Dual-Mode Tumor Imaging Using Probes That Are Responsive to Hypoxia-Induced Pathological Conditions

Hypoxia in solid tumors is associated with poor prognosis, increased aggressiveness, and strong resistance to therapeutics, making accurate monitoring of hypoxia important. Several imaging modalities have been used to study hypoxia, but each modality has inherent limitations. The use of a second modality can compensate for the limitations and validate the results of any single imaging modality. In this review, we describe dual-mode imaging systems for the detection of hypoxia that have been reported since the start of the 21st century. First, we provide a brief overview of the hallmarks of hypoxia used for imaging and the imaging modalities used to detect hypoxia, including optical imaging, ultrasound imaging, photoacoustic imaging, single-photon emission tomography, X-ray computed tomography, positron emission tomography, Cerenkov radiation energy transfer imaging, magnetic resonance imaging, electron paramagnetic resonance imaging, magnetic particle imaging, and surface-enhanced Raman spectroscopy, and mass spectrometric imaging. These overviews are followed by examples of hypoxia-relevant imaging using a mixture of probes for complementary single-mode imaging techniques. Then, we describe dual-mode molecular switches that are responsive in multiple imaging modalities to at least one hypoxia-induced pathological change. Finally, we offer future perspectives toward dual-mode imaging of hypoxia and hypoxia-induced pathophysiological changes in tumor microenvironments.

Nitric oxide and the brain. Part 2: Effects following neonatal brain injury-friend or foe?

Nitric oxide (NO) has critical roles in a wide variety of key biologic functions and has intricate transport mechanisms for delivery to key distal tissues under normal conditions. However, NO also plays important roles during disease processes, such as hypoxia-ischemia, asphyxia, neuro-inflammation, and retinopathy of prematurity. The effects of exogenous NO on the developing neonatal brain remain controversial. Inhaled NO (iNO) can be neuroprotective or toxic depending on a variety of factors, including cellular redox state, underlying disease processes, duration of treatment, and dose. This review identifies key gaps in knowledge that should prompt further investigation into the possible role of iNO as a therapeutic agent after injury to the brain. IMPACT: NO is a key signal mediator in the neonatal brain with neuroprotective and neurotoxic properties. iNO, a commonly used medication, has significant effects on the neonatal brain. Dosing, duration, and timing of administration of iNO can affect the developing brain. This review article summarizes the roles of NO in association with various disease processes that impact neonates, such as brain hypoxia-ischemia, asphyxia, retinopathy of prematurity, and neuroinflammation. The impact of this review is that it clearly describes gaps in knowledge, and makes the case for further, targeted studies in each of the identified areas.

Effect of Magnesium Acetyltaurate and Taurine on Endothelin1-Induced Retinal Nitrosative Stress in Rats

PURPOSE

Retinal ganglion cell apoptosis in glaucoma is associated with elevated levels of endothelin-1 (ET1), a potent vasoconstrictor. ET1-induced retinal ischemia leads to altered expression of nitric oxide synthase (NOS) isoforms leading to increased formation of nitric oxide (NO) and retinal nitrosative stress. Since magnesium (Mg) is known to improve endothelial functions and reduce oxidative stress and taurine (TAU) possesses potent antioxidant properties, we investigated the protective effects of magnesium acetyltaurate (MgAT) against ET1-induced nitrosative stress and retinal damage in rats. We also compared the effects of MgAT with that of TAU alone.

METHODS

Sprague Dawley rats were intravitreally injected with ET1. MgAT and TAU were administered as pre-, co-, or posttreatment. Subsequently, the expression of NOS isoforms was detected in retina by immunohistochemistry, retinal nitrotyrosine level was estimated using ELISA, and retinal cell apoptosis was detected by TUNEL staining.

RESULTS

Intravitreal ET1 caused a significant increase in the expressions of nNOS and iNOS while eNOS expression was significantly reduced compared to vehicle treated group. Administration of both MgAT and TAU restored the altered levels of NOS isoform expression, reduced retinal nitrosative stress and retinal cell apoptosis. The effect of MgAT, however, was greater than that of TAU alone.

CONCLUSIONS

MgAT and TAU prevent ET1-induced retinal cell apoptosis by reducing retinal nitrosative stress in Sprague Dawley rats. Addition of TAU to Mg seems to enhance the efficacy of TAU compared to when given alone. Moreover, the pretreatment with MgAT/TAU showed higher efficacy compared to co- or posttreatment.

Role of Nitric Oxide in Cardiovascular Adaptation to Intermittent Hypoxia

Hypoxia is one of the most frequently encountered stresses in health and disease. The duration, frequency, and severity of hypoxic episodes are critical factors determining whether hypoxia is beneficial or harmful. Adaptation to intermittent hypoxia has been demonstrated to confer cardiovascular protection against more severe and sustained hypoxia, and, moreover, to protect against other stresses, including ischemia. Thus, the direct and cross protective effects of adaptation to intermittent hypoxia have been used for treatment and prevention of a variety of diseases and to increase efficiency of exercise training. Evidence is mounting that nitric oxide (NO) plays a central role in these adaptive mechanisms. NO-dependent protective mechanisms activated by intermittent hypoxia include stimulation of NO synthesis as well as restriction of NO overproduction. In addition, alternative, nonenzymic sources of NO and negative feedback of NO synthesis are important factors in optimizing NO concentrations. The adaptive enhancement of NO synthesis and/or availability activates or increases expression of other protective factors, including heat shock proteins, antioxidants and prostaglandins, making the protection more robust and sustained. Understanding the role of NO in mechanisms of adaptation to hypoxia will support development of therapies to prevent and treat hypoxic or ischemic damage to organs and cells and to increase adaptive capabilities of the organism.

Chronic inhibition of nitric oxide synthase activity by NG-nitro-L-arginine induces nitric oxide synthase expression in the developing rat cerebellum

Studies on chronic inhibition of nitric oxide synthase (NOS) in the CNS suggest a plastic change in nitric oxide (NO) synthesis in areas related to motor control, which might protect the animal from the functional and behavioral consequences of NO deficiency. In the present study, the acute and chronic effect of the substrate analogue inhibitor N(G)-nitro-l-arginine (l-NNA) was examined on NO production, NO-sensitive cyclic guanosine monophosphate (cGMP) levels and the expression of NOS isoforms in the developing rat cerebellum. Acute intraperitoneal administration of the inhibitor (5-200mg/kg) to 21-day-old rats reduced NOS activity and NO concentration dose dependently by 70-90% and the tissue cGMP level by 60-80%. By contrast, chronic application of l-NNA between postnatal days 4-21 diminished the total NOS activity and NO concentration only by 30%, and the tissue cGMP level by 10-50%. Chronic treatment of 10mg/kg l-NNA induced neuronal (n)NOS expression in granule cells, as revealed by in situ hybridization, NADPH-diaphorase histochemistry and Western-blot, but it had no significant influence on tissue cGMP level or on layer formation of the cerebellum. However, a higher concentration (50mg/kg) of l-NNA decreased the intensity of the NADPH-diaphorase reaction in granule cells, significantly reduced cGMP production, and retarded layer formation and induced inducible (i)NOS expression & activity in glial cells. Treatments did not affect endothelial (e)NOS expression. The administration of the biologically inactive isomer D-NNA (50mg/kg) or saline was ineffective. The present findings suggest the existence of a concentration-dependent compensatory mechanism against experimentally-induced cronich inhibition of NOS, including nNOS or iNOS up-regulation, which might maintain a steady-state NO level in the developing cerebellum.

(-)-Epigallocatechin gallate attenuates NADPH-d/nNOS expression in motor neurons of rats following peripheral nerve injury

Background

Oxidative stress and large amounts of nitric oxide (NO) have been implicated in the pathophysiology of neuronal injury and neurodegenerative disease. Recent studies have shown that (-)-epigallocatechin gallate (EGCG), one of the green tea polyphenols, has potent antioxidant effects against free radical-mediated lipid peroxidation in ischemia-induced neuronal damage. The purpose of this study was to examine whether EGCG would attenuate neuronal expression of NADPH-d/nNOS in the motor neurons of the lower brainstem following peripheral nerve crush. Thus, young adult rats were treated with EGCG (10, 25, or 50 mg/kg, i.p.) 30 min prior to crushing their hypoglossal and vagus nerves for 30 seconds (left side, at the cervical level). The treatment (pre-crush doses of EGCG) was continued from day 1 to day 6, and the animals were sacrificed on days 3, 7, 14 and 28. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to assess neuronal NADPH-d/nNOS expression in the hypoglossal nucleus and dorsal motor nucleus of the vagus.

Results

In rats treated with high dosages of EGCG (25 or 50 mg/kg), NADPH-d/nNOS reactivity and cell death of the motor neurons were significantly decreased.

Conclusions

The present evidence indicated that EGCG can reduce NADPH-d/nNOS reactivity and thus may enhance motor neuron survival time following peripheral nerve injury.

Mechanism of increased tyrosine (Tyr(99)) phosphorylation of calmodulin during hypoxia in the cerebral cortex of newborn piglets: the role of nNOS-derived nitric oxide

The present study aims to investigate the mechanism of calmodulin modification during hypoxia and tests the hypothesis that hypoxia-induced increase in Tyr(99) phosphorylation of calmodulin in the cerebral cortex of newborn piglets is mediated by NO derived from nNOS. Fifteen piglets were divided into normoxic (Nx, n = 5), hypoxic (Hx, F(i)O(2) of 0.07 for 1 h, n = 5) and hypoxic-pretreated with nNOSi (Hx-nNOSi, n = 5) groups. nNOS inhibitor I (selectivity >2,500 vs. eNOS and >500 vs. iNOS) was administered (0.4 mg/kg, I.V.) 30 min prior to hypoxia. Cortical membranes were isolated and tyrosine phosphorylation (Tyr(99) and total) of calmodulin determined by Western blot using anti-phospho-(pTyr(99))-calmodulin and anti-pTyr antibodies. Protein bands were detected by enhanced chemiluminescence, analyzed by densitometry and expressed as absorbance. The pTyr(99) calmodulin (ODxmm(2)) was 78.55 +/- 10.76 in Nx, 165.05 +/- 12.26 in Hx (P < 0.05 vs. Nx) and 96.97 +/- 13.18 in Hx-nNOSi (P < 0.05 vs. Hx, P = NS vs. Nx). Expression of total tyrosine phosphorylated calmodulin was 69.24 +/- 13.69 in Nx, 156.17 +/- 16.34 in Hx (P < 0.05 vs. Nx) and 74.18 +/- 3.9 in Hx-nNOSi (P < 0.05 vs. Hx, P = NS vs. Nx). The data show that administration of nNOS inhibitor prevented the hypoxia-induced increased Tyr(99) phosphorylation of calmodulin. Total tyrosine phosphorylation of calmodulin was similar to Tyr(99) phosphorylation. We conclude that the mechanism of hypoxia-induced modification (Tyr(99) phosphorylation) of calmodulin is mediated by NO derived from nNOS. We speculate that Tyr(99) phosphorylated calmodulin, as compared to non-phosphorylated, binds with a higher affinity at the calmodulin binding site of nNOS leading to increased activation of nNOS and increased generation of NO.

Tyrosine phosphorylation of neuronal nitric oxide synthase (nNOS) during hypoxia in the cerebral cortex of newborn piglets: the role of nitric oxide

The present study aims to investigate the mechanism of activation of nNOS during hypoxia and tests the hypothesis that the hypoxia-induced increased tyrosine phosphorylation of nNOS in the cerebral cortical membranes of newborn piglets is mediated by nNOS-derived nitric oxide (NO). Fifteen newborn piglets were divided into normoxic (Nx, n=5), hypoxic (Hx, n=5) and hypoxic-pretreated with nNOS inhibitor I (Hx-nNOSi) groups. Hypoxia was induced by an FiO(2) of 0.07 for 60 min. nNOS inhibitor I (selectivity>2500 vs endothelial NOS and >500 vs inducible NOS) was administered (0.4 mg/kg, i.v.) 30 min prior to hypoxia. Cortical membranes were isolated and tyrosine phosphorylation of nNOS determined by Western blot. Membrane protein was immunoprecipitated with nNOS antibody, separated on 12% SDS-PAGE and blotted with anti-phosphotyrosine antibody. Protein bands were detected by enhanced chemiluminescence, analyzed by densitometry and expressed as absorbance (OD x mm(2)). Density (OD x mm(2)) of tyrosine phosphorylated nNOS was 51.66+/-14.11 in Nx, 118.39+/-14.17 in Hx (p<0.05 vs Nx) and 45.56+/-10.34 in Hx-nNOSi (p<0.05 vs Hx, p=NS vs Nx). The results demonstrate that pretreatment with nNOS inhibitor prevents the hypoxia-induced increased tyrosine phosphorylation of nNOS. We conclude that the mechanism of hypoxia-induced increased tyrosine phosphorylation of nNOS is mediated by nNOS-derived NO.

NO-mediated activation of Src kinase during hypoxia in the cerebral cortex of newborn piglets

The present study aims to investigate the mechanism of Src kinase activation during hypoxia and tests the hypothesis that the hypoxia-induced activation of Src kinase, as determined by Src kinase phosphorylation, in the cerebral cortical membranes of newborn piglets is mediated by NO derived from neuronal nitric oxide synthase (nNOS). Fifteen piglets were divided into normoxic (Nx, n=5), hypoxic (Hx, n=5) and hypoxic-treated with nNOS inhibitor I (Hx-nNOSi) groups. Hypoxia was induced by decreasing FiO(2) to 0.06 for 1h. nNOS inhibitor I (selectivity >2500 vs eNOS and >500 vs iNOS) was administered (0.4 mg/kg, i.v.) 30 min prior to hypoxia. Cortical membranes were isolated and phosphorylation of Src kinase was determined by Western blot analysis. Src kinase activity was determined by radioactive assay using immunopurified enzyme. Membrane proteins were separated by 12% SDS-PAGE and probed with anti-phospho (pTyr(418))-Src kinase antibody. Protein bands were detected, analyzed by densitometry and expressed as absorbance (ODxmm(2)). Density (ODxmm(2)) of phosphorylated Src kinase was 111.7+/-21.1 in Nx, 234.5+/-23.8 in Hx (p<0.05 vs Nx) and 104.7+/-18.1 in Hx-nNOSi (p<0.05 vs Hx, p=NS vs Nx). Src kinase activity (pmol/mgprotein/ h) was 2472+/-75 in Nx, 4556+/-358 in Hx (p<0.05 vs Nx) and 2259+/-207 in Hx-nNOSi (p<0.05 vs Hx, p=NS vs Nx). The data show that pretreatment with nNOS inhibitor prevents the hypoxia-induced increase in tyrosine phosphorylation and the activity of Src kinase. We conclude that the mechanism of hypoxia-induced increased activation of Src kinase is mediated by nNOS derived NO. We propose that NO mediated inhibition of protein tyrosine phosphatases SH-PTP-1 and SH-PTP-2 leads to increased tyrosine phosphorylation and activation of Src kinase in the cerebral cortex of newborn piglets.

Mild hypoxic preconditioning attenuates injury-induced NADPH-d/nNOS expression in brainstem motor neurons of adult rats

Excessive production of nitric oxide (NO) might have detrimental effects on the hypoxia-related neuropathology. This study aimed to test if mild hypoxic preconditioning (MHPC) would attenuate the pathological changes in the brainstem motoneurons having a different functional component after peripheral nerve crush injury (PNCI). Prior to PNCI treatment, young adult rats were caged in the mild hypoxic altitude chamber with 79Torr of the partial oxygen concentration ( pO(2)) (i.e., 0.5atm at 5500m in height) for 4 weeks to adapt the environmental changes. After that, all the animals having successfully crushed both the hypoglossal and vagus nerves (left-side) were allowed to survive for 3, 7, 14, 30 and 60 successive days in normoxic condition. Nicotinamine adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry revealed that MHPC reduces NADPH-d/nNOS expression in the hypoglossal nucleus (HN) and the dorsal motor nucleus of the vagus (DMN) at different time points after PNCI. The morphological findings were further ascertained by Western blot analysis of nNOS and nitrite assay for NO production. Both the morphological and quantitative results peaked at 7 days in HN, whereas for those in DMN were progressively increased up to 60 days following PNCI. The staining intensity of NADPH-d/nNOS(+) neurons, expression of nNOS protein, NO production levels as well as the neuronal loss in HN and DMN of MHPC rats following PNCI were attenuated, especially for those having a longer survival period over 14 days. The MHPC treatment might induce minute amounts of NO to alter the state of milieu of the experimental animals to protect against the PNCI.

Aspects of nitric oxide in health and disease: a focus on hypertension and cardiovascular disease

Nitric oxide (nitrogen monoxide) (NO) plays an important role in a wide range of physiologic processes. A major mediator of endothelial function, NO regulates vasodilatory and antithrombotic actions in the vasculature and plays a role in reproductive functions, bronchodilation, bone formation, memory, insulin sensitivity, and gastrointestinal relaxation. NO is formed from NO synthase. Impaired NO bioactivity is strongly associated with endothelial dysfunction and cardiovascular disease, but is also implicated in a broad range of other disorders, including pulmonary hypertension, insulin resistance, erectile dysfunction, and preeclampsia. Numerous therapies designed to target NO are being investigated and developed, including NO donors and stimulants. The recent African-American Heart Failure Trial (A-HeFT) showed that the NO donor isosorbide dinitrate, combined with the vasodilator hydralazine, significantly reduced morbidity and mortality in black patients with moderate-to-severe heart failure. Antihypertensive drugs, including angiotensin-converting enzyme inhibitors, calcium channel blockers, and third-generation beta-blockers, are NO stimulants that have demonstrated significant improvement of endothelial function and NO bioactivity. Other cardiovascular therapies that may improve NO bioactivity include statins, l-arginine, and nonpharmacologic approaches such as exercise and dietary changes.

Increased cerebral and renal endothelial nitric oxide synthase gene expression after cardiopulmonary bypass in the rat

OBJECTIVE

Hemodilution and endothelial nitric oxide synthase genetic polymorphism may contribute to cerebral and renal injury after cardiopulmonary bypass. This study tested the hypothesis that cardiopulmonary bypass and anemia stimulate an increase in cerebral and renal endothelial nitric oxide synthase gene expression in an experimental model of cardiopulmonary bypass.

METHODS

Anesthetized rats underwent a sham procedure without cardiopulmonary bypass (sham, n = 5), normothermic bypass for 1 hour (CPB, n = 7), or bypass plus hemodilutional anemia (CPB anemia, n = 9). After 24 hours of recovery, RNA was extracted from the cerebral cortex, renal cortex, and renal medulla. Quantitative reverse transcriptase polymerase chain reaction was used to assess endothelial nitric oxide synthase messenger RNA levels in brain and kidney tissues.

RESULTS

The hemoglobin concentration of anemic CPB rats was significantly lower than that of nonanemic rats on bypass (64 +/- 5 vs 99 +/- 8 g x L(-1), P < .001). Cerebral cortical endothelial nitric oxide synthase messenger RNA levels were increased after cardiopulmonary bypass relative to those of the sham group (11.2 +/- 4.2 vs 6.3 +/- 1.5 fg, P = .031), without a further increase in anemic rats. Renal medullary endothelial nitric oxide synthase messenger RNA levels were significantly higher in the CPB anemia group than in the sham and CPB groups (7.1 +/- 4.4 fg vs 1.8 +/- 0.4 fg vs 3.0 +/- 0.6 fg, P < .001). Renal cortical endothelial nitric oxide synthase messenger RNA levels did not change significantly.

CONCLUSIONS

Normothermic cardiopulmonary bypass was associated with higher endothelial nitric oxide synthase messenger RNA levels in kidney and brain than was the sham procedure 24 hours after cardiopulmonary bypass. Anemia accentuated the increase in renal medullary, but not cerebral cortical, endothelial nitric oxide synthase expression. These data provide an approach for exploring potential mechanisms by which endothelial nitric oxide synthase may contribute to renal and cerebral dysfunction after cardiopulmonary bypass and anemia.

Effects of acute hypobaric hypoxia on the nitric oxide system of the rat cerebral cortex: Protective role of nitric oxide inhibitors

Exposure to hypobaric hypoxia produces neuropsychological disorders. The brain nitrergic system was investigated following hypobaric hypoxia in the presence or absence of nitric oxide synthase (NOS) inhibitors. Adult rats were exposed to a simulated altitude of 8325 m (27,000 ft) for 7 h and killed after 0, 1, 3, 5, and 10 days of recovery. In addition to normobaric controls, three experimental groups were studied: i) subjected to hypobaric hypoxia without inhibitors; ii) subjected to hypobaric hypoxia and treated with 7-nitroindazole; iii) subjected to hypobaric hypoxia and treated with N(omega)-nitro-l-arginine methyl ester (l-NAME). Cerebral cortex was assayed by immunohistochemistry, Western blotting, and enzymatic assays. In animals subjected to hypobaric hypoxia without inhibitors, there was an increase in neuronal nitric oxide synthase (nNOS) immunoreactivity and Ca(2+)-dependent NOS activity from 0 to 1 days of reoxygenation. In these animals, inducible nitric oxide synthase (iNOS) expression and Ca(2+)-independent activity were undetectable, but nitrotyrosine immunoreactivity was found in some neurons. Administration of either inhibitor prevented the increase in nNOS immunoreactivity and enzymatic activity provoked by hypobaric hypoxia. Concomitantly, nitrotyrosine immunoreactivity decreased progressively. In conclusion, activation of the nitrergic system constitutes a cortical response to hypobaric hypoxia and the administration of NOS inhibitors could provide new therapeutic avenues to prevent and/or treat the symptoms produced by hypobaric hypoxia.

Increased expression of HIF-1alpha, nNOS, and VEGF in the cerebral cortex of anemic rats

This study tested the hypothesis that specific hypoxic molecules, including hypoxia-inducible factor-1alpha (HIF-1alpha), neuronal nitric oxide synthase (nNOS), and vascular endothelial growth factor (VEGF), are upregulated within the cerebral cortex of acutely anemic rats. Isoflurane-anesthetized rats underwent acute hemodilution by exchanging 50% of their blood volume with pentastarch. Following hemodilution, mean arterial pressure and arterial Pa(O(2)) values did not differ between control and anemic rats while the hemoglobin concentration decreased to 57 +/- 2 g/l. In anemic rats, cerebral cortical HIF-1alpha protein levels were increased, relative to controls (1.7 +/- 0.5-fold, P < 0.05). This increase was associated with an increase in mRNA levels for VEGF, erythropoietin, CXCR4, iNOS, and nNOS (P < 0.05 for all), but not endothelial NOS. Cerebral cortical nNOS and VEGF protein levels were increased in anemic rats, relative to controls (2.0 +/- 0.2- and 1.5 +/- 0.4-fold, respectively, P < 0.05 for both). Immunohistochemistry demonstrated increased HIF-1alpha and VEGF staining in perivascular regions of the anemic cerebral cortex and an increase in the number of nNOS-positive cerebral cortical cells (3.2 +/- 1.0-fold, P < 0.001). The nNOS-positive cells costained with the neuronal marker, Neu-N, but not with the astrocytic marker glial fibrillary acidic protein (GFAP). These nNOS-positive neurons frequently sent axonal projections toward cerebral blood vessels. Conversely, VEGF immunostaining colocalized with both neuronal (NeuN) and astrocytic markers (GFAP). In conclusion, acute normotensive, normoxemic hemodilution increased the levels of HIF-1alpha protein and mRNA for HIF-1-responsive molecules. nNOS and VEGF protein levels were also increased within the cerebral cortex of anemic rats at clinically relevant hemoglobin concentrations.

The intestinal trophic response to enteral food is reduced in parenterally fed preterm pigs and is associated with more nitrergic neurons

In term neonates, total parenteral nutrition (TPN) induces mucosal atrophy, whereas the first intake of milk is followed by intestinal growth. This may be explained in part by an NO-mediated increased blood flow. We hypothesized that the immature gut has an altered response to TPN and enteral nutrition. In Expt. 1, preterm caesarean-delivered pigs were administered elemental nutrients for 3 d, infused parenterally (TPN, n = 7) or enterally (TENT, n = 7). In Expt. 2, preterm pigs were fed sow's colostrum, cow's colostrum, or infant formula for 2 d after a 3-d TPN period (TPN-SOW, TPN-COW, TPN-FORM, n = 8-11). Intestinal morphology and the number of enteric neurons containing nitric oxide synthase-1 (NOS-1) were quantified. Both the TPN and TENT groups had increases in intestinal mass, circumference, and mucosal mass, volume, and surface density, relative to values at birth (+30-50%, P < 0.05). In Expt. 2, the magnitudes of the intestinal trophic responses to feeding were similar to those in Expt. 1, but were also associated with an increased number of nitrergic myenteric neurons and some mucosal damage, most frequently observed for the formula group. We conclude that 1) a short period of TPN does not induce mucosal atrophy in preterm pigs, whereas elemental nutrients infused luminally do not mimic the trophic response seen with milk diets, 2) enteral feeding of preterm pigs after a short period of TPN is associated with a modest, diet-dependent trophic response that may be related in part to the actions of an increased population of enteric NOS-1 neurons.

Tetrahydrobiopterin and nitric oxide synthase dimer levels are not changed following hypoxia-ischemia in the newborn rat

The effect of hypoxia-ischemia on the nitric oxide synthase (NOS) cofactor tetrahydrobiopterin (BH4) and changes in the enzyme dimer state have not previously been studied. Cell-based studies have demonstrated the regulation of nitric oxide (NO) synthesis by intracellular BH4 levels. Activation of NOS requires two NOS polypeptides to form a homodimer. Dimerization results in the creation of high-affinity binding sites for BH4 and L-arginine. Our previous studies have indicated that nNOS activity falls 2 h post-hypoxia-ischemia in the immature rodent model. Thus, the objective of this study was to determine whether changes in nNOS dimeric state could be responsible for the decrease in nNOS activity. Using the immature rat model of HI in conjunction with LT-PAGE and Western blot analysis, we determined the effect of HI on NOS dimer state in hippocampus and cortex and the effects of pharmacologic modulation of NO levels during HI on dimer formation. Using high-performance liquid chromatography (HPLC) and electrospray tandem mass spectrometry (MS-MS), we measured BH4 and L-arginine levels respectively after HI under the same conditions. We found minimal or no changes in either BH4 levels or NOS dimer state at 2 h, 24 h and 7 day recovery from HI on postnatal day 7. In contrast, L-arginine levels were transiently increased in the hypoxic ischemic hemisphere. Thus, our data suggest that the previously described decrease in NOS activity after HI is not associated with depletion of the cofactor BH4, L-arginine substrate or changes in the NOS enzyme dimer state.

Neuronal NADPH-d/NOS expression in the nodose ganglion of severe hypoxic rats with or without mild hypoxic preconditioning

This study aimed to test the hypothesis that mild hypoxic preconditioning (MHPC)-induced NOS expression would attenuate the neuropathological changes in the nodose ganglion (NG) of severe hypoxic exposure (SHE) rats. Thus, the young adult rats were caged in the altitude chamber for 4 weeks prior to SHE for 4 h to gain hypoxic preconditioning. The altitude chamber was used to set the height at the level from 5500 m (0.50 atm; pO2=79 Torr) to 10,000 m (0.27 atm; pO2=43 Torr) for MHPC and SHE, respectively. The experimental animals were allowed to survive for 0, 7, 14, 30 and 60 successive days, respectively. Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to detect NADPH-d/nNOS reactivity in the NG at various time points following hypoxic exposure. The present results showed that about 38% of the neurons in the NG displayed NADPH-d/nNOS positive [NADPH-d/nNOS(+)] in normoxic rats. In SHE rats, a peak in the percentage (71%) and staining intensity (230%) of NADPH-d/nNOS(+) nodose neurons at 0 day, which then gradually decreased at 7-60 days. About 25% of the nodose neurons died 60 days after SHE. However, in MHPC rats subjected to SHE, NADPH-d/nNOS(+) neurons peaked in the percentage (51%) and staining intensity (171%) at 0 day, which then decreased at 7-60 days. In addition, neuronal survival was markedly increased by MHPC. These results suggested that MHPC might have a neuroprotective effect that reduces the susceptibility of the nodose neurons to NOS mediated neuropathy subsequent to SHE.

Role of nitric oxide after brain ischaemia

Ischaemic stroke is the second or third leading cause of death in developed countries. In the last two decades substantial research and efforts have been made to understand the biochemical mechanisms involved in brain damage and to develop new treatments. The evidence suggests that nitric oxide (NO) can exert both protective and deleterious effects depending on factors such as the NOS isoform and the cell type by which NO is produced or the temporal stage after the onset of the ischaemic brain injury. Immediately after brain ischaemia, NO release from eNOS is protective mainly by promoting vasodilation; however, after ischaemia develops, NO produced by overactivation of nNOS and, later, NO release by de novo expression of iNOS contribute to the brain damage. This review article summarizes experimental and clinical data supporting the dual role of NO in brain ischaemia and the mechanisms by which NO is regulated after brain ischaemia. We also review NO-based therapeutic strategies for stroke treatment, not only those directly linked with the NO pathway such as NO donors and NOS inhibitors but also those partially related like statins, aspirin or lubeluzole.

Green tea polyphenol (−)-epigallocatechin gallate attenuates the neuronal NADPH-d/nNOS expression in the nodose ganglion of acute hypoxic rats

Recent studies have shown that (-)-epigallocatechin gallate (EGCG), one of the green tea polyphenols, has a potent antioxidant property. Nitric oxide (NO) plays an important role in the neuropathogenesis induced by brain ischemia/reperfusion and hypoxia. This study aimed to explore the potential neuroprotective effect of EGCG on the ganglionic neurons of the nodose ganglion (NG) in acute hypoxic rats. Thus, the young adult rats were pretreated with EGCG (10, 25, or 50 mg/kg, i.p.) 30 min before they were exposed to the altitude chamber at 10,000 m with the partial pressure of oxygen set at the level of 0.27 atm (pO2=43 Torr) for 4 h. All the animals examined were allowed to survive for 3, 7, and 14 successive days, respectively, except for those animals sacrificed immediately following hypoxic exposure. Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry were carried out to detect the neuronal NADPH-d/nNOS expression in the NG. The present results show a significant increase in the expression of NADPH-d/nNOS reactivity in neurons of the NG at various time intervals following hypoxia. However, the hypoxia-induced increase in NADPH-d/nNOS expression was significantly depressed only in the hypoxic rats treated with high dosages of EGCG (25 or 50 mg/kg). These data suggest that EGCG may attenuate the oxidative stress following acute hypoxia.

Nitric oxide mediates hypoxia-induced changes in paracellular permeability of cerebral microvasculature

Ischemic stroke from a reduction in blood flow to the brain microvasculature results in a subsequent decreased delivery of oxygen (i.e., hypoxia) and vital nutrients to endothelial, neuronal, and glial cells. Hypoxia associated with stroke has been shown to increase paracellular permeability of the blood-brain barrier, leading to the release of cellular mediators and brain tissue injury. Whereas reperfusion does not occur in all ischemic strokes, increased permeability has been seen in posthypoxic reoxygenation. Currently, it is unknown whether these deleterious effects result from cellular mechanisms stimulated by decreased oxygen during stroke or posthypoxic reoxygenation stress. This study used primary bovine brain microvessel endothelial cells (BBMECs) to examine the involvement of nitric oxide (NO) as a mediator in hypoxia-induced permeability changes. Hypoxia-induced increased transport of [14C]sucrose across BBMEC monolayers compared with normoxia was attenuated by either posthypoxic reoxygenation or inhibition of NO synthase (NOS). The hypoxia-induced permeability effect was further reduced when NOS inhibition was combined with posthypoxic reoxygenation. Additionally, a significant increase in total NO was seen in BBMECs after hypoxic exposure. This correlation was supported by the increased [14C]sucrose permeability observed when BBMECs were exposed to the NO donor diethylenetriaamine NONOate. Western blot analyses of NOS isoforms showed a significant increase in the inducible isoform after hypoxic exposure with a subsequent reduction in expression on reoxygenation. Results from this study suggest that hypoxia-induced blood-brain barrier breakdown can be diminished by inhibition of NO synthesis, decreased concentration of NO metabolites, and/or reoxygenation.

Vascular Endothelial Growth Factor Expression and Angiogenesis Induced by Chronic Cerebral Hypoperfusion in Rat Brain

OBJECTIVE

In a rat model, we studied the time courses of vascular endothelial growth factor (VEGF) expression and angiogenesis induced by chronic cerebral hypoperfusion in the brain, and we investigated the histological basis of normal-perfusion pressure breakthrough.

METHODS

Twenty-one Sprague-Dawley rats were randomly divided into a control group (n = 3) and a model group assessed at various time points after the creation of a carotid artery-jugular vein fistula (12 h, n = 3; 24 h, n = 3; 72 h, n = 3; 7 d, n = 3; 21 d, n = 3; 90 d, n = 3). The time courses of the expression of VEGF messenger ribonucleic acid (mRNA) and protein in rat brain were analyzed with semiquantitative reverse transcriptase-polymerase chain reaction and Western blot assays, respectively. Immunohistochemical techniques were used to evaluate VEGF protein localization with rabbit polyclonal anti-rat VEGF, VEGF receptor (VEGFR) expression with rabbit polyclonal antibodies to VEGFR-1 and -2, microvascular density with mouse monoclonal anti-rat CD31, and astrocytic reactivity with polyclonal anti-glial fibrillary acidic protein, in cerebral cortical tissue of the right middle cerebral artery territory.

RESULTS

Three alternative splicing forms, i.e., VEGF(188), VEGF(164), and VEGF(120), were observed in cerebral cortical tissue of the right middle cerebral artery territory in semiquantitative reverse transcriptase-polymerase chain reaction analyses. VEGF(164) mRNA was the predominant isoform expressed in rat brain. VEGF(188) mRNA and VEGF(120) mRNA were also detected but at very low levels (not statistically significant). Low levels of VEGF(164) mRNA were observed in the control brains. However, VEGF(164) mRNA levels were significantly increased in the model brains at 24 hours postoperatively, peaked by 7 days, decreased by 21 days, and returned to basal levels by 90 days after fistula formation. VEGF protein expression, as measured in Western blot assays, was also increased in rat brains in the model group from 24 hours to 21 days postoperatively but returned to control levels by 90 days after fistula formation. VEGF immunohistochemical analyses indicated that this increased expression was mostly associated with endothelial cells. Consistent with the VEGF protein expression findings, up-regulation of VEGFR-1 but not VEGFR-2 expression on endothelial cells in the model brains was observed. Microvascular density in the rat brains began to increase significantly 7 days after fistula formation in the model group, as assessed immunohistochemically, and the increase was maintained for 90 days. Although no prominent astrocytic reactivity was observed in the rat brains throughout the experiments, there was an absence of astrocytic foot processes surrounding some cerebral capillaries 90 days after fistula formation in the model group.

CONCLUSION

These results demonstrated that chronic cerebral hypoperfusion could induce sustained up-regulation of VEGF mRNA and protein expression in rat brain, which was correlated with angiogenesis. An absence of corresponding astrocytic reactivity during angiogenesis may be an important factor accounting for structural deficits of the blood-brain barrier and the occurrence of normal-perfusion pressure breakthrough.

Hemodilutional anemia is associated with increased cerebral neuronal nitric oxide synthase gene expression

Severe hemodilutional anemia may reduce cerebral oxygen delivery, resulting in cerebral tissue hypoxia. Increased nitric oxide synthase (NOS) expression has been identified following cerebral hypoxia and may contribute to the compensatory increase in cerebral blood flow (CBF) observed after hypoxia and anemia. However, changes in cerebral NOS gene expression have not been reported after acute anemia. This study tests the hypothesis that acute hemodilutional anemia causes cerebral tissue hypoxia, triggering changes in cerebral NOS gene expression. Anesthetized rats underwent hemodilution when 30 ml/kg of blood were exchanged with pentastarch, resulting in a final hemoglobin concentration of 51.0 +/- 1.2 g/l (n = 7 rats). Caudate tissue oxygen tension (Pbr(O(2))) decreased transiently from 17.3 +/- 4.1 to 14.4 +/- 4.1 Torr (P < 0.05), before returning to baseline after approximately 20 min. An increase in CBF may have contributed to restoring Pbr(O(2)) by improving cerebral tissue oxygen delivery. An increase in neuronal NOS (nNOS) mRNA was detected by RT-PCR in the cerebral cortex of anemic rats after 3 h (P < 0.05, n = 5). A similar response was observed after exposure to hypoxia. By contrast, no increases in mRNA for endothelial NOS or interleukin-1beta were observed after anemia or hypoxia. Hemodilutional anemia caused an acute reduction in Pbr(O(2)) and an increase in cerebral cortical nNOS mRNA, supporting a role for nNOS in the physiological response to acute anemia.

Potentiation of intracellular Ca2+ mobilization by hypoxia-induced NO generation in rat brain striatal slices and human astrocytoma U-373 MG cells and its involvement in tissue damage

The relationship between nitric oxide (NO) and intracellular Ca2+ in hypoxic-ischemic brain damage is not known in detail. Here we used rat striatal slices perfused under low-oxygen and Ca2+-free conditions and cultured human astrocytoma cells incubated under similar conditions as models to study the dynamics of intracellular NO and Ca2+ in hypoxia-induced tissue damage. Exposure of rat striatal slices for 70 min to low oxygen tension elicited a delayed and sustained increase in the release of 45Ca2+. This was potentiated by the NO donors sodium nitroprusside (SNP) and spermine-NO and inhibited by N-omega-nitro-L-arginine methyl ester (L-NAME) or by the NO scavenger 2-phenyl-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (PTIO). A membrane-permeant form of heparin in combination with either ruthenium red (RR) or ryanodine (RY) also inhibited 45Ca2+ release. In human astrocytoma U-373 MG cells, hypoxia increased intracellular Ca2+ concentration ([Ca2+]i) by 67.2 +/- 13.1% compared to normoxic controls and this effect was inhibited by L-NAME, PTIO or heparin plus RR. In striatal tissue, hypoxia increased NO production and LDH release and both effects were antagonized by L-NAME. Although heparin plus RR or RY antagonized hypoxia-induced increase in LDH release they failed to counteract increased NO production. These data therefore indicate that NO contributes to hypoxic damage through increased intracellular Ca2+ mobilization from endoplasmic reticulum and suggest that the NO-Ca2+ signalling might be a potential therapeutic target in hypoxia-induced neuronal degeneration.

Enhanced neuronal nitric oxide synthase expression is central to cardiac vagal phenotype in exercise-trained mice

We investigated whether enhanced cardiac vagal responsiveness elicited by exercise training is dependent on neuronal nitric oxide synthase (NOS-1), since the NO-cGMP pathway facilitates acetylcholine release. Isolated atria with intact right vagal innervation were taken from male mice (18-22 weeks old) after a period of 10 weeks voluntary wheel-running (+EX, n = 27; peaked 9.8 +/- 0.6 km day(-1) at 5 weeks), and from mice housed in cages without wheels (-EX, n = 27). Immunostaining of whole atria for NOS-1 identified intrinsic neurones, all of which co-localized with choline acetyltransferase-positive ganglia. Western blot analysis confirmed that NOS-1 protein level was significantly greater in +EX compared to -EX atria (P < 0.05, unpaired t test). Basal heart rates (HR) were slower in +EX than in -EX atria (322 +/- 6 versus 360 +/- 7 beats min(-1); P < 0.05, unpaired t test) However, in +EX atria, HR responses to vagal stimulation (VNS, 3 and 5 Hz) were significantly enhanced compared to -EX atria (3 Hz, +EX: -76 +/- 8 beats min(-1) versus -EX: -62 +/- 7 beats min(-1); 5 Hz, +EX: -106 +/- 4 beats min(-1) versus -EX: -93 +/- 3 beats min(-1); P < 0.01, unpaired t test). Inhibition of NOS-1 with vinyl-L-N-5-(1-imino-3-butenyl)-L-ornithine (L-VNIO, 100 microM) or soluble guanylyl cyclase with 1H-[1, 2, 4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ, 10 microM) abolished the difference in HR responses to VNS between +EX and -EX atria, and effects of L-VNIO were reversed by excess L-arginine (1 mM; P < 0.01, ANOVA). There were no differences between the HR responses to the bath-applied acetylcholine analogue carbamylcholine chloride in +EX and -EX atria (IC(50) concentrations were 5.9 +/- 0.4 microM (-EX) and 5.7 +/- 0.4 microM (+EX)), suggesting that the changes in vagal responsiveness resulted from presynaptic facilitation of neurotransmission. In conclusion, NOS-1 appears to be a key protein in generating the cardiac vagal gain of function elicited by exercise training.

Prolonged mild fetal hypoxia up-regulates type I nitric oxide synthase expression in discrete areas of the late-gestation fetal sheep brain

OBJECTIVE

Our purpose was to study the effects of prolonged mild hypoxemia on type I nitric oxide synthase (NOS) messenger RNA, protein, and enzymatic activity in the fetal sheep brain.

STUDY DESIGN

Pregnant sheep were randomly allocated to receive maternal nitrogen (n = 8) or compressed air (controls, n = 5) to reduce fetal brachial artery PO(2) by 25% for 5 days. Type I NOS mRNA (determined by ribonuclease protection assay) protein (determined by Western blot) and enzymatic activity (determined by citrulline assay) were measured in the hippocampus, striatum, cerebellum, and frontal cortex. Data are presented as mean +/- SEM and were compared by means of one-way analysis of variance or two-sample t test.

RESULTS

The reduction in maternal inspired oxygen concentration decreased fetal PO(2) by 26% and oxygen content by 25% without an associated change in PCO(2) or pH. Fetal hypoxemia increased type I NOS mRNA by threefold in the striatum and by 2-fold in the frontal cortex and cerebellum, but it did not change mRNA expression in the hippocampus (P <.05). Type I NOS protein and catalytic activity increased only in the striatum (P <.05).

CONCLUSION

Prolonged mild hypoxemia has a differential effect on type I NOS mRNA in fetal sheep brain areas. Type I NOS protein and catalytic activity significantly increased only in the striatum. Our data suggest that fetal type I NOS gene expression is regulated at transcriptional, post-transcriptional, and translational levels.

Cerebral microvascular changes in permeability and tight junctions induced by hypoxia-reoxygenation

Cerebral microvessel endothelial cells that form the blood-brain barrier (BBB) have tight junctions (TJ) that are critical for maintaining brain homeostasis and low permeability. Both integral (claudin-1 and occludin) and membrane-associated zonula occluden-1 and -2 (ZO-1 and ZO-2) proteins combine to form these TJ complexes that are anchored to the cytoskeletal architecture (actin). Disruptions of the BBB have been attributed to hypoxic conditions that occur with ischemic stroke, pathologies of decreased perfusion, and high-altitude exposure. The effects of hypoxia and posthypoxic reoxygenation in cerebral microvasculature and corresponding cellular mechanisms involved in disrupting the BBB remain unclear. This study examined hypoxia and posthypoxic reoxygenation effects on paracellular permeability and changes in actin and TJ proteins using primary bovine brain microvessel endothelial cells (BBMEC). Hypoxia induced a 2.6-fold increase in [(14)C]sucrose, a marker of paracellular permeability. This effect was significantly reduced (~58%) with posthypoxic reoxygenation. After hypoxia and posthypoxic reoxygenation, actin expression was increased (1.4- and 2.3-fold, respectively). Whereas little change was observed in TJ protein expression immediately after hypoxia, a twofold increase in expression was seen with posthypoxic reoxygenation. Furthermore, immunofluorescence studies showed alterations in occludin, ZO-1, and ZO-2 protein localization during hypoxia and posthypoxic reoxygenation that correlate with the observed changes in BBMEC permeability. The results of this study show hypoxia-induced changes in paracellular permeability may be due to perturbation of TJ complexes and that posthypoxic reoxygenation reverses these effects.

NMDA receptor and neonatal hypoxic brain injury

The NMDA-type glutamate receptor is a predominant mediator of excitotoxicity in the immature brain due to overexpression of the receptor in the developing brain. Within the development period however, the extent of NMDA receptor mediated processes including hypoxia-induced excitotoxicity may depend on the ontogeny of the NMDA receptor recognition and modulation sites, and subunits leading to altered function of the ion-channel comples. The function of the receptor may be modified by intracellular mechanisms such as phosphorylation/dephosphorylation, nitration, and generation of free radicals including nitric oxide. The susceptibility of the developing brain to hypoxia depends on several factors: the lipid composition of the brain cell membrane; the rate of membrane lipid peroxidation and the status of anti-oxidant defenses; the development and modulation of the NMDA receptor sites; the intracellular Ca(2+) influx mechanisms; expression of apoptotic and antiapoptotic genes such as Bax and Bcl-2; and the activation of initiator caspases and caspase-3, the "executioner" of cell death. The developmental status of these cellular mechanisms and their response to hypoxia determine the fate of the hypoxic cell in the developing brain in the fetus and the newborn.

Melatonin attenuates the neuronal NADPH-d/NOS expression in the nodose ganglion of acute hypoxic rats

Excessive production of nitric oxide (NO) may play a detrimental role in the process of hypoxia-related neuropathology. This study explored whether treatment with melatonin would attenuate the neuropathological changes in the vagal ganglia following a severe hypoxic insult. Thirty minutes prior to hypoxia treatment, young adult rats were pre-treated with melatonin at 5. 25 or 100 mg/kg injected intraperitoneally. Hypoxia was achieved by subjecting the rats to a barometric pressure of 0.2 atm (PO2 = 43 Torr) for 4 hr in an altitude chamber. Nicotinamine adenine dinucleotide phosphatediaphorase (NADPH-d) histochemistry combined with the neuronal nitric oxide synthase (nNOS) immunohistochemistry were used to detect the NADPH-d/nNOS reactivity in the nodose ganglion (NG) at various time points following the hypoxic exposure. In normal untreated rats, about 43% of the neurons in the NG displayed NADPH-d/nNOS reactivity. Following hypoxic exposure, both the percentage and the staining intensity of NADPH-d/nNOS positive neurons in the NG were markedly increased, but these were reduced in longer surviving animals. Quantitative analysis of cell counts revealed that about 17% of the neurons died at 14 days after hypoxia treatment. However, in hypoxic rats given different doses of melatonin pretreatment, neuronal death as well as the frequency and staining intensity of NADPH-d/nNOS reactivity of the nodose neurons were significantly decreased. The effect of melatonin on neuronal survival and NADPH-d/ nNOS expression was dose-dependent. It is therefore suggested that melatonin exerts a neuroprotective effect and may serve as a potential therapeutic strategy for prevention and/or reducing the susceptibility of nodose neurons to NO-mediated hypoxic neuropathy.

Sepsis is associated with reciprocal expressional modifications of constitutive nitric oxide synthase (NOS) in human skeletal muscle: down-regulation of NOS1 and up-regulation of NOS3

OBJECTIVE

To study the expression (mRNA and protein) and activity of the constitutive isoforms of nitric oxide synthase (NOS1 and NOS3) in a skeletal muscle of septic patients.

DESIGN

Prospective study.

SETTING

An adult trauma/surgical intensive care unit in an urban teaching hospital.

PATIENTS

Sixteen septic patients and 21 controls.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Samples of the rectus abdominis muscle were obtained during surgical procedure. NOS mRNA, protein, and activity were detected by reverse-transcriptase polymerase chain reaction, Western blot, and the conversion of [3H]L-arginine to [3H]L-citrulline, respectively. The main results of this study are as follows: a) Levels of NOS1 mRNA and protein were significantly higher than those of NOS3 in the rectus abdominis muscle of control patients; b) NOS1 expression was down-regulated in septic patients, whereas NOS3 was up-regulated; c) these modulations were associated with a reduction in constitutive NOS activity; and d) modifications of NOS1 and NOS3 protein expression were correlated significantly with the severity of sepsis, assessed by the Simplified Acute Physiology Score II.

CONCLUSIONS

Sepsis induces reciprocal expressional modifications of NOS1 and NOS3 in human skeletal muscle, which decreases muscular constitutive NOS activity. These modifications may have implications for muscle impairment in septic patients.

Nitric oxide synthase activity in brain tissues from llama fetuses submitted to hypoxemia

The fetal llama (Lama glama; a species adapted to live in chronic hypoxia in the highlands of the Andes) did not increase cerebral blood flow and reduce the brain oxygen uptake during hypoxemia. Although nitric oxide (NO) is a normal mediator in the regulation of vascular tone and synaptic transmission, NO overproduction by hypoxemia could produce neuronal damage. We hypothesized that nitric oxide synthase (NOS) activity is either maintained or reduced in the central nervous system of the llama fetuses submitted to chronic hypoxemia. Approximately 85% of the Ca(2+)-dependent NOS activity was soluble, at least 12% was associated with the mitochondrial fraction, and less than 5% remains associated with microsomes. To understand the role of NO in chronic hypoxemia, we determined the effect of 24-h hypoxemia on NOS activity in the central nervous system. No changes in activity or the subcellular distribution of NOS activity in brain tissues after hypoxemia were found. We proposed that the lack of changes in NOS activity in the llama under hypoxemia could be a cytoprotective mechanism inherent to the llama, against possible toxic effects of NO.

Differential nitric oxide synthase activity, cofactor availability and cGMP accumulation in the central nervous system during anaesthesia

We investigated the effects of anaesthesia on dynamic nitric oxide production, concentrations of tetrahydrobiopterin and the accumulation of cyclic GMP (cGMP) in the rat central nervous system (CNS). Rats were assigned to anaesthesia with halothane, isoflurane, pentobarbital, diazepam, ketamine or xenon (n=6 per group). After 30 min, [14C]L-arginine (i.v.) was given and, after a further 60 min of anaesthesia, rats were killed and exposed immediately to focused microwave radiation. After removal of the brain and spinal cord, nitric oxide production from radiolabelled arginine (and hence nitric oxide synthase activity during anaesthesia) was measured as [14C]L-citrulline by scintillation counting. cGMP was determined by enzyme immunoassay and tetrahydrobiopterin by fluorescence HPLC, in brain regions and the spinal cord. Nitric oxide synthase activity was similar in all brain regions but was lower in the spinal cord, and was unaffected by anaesthesia. cGMP was similar in all areas of the CNS and was significantly decreased in rats anaesthetized with halothane. Isoflurane produced similar effects. In contrast, ketamine and xenon anaesthesia increased cGMP in the spinal cord, brainstem and hippocampus. Diazepam and pentobarbital had no effect. Tetrahydrobiopterin concentrations were similar in all areas of the CNS and were increased in the cortex and hippocampus after anaesthesia. We have shown profound differential effects of anaesthesia on the nitric oxide pathway in the rat CNS.

Regulation of diaphragmatic nitric oxide synthase expression during hypobaric hypoxia

Nitric oxide (NO) is normally synthesized inside skeletal muscle fibers by both endothelial (eNOS) and neuronal (nNOS) nitric oxide synthases. In this study, we evaluated the influence of hypobaric hypoxia on the expression of NOS isoforms, argininosuccinate synthetase (AS), argininosuccinate lyase (AL), and manganese superoxide dismutase (Mn SOD) in the ventilatory muscles. Rats were exposed to hypobaric hypoxia ( approximately 95 mmHg) from birth for 60 days or 9-11 mo. Age-matched control groups of rats also were examined. Sixty days of hypoxia elicited approximately two- and ninefold increases in diaphragmatic eNOS and nNOS protein expression (evaluated by immunoblotting), respectively, and about a 50% rise in diaphragmatic NOS activity. In contrast, NOS activity and the expression of these proteins declined significantly in response to 9 mo of hypoxia. Hypoxia elicited no significant alterations in AS, AL and Mn SOD protein expression. Moreover, the inducible NOS (iNOS) was not detected in normoxic and hypoxic diaphragmatic samples. We conclude that diaphragmatic NOS expression and activity undergo significant adaptations to hypobaric hypoxia and that iNOS does not participate in this response.

Vascular endothelial growth factor upregulation in transient global ischemia induced by cardiac arrest and resuscitation in rat brain

This study examined vascular endothelial growth factor (VEGF) expression in rat brain after reversible global cerebral ischemia produced by cardiac arrest and resuscitation. Three alternative splicing forms, VEGF(188), VEGF(164) and VEGF(120), were observed in cortex, hippocampus and brainstem by RT-PCR analysis. After 24 h of recovery from cardiac arrest, mRNA levels corresponding to VEGF(188) and VEGF(164) were significantly increased by about double in all the regions analyzed. These mRNA levels remained elevated at 24 and 48 h of recovery but returned to basal expression after 7 days of recovery. Changes in VEGF(120) expression after cardiac arrest did not reach statistical significance. VEGF protein expression measured by Western blot was also increased by about double at 24 and 48 h of recovery but returned to control levels after 7 days of recovery. VEGF immunohistochemistry localized this increased expression mostly associated with astrocytes. Considering its biological activity, VEGF induction after cardiac arrest and resuscitation may be responsible for the increased vascular permeability and the resultant vasogenic edema, found 24-48 h after reversible global ischemia.

Roles of nitric oxide in brain hypoxia-ischemia

A large body of evidence has appeared over the last 6 years suggesting that nitric oxide biosynthesis is a key factor in the pathophysiological response of the brain to hypoxia-ischemia. Whilst studies on the influence of nitric oxide in this phenomenon initially offered conflicting conclusions, the use of better biochemical tools, such as selective inhibition of nitric oxide synthase (NOS) isoforms or transgenic animals, is progressively clarifying the precise role of nitric oxide in brain ischemia. Brain ischemia triggers a cascade of events, possibly mediated by excitatory amino acids, yielding the activation of the Ca2+-dependent NOS isoforms, i.e. neuronal NOS (nNOS) and endothelial NOS (eNOS). However, whereas the selective inhibition of nNOS is neuroprotective, selective inhibition of eNOS is neurotoxic. Furthermore, mainly in glial cells, delayed ischemia or reperfusion after an ischemic episode induces the expression of Ca2+-independent inducible NOS (iNOS), and its selective inhibition is neuroprotective. In conclusion, it appears that activation of nNOS or induction of iNOS mediates ischemic brain damage, possibly by mitochondrial dysfunction and energy depletion. However, there is a simultaneous compensatory response through eNOS activation within the endothelium of blood vessels, which mediates vasodilation and hence increases blood flow to the damaged brain area.

Neuronal-type NO synthase: transcript diversity and expressional regulation

Of the three established isoforms of NO synthase, the gene for the neuronal-type enzyme (NOS I) is by far the largest and most complicated one. The genomic locus of the human NOS I gene is located on chromosome 12 and distributed over a region greater than 200 kb. The nucleotide sequence corresponding to the major neuronal mRNA transcript is encoded by 29 exons. The full-length open reading frame codes for a protein of 1434 amino acids with a predicted molecular weight of 160.8 kDa. However, both in rodents and in humans, multiple, tissue-specific or developmentally regulated NOS I mRNA transcripts have been reported. They arise from the initiation by different transcriptional units containing alternative promoters (at least eight in the human gene), cassette exon deletions or insertions, and/or the usage of alternate polyadenylation signals. Depending on the insertions and deletions, translation results in functional or nonfunctional proteins. The use of alternative promoters can influence gene expression by various means. Indeed, NOS I is not a static, constitutively expressed enzyme, but subject to expressional regulation by various compounds and conditions. The molecular mechanisms underlying these regulations are currently being studied in several laboratories including our own.

The generation of nitric oxide by G protein-coupled receptors

The highly reactive free radical gas, nitric oxide, serves a variety of biomodulatory functions and has been implicated in a growing array of physiological and pathophysiological states. The striking differences between this labile substance and other, more conventional, signaling molecules highlight the tight degree of nitric oxide regulation that is required in order to maintain appropriate cellular homeostasis. The generation of nitric oxide represents a common component of the signal transduction pathways of a number of chemical signaling molecules that act via binding to G protein-coupled receptors. This review focuses on the relationship between this receptor superfamily, the generation of nitric oxide via the actions of the nitric oxide synthases and some of the inter- and intracellular roles of nitric oxide.

Episodic hypoxia enhances late hypoxic ventilation in developing rat: putative role of neuronal NO synthase

Nitric oxide (NO) is an excitatory neurotransmitter in the hypoxic ventilatory response (HVR). Furthermore, neuronal NO synthase (nNOS) activity in the developing rat correlates with the magnitude of late hypoxic ventilatory depression. To test the hypothesis that repeated short exposures to hypoxia may modify late HVR characteristics in young rats, we conducted 30-min hypoxic challenges in 2- to 3-day-old rat pups, before (Pre) and 6 h after (Post) they completed a series of eight cycles consisting of 5 min of hypoxia and 10 min of normoxia (Hyp-Norm) or normoxia throughout (Norm-Norm). In an additional group, similar challenges were performed after administration of either intraperitoneal vehicle or 25 mg/kg 7-nitroindazole (7-NI). Ventilation (VE) was measured using whole body plethysmography. Although no changes in peak VE responses occurred with episodic hypoxia (Pre vs. Post, P = not significant), late VE reductions were markedly attenuated in Post (DeltaVE from early to late: 7.2 +/- 1.5 ml/min in Pre vs. 4.5 +/- 1.1 ml/min in Post; P < 0.002). Furthermore, 7-NI treatment of Post animals was associated with late VE reductions to Pre levels in Hyp-Norm-exposed animals. Western blots of protein equivalents from the caudal brain stem revealed increased nNOS expression in Hyp-Norm compared with Norm-Norm (P < 0.01). Current findings suggest that repeated short hypoxic exposures improve the ability to sustain VE, which appears to be mediated by increased nNOS expression and activity in brain stem respiratory regions. We postulate that changes in nNOS may play a role in respiratory control plasticity.

Effect of chronic hypoxemia on the regulation of nitric-oxide synthase in the fetal sheep brain

We tested the hypothesis that chronic hypoxemia modulates NO production of the fetal brain by altering its gene and protein expression. Chronically instrumented preterm fetal sheep were made hypoxemic by placental embolization for 21 days. Fetal oxygen content was measured to determine the level of hypoxemia. The expression of both eNOS and nNOS proteins and mRNA and enzyme activities of fetal sheep cerebrum were measured and compared between normoxic and hypoxemic animals. Our results show that in utero hypoxemia downregulates both Ca2+ dependent NOS activity and expression of eNOS protein and mRNA in the fetal sheep brain. In contrast, hypoxemia increased nNOS protein and mRNA levels in the cerebrum. This suggests that chronic hypoxemia has an opposing effect on eNOS and nNOS gene regulation. We propose that increased nNOS activity during chronic hypoxemia may excessively stimulate the neurons and contribute to fetal brain injury. On the other hand, downregulation of eNOS activity and expression may compromise the neuroprotective effect of eNOS and, therefore, further exacerbate brain injury.

Regulation of nitric oxide production in response to skeletal muscle activation

Nitric oxide (NO) is synthesized in normal muscle fibers by the neuronal (nNOS) and the endothelial (ecNOS) isoforms of nitric oxide synthase (NOS). NO contributes to the regulation of several processes such as excitation-contraction coupling and mitochondrial respiration. We assessed in this study whether NO production is regulated in response to an acute increase in muscle activation. Three groups of anesthetized, tracheostomized, spontaneously breathing rats were examined after an experimental period of 3 h. Group 1 served as a control (no loading), whereas groups 2 and 3 were exposed to moderate and severe inspiratory resistive loads, respectively, which elicited tracheal pressures of 30 and 70% of maximum, respectively. Ventilatory (diaphragm, intercostal, and transverse abdominis) and limb (gastrocnemius) muscles were excised at the end of the experimental period and examined for NOS activity and NOS protein expression. Neither submaximal nor maximum tracheal pressures were altered after 3 h of resistive loading. Diaphragmatic and intercostal muscle NOS activities declined significantly in response to moderate and severe loading, whereas those of transverse abdominis and gastrocnemius muscles remained unchanged. On the other hand, resistive loading had no significant effect on ventilatory and limb muscle NOS isoform expression. We propose that a contraction-induced decline in muscle NOS activity represents a compensatory mechanism through which muscle contractility and mitochondrial function are protected from the inhibitory influence of NO.

Endotoxin-induced skeletal muscle contractile dysfunction: contribution of nitric oxide synthases

The aims of this study were to assess the role of nitric oxide (NO) and the contribution of different NO synthase (NOS) isoforms in skeletal muscle contractile dysfunction in septic shock. Four groups of conscious rats were examined. Group 1 served as control; group 2, 3, and 4 were injected with Escherichia coli endotoxin [lipopolysaccharide (LPS), 20 mg/kg i.p.] and killed after 6, 12, and 24 h, respectively. Protein expression was assessed by immunoblotting and immunostaining. LPS injection elicited a transient expression of the inducible NOS isoform, which peaked 12 h after LPS injection and disappeared within 24 h. This expression coincided with a significant increase in nitrotyrosine formation (peroxynitrite foot-print). Muscle expression of the endothelial and neuronal NOS isoforms, by comparison, rose significantly and remained higher than control levels 24 h after LPS injection. In vitro measurement of muscle contractility 24 h after LPS injection showed that incubation with NOS inhibitor (S-methyliosothiourea) restored the decline in submaximal force generation, whereas maximal muscle force remained unaffected. We conclude that NO plays a significant role in muscle contractile dysfunction in septic animals and that increased NO production is due to induction of the inducible NOS isoform and upregulation of constitutive NOS isoforms.