Vasodilator tone in the llama fetus: the role of nitric oxide during normoxemia and hypoxemia

The fetal llama responds to hypoxemia, with a marked peripheral vasoconstriction but, unlike the sheep, with little or no increase in cerebral blood flow. We tested the hypothesis that the role of nitric oxide (NO) may be increased during hypoxemia in this species, to counterbalance a strong vasoconstrictor effect. Ten fetal llamas were operated under general anesthesia. Mean arterial pressure (MAP), heart rate, cardiac output, total vascular resistance, blood flows, and vascular resistances in cerebral, carotid and femoral vascular beds were determined. Two groups were studied, one with nitric oxide synthase (NOS) blocker N(G)-nitro-L-arginine methyl ester (L-NAME), and the other with 0.9% NaCl (control group), during normoxemia, hypoxemia, and recovery. During normoxemia, L-NAME produced an increase in fetal MAP and a rapid bradycardia. Cerebral, carotid, and femoral vascular resistance increased and blood flow decreased to carotid and femoral beds, while cerebral blood flow did not change significantly. However, during hypoxemia cerebral and carotid vascular resistance fell by 44% from its value in normoxemia after L-NAME, although femoral vascular resistance progressively increased and remained high during recovery. We conclude that in the llama fetus: 1) NO has an important role in maintaining a vasodilator tone during both normoxemia and hypoxemia in cerebral and femoral vascular beds and 2) during hypoxemia, NOS blockade unmasked the action of other vasodilator agents that contribute, with nitric oxide, to preserving blood flow and oxygen delivery to the tissues.     

Role of nitric oxide in the regulation of digital pulse volume amplitude in humans.

Measurement of the increase in digital pulse volume amplitude (PVA) during reactive hyperemia relative to baseline (PVA-RH) is being applied widely as a convenient test of nitric oxide bioavailability. However, evidence linking digital PVA-RH to nitric oxide is currently lacking. Accordingly, we investigated whether nitric oxide is responsible for the increase in digital PVA. During reactive hyperemia, we used a peripheral arterial tonometer to record digital PVA at baseline and during reactive hyperemia. The role of nitric oxide in these responses was investigated in 19 healthy subjects by inhibiting nitric oxide synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME). Ten subjects underwent the identical protocol with saline and five with phenylephrine, a nonspecific vasoconstrictor, instead of L-NAME. The change in digital PVA after drug administration was compared between the three groups. Relative to the response with saline (-5 +/- 2%), baseline PVA was unchanged by L-NAME infusion (-10 +/- 2%), but it decreased significantly with phenylephrine (-50 +/- 12%; P = 0.003). PVA-RH increased slightly with saline infusion (9 +/- 4%). In comparison, PVA-RH was significantly blunted by L-NAME administration (-46 +/- 21%; P = 0.002) and was relatively unchanged by phenylephrine (20 +/- 9%). The present study establishes a central role for nitric oxide in the augmentation of PVA during reactive hyperemia. The measurement of digital PVA-RH may indeed provide a simple means of assessing endothelial function in humans.     

Activity of nitric oxide synthase in the ventilatory muscle vasculature

We evaluated in the in situ vascularly isolated canine diaphragm the role of nitric oxide (NO) in the regulation of basal vascular resistance and vascular responses to increased muscle activity (active hyperemia), brief occlusions of the phrenic artery (reactive hyperemia), and changes in arterial pressure. The vasculature of the left hemidiaphragm was either pump-perfused at a fixed flow rate or autoperfused with arterial blood from the femoral artery. Endothelial nitric oxide synthase (NOS) activity was inhibited by intraphrenic infusion of L-arginine analogues such as N(G)-nitro-L-arginine, N(G)-nitro-L-arginine methyl ester and argininosuccinic acid. Active hyperemia was produced by low (2 Hz) frequency stimulation of the left phrenic nerve. Reactive hyperemia was measured in response to 10, 20, 30, 60, and 120 sec duration occlusions of the left phrenic artery and was quantified in terms of postocclusive blood flow, vascular resistance, hyperemic duration, and hyperemic volume. Infusion of NOS inhibitors into the vasculature of the resting diaphragm increased phrenic vascular resistance significantly and to a similar extent. Reactive hyperemic volume and reactive hyperemic duration were also significantly attenuated after NOS inhibition, however, peak reactive hyperemic dilation was not influenced by NOS inhibition. It was also found that enhanced NO release contribute by about 41% to active dilation elicited by continuous 2 Hz stimulation. In addition, NOS inhibition had no effect on O2 consumption of the resting diaphragm, but significantly attenuated the rise in diaphragmatic O2 consumption during during 2 Hz stimulation. The decline in diaphragmatic O2 consumption was due to reduction in blood flow. These results indicate that NO release plays a significant role in the regulation of diaphragmatic vascular tone and O2 consumption.     

Long-term inhibition of nitric oxide synthesis increases arterial thrombogenecity in rat carotid artery

Reduced activity of endothelial nitric oxide (NO) may be involved in thrombus formation on atherosclerotic plaques, a major cause of acute coronary syndrome. However, mechanisms of such increase in arterial thrombogenecity have not been fully understood. We previously reported that long-term inhibition of NO synthesis by administration of N(G)-nitro-L-arginine methyl ester (L-NAME) causes hypertension and activates vascular tissue angiotensin-converting enzyme (ACE) activity. We used this model to investigate the mechanism by which long-term impairment of NO activity increases arterial thrombogenecity. We observed cyclic flow variations (CFVs), a reliable marker of platelet thrombi, after the production of stenosis of the carotid artery in rats treated with L-NAME for 4 wk. The thrombin antagonist argatroban suppressed the CFVs. The CFVs were detected in rats receiving L-NAME plus hydralazine but not in rats receiving L-NAME plus an ACE inhibitor (imidapril). Treatment with the ACE inhibitor imidapril, but not with hydralazine, prevented L-NAME-induced increases in carotid arterial ACE activity and attenuated tissue factor expression. These results suggest that long-term inhibition of endothelial NO synthesis may increase arterial thrombogenecity at least in part through angiotensin II-induced induction of tissue factor and the resultant thrombin generation. These data provide a new insight as to how endothelial NO exhibits antithrombogenic properties of the endothelium.     

Blood flow of the right and left submandibular gland during unilateral carotid artery occlusion in rat: role of nitric oxide

The aim of the present study was to investigate the effect of unilateral carotid artery occlusion on the blood flow of submandibular gland in anesthetized rats and identify the role of nitric oxide (NO) in blood flow changes after the artery occlusion. L-NAME (N omega-nitro-L-arginine-methyl-ester; 10 mg/kg/day, per os) dissolved in tap water was used to block nitric oxide synthase. Glandular blood flow was measured using Sapirstein's indicator (86Rb) distribution technique. In the control animals the blood flow of left (ligated side) submandibular gland was lower than in the right (unligated side) one (right: 76.4+/-15.4 ml/min/100 g, 64.1+/-13.4 ml/min/100 g, p<0.01). The blood flow of submandibular glands decreased in NOS blocked group versus control. The vascular resistance after L-NAME treatment was elevated (control: 11+/-2.3 R/kg, L-NAME: 17.5+/-4.1 R/kg, p<0.001). In L-NAME group the difference between blood flow value of the left and right submandibular gland was significantly lower than in the control group (control: -16%, NAME: -8%, p<0.01). Conclusion: The maintenance of the blood flow in the left submandibular gland during ligation of the left common carotid artery could be due to the good vascular anastomotic system at these regions and adaptation of the submandibular vessels to the decreased perfusion pressure. Nitric oxide may have a role in the regulation of blood flow tinder this condition.     

Voltage-dependent K+ channels regulate the duration of reactive hyperemia in the canine coronary circulation

We previously demonstrated a role for voltage-dependent K(+) (K(V)) channels in coronary vasodilation elicited by myocardial metabolism and exogenous H(2)O(2), as responses were attenuated by the K(V) channel blocker 4-aminopyridine (4-AP). Here we tested the hypothesis that K(V) channels participate in coronary reactive hyperemia and examined the role of K(V) channels in responses to nitric oxide (NO) and adenosine, two putative mediators. Reactive hyperemia (30-s occlusion) was measured in open-chest dogs before and during 4-AP treatment [intracoronary (ic), plasma concentration 0.3 mM]. 4-AP reduced baseline flow 34 +/- 5% and inhibited hyperemic volume 32 +/- 5%. Administration of 8-phenyltheophylline (8-PT; 0.3 mM ic or 5 mg/kg iv) or N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/min ic) inhibited early and late portions of hyperemic flow, supporting roles for adenosine and NO. 4-AP further inhibited hyperemia in the presence of 8-PT or L-NAME. Adenosine-induced blood flow responses were attenuated by 4-AP (52 +/- 6% block at 9 microg/min). Dilation of arterioles to adenosine was attenuated by 0.3 mM 4-AP and 1 microM correolide, a selective K(V)1 antagonist (76 +/- 7% and 47 +/- 2% block, respectively, at 1 microM). Dilation in response to sodium nitroprusside, an NO donor, was attenuated by 4-AP in vivo (41 +/- 6% block at 10 microg/min) and by correolide in vitro (29 +/- 4% block at 1 microM). K(V) current in smooth muscle cells was inhibited by 4-AP (IC(50) 1.1 +/- 0.1 mM) and virtually eliminated by correolide. Expression of mRNA for K(V)1 family members was detected in coronary arteries. Our data indicate that K(V) channels play an important role in regulating resting coronary blood flow, determining duration of reactive hyperemia, and mediating adenosine- and NO-induced vasodilation.     

Nitric oxide synthase inhibition does not alter the reactive hyperemic response in the cutaneous circulation.

Reactive hyperemia is the sudden rise in blood flow after release of an arterial occlusion. Currently, the mechanisms mediating this response in the cutaneous circulation are poorly understood. The purpose of this study was to 1). characterize the reactive hyperemic response in the cutaneous circulation and 2). determine the contribution of nitric oxide (NO) to reactive hyperemia. Using laser-Doppler flowmetry, we characterized reactive hyperemia after 3-, 5-, 10-, and 15-min arterial occlusions in 10 subjects. The total hyperemic response was calculated by taking the area under the curve (AUC) of the hyperemic response minus baseline skin blood flow (SkBF) [i.e., total hyperemic response = AUC - [baseline SkBF as %maximal cutaneous vascular conductance (CVC(max) x duration of hyperemic response in s]]. For the characterization protocol, the total hyperemic response significantly increased as the period of ischemia increased from 5 to 15 min (P < 0.05). However, the 3-min response was not significantly different from the 5-min response. In the NO contribution protocol, two microdialysis fibers were placed in the forearm skin of eight subjects. One site served as a control and was continuously perfused with Ringer solution. The second site was continuously perfused with 10 mM NG-nitro-l-arginine methyl ester (l-NAME) to inhibit NO synthase. CVC was calculated as flux/mean arterial pressure and normalized to maximal blood flow (28 mM sodium nitroprusside). The total hyperemic response in control sites was not significantly different from l-NAME sites after a 5-min occlusion (3261 +/- 890 vs. 2907 +/- 531% CVC(max. s). Similarly, total hyperemic responses in control sites were not different from l-NAME sites (9155 +/- 1121 vs. 9126 +/- 1088% CVC(max. s) after a 15-min arterial occlusion. These data suggest that NO does not directly mediate reactive hyperemia and that NO is not produced in response to an increase in shear stress in the cutaneous circulation.     

Role of nitric oxide in capillary perfusion and oxygen delivery regulation during systemic hypoxia

The role of nitric oxide (NO) and reactive oxygen species (ROS) in regulating capillary perfusion was studied in the hamster cheek pouch model during normoxia and after 20 min of exposure to 10% O2-90% N2. We measured PO2 by using phosphorescence quenching microscopy and ROS production in systemic blood. Identical experiments were performed after treatment with the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) and after the reinfusion of the NO donor 2,2'-(hydroxynitrosohydrazono)bis-etanamine (DETA/NO) after treatment with L-NMMA. Hypoxia caused a significant decrease in the systemic PO2. During normoxia, arteriolar intravascular PO2 decreased progressively from 47.0 +/- 3.5 mmHg in the larger arterioles to 28.0 +/- 2.5 mmHg in the terminal arterioles; conversely, intravascular PO2 was 7-14 mmHg and approximately uniform in all arterioles. Tissue PO2 was 85% of baseline. Hypoxia significantly dilated arterioles, reduced blood flow, and increased capillary perfusion (15%) and ROS (72%) relative to baseline. Administration of L-NMMA during hypoxia further reduced capillary perfusion to 47% of baseline and increased ROS to 34% of baseline, both changes being significant. Tissue PO2 was reduced by 33% versus the hypoxic group. Administration of DETA/NO after L-NMMA caused vasodilation, normalized ROS, and increased capillary perfusion and tissue PO2. These results indicate that during normoxia, oxygen is supplied to the tissue mostly by the arterioles, whereas in hypoxia, oxygen is supplied to tissue by capillaries by a NO concentration-dependent mechanism that controls capillary perfusion and tissue PO2, involving capillary endothelial cell responses to the decrease in lipid peroxide formation controlled by NO availability during low PO2 conditions.     

Cyclooxygenase and nitric oxide synthase dependence of cutaneous reactive hyperemia in humans

We tested the hypothesis that cyclooxygenases (COXs) or COX products inhibit nitric oxide (NO) synthesis and thereby mask potential effects of NO on reactive hyperemia in the cutaneous circulation. We performed laser-Doppler flowmetry (LDF) with intradermal microdialysis in 12 healthy volunteers aged 19-25 yr. LDF was expressed as the percent cutaneous vascular conduction (%CVC) or as the maximum %CVC (%CVC(max)) where CVC is LDF/mean arterial pressure. We tested the effects of the nonisoform-specific NO synthase inhibitor nitro-L-arginine (NLA, 10 mM), the nonspecific COX inhibitor ketorolac (Keto, 10 mM), combined NLA + Keto, and NLA + sodium nitroprusside (SNP, 28 mM) on baseline and reactive hyperemia flow parameters. We also examined the effects of isoproterenol, a beta-adrenergic agonist that causes prostaglandin-independent vasodilation to correct for the increase in baseline flow caused by Keto. When delivered directly into the intradermal space, Keto greatly augments all aspects of the laser-Doppler flow response to reactive hyperemia: peak reactive hyperemic flow increased from 41 +/- 5 to 77 +/- 7%CVC(max), time to peak flow increased from 17 +/- 3 to 56 +/- 24 s, the area under the reactive hyperemic curve increased from 1,417 +/- 326 to 3,376 +/- 876%CVC(max).s, and the time constant for the decay of peak flow increased from 100 +/- 23 to 821 +/- 311 s. NLA greatly attenuates the Keto response despite exerting no effects on baseline LDF or on reactive hyperemia when given alone. Low-dose NLA + SNP duplicates the Keto response. Isoproterenol increased baseline and peak reactive flow. These results suggest that COX inhibition unmasks NO dependence of reactive hyperemia in human cutaneous circulation.     

The effect of local nitric oxide synthase inhibition on the diameter of pulpal arteriole in dental bond material-induced vasodilation in rat

Local application of dental bond materials can cause pulpal vasodilation and hyperemia. Such local hemodynamic changes may be mediated by alterations in the levels of locally generated nitric oxide (NO). In different species systemic administration of NO synthase inhibitors leads to a decrease in pulpal blood flow. In contrast, the local administration of these inhibitors has not been tested yet. Therefore, the effect of locally blocked NO synthase on the internal diameter of rat pulpal arterioles under basal conditions and immediately after dental bond material application was studied by using vitalmicroscopic technique. The NO synthase blocker (L-NAME) was locally administered on a thinned dentine layer of the left lower incisor. L-NAME reduced the diameter of the pulpal arteriole both in basal and after bond material-induced hyperaemic conditions. These data suggest that the local formation of NO may have a significant role in the acute vasodilation induced by bond material application and also in maintenance of basal pulpal arteriolar tone.     

Direct detection of nitric oxide and its roles in maintaining gastric mucosal integrity following ethanol-induced injury in rats

In gastric mucosal injury, nitric oxide (NO) plays both cytoprotective and cytotoxic roles, and the NO level is one determinant of these dual roles. We employed electron paramagnetic resonance (EPR)-spectrometry combined with an NO-trapping technique to directly evaluate NO production in ethanol-induced gastric injury in rats. The rat stomach, mounted on an ex vivo chamber, was perfused with ethanol (12.5 and 43%), and NO levels in mucosal tissues were measured during perfusion. Luminal nitrite/nitrate (NO_x) content, mucosal blood flow, area of mucosal injury, transmucosal potential difference (PD), and luminal pH were simultaneously monitored with/without preadministration of the NO synthase inhibitor, N^G-nitro-L-arginine methyl ester (L-NAME). NO levels in the gastric tissue increased during ethanol perfusion, and luminal NO_x levels increased after the perfusion, accompanying an increase in the area of mucosal injury and changes in physiological parameters. Preadministration of L-NAME aggravated the gastric mucosal damage and suppressed increases in mucosal blood flow in a dose-dependent manner. These results demonstrate that endogenous NO produced in ethanol-induced gastric injury contributes to maintenance of mucosal integrity via regulation of mucosal blood flow.     

High-calcium diet enhances vasorelaxation in nitric oxide-deficient hypertension

Because the effects of calcium supplementation on arterial tone in nitric oxide-deficient hypertension are unknown, we investigated the influence of elevating dietary calcium from 1.1 to 3.0% in Wistar rats treated with N(G)-nitro-L-arginine methyl ester (L-NAME; 20 mg. kg(-1). day(-1)) for 8 wk. A high-calcium diet attenuated the development of hypertension induced by L-NAME and abrogated the associated impairments of endothelium-independent mesenteric arterial relaxations to nitroprusside, isoproterenol, and cromakalim. Endothelium-dependent relaxations to acetylcholine during nitric oxide synthase inhibition in vitro were decreased in L-NAME rats and improved by calcium supplementation. The inhibition of cyclooxygenase by diclofenac augmented the responses to acetylcholine in L-NAME rats but not in calcium + L-NAME rats. When hyperpolarization of smooth muscle was prevented by KCl precontraction, the responses to acetylcholine during combined nitric oxide synthase and cyclooxygenase inhibition were similar in all groups. Furthermore, superoxide dismutase enhanced the acetylcholine-induced relaxations in L-NAME rats but not in calcium + L-NAME rats. In conclusion, calcium supplementation reduced blood pressure during chronic nitric oxide synthase inhibition and abrogated the associated impairments in endothelium-dependent and -independent arterial relaxation. The augmented vasorelaxation after increased calcium intake in L-NAME hypertension may be explained by enhanced hyperpolarization and increased sensitivity to nitric oxide in arterial smooth muscle and decreased vascular production of superoxide and vasoconstrictor prostanoids.     

Nitric oxide regulation of lingual blood flow in the rat.

The purpose of this study was to determine the role of nitric oxide in the maintenance of basal lingual blood flow in the anesthetized rat. By using laser-Doppler flowmetry, blood flow was measured from the tongue before and after treatment with the nonselective inhibitor of nitric oxide synthase, L-NAME (0.2, 2.0, and 20 mg/kg), or the selective neuronal nitric oxide synthase inhibitor, 7-nitroindazole (40 mg/kg). Other groups of rats were treated with saline, D-NAME (2.0 mg/kg), L-arginine (200 mg/kg), L-arginine + L-NAME (200 + 2.0 mg/kg), or the 7-nitroindazole vehicle. L-NAME produced a dose-related depression in blood flow in the tongue (concurrent with increased arterial blood pressure), which was attenuated by prior administration of L-arginine. Lingual blood flow depression was not seen after administration of the inactive stereoisomer, D-NAME. In addition, the neuronally specific nitric oxide synthase inhibitor, 7-nitroindazole, failed to produce a significant depression of lingual blood flow. These results suggest that the tonic release of nitric oxide from the vascular endothelium plays an important role in maintaining basal blood flow in the tongue and that neuronally released nitric oxide is not involved in maintaining basal circulation in this vascular bed.     

Effect of sildenafil on coronary active and reactive hyperemia

Sildenafil, a selective inhibitor of phosphodiesterase type 5, produces relaxation of isolated epicardial coronary artery segments by causing accumulation of cGMP. Because shear-induced nitric oxide-dependent vasodilation is mediated by cGMP, this study was performed to determine whether sildenafil would augment the coronary resistance vessel dilation that occurs during the high-flow states of exercise or reactive hyperemia. In chronically instrumented dogs, sildenafil (2 mg/kg per os) augmented the vasodilator response to acetylcholine, with a leftward shift of the dose-response curve relating coronary flow to acetylcholine dose. Sildenafil caused a 6. 7 +/- 2.1 mmHg decrease of mean aortic pressure, which was similar at rest and during treadmill exercise (P < 0.05), with no change of heart rate, left ventricular (LV) systolic pressure, or LV maximal first time derivative of LV pressure. Sildenafil tended to increase myocardial blood flow at rest and during exercise (mean increase = 14 +/- 3%; P < 0.05 by ANOVA), but this was associated with a significant decrease in hemoglobin, so that the relationship between myocardial oxygen consumption and oxygen delivery to the myocardium (myocardial blood flow x arterial O(2) content) was unchanged. Furthermore, sildenafil did not alter coronary venous PO(2), indicating that the coupling between myocardial blood flow and myocardial oxygen demands was not altered. In addition, sildenafil did not alter the peak coronary flow rate, debt repayment, or duration of reactive hyperemia that followed a 10-s coronary occlusion. The findings suggest that cGMP-mediated resistance vessel dilation contributes little to the increase in myocardial flow that occurs during exercise or reactive hyperemia.     

Nitric oxide modulates oxygen consumption by arteriolar walls in rat skeletal muscle

To study the role of nitric oxide (NO) in regulating oxygen consumption by vessel walls, the oxygen consumption rate of arteriolar walls in rat cremaster muscle was measured in vivo during flow-induced vasodilation and after inhibiting NO synthesis. The oxygen consumption rate of arteriolar walls was calculated based on the intra- and perivascular PO2 values measured by phosphorescence quenching laser microscopy. The perivascular PO2 value of the arterioles during vasodilation was significantly higher than under control conditions, although the intravascular PO2 values under both conditions were approximately the same. Inhibition of NO synthesis, on the other hand, caused a significant increase in arterial blood pressure and a significant decrease in arteriolar diameter. Inhibition of NO synthesis also caused a significant decrease in both the intra- and perivascular PO2 values of the arterioles. Inhibition of NO synthesis increased the oxygen consumption rate of the vessel walls by 42%, whereas enhancement of flow-induced NO release decreased it by 34%. These results suggest that NO plays an important role not only as a regulator of peripheral vascular tone but also as a modulator of tissue oxygenation by reducing oxygen consumption by vessel walls. In addition, enhancement of NO release during exercise may facilitate efficient oxygen supply to the surrounding high metabolic tissue.     

The effect of L-NAME administrations after oral mucosal incision on wound NO level in rabbit

The aim of the current study was to comparatively investigate the effect of inhibition of nitric oxide (NO) production by N-nitro-L-arginine methyl ester (L-NAME), an isoform non-specific inhibitor of nitric oxide synthase (NOS), after oral mucosal incision on wound tissue NO levels. A standard incision was applied to the oral mucosa of rabbits. After oral mucosal incision, rabbits were divided into five groups as follows: (1) Untreated incisional group (control); (2) Titanium (Ti) implanted group; (3) Ti + Polyethylene glycol (PEG) 4000 implanted group; (4) Ti + PEG 4000 + L-NAME (2 × 10⁻⁴ M) implanted group and (5) i.p. L-NAME administrated group (10 mg/kg). At 5 days after oral incision operations, wound tissue strips and plasma were obtained from rabbits. Oral wound tissue and plasma nitric oxide, plasma thiobarbituric acid reactive substances (TBARS) and total sulfhydryl group (RSH) levels were investigated. Plasma TBARS and NOx levels decreased after i.p. L-NAME administration. Total RSH group levels were not changed in all groups (p>0.05). This means that L-NAME inhibits the deteriorating effects of free radicals without affecting healing. L-NAME in PEG and titanium also has no effect on tissue and plasma NOx levels. These findings indicate that NO generation will not be affected both Ti and local nitric oxide synthase (NOS) inhibitor. (Mol Cell Biochem 278: 65–69, 2005)     

Contribution of anaerobic metabolism to reactive hyperemia in skeletal muscle

Elevated blood flow (reactive hyperemia) is seen in many organs after a period of blood flow stoppage. This hyperemia is often considered to be due in part to a shift to anaerobic metabolism during tissue hypoxia. The aim of our study was to test this hypothesis in skeletal muscle. For this purpose we measured NADH fluorescence at localized tissue areas in cat sartorius muscle during and after arterial occlusions of 5-300 s. In parallel studies, red blood cell (RBC) velocity was measured in venules. Tissue NADH fluorescence rose significantly with occlusions of 45 s or greater, reaching a maximum of 44% above control at 180 s. Peak RBC velocity rose to four times control as occlusion duration was increased from 5 to 45 s, but hyperemia duration was stable at approximately 70 s. With occlusions of 45-240 s, hyperemia duration increased progressively to 210 s while peak flow was unchanged. However, after 300-s occlusions, peak flow rose to six times above control and hyperemia duration fell to 140 s. With occlusions of 45-300 s the time integral both of increased NADH fluorescence and of reduced fluorescence following occlusion release showed a high degree of correlation with the additional hyperemia. We conclude that in this muscle anaerobic vasodilator metabolites are responsible for the increase in reactive hyperemia with arterial occlusions longer than 45 s. Since the durations of reactive hyperemia and reduced fluorescence are substantially different, vasodilator metabolite removal may be due to washout by the bloodstream rather than metabolic uptake.     

Cerebral blood flow modulates hyperbaric oxygen induced neurotoxicity by neuronal and endothelial nitric oxide

The goal of work was to reveal changes in microcirculation of the rat brain and the role of nitric oxide (NO) in development of seizures at hyperbaric oxygen exposure. The Wistar rats with implanted paired platinum electrodes in left and right striatum were used for experiments. The latency of seizures was defined by the EEG, the cerebral blood flow (CBF) was measured by hydrogen clearance. One group of animals was exposed to a 5-ata oxygen, while the others before oxygen treatment were injected with: Nw-nitro-L-arginine methyl ester (L-NAME), blockator of constitutive NO synthase; 7-nitroindozol (7NI), specific inhibitor of neural NO synthase. The latency of seizures was 41 +/- 1.9 min at 5 ata oxygen exposure. CBF was decreased to 10-14% but before seizures it increased to 23 +/- 9%. L-NAME and 7NI prevented development of hyperoxygen hyperemia and onset of seizures. The results indicate occurrence of hyperbaric oxygen changes of the CBF that modulate neurotoxic effects of NO in neurons as well as in cerebral vessels.     

Preservation of neurological functions by nitric oxide synthase inhibitors in conscious rats following delayed hemorrhagic shock

Excessive production of nitric oxide (NO) as result of inducible nitric oxide synthase (iNOS) induction has been implicated in the pathophysiology of hemorrhagic shock. Our aim was to study the effects of NOS inhibitors, aminoguanidine (AG) and NG-nitro-L-arginine methyl ester (L-NAME), on survival rate, mean arterial blood pressure (MABP), temporal evolution of infarct volume, nitric oxide (NO) production and neurological deficit in a model of delayed hemorrhagic shock (DHS) in conscious rats. Our results showed that the NOS inhibitors significantly improved survival rate, MABP, and attenuated brain NO overproduction 24, 48 h and 72 h after DHS. AG reduced brain infarct volume and improved the neurological performance evaluated by the rotameric and grip strength tests while L-NAME did not show protective effect in rats following DHS. These findings suggest that NO formation via iNOS activation may contribute to organ damage and that the selective iNOS inhibitor, AG, may be of interest as a therapeutic agent for neurological recovery following DHS.     

Local inhibition of nitric oxide temporarily stimulates aldosterone secretion in conscious sheep in vivo

The effect of localized blockage of endogenous nitric oxide (NO) on basal aldosterone secretion was studied in conscious sheep with autotransplanted adrenal glands. We have shown that infusion of the NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 130 microg/l blood flow) significantly stimulated basal aldosterone secretion rate (ASR). This stimulatory effect was seen up to 4 h of infusion. Beyond this time point, however, the elevated ASR level was not sustained, and it was seen to drop markedly to lower than control values at 5 h. L-NAME had no effect on cortisol secretion rate (FSR) during the first 4 h of infusion, but a significant reduction in FSR was seen by the 8-h time point. Adrenal blood flow was consistently decreased in association with long L-NAME infusion. Additionally, L-NAME was shown to have no effect on aldosterone secretion when infused systemically. We conclude that the relationship between NO and aldosterone secretion is an inhibitory one, in which NO seems to have a negative effect on basal aldosterone secretion.