caffeine, an adenosine receptor antagonist, on intracerebral arterioles in vitro and subsequently, on the pial circulation in vivo during cortical activation induced by contralateral sciatic nerve stimulation (SNS). In our in vitro studies, we utilized isolated intracerebral arterioles to determine the effects of caffeine (10 or 50 micromol/L) on adenosine-induced vasodilatation. At the lower concentration, caffeine was without effect, but at the higher concentration, caffeine produced significant attenuation. In our in vivo studies, we determined the cerebrospinal fluid (CSF) caffeine concentrations at 15, 30, and 60 mins after intravenous administration of 5, 10 and 40 mg/kg. At the latter two concentrations, CSF levels exceeded 10 micromol/L. We then evaluated the pial arteriolar response during cortical activation caused by contralateral SNS after administering caffeine intravenously (0, 5, 10, 20 30, and 40 mg/kg). The pial circulation was observed through a closed cranial window in chloralose-anesthetized Sprague-Dawley rats. The contralateral sciatic nerve was isolated, positioned on silver electrodes and stimulated for 20 secs (0.20 V, 0.5 ms, and 5 Hz). Arteriolar diameter was quantified using an automated video dimension analyzer. Contralateral SNS resulted in a 23.8% +/-3.9% increase in pial arteriolar diameter in the hindlimb sensory cortex under control conditions. Intravenous administration of caffeine at the lowest dose studied (5 mg/kg) had no effect on either resting arteriolar diameter or SNS-induced vasodilatation. However, at higher doses (10, 20, 30, and 40 mg/kg, intravenously), caffeine significantly (P < 0.05; n = 6) attenuated both resting diameter and cerebral blood flow (CBF) responses to somatosensory stimulation. Intravenous administration of theophylline (10, 20, and 40 mg/kg), another adenosine receptor antagonist, also significantly reduced SNS-induced vasodilatation in a dose-dependent manner. Hypercarbic vasodilatation was unaffected by either caffeine or theophylline. The results of the present study show that caffeine significantly reduces cerebrovascular responses to both adenosine and to somatosensory stimulation and supports a role of adenosine in the regulation of CBF during functional neuronal activity.     

Nitric oxide synthase inhibitor augments post-ischemic leukocyte adhesion in the cerebral microcirculation in vivo.

The objective was to examine the effect of the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME) on leukocyte adhesion in the cerebral microcirculation during reperfusion following partial forebrain ischemia in the rat. Intravital fluorescence video-microscopy through a closed cranial window was used to visualize leukocyte-endothelium interaction in small pial veins of 15-100 microns diameter. Forebrain ischemia was produced by the ligation of both common carotid arteries plus elevation of the intracranial pressure to 20 mmHg for 60 min. The number of leukocytes adhering to the endothelium for longer than 3 sec was determined during ischemia (5 min and 60 min) and during reperfusion (5 min and 60 min). Two experimental groups were treated with either L-NAME or its inactive enantiomer D-NAME (20 mg kg-1 i.v.) 30 min prior to reperfusion. In a third group, also treated with D-NAME, post-ischemic hyperemia was prevented by lowering the ICP without removing the occlusion of common carotid arteries (partial reperfusion). The velocity of flow adjacent to the endothelial surface of pial veins was measured by tracking the movement of fluorescently labeled red blood cells as flow markers before and after ischemia. During ischemia, the number of adhering leukocytes increased approximately two-fold at 5 min, and three-fold at 60 min. In the D-NAME-treated group with complete reperfusion, leukocyte adhesion returned to the baseline level by 60 min of reperfusion. However, in the L-NAME-treated group, leukocyte adhesion remained elevated at 60 min of reperfusion. Post-ischemic flow velocity was significantly decreased (-66%) from control after L-NAME treatment whereas it was increased (+53%) in the D-NAME-treated group. In the partial reperfusion group, leukocyte adhesion continued to increase after the first hour of ischemia and reached a level 2.7-fold over baseline at 60 min reperfusion. Flow velocity remained below control (-26%) at 60 min reperfusion. Leukocyte adhesion was absent in pial arteries and no plugging by leukocytes was observed in cortical capillaries. The results suggest that leukocyte adhesion in small pial veins increases during 1 h forebrain ischemia and continues to increase during reperfusion if the velocity of flow or shear rate is low. The increase in leukocyte adhesion is reversible if flow velocity is elevated during reperfusion. L-NAME prevents post-ischemic hyperemia and augments leukocyte adhesion principally via a decrease in velocity or shear rate.     

Stable prostacyclin analogue preventing microcirculatory derangement in experimental cerebral ischemia in cats

We evaluated the effect of a stable synthetic prostacyclin analogue, TRK-100, on the microcirculatory derangement occurring in feline pial vessels with endothelial damage after middle cerebral artery occlusion. Fifteen adult cats were divided into an untreated group (Group 1, n = 8) and a treated group (Group 2, n = 7). Thirty minutes after 10 minutes of ultraviolet irradiation, which selectively damaged endothelium in the pial vessels, the middle cerebral artery was occluded in both groups and maintained for 30 minutes. In Group 2, 50 ng/kg/min TRK-100 was continuously infused intravenously following ultraviolet irradiation. In both the pial arteries and veins, platelet aggregate adhesion to the endothelium with subsequent thrombus formation was significantly (p less than 0.01 and p less than 0.05, respectively) inhibited during middle cerebral artery occlusion in Group 2 compared with Group 1. Similarly, blood flow stasis in the pial veins was effectively prevented in Group 2 during occlusion. Furthermore, the pial artery diameter returned to the control level during the late period of occlusion, whereas in Group 1 the pial artery remained constricted. Our data suggest that TRK-100 can prevent microcirculatory derangement in the acute stage of ischemic stroke.     

Transient cerebral vasodilatory effect of neuropeptide y mediated by nitric oxide

The effects of intracarotidly injected neuropeptide Y (NPY; 0.1 μg/kg) on the local cerebral blood volume (CBV) and blood flow (CBF) in the parieto-temporal cortex were examined by the photoelectric method in 17 anesthetized cats. CBV reflects the cumulative crosssectional area of the cerebral microvascular beds. NPY immediately caused transient but significant increases in CBV and CBF, which lasted for less than 5 min. Thereafter, CBV returned to and remained at the control level, although CBF was decreased by 30–40% for 60 min during the monitoring period. The CBV increases after NPY were prevented by a 15-min preinjection of 0.35 mg/kg/min of N^g-monomethyl-L-arginine (L-NMMA), which is a competitive blocker of nitric oxide synthesis. The CBV increases after NPY reappeared following a 15-min administration of 0.25 mg/kg/min of L-arginine, which is a precursor of nitric oxide. We conclude that NPY administered in vivo exerts a previously unreported effect of transient vasodilatation on the cerebral microvessels. This action appears to be mediated by nitric oxide, which is a major candidate as an endothelium-derived relaxing factor (EDRF).     

Interactions between the endothelium-derived relaxing factor/nitric oxide system and the endogenous opiate system in the modulation of cerebral and spinal vascular CO2 responsiveness.

The role of the L-arginine-nitric oxide (NO) system, the role of the endogenous morphine-like substances (endorphins), and the possible interaction between these two systems in the modulation of regional cerebral and spinal CO2 responsiveness was investigated in anesthetized, ventilated, normotensive, normoxic cats. Regional cerebral blood flow was measured with radiolabeled microspheres in hypocapnic, normocapnic, and hypercapnic conditions in nine individual cerebral and spinal cord regions. General opiate receptor blockade by 1 mg/kg naloxone intravenously alone or NO synthase blockade by 3 mg/kg N(omega)-nitro-L-arginine-methyl ester (L-NAME) intravenously alone caused no changes in regional CO2 responsiveness. Combined administration of these two blocking agents in the very same doses, however, resulted in a strong potentiation, with a statistically significant reduction of the CO2 responsiveness observed. Separation of the blood flow response to hypercapnia and hypocapnia indicates that this reduction occurs only during hypercapnia. Specific mu and delta opiate receptors were blocked by 0.5 mg kg(-1) IV beta-funaltrexamine and 0.4 mg kg(-1) IV naltrindole, respectively. The role of specific mu and delta opiate receptors in the NO-opiate interaction was found to be negligible because neither mu nor delta receptor blockade along with simultaneous NO blockade were able to decrease CO2 responsiveness. The current findings suggest a previously unknown interaction between the endothelium-derived relaxing factor/nitric oxide (EDRF/NO) system and the endogenous opiate system in the cerebrovascular bed during hypercapnic stimulation, with the phenomenon not mediated by mu or delta opiate receptors.     

Adrenergic vasoconstrictor activity in the cerebral circulation after inhibition of nitric oxide synthesis in conscious goats

The interaction between nitric oxide (NO) and adrenergic activity in the cerebral circulation was studied using conscious goats, where blood flow to one brain hemisphere (cerebral blood flow) was electromagnetically measured, and the effects of phentolamine and hexamethonium on cerebrovascular resistance were evaluated before (control) and after inhibition of NO synthesis with NW-nitro-L-arginine methyl ester (L-NAME). L-NAME (12 goats, 40 mg kg(-1) administered i.v.) reduced cerebral blood flow from 62 +/- 3 to 44 +/- 2 ml min(-1), increased mean systemic arterial pressure from 100 +/- 3 to 126 +/- 4 mm Hg, decreased heart rate from 79 +/- 5 to 50 +/- 4 beats min(-1) and increased cerebrovascular resistance from 1.63 +/- 0.08 to 2.91 +/- 0.016 mm Hg ml(-1)min(-1) (all P < 0.01). These hemodynamic variables normalized 48-72 h after L-NAME administration. Phentolamine (six goats, 1 mg), injected into the cerebral circulation. increased cerebral blood flow without changing systenic arterial pressure, but its cerebrovascular effects were augmented for about 24 h after L-NAME. The decrements in cerebrovascular resistance induced by phentolamine, in mm Hg ml(-1) min(-1), were: under control, 0.42 +/- 0.05; immediately after L-NAME, 1.38 +/- 0.09 (P < 0.01 compared with control); by about 24 h after L-NAME, 0.71 +/- 0.09 (P < 0.05 compared with control); and by about 48 h after L-NAME, 0.40 +/- 0.07 (P > 0.05 compared with control). Hexamethonium (six goats, 0.5-1 mg kg(-1) min(-1) i.v.) decreased mean systemic arterial pressure to about 75 mm Hg and caused tachycardia similarly before and after L-NAME, but the decrements in cerebrovascular resistance were augmented for about 24 h after L-NAME. The decrements in cerebrovascular resistance induced by hexamethonium, in mm Hg ml(-1).min(-1), were: under control. 0.61 +/- 0.09, immediately after L-NAME, 1.33 +/- 0.16 (P < 0.01 compared with control); by about 24 h after L-NAME, 1.18 +/- 0.10 (P < 0.01 compared with control): and by about 48 h after L-NAME, 0.99 +/- 0.10 (P > 0.05 compared with control). Therefore, these results suggest that adrenergic vasoconstrictor tone in cerebral vasculature may be augmented after inhibition of NO synthesis, and that this increment may contribute to the reduction of cerebral blood flow after inhibition of NO formation.     

Melatonin inhibits cortical spreading depression-evoked trigeminal nociception

This study was conducted to determine the effect of melatonin on cortical spreading depression-evoked trigeminovascular nociception. Melatonin (20 or 40 mg/kg) or saline was given to Wistar rats. KCl was placed on the cortical surface to elicit the cortical spreading depression. Cortical blood flow was monitored and ultrastructure of cerebral microvessels was studied. The medulla and cervical cord were removed for Fos and nitric oxide synthase immunohistochemical study. The results showed that melatonin pretreatment significantly minimized the cortical spreading depression-evoked cerebral hyperaemia and attenuated the cortical spreading depression-induced microvascular changes. Pretreatment with melatonin also reduced the number of Fos and nitric oxide synthase immunoreactive cells in the trigeminal nucleus caudalis. The results of this study suggest that melatonin can attenuate the process of trigeminovascular nociception induced by cortical spreading depression.     

Superoxide dismutase decreases mortality, blood pressure, and cerebral blood flow responses induced by acute hypertension in rats

Oxygen radicals are known to be produced by the cerebral vasculature during acute, pressor-induced hypertension and are also known to inactivate endothelium-derived relaxing factor. The objective of our present study was to determine if the oxygen radical scavenger superoxide dismutase (24,000 units/kg plus 1,600 units/kg/min) alters the pressor, cerebral blood flow, and mortality responses to systemic norepinephrine in rats. Increasing doses (0.01-30 micrograms/kg i.v.) of norepinephrine were given by bolus injection to eight rats, and changes in the cortical microcirculatory blood flow were measured by laser-Doppler flowmetry. Superoxide dismutase shifted the norepinephrine-blood pressure and -cerebral blood flow dose-response curves moderately, but significantly, to the right such that it took more norepinephrine to reach a given blood pressure. However, superoxide dismutase had no effect on the autoregulation of cerebral blood flow. Additionally, whereas five (63%) of the eight control rats died after the 10 micrograms/kg norepinephrine dose, all eight rats treated with superoxide dismutase survived this dose. The mechanism by which superoxide dismutase reduced mortality is uncertain. The blood pressure and cerebral blood flow results suggest that superoxide dismutase prevents oxygen radicals from destroying endothelium-derived relaxing factors, which reduce the pressor effects of norepinephrine.     

Role of nitric oxide in the cerebral circulation during hypotension after hemorrhage, ganglionic blockade and diazoxide in awake goats

The role of nitric oxide in cerebrovascular response to hypotension was analyzed by evaluating the changes in cerebrovascular resistance after inhibition of nitric oxide synthesis with Nw-nitro-L-arginine methyl ester (L-NAME) during three types of hypotension in conscious goats. Blood flow to one brain hemisphere was electromagnetically measured, hypotension was induced by controlled bleeding, and by i.v. administration of hexametonium (ganglionic blocker) or of diazoxide (vasodilator drug), and L-NAME was injected by i.v. route (35 mg kg-1). Under control conditions (13 goats), L-NAME increased arterial pressure from 98 +/- 3 to 123 +/- 4 mmHg and decreased cerebral blood flow from 65 +/- 3 to 40 +/- 3 ml min-1 (all P < 0.001); cerebrovascular resistance increased from 1.52 +/- 0.04 to 3.09 +/- 0.013 mmHg ml-1 min-1 (P < 0.01) (delta = 1.59 +/- 0.12 mmHg ml-1 min-1). After bleeding (five goats), mean arterial pressure decreased to 60 +/- 4 mmHg and cerebral blood flow decreased to 37 +/- 4 ml min-1 (all P < 0.01); cerebrovascular resistance did not change (1.56 +/- 0.14 vs. 1.54 +/- 0.12 mmHg ml-1 min-1, P > 0.05). During this hypotension, L-NAME increased arterial pressure to reach the normotensive values an did not affect the hypotensive values for cerebral blood flow; cerebrovascular resistance increased from the hypotensive values to 2.91 +/- 0.19 mmHg ml-1 min-1 (P < 0.01) (delta = 1.37 +/- 0.16 mmHg ml-1 min-1), and this increment is comparable to that under control conditions (P > 0.05). Ganglionic blockade (six goats) decreased arterial pressure to 67 +/- 2 mmHg) and did not affect significantly cerebral blood flow; cerebrovascular resistance decreased from 1.71 +/- 0.11 to 1.05 +/- 0.09 mmHg ml-1 min-1 (P < 0.01). During this hypotension, L-NAME increased arterial pressure to 103 +/- 6 mmHg (P < 0.001), and did not affect cerebral blood flow; cerebrovascular resistance increased from the hypotensive values to 1.68 +/- 0.18 mmHg ml-1 min-1 (P < 0.01) (delta = 0.63 +/- 0.10 mmHg ml-1 min-1), and this increment was lower than under control conditions (P < 0.01). Diazoxide (six goats) decreased arterial pressure to 69 +/- 5 mmHg (P < 0.01) without changing cerebral blood flow; cerebrovascular resistance decreased from 1.89 +/- 0.11 to 1.16 +/- 0.14 mmHg ml-1 min-1 (P < 0.01). During this hypotension, L-NAME increased arterial pressure to 87 +/- 6 mmHg (P < 0.05) and did not affect the hypotensive values for cerebral blood flow (P > 0.05); cerebrovascular resistance increased from the hypotensive values to 1.53 +/- 0.13 mmHg ml-1 min-1 (P < 0.05) (delta = 0.36 +/- 0.06 mmHg-1 ml-1 min-1), and this increment was lower than under control conditions (P < 0.01). Therefore, the role of nitric oxide in cerebrovascular response to hypotension may differ in each type of hypotension, as this role during hemorrhagic hypotension may not change and during hypotension by ganglionic blockade or diazoxide may decrease. These differences may be related to changes in nitric oxide release as stimuli on the endothelium (shear stress and sympathetic activity) may vary in each type of hypotension.     

Dynamic cerebral microcirculatory changes in transient forebrain ischemia in rats: involvement of type I nitric oxide synthase.

The diameter of surface microvessels and the erythrocyte velocity and flux through intraparenchymal capillaries in the parietal cortex were measured during transient global cerebral ischemia and reperfusion using laser-scanning confocal fluorescence microscopy in anesthetized rats. The role of nitric oxide (NO) from neurons in the microcirculatory changes was also investigated using 7-nitro-indazole (7-NI, 25 mg/kg, i.p.). Wistar rats (4 per group) equipped with a closed cranial window were given fluorescein isothiocyanate (FITC)-Dextran and FITC-labeled erythrocytes intravenously to respectively visualize the microvessels and the erythrocytes in the capillaries. Experiments were videorecorded on-line. Forebrains were made ischemic for 15 minutes and then reperfused for 120 minutes under the microscope. Ischemia was associated with a flattened EEG, a low persistent blood flow, and a transient leakage of fluorescein across the arteriole wall. Unclamping the carotid arteries led to immediate high blood flow in the arterioles, but it was not until 5 minutes later that the arterioles dilated significantly (181% +/- 27%) and erythrocyte velocity in the capillaries increased significantly (460% +/- 263%). Neither nonperfused capillaries nor erythrocyte capillary recruitment occurred. 7-Nitro-indazole significantly reduced the arteriole dilatation and prevented the increase in erythrocyte velocity and flux through capillaries in early reperfusion. 7-Nitroindazole had no influence on the fluorescein leakage. The current study suggests a partial role for NO released from neurons in the postischemic microcirculatory changes and provides new findings on the timing of arteriole dilatation and blood-brain barrier opening, and on erythrocyte capillary circulation in global ischemia.     

Endothelial nitric oxide synthase-dependent cerebral blood flow augmentation by L-arginine after chronic statin treatment.

Nitric oxide, a product of nitric oxide synthase activity, relaxes vascular smooth muscle and elevates brain blood flow. We evaluated the importance of eNOS to cerebral blood flow augmentation after L-arginine infusion and increases in flow after eNOS upregulation in SV-129 mice. Blood flow was measured by laser-Doppler flowmetry before and after L-arginine infusion (450 mg/kg during a 15-minute period) or measured by 14C-iodoamphetamine indicator fractionation or 14C-iodoantipyrine tissue equilibration techniques. rCBF increased by 26% (laser Doppler flowmetry) after L-arginine infusion but did not change in mutant mice deficient in eNOS expression. After eNOS upregulation by chronic simvastatin treatment (2 mg/kg subcutaneously, daily for 14 days), L-arginine amplified and sustained the hyperemia (38%) and increased absolute brain blood flow from 86 +/- 7 to 119 +/- 10 mL/100 g per minute. Furthermore, pretreatment with simvastatin enhanced blood flow within ischemic brain tissue after middle cerebral artery occlusion. Together, these findings suggest that eNOS activity is critical for blood flow augmentation during acute L-arginine infusion, and chronic eNOS upregulation combined with L-arginine administration provides a novel strategy to elevate cerebral blood flow in the normal and ischemic brain.     

Antithrombotic role of nitric oxide in rats under physiological conditions

Although sepsis-induced release of nitric oxide (NO) is known to have an antithrombotic effect, it is unknown if NO exerts this same effect under physiological conditions.We have there-fore attempted to determine whether or not NO protects against thrombus formation in normal Wistar rats injected with various amounts (0.8, 4.0, 20.0 and 100 mg/kg/4 hr) of L-NAME (N (omega)-nitro-l-arginine methyl ester), an NO synthase inhibitor, via the tail vein. Plasma levels of D-dimer fragments of fibrin were significantly increased in rats receiving L-NAME (0.21+/-0.04, 0.22+/-0.05, 0.26+/-0.07, 0.59+/-0.17 micro g/mL, means+/-SE; p<0.05, 0.05, 0.05, 0.01: L-NAME 0.8, 4, 20, 100, respectively, compared with control levels: <0.06 micro g/mL), and thrombin-anti-thrombin complex (TAT) levels were significantly increased in rats receiving 20mg/kg/4 hr or greater doses of L-NAME (4.5+/-1.1, 4.7+/-1.4, 18.7+/-4.9, 42.5+/-4.0 ng/mL, NS, NS, p<0.05, 0.01, respectively, compared with control levels: 3.8+/-1.2 ng/mL). Glomerular fibrin deposition was increased in a dose-dependent manner in rats receiving L-NAME (6.8+/-1.5, 13.9+/-1.6, 32.4+/-2.6, 49.2+/-5.2%, p<0.05, 0.05, 0.01, 0.01, respectively, com-pared with control levels: 0.0+/-0.0%). Renal dysfunction and hepatic dysfunction were observed in rats receiving 20mg/kg/4 hr or greater, or 100mg/kg/4 hr, doses of L-NAME, respectively. Mean blood pressure was also elevated in rats receiving L-NAME in a dose-dependent manner. These findings suggest that NO, in addition to regulating blood pressure, is involved in prevention of thrombus formation under physiological circumstances.     

Endothelium-derived relaxing factor inhibits constrictor responses of large cerebral arteries to serotonin.

Endothelium-derived relaxing factor [EDRF, nitric oxide (NO) or a NO-containing compound] influences basal tone of cerebral blood vessels and mediates vasodilation in response to several stimuli. It is not known whether EDRF also modulates responses to cerebral vasoconstrictor stimuli in vivo. Our goal was to determine whether formation of EDRF inhibits constrictor responses of large cerebral arteries to serotonin. We measured cerebral blood flow (microspheres) and pial microvascular pressure (servo null) in anesthetized rabbits and calculated resistance of large cerebral arteries. Responses to an inhibitor of NO formation, NG-nitro-L-arginine (L-NNA, 3 mg/kg i.v.), were examined. L-NNA produced an increase in resistance of large arteries and total cerebral vascular resistance of approximately 15% (p less than 0.05 for both variables) and a small decrease in cerebral blood flow (35 +/- 9 vs. 32 +/- 7 ml min-1 100 g-1, mean +/- SD, p less than 0.05). Under control conditions, infusion of serotonin (10 micrograms kg-1 min-1, into the left atrium) produced an increase in resistance of large arteries. Following treatment with L-NNA, the change in resistance of large arteries in response to serotonin was increased more than twofold (0.20 +/- 0.17 vs. 0.43 +/- 0.21 mm Hg ml-1 min 100 g, p less than 0.05). In contrast, L-NNA did not alter the increase in resistance of large arteries during hypocapnia. L-arginine inhibited the effects of L-NNA on baseline cerebral vascular resistance and on responses of large arteries to serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nicotine-induced NO-mediated increase in cortical cerebral blood flow is blocked by beta2-adrenoceptor antagonists in the anesthetized rats

Involvement of nitric oxide (NO) and beta-adrenoceptors in an increase in the cortical cerebral blood flow (CBF) following an intravenous (i.v.) injection of a small dose of nicotine which did not affect the systemic blood pressure in the rats was investigated. I.v. injection of nicotine (30 microg/kg) for 1 min produced a significant increase in CBF lasting for more than 20 min without a significant effect on the systemic blood pressure. I.v. injection of L-N(G)-nitroarginine methylester (30 mg/kg) significantly attenuated nicotine-induced increase in the cortical CBF. The attenuation was reversed by i.v. injection of L-arginine (300 mg/kg), suggesting an intimate role of nitric oxide (NO) in nicotine-induced increase in the cortical CBF. The nicotine-induced increase in the cortical CBF was significantly attenuated by propranolol (10 mg/kg, i.v.) and ICI 118,551 (a beta2-adrenoceptor antagonist, 10 mg/kg, i.v.) but not by metoprolol (a beta1-adrenoceptor antagonist, 10 mg/kg, i.v.). Beta2-adrenoceptors on presynaptic nitrergic nerves may be involved in nicotine-induced NO-mediated increase in the cortical CBF.