The Roles of Nitric Oxide and Carbon Dioxide Gas in the Neurotoxic Actions of Oxygen under Pressure

The hypothesis that in conditions of hyperbaric oxygenation, nitric oxide (NO) modulates the vasodilatory effect of CO₂ in the brain and thus accelerates the neurotoxic action of oxygen was verified experimentally. Conscious rats breathed atmospheric air or oxygen at 5 atm and blood flow in the striatum was measured before and after inhibition of carbonic anhydrase with acetazolamide, which causes retention of CO₂ in the brain. Acetazolamide (35 mg/kg) increased blood flow in the animals when breathing air by 38 ± 7.4% (p < 0.01), while preliminary inhibition of NO synthase with N^ω-nitro-L-arginine-methyl ester (L-NAME, 30 mg/kg) significantly weakened its vasodilatory action. Inhibition of carbonic anhydrase in animals breathing hyperbaric oxygen at 5 atm prevented cerebral vasoconstriction, increased brain blood flow, and accelerated the development of oxygen convulsions. The vasodilatory effect of acetazolamide in hyperbaric oxygenation was significantly reduced in animals pretreated with the NO synthase inhibitor, such that the latent period of convulsions increased. The results obtained here provide evidence that in conditions of extreme hyperoxia, NO modulates the cerebral hyperemia developing in conditions of CO₂ retention in the brain and accelerates the development of the neurotoxic actions of hyperbaric oxygen.__________Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 90, No. 4, pp. 428–436, April, 2004.     

Hyperoxic Vasoconstriction in the Brain Is Mediated by Inactivation of Nitric Oxide by Superoxide Anions

The hypothesis that decreases in brain blood flow during respiration of hyperbaric oxygen result from inactivation of nitric oxide (NO) by superoxide anions (O_{2 –}) is proposed. Changes in brain blood flow were assessed in conscious rats during respiration of atmospheric air or oxygen at a pressure of 4 atm after dismutation of O_{2 –} with superoxide dismutase or suppression of NO synthesis with the NO synthase inhibitor L-NAME. I.v. administration of superoxide dismutase increased brain blood flow in rats breathing air but was ineffective after previous inhibition of NO synthase. Hyperbaric oxygenation at 4 atm induced decreases in brain blood flow, though prior superoxide dismutase prevented hyperoxic vasoconstriction and increased brain blood flow in rats breathing hyperbaric oxygen. The vasodilatory effect of superoxide dismutase in hyperbaric oxygenation was not seen in animals given prior doses of the NO synthase inhibitor. These results provide evidence that one mechanism for hyperoxic vasoconstriction in the brain consists of inactivation of NO by superoxide anions, decreasing its basal vasorelaxing action.     

Effect of indomethacin on cerebral blood flow and development of oxygen convulsions

Hyperbaric oxygenation modulates cerebral blood flow affecting the development of oxygen convulsions. Before hyperbaric oxygenation-induced convulsions in rats the initial decrease in blood flow gave place to hyperemia, Po₂ increased. In rats receiving cyclooxygenase inhibitor indomethacin no convulsions were observed, blood flow and Po₂ were lower than in controls. Our results indicate that indomethacin prevents hyperemia and alleviates oxygen convulsions under conditions of hyperbaric oxygenation. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 142, No. 7, pp. 31–33, July, 2006     

Effect of hyperbaric oxygen treatment on nitric oxide and oxygen free radicals in rat brain

Oxygen (O(2)) at high pressures acts as a neurotoxic agent leading to convulsions. The mechanism of this neurotoxicity is not known; however, oxygen free radicals and nitric oxide (NO) have been suggested as contributors. This study was designed to follow the formation of oxygen free radicals and NO in the rat brain under hyperbaric oxygen (HBO) conditions using in vivo microdialysis. Male Sprague-Dawley rats were exposed to 100% O(2) at a pressure of 3 atm absolute for 2 h. The formation of 2,3-dihydroxybenzoic acid (2, 3-DHBA) as a result of perfusing sodium salicylate was followed as an indicator for the formation of hydroxyl radicals. 2,3-DHBA levels in hippocampal and striatal dialysates of animals exposed to HBO conditions were not significantly different from controls. However, rats treated under the same conditions showed a six- and fourfold increase in nitrite/nitrate, break down products of NO decomposition, in hippocampal and striatal dialysates, respectively. This increase was completely blocked by the nitric oxide synthase (NOS) inhibitor L-nitroarginine methyl ester (L-NAME). Using neuronal NOS, we determined the NOS O(2) K(m) to be 158 +/- 28 (SD) mmHg, a value which suggests that production of NO by NOS would increase approximately four- to fivefold under hyperbaric O(2) conditions, closely matching the measured increase in vivo. The increase in NO levels may be partially responsible for some of the detrimental effects of HBO conditions.     

Decreased renal haemodynamic response to inhibition of nitric oxide synthase in subtotally nephrectomized rats

To assess the renal haemodynamic response to manipulations of the nitric oxide (NO) system, we examined subtotally nephrectomized (SNX) rats and control rats (CON) 28 days after their operation. Bolus infusions of the NO synthase inhibitor N ^G-nitro-l-arginine (l-NA) were given intravenously at doses of 2 mg/kg and 10 mg/kg. Blood pressure was measured intra-arterially, glomerular filtration rate was measured by inulin clearance and fractional changes in renal blood flow (RBF) were determined by a Doppler flow probe. Both doses of l-NA caused a similar and dose dependent increase in mean blood pressure in both SNX and CON rats. In contrast, the decrease in RBF and the increase in the renovascular resistance index (RVRI) was less in SNX rats as compared to CON rats (RBF = –70.1±2.2% of baseline vs –52.7±5.2%, P<0.01; RVRI = +177±9% of baseline vs +243±24%, P<0.05). These changes were not affected by autonomic blockade (hexamethonium), or by blockade of the angiotensin II receptor (Losartan). The exogenous NO donor sodium nitroprusside (0.5 and 1.5 g · kg^–1 · min^–1) lowered mean blood pressure to a similar degree in SNX and CON rats; in contrast, RVRI decreased less in SNX rats (86.9±9.2% of baseline) than in CON rats (68.2±4.6%, P<0.05). We conclude that the reaction of the renal vasculature to manipulations of the NO system is altered in the SNX rats. The data suggest that in the remnant kidney, renovascular resistance is less dependent on endogenous NO and the vascular bed is less sensitive to exogenous NO.     

Endogenous nitric oxide and pulmonary circulation response to hypoxia in high-altitude adapted Tibetan sheep

Nitric oxide (NO) is important for the pulmonary circulation response to acute and chronic hypoxia. We examined effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary vascular tone in response to hypoxia in Tibetan sheep dwelling at 3,000 m above sea level using a pressure chamber. Unanaesthetized male sheep living at 2,300 m above sea level (n=7) were prepared for vascular monitoring. Pulmonary artery (P_pa), pulmonary artery wedge (P_cwp) and systemic artery pressures together with cardiac output (CO) were measured, and pulmonary vascular resistance (PVR) was calculated as (P_pa–P_cwp)/CO. A non-selective NOS inhibitor, N-nitro-l-arginine (NLA; 20 mg kg^–1), and a selective NOS inhibitor, ONO-1714 (0.1 mg kg^–1), were used and measurements were made at 0 m, 2,300 m, and 4,500 m, with and without the NOS inhibitors. After NLA, P_pa increased slightly and CO decreased in animals at baseline (2,300 m). The increased PVR after NLA at 4,500 m was greater than that at 2,300 m (P<0.05). Selective NOS inhibition increased PVR at baseline, but not at 4,500 m. The enhanced pulmonary vasoconstriction after NO inhibition at basal and hypoxic conditions suggests a modulating role of NOS bioactivity in the pulmonary circulation and that augmented endothelial NOS plays a counterregulatory role in the pulmonary vasoconstrictor response to acute hypoxia in high-altitude adapted Tibetan sheep.     

Role of nitric oxide and adenosine in the onset of vasodilation during dynamic forearm exercise

We tested the hypothesis that nitric oxide (NO) and adenosine contribute to the onset of vasodilation during dynamic forearm exercise. Twenty-two subjects performed rhythmic forearm exercise (20 % of maximum) during control and NO synthase (NOS) inhibition (N ^G-monomethyl-l-arginine; l-NMMA) trials. A subset of subjects performed a third trial of forearm exercise during combined inhibition of NOS and adenosine (aminophylline; n = 9). Additionally, a separate group of subjects (n = 7) performed rhythmic forearm exercise during control, inhibition of adenosine alone and combined inhibition of adenosine and NOS. Forearm vascular conductance (FVC; ml min^?1 · 100 mmHg^?1) was calculated from blood flow and mean arterial pressure (mmHg). The onset of vasodilation was assessed by calculating the slope of the FVC response for every duty cycle between baseline and steady state, and the number of duty cycles (1-s contraction/2-s relaxation) to reach steady state. NOS inhibition blunted vasodilation at the onset of exercise (11.1 ± 0.8 vs. 8.5 ± 0.6 FVC units/duty cycle; P < 0.001 vs. control) and increased the time to reach steady state (25 ± 1 vs. 32 ± 1 duty cycles; P < 0.001 vs. control). Vasodilation was blunted further with combined inhibition of NOS and adenosine (7.5 ± 0.6 vs. 6.2 ± 0.8 FVC units/duty cycle; P < 0.05 vs. l-NMMA alone), but not with aminophylline alone (16.0 ± 2.2 vs. 14.7 ± 2.0 FVC units/duty cycle; P = 0.67 vs. control). Our data indicate that NO and adenosine (in the absence of NO) contribute to the onset of vasodilation during dynamic forearm exercise.     

Adenosine and nitric oxide in exercise‐induced human skeletal muscle vasodilatation

The vasoactive substances adenosine and nitric oxide (NO) are credible candidates in the local regulation of skeletal muscle blood flow. Adenosine and NO have both been shown to increase in skeletal muscle cells and interstitial fluid during exercise and the enzymes responsible for their formation, AMP 5′‐nucleotidase and NO synthase (NOS), have been shown to be activated upon muscle contraction. In vitro as well as in vivo evidence suggest that the contraction‐induced increase in interstitial adenosine concentration largely stems from extracellular formation via the membrane‐bound ecto‐form of AMP 5′‐nucleotidase. It remains unclear whether the exercise‐induced NO formation in muscle originates from endothelial NOS in the microvascular endothelium, or from neuronal NOS (nNOS) in nerve cells and muscle fibres. Functional evidence for the role of adenosine in muscle blood flow control stems from studies using adenosine receptor agonists and antagonsits, adenosine deaminase or adenosine uptake inhibitors. The majority of these studies have been performed on laboratory animals and, although the results show some discrepancy, the majority of studies indicate that adenosine does participate in the regulation of muscle blood flow. In humans, evidence is lacking. The role of NO in the regulation of skeletal muscle blood flow has mainly been studied using NOS inhibitors. Despite a large number of studies in this area, the role of NO for the contraction‐induced increase in skeletal muscle blood flow is uncertain. The majority, but not all, human and animal studies show that, whereas blockade of NOS reduces muscle blood flow at rest and in recovery from exercise, there is no effect on the exercise‐induced increase in muscle perfusion. Conclusive evidence for the mechanisms underlying the precise regulation of the multiphased increase in skeletal muscle blood flow during exercise and the role and potency of various vasoactive substances, remain missing.     

Non-lethal Sepsis and Nitric Oxide Inhibition in the Guinea Pig

Nitric oxide (NO) is synthesized from arginine by three distinct isoforms of nitric oxide synthase (NOS). Two of these isoforms, endothelial NOS (eNOS, type III NOS) and neuronal NOS (nNOS, type I NOS) are expressed in a constitutive manner and are responsible for regulating physiological functions. The induction of the third isoform of nitric oxide synthase (iNOS) by inflammatory processes and the subsequent overproduction of NO is thought to contribute to the tissue damage that occurs in a number of diseases having an inflammatory component, such as sepsis. As a model of sepsis, non-lethal endotoxemia in Hartley guinea pigs was induced using three serotypes of lipopolysaccharide (LPS, 30 mg/kg, i.p.). Each stimulated an increase in total plasma nitrites 6 h following their administration. Treatment orally with two inhibitors of NOS, L-nitroarginine methyl ester (L-NAME), a non-selective inhibitor and N-iminoethyl-L-lysine (L-NIL), a selective iNOS inhibitor, 30 min after the induction of sepsis, inhibited (p < 0.05) the increase in plasma nitrites with ED₅₀ values of 7 ± 1 and 0.4 ± 0.03 mg/kg, respectively, suggesting that NOS inhibitors may be useful in the treatment of human sepsis, or other diseases where excess NO levels have been implicated, such as asthma, arthritis and inflammatory bowel disease.     

Development of the nitric oxide/cGMP system in the embryonic and juvenile pond snail,Lymnaea stagnalis L. A comparative in situ hybridization,histochemical and immunohistochemical study

Recent studies have indicated that nitric oxide (NO)-induced cGMP synthesis is involved in different steps of neurogenesis in invertebrates. The development of putative NO synthetising elements was described earlier in the embryonic and juvenile pond snail, Lymnaea stagnalis, applying NADPH-diaphorase histochemistry (Serfz et al., 1998). In the present study, we examined the distribution of NO synthase (NOS) during Lymnaea development by in situ hybridization for Lymnaea-NOS mRNA, histochemical, and immunohistochemical techniques for the NOS and NO-stimulated cGMP. Peripheral fibers projecting to the CNS and terminating in the ganglionic neuropils showed NOS immunoreactivity from 85% of embryonic development. At the same time, a fine dot-like, immunostaining indicated the presence of cGMP in the neuropil area. In the CNS, Lymnaea-NOS mRNA positive, as well as NOS and cGMP immunoreactive perikarya were detected first during postembryonic development; their number significantly increased from P3 juvenile stage. Some of the cell groups in the CNS containing NOS immunoreactive material also displayed Lymnaea-NOS mRNA hybridization signal and were cGMP-positive. However, in the subesophageal ganglia, the distribution of Lymnaea-NOS mRNA positive cell groups did not correspond to that of the NOS immunoreactive cells. Neurons revealing transient NOS and cGMP immunoreactivity, respectively, could also be detected in this part of the CNS. In most of the ganglia the number of Lymnaea-NOS mRNA containing and cGMP immunopositive neurons, respectively, exceeded that of the NOS immunoreactive cells from P4 juvenile stage. The localization of NADPH-diaphorase reaction also correlated well with that of the NOS immunoreactivity in the developing CNS. At the periphery, colocalization of Lymnaea-NOS mRNA signal, NOS and cGMP immunoreactivities were observed in the epithelial cells of the esophagus and mantle after hatching. The findings suggest the functional maturity of the NO/cGMP signal transduction pathway at both central and peripheral levels during the development of the snail, Lymnaea stagnalis. The differences in the localization of Lymnaea-NOS mRNA labeling and NOS immunoreactivity in the CNS and PNS can be explained by the existence of different NOS isoforms, posttranslational regulation of NOS, and/or some non-specific antibody labeling.     

高压氧对脑缺血再灌注小鼠明胶酶、一氧化氮合酶及血脑屏障的影响

目的:通过检测脑缺血再灌注小鼠明胶酶A(MMP－2)、明胶酶B(MMP－9)及一氧化氮合酶(NOS)活性,探讨高压氧(HBO)对脑缺血再灌注小鼠明胶酶、NOS及血脑屏障(BBB)通透性的影响。方法:复制脑缺血再灌注模型,并于再灌注期间经0.25MPaHBO治疗5次,采用明胶酶谱分析法及比色法,检测脑组织海马区明胶酶及NOS的活性,同时经尾静脉注射2%伊文思兰(EB),检测脑组织EB含量。结果:脑缺血再灌注后明胶酶(A、B)活性增加,HBO+脑缺血再灌注组明胶酶B(MMP－9)活性明显低于脑缺血再灌注组;而HBO对明胶酶A(MMP－2)作用不显著。脑缺血再灌注后NOS活性增加,HBO+脑缺血再灌注组NOS含量显著低于脑缺血再灌注组(P＜0.01)。脑组织EB含量以再灌注4h最高,11h、23h、48h、72h脑组织EB含量逐渐减少。高压氧+脑缺血再灌注组脑组织EB含量明显低于脑缺血再灌注组(P＜0.05,P＜0.01)。结论:高压氧具有降低脑缺血再灌注小鼠明胶酶B及NOS活性,降低血脑屏障的通透性的作用。     

Effects of 7-nitroindazole and N-nitro-l-arginine methyl ester on changes in cerebral blood flow and nitric oxide production preceding development of hyperbaric oxygen-induced seizures in rats

Hyperbaric oxygen (HBO(2)) exposure induces increases in cerebral blood flow (CBF) and extracellular concentrations of nitric oxide (NO) that precede the appearance of central nervous system toxicity, which may manifest as convulsions. To elucidate the origins of NO production during HBO(2) exposure, we examined the effects of the selective neuronal NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), and the non-selective NOS inhibitor, N-nitro-l-arginine methyl ester (l-NAME), on changes in CBF and NO metabolites (NO(x), nitrite and nitrate) using a laser Doppler flow probe and in vivo microdialysis techniques, respectively. Rats were anesthetized, artificially ventilated, and pressurized to 5 atmosphere absolute (ATA) with pure oxygen for 60 min. In rats treated with vehicle, CBF and NO(x) levels in the cortex increased to 201% and 239% of basal levels, respectively, before the onset of electrical discharges, measured by electroencephalogram. The increase in CBF and NO(x) was completely inhibited by 7-NI and l-NAME. Both drugs also inhibited the appearance of electrical discharges for 60 min. Dynamic changes in CBF and NO(x) were not significantly different between 7-NI and l-NAME. These findings suggest that neuronal NOS is the main mediator of NO production associated with increase in CBF leading to the appearance of electrical discharge during HBO(2) exposure.     

Oral contraceptive administration aggravates nitric oxide synthesis inhibition-induced high blood pressure in female rats

The use of estrogen–progestogen oral contraceptive (OC) is associated with high blood pressure, although mechanisms responsible are still unclear. This study sought to investigate the effects of administration of OC on high blood pressure resulting from nitric oxide (NO) synthesis inhibition in female Sprague–Dawley rats. Rats were given ethinyl estradiol in combination with norgestrel and were treated with NO synthase inhibitor, N^G-nitro-l-arginine methyl ester (l-NAME) in the drinking water or drinking water alone for 6 weeks. OC treatment alone led to a significant increase in blood pressure and positive water balance. Treatment with l-NAME alone resulted in a significant elevation of blood pressure without significant positive water balance. Concomitant treatment with OC and l-NAME produced significant increases in blood pressure and water balance. These magnitudes of increases were significantly greater than those observed in rats treated with OC or l-NAME alone. Treatment with OC did not affect NO biosynthesis with or without concurrent l-NAME treatment. Treatment with OC and/or l-NAME did not significantly affect body weight, food intake, heart rate, cardiac weight/body weight ratio, plasma sodium, glomerular filtration rate and urinary sodium output. Conclusion: These data demonstrate that OC administration resulted in a modest increased blood pressure via enhanced water retention that was not associated with impaired NO synthesis. On the other hand, these results showed that increased blood pressure induced by inhibition of NO synthesis was not associated with water retention. The study also indicated that OC administration aggravated increase in blood pressure during NO synthesis inhibition, via enhanced water retention.     

Pharmacological interventions in the newborn piglet in the first 24 h after hypoxia-ischemia. A hemodynamic and electrophysiological perspective

The purpose of this study was to investigate whether combined inhibition of neuronal and inducible nitric oxide synthase (NOS) by 2-iminobiotin, free radical scavenging by allopurinol, and non-protein-bound iron chelation with deferoxamine improved cerebral oxygenation, electrocortical brain activity, and brain energy status during the first 24 h after hypoxia-ischemia (HI) in the newborn piglet. Forty-three newborn piglets were subjected to 1 h of severe HI by occluding both carotid arteries and phosphorous magnetic resonance spectroscopy (³¹P-MRS)-guided hypoxia, whereas five served as sham-operated controls. Upon reperfusion, piglets received vehicle (n=12), 2-iminobiotin (n=11), allopurinol (n=10), or deferoxamine (n=10). Cerebral oxygenation was recorded with near-infrared spectrophotometry (NIRS), electrocortical brain activity was assessed with amplitude-integrated EEG (aEEG), and cerebral energy status with ³¹P-MRS. The oxygenated hemoglobin (HbO₂) and total hemoglobin (tHb) were significantly increased in vehicle-treated piglets compared with 2-iminobiotin-treated and deferoxamine-treated piglets. No change in deoxygenated Hb (HHb) was demonstrated over time. The aEEG was significantly preserved in 2-iminobiotin- and deferoxamine-treated piglets compared with vehicle-treated piglets. Allopurinol treatment was not as effective as 2-iminobiotin treatment after HI. Phosphocreatine/inorganic phosphate ratios (PCr/P_i) were significantly decreased for vehicle-treated piglets at 24 h post-HI, whereas 2-iminobiotin, allopurinol, and deferoxamine prevented the development of secondary energy failure. We speculate that the beneficial effects, especially of 2-iminobiotin, but also of deferoxamine, are due to reduced peroxynitrite-mediated oxidation. Electronic Publication     

Blood flow velocity in the common carotid artery in humans during graded exercise on a treadmill

Cerebral blood volume flow and flow velocity have been reported to increase during dynamic exercise, but whether the two increase in parallel and whether both increases occur as functions of exercise intensity remain unsettled. In this study, blood flow velocity in the common carotid artery was measured using the Doppler ultrasound method in eight healthy male students during graded treadmill exercise. The exercise consisted of stepwise progressive increases and decreases in exercise intensity. The peak intensity corresponded to approximately 85% of maximal oxygen consumption. During this exercise, the heart rate (f _c), mean blood pressure (BP) in the brachial artery and mean blood flow velocity (_cc) in the common carotid artery increased as functions of exercise intensity. At the peak exercise intensity, (f _c), BP and _cc increased by 134.5%, 20.5% and 51.8% over the control levels before exercise (P < 0.01), respectively. The resistance index (RI) and pulsatility index (PI) were determined from the velocity profile and were expected to reflect the distal cerebral blood flow resistance. The RI and PI increased during the graded exercise, but tended to decrease at the highest levels of exercise intensity. As _cc increased with increases in exercise intensity it would be expected that cerebral blood flow would also increase at these higher intensities. It is also suggested that blood flow velocity in the cerebral artery does not proportionately reflect the cerebral blood flow during dynamic exercise, since the cerebral blood flow resistance changes.     

Competitive metabolism of L -arginine: arginase as a therapeutic target in asthma

Exhaled breath nitric oxide (NO) is an accepted asthma biomarker.Lung concentrations of NO and its amino acid precursor,L-arginine,are regulated by the relative expressions of the NO synthase (NOS) and arginase isoforms.Increased expression of arginase I and NOS2 occurs in murine models of allergic asthma and in biopsies of asthmatic airways.Although clinical trials involving the inhibition of NO-producing enzymes have shown mixed results,small molecule arginase inhibitors have shown potential as a therapeutic intervention in animal and cell culture models.Their transition to clinical trials is hampered by concerns regarding their safety and potential toxicity.In this review,we discuss the paradigm of arginase and NOS competition for their substrate L-arginine in the asthmatic airway.We address the functional role of L-arginine in inflammation and the potential role of arginase inhibitors as therapeutics.     

Nitric oxide in the pathogenesis of vascular disease

Nitric oxide (NO) is synthesized by at least three distinct isoforms of NO synthase (NOS). Their substrate and cofactor requirements are very similar. All three isoforms have some implications, physiological or pathophysiological, in the cardiovascular system. The endothelial NOS III is physiologically important for vascular homeostasis, keeping the vasculature dilated, protecting the intima from platelet aggregates and leukocyte adhesion, and preventing smooth muscle proliferation. Central and peripheral neuronal NOS I may also contribute to blood pressure regulation. Vascular disease associated with hypercholesterolaemia, diabetes, and hypertension is characterized by endothelial dysfunction and reduced endothelium-mediated vasodilation. Oxidative stress and the inactivation of NO by superoxide anions play an important role in these disease states. Supplementation of the NOS substrate L-arginine can improve endothelial dysfunction in animals and man. Also, the addition of the NOS cofactor (6R)-5,6,7, 8-tetrahydrobiopterin improves endothelium-mediated vasodilation in certain disease states. In cerebrovascular stroke, neuronal NOS I and cytokine-inducible NOS II play a key role in neurodegeneration, whereas endothelial NOS III is important for maintaining cerebral blood flow and preventing neuronal injury. In sepsis, NOS II is induced in the vascular wall by bacterial endotoxin and/or cytokines. NOS II produces large amounts of NO, which is an important mediator of endotoxin-induced arteriolar vasodilatation, hypotension, and shock.     

Combined inhibition of nitric oxide and prostaglandins reduces human skeletal muscle blood flow during exercise

The vascular endothelium is an important mediator of tissue vasodilatation, yet the role of the specific substances, nitric oxide (NO) and prostaglandins (PG), in mediating the large increases in muscle perfusion during exercise in humans is unclear. Quadriceps microvascular blood flow was quantified by near infrared spectroscopy and indocyanine green in six healthy humans during dynamic knee extension exercise with and without combined pharmacological inhibition of NO synthase (NOS) and PG by l -NAME and indomethacin, respectively. Microdialysis was applied to determine interstitial release of PG. Compared to control, combined blockade resulted in a 5- to 10-fold lower muscle interstitial PG level. During control incremental knee extension exercise, mean blood flow in the quadriceps muscles rose from 10 ± 0.8 ml (100 ml tissue)⁻¹ min⁻¹ at rest to 124 ± 19, 245 ± 24, 329 ± 24 and 312 ± 25 ml (100 ml tissue)⁻¹ min⁻¹ at 15, 30, 45 and 60 W, respectively. During inhibition of NOS and PG, blood flow was reduced to 8 ± 0.5 ml (100 ml tissue)⁻¹ min⁻¹ at rest, and 100 ± 13, 163 ± 21, 217 ± 23 and 256 ± 28 ml (100 ml tissue)⁻¹ min⁻¹ at 15, 30, 45 and 60 W, respectively ( P < 0.05 vs. control). In conclusion, combined inhibition of NOS and PG reduced muscle blood flow during dynamic exercise in humans. These findings demonstrate an important synergistic role of NO and PG for skeletal muscle vasodilatation and hyperaemia during muscular contraction.     

Disinhibition of hippocampal pyramidal cells during the transition into theta rhythm

The activity of hippocampal complex-spike cells (presumed pyramidal cells) and theta cells (presumed interneurons) was examined during transitions from non-theta electroencephalogram (EEG) states to theta EEG states in freely moving and sleeping rats. Theta cell firing rates were significantly depressed in a 1-s period centered on the EEG transition relative to the surrounding 1-s periods (normalized rates±SEM): 1.05±0.02 for the non-theta period, 0.59±0.03 for the transition period, and 1.36±0.04 for the theta period (n = 26 cells). Conversely, complex-spike cell firing was significantly increased during the transition period: 0.51±0.11 for the non-theta period, 2.24±0.19 for the transition period, and 0.24±0.04 for the theta period (n = 27 cells). This diametrically altered activity indicates that theta cells must be actively inhibited during the transition. The increased activity in complex-spike cells during the transition may be simply a release from inhibitory control by interneurons. The pattern of theta cell inhibition together with increased complex-spike cell activity appears to be a general property of transitions into the theta EEG state, irrespective of behavior. It is suggested that increased activity in septal afferents (GABAergic cell activity greater than cholinergic cell activity) initially inhibits hippocampal interneurons. The inhibition is not sustained because of an activity-dependent decrease in the potency of the septointerneuronal inhibition, leaving the rhythmic excitatory (cholinergic) septointerneuronal inputs, together with principal cell inputs, to increase interneuron firing rates.     

Nitric oxide mediates the anti-adrenergic effect of adenosine on calcium current in isolated rabbit atrioventricular nodal cells

The aim of this study was to determine if adenosine exerts an anti-adrenergic effect on rabbit isolated atrioventricular (AV) nodal cells and, if so, the dependence of this effect on nitric oxide (NO) production. Inward Ca current,I _Ca, was measured in AV nodal cells, enzymatically isolated from rabbit hearts. Isoprenaline (0.1 M) increasedI _Ca from 676 ± 59 to 1102 ± 86 pA (n = 25). This isoprenaline-induced increase inI _Ca, (178 ± 15 % of control) was abolished in the presence of 10 M adenosine (I _Ca 100 ± 2 % of control,n = 9, P < 0.05). This effect of adenosine was completely blocked by the A₁ receptor antagonist CPDPX (8-cyclopentyl 1, 3-dipropylxanthine, 0.1 M). In cells pre-treated with the NO synthase inhibitor,l-nitro-arginine methyl ester (l-NAME, 1 mM) the isoprenaline-induced increase inI _Ca(208 ± 39 % of control,n = 7) was not reduced by the addition of 10 M adenosine (195 ± 32% of control). Co-incubation of cells inl-NAME withl--arginine (1 mM, the endogenous substrate of NO synthase) restored the adenosine-induced attenuation ofI _Ca. In these cells, isoprenaline increasedI _Ca (157 ± 7% of control,n = 6), and, following addition of adenosine (10 M)I _Ca was reduced to 107 ± 8% (P < 0.05). The NO-releasing agent SIN-1 (3-morpholino-sydnonimine, 100 M) inhibitedI _Ca augmented by isoprenaline (n = 5). It is concluded that adenosine exerts an anti-adrenergic effect on the AV node via A, receptors to attenuate a catecholamine-stimulated increase inI _Ca and that this action involves the intracellular production of NO.