L-arginine is the physiological precursor for the formation of nitric oxide in endothelium-dependent relaxation

L-arginine improves endothelium-dependent vasodilation in hypercholesterolemic humans

Endothelium-dependent vasodilation is impaired in hypercholesterolemia, even before the development of atherosclerosis. The purpose of this study was to determine whether infusion of L-arginine, the precursor of the endothelium-derived relaxing factor, nitric oxide, improves endothelium-dependent vasodilation in hypercholesterolemic humans. Vascular reactivity was measured in the forearm resistance vessels of 11 normal subjects (serum LDL cholesterol = 2.76 +/- 0.10 mmol/liter) and 14 age-matched patients with hypercholesterolemia (serum LDL cholesterol = 4.65 +/- 0.36 mmol/liter, P < 0.05). The vasodilative response to the endothelium-dependent vasodilator, methacholine chloride, was depressed in the hypercholesterolemic group, whereas endothelium-independent vasodilation, induced by nitroprusside, was similar in each group. Intravenous administration of L-arginine augmented the forearm blood flow response to methacholine in the hypercholesterolemic individuals, but not in the normal subjects. L-arginine did not alter the effect of nitroprusside in either group. D-arginine had no effect on forearm vascular reactivity in either group. It is concluded that endothelium-dependent vasodilation is impaired in hypercholesterolemic humans. This abnormality can be improved acutely by administration of L-arginine, possibly by increasing the synthesis of endothelium-derived relaxing factor.

Effects of L-arginine on forearm vessels and responses to acetylcholine

This study was designed to investigate the effects of L-arginine (the substrate of endothelium-derived nitric oxide) in human forearm vessels. We examined whether intra-arterial infusion of L-arginine dilated forearm vessels and augmented vasodilatory responses to acetylcholine in young, healthy humans. The left brachial artery was cannulated for drug infusions and direct measurement of arterial pressure. Forearm blood flow was measured by a strain gauge plethysmograph. Intra-arterial infusions of L-arginine at 10, 20, 40, and 60 mg/min increased forearm blood flow from 4.7 +/- 0.6 to 4.9 +/- 0.5, 5.7 +/- 0.5, 7.2 +/- 0.8, and 8.2 +/- 0.9 ml.min-1.100 ml-1, respectively (n = 8, p less than 0.01), whereas D-arginine at the same doses did not alter forearm blood flow (n = 7). Intra-arterial infusions of acetylcholine (n = 7) (4, 8, 16, and 24 micrograms/min) and sodium nitroprusside (n = 5) (0.2, 0.4, 0.8, and 1.2 micrograms/min) increased forearm blood flow dose dependently (p less than 0.01 for both). Arterial pressure was not altered with infusions of these drugs. Responses to acetylcholine were augmented with simultaneous intra-arterial infusion of L-arginine at 10 mg/ml (p less than 0.01) but not with D-arginine. Responses to sodium nitroprusside were not altered by L-arginine. These results in human forearm resistance vessels support the notion that vasodilation induced by acetylcholine is a result of the conversion from L-arginine to endothelium-derived nitric oxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-dependent modulation of resistance vessel contraction: studies with NG-nitro-L-arginine methyl ester and NG-nitro-L-arginine

1. The effect of NG-nitro-L-arginine methyl ester (L-NAME) and NG-nitro-L-arginine (L-NOARG) on noradrenaline (NA)-induced contractility and acetylcholine (ACh)-induced endothelium-dependent relaxation was studied in rat mesenteric resistance arteries. 2. Third order branches of mesenteric arteries were dissected and mounted on two forty micron wires in a Mulvany myograph. 3. Incubation with L-NAME and L-NOARG (10 microM) caused a time-dependent shift in the 50% response to NA (ED50) (0.01 microM-10 microM) but was not associated with an increase in the maximum contractile response. 4. L-NAME and L-NOARG (10 microM) caused a time-dependent inhibition of ACh (1 microM)-induced relaxation with a maximum effect after 120 min. 5. Following endothelium removal, incubation with either L-NAME or L-NOARG caused no significant shift in the ED50, although the residual relaxation response to ACh (1 microM) was further attenuated. 6. Incubation with the cyclo-oxygenase inhibitor, indomethacin, enhanced the relaxation to ACh and reduced the inhibitory effects of L-NAME and L-NOARG. 7. In conclusion, L-NAME and L-NOARG are potent inhibitors of acetylcholine-induced endothelium-dependent relaxation in mesenteric resistance arteries. The shift in ED50 associated with these inhibitors suggests a probable role for the endothelium in modulating the contractility of the resistance vasculature.

Endothelial modulation of the ouabain-induced contraction in human placental vessels

Ouabain (1 x 10(-7)-3 x 10(-5) M) elicited concentration-dependent vasoconstriction in human placental arteries and veins. These responses, but not those produced by 10(-6) M 5-hydroxytryptamine, were increased after the removal of vascular endothelium. In placental arteries, the respective blockade of cyclooxygenase or lipoxygenase with indomethacin of 5,8,11,15-eicosatetraynoic acid as well as the inactivation of nitric oxide with phenidone or oxyhemoglobin and the inhibition of nitric oxide synthesis with NG-monomethyl L-arginine (all at 10(-5) M) did not mimic the effects of endothelial denudation on ouabain-evoked contractions. Bioassay experiments suggested that the above-mentioned endothelial effects are mediated by diffusible factors. 86Rb+ uptake, a method to measure sodium pump activity, was significantly reduced by the removal of endothelium. These results suggest the existence of an inhibitory modulation by the endothelium of the contractions induced by ouabain, likely mediated by a diffusible factor(s) released from these cells. The nature of this substance is unknown but is not related to prostaglandins or leukotrienes, and neither is it a nitric oxide-related compound. Its mechanism of action could be stimulating the activity of vascular sodium pump and/or antagonizing its inhibition by ouabain.

Role of basal release of nitric oxide on coronary flow and mechanical performance of the isolated rat heart

1. The role of nitric oxide (NO) in the regulation of coronary flow and mechanical function was studied in isolated working rat hearts. 2. The inhibition of basal release of NO by L-Ng-monomethylarginine (L-NMMA; 500 microM), a specific inhibitor of its synthesis, induced a reduction of coronary flow to 39.1% (+/- 3.4) of its basal level. 3. Associated with this was a drop of cardiac output to 58.10% (+/- 5.42) of control values. 4. This mechanical dysfunction, which was more pronounced in hypertrophied hearts, appears to be related to ischaemia, as it was prevented by simultaneously administered glyceryl trinitrate. Furthermore, L-NMMA did not alter the contractile activity of isolated cardiac myocytes, thereby excluding a direct toxic effect. 5. These findings provide evidence that NO plays a crucial role in the maintenance of basal coronary flow and appears to be essential for sustaining mechanical activity.

The non specificity of specific nitric oxide synthase inhibitors

L-NAME (Nw-Nitro-L-arginine methylester) and L-NMMA (NG- Monomethyl-L-arginine, monoacetate) are used widely as nitric oxide (NO) synthase inhibitors. Because of their functional groups (alcohols, amines and carboxylates), it appeared that they could interact with iron in a variety of systems. Using three in vitro models we observed these two compounds had inhibitory effects on cytochrome C reduction by ferrous iron, by ferrous iron accelerated by an unsaturated fatty acid or by epinephrine. This suggests that L-NAME and L-NMMA could have effects in iron containing systems found intracellularly apart from their inhibition of (NO) synthesis.

Nitric oxide-mediated gastric hyperemia decreases ethanol-induced gastric mucosal injury in uremic rats

We investigated whether the recently described endothelium-derived nitric oxide-mediated gastric hyperemia in the uremic rat protects the gastric mucosa against ethanol injury. Uremia was induced by subtotal nephrectomy. Basal gastric mucosal blood flow, measured by a hydrogen gas clearance technique, was significantly higher in uremic than control rats. Continuous intragastric perfusion with 40% ethanol produced significantly less gross and histological lesions in uremic than in control rats. The administration of 3 mg/kg of NW-nitro-L-arginine methyl ester, a specific inhibitor of nitric oxide biosynthesis, decreased resting gastric mucosal blood flow to control levels in uremic rats, but had no effect on basal gastric blood flow in control rats. This pretreatment with the inhibitor of nitric oxide biosynthesis increased 40% ethanol-induced gastric mucosal lesions in uremic rats to the same level as that observed in control rats, but had no effect on lesions in control rats. In conclusion, this study suggests that in the uremic rat, gastric hyperemia, mediated by increased endothelium-derived nitric oxide, attenuates ethanol-induced gastric mucosal injury.

Production of reactive nitrogen intermediates by bone marrow-derived macrophages on treatment with cisplatin in vitro

L929 culture medium (a source of macrophage colony stimulating factor (M-CSF) or recombinant granulocyte-macrophage colony stimulating factor (rGM-CSF)-derived bone marrow macrophages treated with cisplatin or lipopolysaccharide (LPS) (10 micrograms/ml) were effective in the production of L-arginine-dependent reactive nitrogen intermediates (RNI) and generation of tumouricidal activity. The abilities of RNI secretion and related tumouricidal activity against P815 mastocytoma cells were compared. These parameters were found to be closely correlated in various experiments. RNI secretion and generation of bone marrow macrophage-mediated tumouricidal activity were significantly inhibited by L-N-monomethyl arginine (L-NMMA), a specific inhibitor of the L-arginine pathway, but L-NMMA did not inhibit macrophage-mediated killing of tumour necrosis factor (TNF)-sensitive Wehi cells, suggesting that activated macrophages exhibit at least two cytolytic mechanisms, one by L-arginine-dependent nitric oxide pathway and another by TNF-mediated killing. The present findings suggest that the mechanism of tumour cell killing by activated macrophages may differ, depending on the tumour cell type, and reactive nitrogen intermediates play a major role in cisplatin-mediated activation of bone marrow-derived macrophages.

Estrogen-induced uterine vasodilatation is antagonized by L-nitroarginine methyl ester, an inhibitor of nitric oxide synthesis

OBJECTIVES

Our study was designed to determine whether nitric oxide mediates estrogen-induced increases in uterine blood flow.

STUDY DESIGN

Six nonpregnant oophorectomized ewes were instrumented with uterine artery flow probes and catheters. Ewes received estradiol-17 beta 1 microgram/kg, which maximally increased uterine blood flow by 120 minutes. Each animal then received local bolus injections of the nitric oxide synthetase inhibitor L-nitroarginine methyl ester.

RESULTS

Estradiol-17 beta increased uterine blood flow from 16 +/- 6 to 139 +/- 32 ml/min by 120 minutes. Local uterine artery administration of L-nitroarginine methyl ester (1 to 30 mg) caused a dose-related decrease in uterine blood flow, which reached a maximum of 59% +/- 6% inhibition. Higher doses of L-nitroarginine methyl ester less than or equal to 10 mg/kg (330 to 460 mg) given locally led to a maximum inhibition of 79% +/- 3% but showed systemic responses.

CONCLUSION

Estradiol-17 beta-induced increases in uterine blood flow are mediated mainly by nitric oxide; the observed vasodilation can be antagonized by the intraaterial administration of nitric oxide synthetase inhibitor L-nitroarginine methyl ester.

Regulation of regional cerebral blood flow by cholinergic fibers originating in the basal forebrain

We review mainly recent studies on vasodilative regulation of cortex and hippocampus by central cholinergic nerves originating in the basal forebrain. We also briefly review the influence of other central noradrenergic fibers originating in the locus ceruleus, serotonergic fibers originating in the dorsal raphe nucleus, dopaminergic fibers originating in the substantia nigra, and peripheral sympathetic and parasympathetic nerve fibers upon regulation of regional cerebral blood flow. Local metabolites have long been considered to play an important physiological role in regulating regional cerebral blood flow. However, the evidence reviewed here emphasizes that the regulation of regional cerebral blood flow by these central cholinergic nerves is independent of regional metabolism. We propose through this review that although studies investigating neural regulation of cortical and hippocampal blood flow by cholinergic fibers originating in the basal forebrain have added much to the understanding of regulation of regional cerebral blood flow further studies are needed to determine the physiological relevance of regional cerebral blood flow in relation to higher nervous functions such as memory, learning, and personality, and changes in these cognitive functions with aging and pathology such as Alzheimer's disease.

Inhibition of long-term potentiation by NMDA-mediated nitric oxide release

Activation of N-methyl-D-aspartate (NMDA) receptors before tetanic stimulation blocks long-term potentiation (LTP) in the CA1 region of the hippocampus. This NMDA-mediated inhibition of LTP can be reversed by the nitric oxide (NO) inhibitors L-NG-monomethyl-arginine or hemoglobin and mimicked by sodium nitroprusside. These results indicate that the timing of NO release relative to high-frequency activation of CA1 synapses may be an important determinant of LTP generation and suggest that NO may play a positive or negative modulatory role in LTP depending on prior events at the tetanized synapse and the ambient concentration of excitatory amino acids.

The quantitative contribution of nitric oxide and sensory nerves to bradykinin-induced inflammation in rat skin microvasculature

Using a blister model in the rat hind footpad, the present study undertook to examine the relative contribution of sensory nerves and nitric oxide (NO) to the inflammatory response induced by bradykinin (BK). Using this model, combined with laser Doppler flowmetry, we were able to simultaneously monitor two parameters of the inflammatory response, namely vasodilatation (VD) and plasma extravasation (PE). Perfusion of BK (1, 10 or 100 microM) over the blister base elicited both VD and PE responses which were dose-dependent. The VD response was of rapid onset, sustained at the lowest concentration (1 microM), and showed tachyphylaxis at the highest two concentrations (10 and 100 microM). The PE response, however, was delayed in onset at the lower concentration but the response was maintained at all concentrations. The endothelium-independent vasodilator, sodium nitroprusside. (SNP, 100 microM), was used as an internal control and elicited a rapid maintained VD response. In rats pretreated as neonates with capsaicin to destroy primary sensory afferents, the inflammatory response to 10 microM BK was significantly smaller (50% and 64% decrease in VD and PE, respectively). The selective inhibitor of NO synthase, NG-nitro-L-arginine (L-NORAG) at 100 microM significantly attenuated the inflammatory response to BK in control rats (76% and 60% decrease in VD and PE, respectively) with a further decrease in the response in capsaicin pretreated rats. The inactive stereoisomer NG-nitro-D-arginine (D-NORAG) (100 microM) did not affect the inflammatory response to BK. The vasodilator response to SNP was intact in capsaicin pretreated rats and was not affected by either L-NORAG or D-NORAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-accelerated hyporesponsiveness of norepinephrine-elicited contraction of rat aorta in the presence of bacterial lipopolysaccharide

The role of the endothelium in the hyporesponsiveness of alpha-adrenoceptor-mediated contractions of the rat aorta was investigated. The norepinephrine-induced maximal contraction was diminished after repeated addition of the agonist. The hyporesponsiveness of the maximal contraction was endothelium dependent, being prevented by NG-monomethyl-L-arginine (0.5 mM), L-argininosuccinic acid (0.5 mM), puromycin (IC50 = 100 microM), actinomycin D (IC50 = 80 nM) but not by indomethacin, which suggests that nitric oxide (NO) synthase is induced. The sensitivity of the rings to NO-induced relaxation remained unchanged. The above-mentioned hyporesponsiveness of norepinephrine-induced maximal contractions of aorta rings was also observed after a 5-h incubation without norepinephrine. The agonist-independent hyporesponsiveness was also prevented by NG-monomethyl-L-arginine, puromycin and actinomycin D, which suggests that NO synthase is induced. Moreover, the norepinephrine-independent hyporesponsiveness was prevented by polymyxin B (10 micrograms/ml), which suggests that bacterial lipopolysaccharide (LPS) might be involved. The concentration of contaminating LPS was 89 +/- 11 ng/ml. When the concentration of contaminating LPS was reduced to 40-70 pg/ml, the hyporesponsiveness of the maximal contraction did not occur after repeated addition of norepinephrine or alter a 5-h incubation without the agonist. An addition of 30 or 100 ng/ml of E. coli lipopolysaccharide to the organ bath reproduced the hyporesponsiveness of the maximal contraction. After a 5-h incubation of aortic rings with 30 ng/ml LPS, only the endothelium-intact ring showed a reduced contraction. However, a 24-h incubation reduced the contraction even in the absence of endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Delayed increase of extracellular arginine, the nitric oxide precursor, following electrical white matter stimulation in rat cerebellar slices

Amino acid levels were measured in perfusates from biplanar slices of rat cerebellum installed in a Krebs-filled three-compartment chamber. The two lateral compartments housed the white matter and a section containing parallel fibres respectively. The central compartment housed cortical structures, including the Purkinje cell and granule cell bodies. This arrangement allows selective electrical stimulation of the parallel fibre or mossy fibre pathways, recording of the evoked responses to such stimulation and collection of the perfusion medium passing through the central chamber for amino acid analysis using high-pressure liquid chromatography (HPLC). Both, 2-Hz and 5-Hz stimulation of white matter caused a delayed increase in arginine levels in the perfusate. Since L-arginine is the physiological precursor of nitric oxide, a neuronal messenger in the brain, the data suggest that physiological stimuli can result in the release of this precursor, possibly to supply the nitric oxide synthase.

Effects of nitric oxide-related compounds and carperitide on hemodynamics and hematocrit in anesthetized rats

Sodium nitroprusside, nitroglycerin and carperitide (alpha-human ANP) all reduced mean blood pressure, but only carperitide increased the hematocrit in rats with bilateral renal artery- and ureter-ligation. NG-Monomethyl-L-arginine, a selective inhibitor of nitric oxide synthesis, elevated the mean blood pressure but did not change the hematocrit significantly. These findings suggest that ANP has a physiological role in regulating circulatory blood volume distinct from that of NO, although both increase intracellular cyclic GMP in the vasculature.

Inhibition of cytokine-induced nitric oxide production by transforming growth factor-beta 1 in human smooth muscle cells

1. Experiments were performed to investigate the effects of human recombinant interleukin-1 beta on the production of vasoactive substances by human aortic smooth muscle cells in culture. Smooth muscle cells were cultured either on microcarrier beads for bioassay experiments, or in multiwell plates for the determination of nitrite levels. 2. Cells were grown on microcarrier beads, treated with interleukin-1 beta or vehicle (control) for 24 h, and packed in a column which was perfused with oxygenated Krebs-Ringer solution in the presence of indomethacin. The activity of the perfusates was bioassayed by measuring the changes in tension of a contracted ring of Wistar rat aorta without endothelium, and by evaluating the modulation of thrombin-induced platelet aggregation. 3. Perfusates from interleukin-1 beta treated cells evoked relaxations of the contracted detector tissues, and microcarrier beads covered with treated cells inhibited thrombin-induced platelet aggregation. Superoxide dismutase enhanced these effects whereas Methylene Blue abolished them. Control cells evoke neither relaxation nor inhibition of platelet aggregation. Interleukin-1 beta induced a time- and concentration-dependent production of nitrite. Cycloheximide and nitro-L-arginine inhibited the relaxations and the production of nitrite evoked by interleukin-1 beta-treated cells. L-Arginine but not D-arginine overcame the blockade elicited by nitro-L-arginine. Transforming growth factor-beta 1 reduced the interleukin-1 beta-dependent generation of nitrite by cultured smooth muscle cells and relaxation of contracted bioassay tissues. 4. Interleukin-1 beta, transforming growth factor-beta 1, Methylene Blue and L-arginine-related compounds did not induce significant variations of tension of the detector rings. 5. These data demonstrate that the inflammatory and immunological mediator interleukin-1 can stimulate the production of a nitric oxide-like substance(s) in cultured human smooth muscle cells leading to the activation of soluble guanylate cyclase. Liberation of transforming growth factor-beta by activated platelets may inhibit these reactions.

Involvement of the nitric oxide-cyclic GMP pathway in the desensitization of bradykinin responses of cultured rat sensory neurons

Bradykinin (BK) excites a subset of dorsal root ganglion neurons by inducing an inward cation current (IBK) that strongly desensitizes and is accompanied by elevations in cGMP. We have examined the links between cGMP metabolism and IBK. The BK dose dependencies of IBK activation, desensitization, and cGMP production are comparable. Stimulation (with sodium nitroprusside [NP] or 8-bromo-cGMP [8Br-cGMP]) or inhibition (with methylene blue, hemoglobin, and nitric oxide synthase [NOS] inhibitors) of cGMP levels did not mimic or diminish IBK. However, desensitization was affected by the following agents: first, desensitization was enhanced by NP and reduced by NOS inhibitors. Second, the effects of NOS inhibitors could be overcome by 8Br-cGMP or L-arginine. Third, 8Br-cGMP modification of desensitization required receptor occupancy. We conclude that the NO-cGMP pathway affects a component of IBK desensitization at the receptor or G protein level.

Biosynthesis of nitric oxide and citrulline from L-arginine by constitutive nitric oxide synthase present in rabbit corpus cavernosum

The objective of this study was to determine whether a constitutive isoform of nitric oxide (NO) synthase is present in rabbit corpus cavernosum that could account for the involvement of the L-arginine-NO pathway in neurogenically-elicited relaxation of the corpus cavernosum and, therefore, penile erection. Citrulline was determined by monitoring the formation of 3H-citrulline from 3H-L-arginine. NO was determined by monitoring the formation of total NO(x) (NO+nitrite [NO2-]+nitrate [NO3-]) by chemiluminescence after reduction of NO(x) to NO by acidic vanadium (III). Equimolar quantities of NO plus citrulline were generated from L-arginine and the formation of both products was time-dependent at 37 degrees C. NO synthase activity was distributed almost entirely to the cytosolic fraction. Enzymatic activity was completely dependent on NADPH, calmodulin, and calcium. Addition of tetrahydrobiopterin increased NO synthase activity by about 30 percent. The NO synthase inhibitor NG-nitro-L-arginine, abolished enzymatic activity. The Km for L-arginine was 17 microM and the Vmax of the reaction was 18 pmol/min/mg protein. These observations indicate that a cytosolic, constitutive isoform of NO synthase, like that found in brain neuronal tissue, is present in rabbit corpus cavernosum.

Protamine releases endothelium-derived relaxing factor from systemic arteries. A possible mechanism of hypotension during heparin neutralization

BACKGROUND

When used to reverse the anticoagulant effect of heparin, protamine sulfate often causes vasodilation that can lead to systemic hypotension. Protamine is rich in the basic amino acid arginine, which is the precursor of endothelial cell synthesis of nitric oxide, and nitric oxide is the active component of endothelium-derived relaxing factor (EDRF).

METHODS AND RESULTS

To determine whether the hypotensive effect of protamine could be due to stimulated release of EDRF, we studied rings (4-5 mm) of canine coronary, femoral, and renal artery suspended in organ chambers containing physiological salt solution (37 degrees C and 95% O2-5% CO2). Arterial rings with and without endothelium were contracted with prostaglandin F2 alpha (2 x 10(-6) M) and exposed to increasing concentrations of protamine (final organ bath concentration, 40-400 micrograms/ml). In arterial segments without endothelium, protamine caused only a modest decrease in tension. However, protamine induced concentration-dependent relaxation in all arterial segments with endothelium, which was significantly greater than in segments without endothelium (p less than 0.05). The endothelium-dependent relaxation induced by protamine was inhibited by NG-monomethyl-L-arginine (L-NMMA) (10(-5) M), but L-NMMA had no effect on rings without endothelium. The action of L-NMMA could be reversed by L-arginine (10(-4) M) but not D-arginine (10(-4) M).

CONCLUSIONS

This study demonstrates that protamine stimulates the release of EDRF from arterial endothelium, and that endothelium-dependent vasodilation may be an important cause of systemic hypotension during protamine infusion.

The role of L-arginine in ameliorating reperfusion injury after myocardial ischemia in the cat

BACKGROUND

Myocardial ischemia followed by reperfusion results in endothelial dysfunction characterized by a reduced release of endothelium-derived relaxing factor (EDRF). Because EDRF has been characterized as nitric oxide, we examined the ability of L-arginine, the substrate for nitric oxide synthesis, to protect in a feline model of myocardial ischemia plus reperfusion.

METHODS AND RESULTS

The effects of L-arginine were investigated in a 6-hour model of myocardial ischemia and reperfusion in pentobarbital-anesthetized cats. A bolus administration (30 mg/kg) of L-arginine, or its enantiomer D-arginine, was given followed by a continuous infusion of 10 mg/kg/min for 1 hour starting 10 minutes before reperfusion. Myocardial ischemia plus reperfusion in cats receiving D-arginine resulted in severe myocardial injury and endothelial dysfunction characterized by marked myocardial necrosis, high cardiac myeloperoxidase activity in ischemic cardiac tissue, and loss of acetylcholine- and A-23187-induced endothelium-dependent relaxation in coronary artery rings. In contrast, myocardial ischemia plus reperfusion cats treated with L-arginine exhibited a reduced area of cardiac necrosis (16 +/- 2% versus 41 +/- 5% of area at risk, p less than 0.01), lower myeloperoxidase activity in the ischemic region (0.3 +/- 0.08 versus 0.8 +/- 0.10 units/100 mg tissue, p less than 0.05), and significant preservation of acetylcholine- (p less than 0.01) and A-23187- (p less than 0.01) induced endothelial-dependent relaxation.

CONCLUSIONS

These results demonstrate the ability of L-arginine to reduce necrotic injury in a cat model of myocardial ischemia plus reperfusion, and this reduction in infarct size is associated with the preservation of endothelial function and attenuation of neutrophil accumulation in ischemic cardiac tissue.

Effects of inhibition of nitric oxide formation on regional blood flow in experimental myocardial infarction

BACKGROUND

Large myocardial infarction is associated with reactive hypertrophy and dilation of the left ventricle, depressed coronary flow reserve, and the development of heart failure including systemic vasoconstriction. We hypothetized that changes in endothelial function, e.g., in the synthesis or action of nitric oxide in the coronary and peripheral vasculatures, might be involved in the depressed coronary flow reserve and increased systemic vascular resistance observed in postinfarction myocardial hypertrophy and failure.

METHODS AND RESULTS

The regional blood flow changes that occur as a result of inhibiting the basal release of nitric oxide with NG-monomethyl-L-arginine (L-NMMA) and how this regional pattern may be altered in large MI (infarct size, 30-51% of left ventricle) were examined. Measurements were made 24 hours and 8 weeks after myocardial infarction or sham operation in conscious rats. The left ventricular end-diastolic pressure and effects of L-NMMA on left ventricular end-diastolic pressure was similar 24 hours and 8 weeks after myocardial infarction. The effects of L-NMMA (30 mg/kg i.v.) on heart rate and blood pressure were similar in infarcted and sham animals. L-NMMA exerted a marked vasoconstriction in the renal, splanchnic, cutaneous, and cerebral circulations of similar magnitude in sham-operated rats and animals with myocardial infarction. The coronary vasoconstrictor effect of L-NMMA was attenuated significantly in the hypertrophied right and noninfarcted left ventricle of 8-week-old infarcted rats (p less than 0.01 versus sham-operated animals) but not 24 hours after induction of myocardial infarction when cardiac hypertrophy has not yet developed. The increase in left ventricular coronary resistance in 8-week-old infarcted animals was inversely related to infarct size (r = -0.787, p = 0.012, n = 9). Nitroglycerin exerted similar increases in coronary blood flow in rats with chronic myocardial infarction and sham-operated animals, arguing against a reduced vascular responsiveness to nitric oxide. Transmission electron microscopy of coronary resistance vessels in 8-week-old infarcted animals did not reveal endothelial abnormalities.

CONCLUSIONS

These data suggest that the basal release of nitric oxide in the renal, intestinal, and cutaneous circulations is not affected adversely in this model of myocardial infarction and failure. However, the blunted coronary vasoconstrictor effect of L-NMMA late after large myocardial infarction supports the view that the basal release of nitric oxide is impaired in postinfarction reactive cardiac hypertrophy.

Nitric oxide and sensory nerves are involved in the vasodilator response to acetylcholine but not calcitonin gene-related peptide in rat skin microvasculature

1. The contributions of sensory nerves and nitric oxide (NO) to vasodilator responses to acetylcholine (ACh) and calcitonin gene-related peptide (CGRP) were examined in rat skin microvasculature with a laser Doppler flowmeter to monitor relative blood flow. 2. Perfusion of ACh (100 microM; for 30 min) over a blister base on the rat hind footpad elicited microvascular vasodilatation and this response was not sustained. CGRP (1 microM; 10 min perfusion) also elicited vasodilatation and this response was maintained even when CGRP was no longer in contact with the blister base. 3. The vasodilator response to ACh was significantly smaller in rats pretreated as neonates with capsaicin to destroy primary sensory afferents than it was in age-matched controls. The vasodilator response to CGRP was unaffected by capsaicin pretreatment. 4. Selective inhibitors of NO synthase, NG-nitro-L-arginine (L-NOARG) and NG-monomethyl-L-arginine (L-NMMA) (both at 100 microM) attenuated the vasodilator response to ACh in control rats, but had no effect on the vasodilator response to CGRP. There was a significant L-NOARG-resistant component in control rats while in capsaicin-treated rats the vasodilator response to ACh was virtually abolished by L-NOARG. The inactive stereoisomer NG-monomethyl-D-arginine (100 microM) did not affect the vasodilator response to ACh. 5. The efficacy of L-NOARG and L-NMMA as inhibitors of endothelium-dependent responses was confirmed by use of an endothelium-dependent vasodilator, the calcium ionophore A23187 (100 microM; 10 min perfusion). Vasodilatation to A23187 was strongly attenuated by both L-NOARG and L-NMMA.6. These results suggest that sensory nerves and NO are both involved in the dilatation produced by ACh in rat skin microvasculature. A component of the vasodilator response elicited by ACh involves a direct action on the microvascular endothelium with subsequent generation of NO, while an additional component is elicited via activation of sensory nerves. The vasodilator mediator(s) released by ACh from sensory nerves acts largely independently of NO.7. The vasodilator response to CGRP is independent of a prejunctional action on sensory nerves and of NO.

Interaction between microvascular alpha 1- and alpha 2-adrenoceptors and endothelium-derived relaxing factor

Intravital microscopy was used to study the effect of endothelium-derived relaxing factor (EDRF) on microvascular adrenoceptor sensitivity in rat cremaster skeletal muscle. NG-Monomethyl L-arginine (L-NMMA, 1-300 microM), an inhibitor of EDRF formation, produced concentration-dependent constriction of arterioles and venules. When an intermediate amount of alpha 1- versus alpha 2-adrenoceptor tone was first produced with bath-added norepinephrine (NE) in the presence of rauwolscine or prazosin, L-NMMA caused constriction with greater potency and efficacy during alpha 2 than during alpha 1 tone. During localized alpha 1 or alpha 2 constriction along an arteriole by perivascular micropipette suffusion of NE in the presence of rauwolscine or prazosin, again, bath-added L-NMMA produced constriction with greater potency during alpha 2 than during alpha 1 constriction. Like L-NMMA, disruption of EDRF release by microembolization caused baseline arteriole constriction and selectively increased alpha 2 sensitivity 75-fold. Although these findings support the hypothesis that endothelial cells possess alpha 2-adrenoceptors that promote EDRF release, a greater susceptibility of alpha 2 than alpha 1 constriction to EDRF inhibition could also account for the results. In support of this latter possibility, alpha 2 constriction was approximately 50-fold more susceptible than alpha 1 constriction to inhibition by the EDRF-like nitrodilator nitroprusside. The similarity in magnitude of this difference in sensitivity with the difference obtained in the embolization experiments does not support the hypothesis that microvascular endothelial cells in skeletal muscle possess EDRF-promoting alpha 2-adrenoceptors. However, these data do suggest that endogenous EDRF release modulates basal arteriole and venule tone and that alpha 2-adrenoceptor constriction is more sensitive than alpha 1 constriction to inhibition by EDRF.

Analysis of the depressant effect of the endothelium on contractions of rabbit isolated basilar artery to 5-hydroxytryptamine

1. The effects of endothelium removal and of a number of pharmacological agents known to modify endothelial cell function on the contractile response of rabbit isolated basilar arteries to 5-hydroxytryptamine (5-HT) and other vasoconstrictors were studied. 2. Endothelium removal slightly reduced the contractile response to potassium chloride (40 mM) but markedly augmented and potentiated contractions to 5-HT (1 nM-10 microM). 3. L-NG-nitro-arginine (L-NOARG, 1-30 microM), an inhibitor of nitric oxide formation in vascular endothelial cells, evoked endothelium-dependent contraction, and augmented and potentiated contractions to 5-HT in endothelium-intact but not endothelium-denuded tissues. Prior incubation with L-arginine (1 mM), but not D-arginine (1 mM), abolished these effects of L-NOARG (1 microM). L-NOARG (30 microM) also augmented contractions of endothelium-intact tissues to noradrenaline, prostaglandin F2 alpha, and to a lesser degree endothelin-1. 4. Neither glibenclamide (3 microM) nor N-ethylmaleimide (1 microM), putative inhibitors of the effects of endothelium-derived hyperpolarizing factor (EDHF) and of agonist-stimulated endothelium-derived relaxing factor (EDRF) release respectively, had any effect on either resting tension or the contractile response to 5-HT. In some tissues indomethacin (3 microM), a cyclo-oxygenase inhibitor, produced a small contraction and augmented the contractile response to 5-HT, but in most cases indomethacin was without effect. 5. In endothelium-intact tissues precontracted with uridine 5'-triphosphate (UTP; 100 microM), 5-HT did not evoke relaxation but rather caused further contraction. Under the same conditions acetylcholine (0.01-10 microM) evoked endothelium-dependent relaxation.6. These data demonstrate that the endothelium profoundly depresses contractions of rabbit isolated basilar artery to 5-HT, and that this phenomenon can be fully accounted for by the release of an L-NOARG-sensitive relaxing factor. Neither glibenclamide-sensitive EDHF nor cyclo-oxygenase products plays a major role. As we could find no evidence that 5-HT stimulates the production of EDRF per se, and L-NOARG caused endothelium-dependent contraction and augmented contractions to other vasoconstrictor agents, it seems likely that a basal release of EDRF underlies this phenomenon.

Nitric oxide synthase from cerebellum catalyzes the formation of equimolar quantities of nitric oxide and citrulline from L-arginine

This study examined whether constitutive nitric oxide (NO) synthase from rat cerebellum catalyzes the formation of equimolar amounts of NO plus citrulline from L-arginine under various conditions. Citrulline was determined by monitoring the formation of 3H-citrulline from 3H-L-arginine. NO was determined by monitoring the formation of total NOx (NO+nitrite [NO2-] + nitrate [NO3-]) by chemiluminescence after reduction of NOx to NO by acidic vanadium (III). Equal quantities of NO plus citrulline were generated from L-arginine and the formation of both products was linear for about 20 min at 37 degrees C provided L-arginine was present in excess to maintain a zero order reaction rate. Deletion of NADPH, addition of the calmodulin antagonist calmidazolium, or addition of NO synthase inhibitors (NG-methyl-L-arginine, NG-amino-L-arginine) abolished or markedly inhibited the formation of both NO and citrulline. The Km for L-arginine (14 microM; 18 microM) and the Vmax of the reaction (0.74 nmol/min/mg protein; 0.67 nmol/min/mg protein) were the same whether NO or citrulline formation, respectively, was monitored. These observations indicate clearly that NO and citrulline are formed in equimolar quantities from L-arginine by the constitutive isoform of NO synthase from rat cerebellum.

Importance of NO/EDRF for glomerular and tubular function: studies in the isolated perfused rat kidney

The effect of the addition of N omega-nitro-L-arginine (L-NNA, 10 and 100 microM) to isolated rat kidneys perfused with a complex medium containing 21 amino acids has been studied. A cyclooxygenase inhibitor was added throughout to block prostaglandin synthesis. L-NNA caused significant reductions in renal perfusion flow rate (PFR, 9.8 +/- 1.4 vs. 15.9 +/- 1.1 ml.min-1.g kidney wt-1, P less than 0.0001), glomerular filtration rate (GFR, 566 +/- 57 vs. 705 +/- 47 microliters.min-1.g kidney wt-1, P less than 0.05) and an increase in the relative filtration fraction (%FF, 7.0 +/- 0.6 vs. 5.2 +/- 0.4%, P less than 0.05) compared to control kidneys. L-NNA perfused kidneys had a lower absolute sodium (72 +/- 9 vs. 88 +/- 4 mumol.min-1.g kidney wt-1, P less than 0.05) and glucose reabsorption (3.5 +/- 0.5 vs. 5.4 +/- 0.4 mumol.min-1.g kidney wt-1, P less than 0.05), corresponding mainly to a lower sodium and glucose filtration. However, the relative reabsorption of sodium and glucose in the presence of L-NNA was attenuated, too (82.8 +/- 2.0 vs. 87.0 +/- 3.3% P less than 0.05 and 91.3 +/- 1.1 vs. 94.1 +/- 0.5%, P less than 0.05). Potassium handling and protein excretion were not changed significantly; fractional protein excretion increased slightly with the addition of L-arginine (47 +/- 5 vs. 55 +/- 7 ng.microliters-1, P less than 0.05). The differences between control and L-NNA treated kidneys (with the exception of differences in FRGluc) could be fully (L-NNA, 10 microM) or partially (L-NNA 100 microns) reversed by adding L-arginine (1 mM) to the perfusion medium. The observed results could be obtained in two different rat strains (Sprague-Dawley and Wistar). Only L-NNA and L-arginine caused the observed changes, while D-NNA and D-arginine were without effect. It is concluded that NO/EDRF is basally released from the isolated perfused rat kidney, and is of importance not only in the regulation of renal hemodynamics but also in the regulation of renal tubular function.

Nitric oxide modulates coronary autoregulation in the guinea pig

A guinea pig heart Langendorff preparation was used in the present study to test the hypothesis that the coronary endothelium modulates coronary autoregulation through the production of nitric oxide (NO). Pacing at 250 beats per minute and venting the left ventricle to ensure that the hearts did no external work were performed in an attempt to reduce the metabolic stimulus to coronary vasomotion and keep it constant. We measured the responses of coronary flow and oxygen metabolism to stepwise changes of the perfusion pressure over the range between 18 and 85 mm Hg. The hearts exhibited autoregulation between 25 and 55 mm Hg and active vasodilation at perfusion pressures above that range. Perfusion with 100 microM NG-nitro-L-arginine (NNLA), an inhibitor of NO synthase, decreased coronary flow over the entire range of perfusion pressures and abolished active vasodilation over 65 mm Hg, thus widening the autoregulatory range. The administration of 200 microM L-arginine, but not D-arginine, reversed the action of NNLA. Inhibition of the cyclooxygenase pathway by 10 microM indomethacin did not affect autoregulation. Perfusion with 1 nM arginine vasopressin, a direct smooth muscle constrictor, lowered coronary flow rate to the same extent as NNLA at 55 mm Hg but did not prevent the pressure-dependent increase in flow above that pressure. These observations suggest that 1) the coronary endothelium actively modulates coronary autoregulation through the production of NO but not prostanoids, 2) mechanical stress (shear stress and/or stretching secondary to vasodilation) may be the stimulus to NO production, especially above the autoregulatory range, and 3) autoregulatory tone is likely to be myogenic in origin rather than mediated by extrinsic vasoconstrictors.

Nitric oxide functions as an inhibitor of platelet adhesion under flow conditions

BACKGROUND

Nitric oxide (NO) has been identified as endothelium-derived relaxing factor (EDRF), which, in addition to its relaxant effects on vascular smooth muscle cells, is also a potent inhibitor of platelet aggregation. An inhibitory role on platelet adhesion has been suggested from experiments with washed platelets under static conditions. We have determined whether endothelium-derived and exogenous NO also regulates platelet adhesion in whole blood under flow conditions.

METHODS AND RESULTS

The effect of endothelium-derived NO was studied by the addition of specific inhibitors of NO production, L-N-monomethyl arginine (L-NMMA) and N-iminoethyl-L-ornithine (L-NIO), to a perfusion system in which both endothelial cells and their matrices were present. A concentration-dependent increase in platelet adhesion to the matrix was found with a maximum inhibition at a concentration of 2 mM L-NMMA and 0.1 mM L-NIO. The effect was dependent on the presence of endothelial cells, because no increase in platelet adhesion was observed in their absence. The effect of exogenous NO was tested in a specially devised perfusion system in which the NO was introduced at the site of adhesion by means of a porous membrane on which an extracellular matrix of endothelial cells was present. Inhibition of platelet adhesion by NO was found at all shear rates tested and after all perfusion periods.

CONCLUSIONS

These results demonstrate that NO is a potent inhibitor of platelet adhesion under flow conditions and thereby contributes to the regulatory role of vascular endothelial cells on platelet-vessel wall interaction.

Attenuated afferent arteriolar response to acetylcholine in Goldblatt hypertension

We tested the hypothesis that endothelium-dependent afferent arteriolar vasodilation is impaired in the nonclipped kidney of two-kidney, one clip Goldblatt hypertensive rats relative to sham-operated controls. Five to six weeks after positioning of a 0.25-mm clip on the left renal artery, systolic pressure averaged 173 +/- 10 mm Hg in Goldblatt rats and 118 +/- 4 mm Hg in controls (p less than 0.01). The right kidney was harvested for videometric study of the microvasculature using the in vitro blood-perfused juxtamedullary nephron technique. Kidneys from Goldblatt and control rats were perfused at renal arterial pressures of 150 and 110 mm Hg, respectively. Afferent arteriolar inside diameter did not differ between control (20.3 +/- 0.7 microns) and Goldblatt (21.1 +/- 1.7 microns) kidneys. Determination of afferent responses to increasing concentrations of the endothelium-dependent vasodilator acetylcholine (1 nM to 10 microM) in the bathing solution unveiled a shift to the right in the dose-response relation in Goldblatt rats. Afferent arterioles from control kidneys dilated significantly when exposed to 1 nM acetylcholine, whereas a 1,000-fold higher concentration was required to dilate arterioles from Goldblatt rats. Sodium nitroprusside, an endothelium-independent vasodilator, increased afferent diameter to a similar extent in both groups. In a separate group of normal kidneys, vasodilator responses to 10 microM acetylcholine were completely blocked by 1,000 microM nitro-L-arginine, an inhibitor of nitric oxide synthesis. Thus, endothelium-dependent afferent vasodilation appears to be impaired in the nonclipped kidney of Goldblatt hypertensive rats. This phenomenon could contribute to the altered renal hemodynamic status characteristic of Goldblatt hypertension.

Nitric oxide mediates relaxation in rabbit and human corpus cavernosum smooth muscle

We investigated in vitro the relaxant effect of exogenous acetylcholine (ACh) and electric-field stimulation (EFS) on rabbit and human corpus cavernosum smooth muscle strips (CC) precontracted with phenylephrine. The effects of EFS and ACh were monitored alone, after muscarinic receptor blockade and after inhibition of nitric oxide (NO) formation with L-N-nitro-arginine (L-NOARG). In rabbit and human CC, both atropine and L-NOARG abolished the relaxant effects of ACh. The relaxant effects of EFS, however, were only slightly reduced by atropine to 97.5 +/- 17.5% in human CC and to 89.0 +/- 6.1% in rabbit CC. L-NOARG further reduced the EFS effects to 0.8 +/- 1.7% in human CC and to 16.2 +/- 8.7% in rabbit CC. In strips obtained from impotent patients with diabetes mellitus, the relaxant effects appeared to be significantly less than in strips from nondiabetic impotent men. Tetrodotoxin blocked the relaxant EFS effects in human and rabbit strips completely. The data indicate the important role of NO in cholinergically induced relaxation of cavernous smooth muscle in rabbits and humans. Our findings support the idea of NO as the nonadrenergic noncholinergic neurotransmitter in penile erection in both species. Rabbit erectile tissue might serve as an in vitro animal model for further investigation.

The role of induction of nitric oxide synthesis in the altered responses of jugular veins from endotoxaemic rabbits

1. Endotoxaemia is characterized by hypotension, peripheral vasodilatation and a reduced response to vasoconstrictors. Clinical studies have indicated that venodilatation contributes to the haemodynamic changes, although there is no direct evidence for abnormal venous reactivity. In the present study, the role of nitric oxide (NO) in modifying the responses of rabbit isolated jugular veins was examined in vitro, 4 h after intravenous injection of endotoxin. 2. Treatment with endotoxin reduced the contractile response to the thromboxane-mimetic, 9,11-dideoxy-11 alpha, 9 alpha-epoxymethano-prostaglandin F2 alpha (U-46619). This affect was endothelium-independent. The response was partially restored by the NO synthase inhibitor. NG-monomethyl-L-arginine (L-NMMA 300 microM). 3. Jugular veins from control animals did not contract to L-NMMA whereas those from endotoxin-treated animals showed concentration-dependent contractions to L-NMMA. The contractions produced by L-NMMA were reversed by L-arginine but not by D-arginine. Treatment of the animals with dexamethasone (4 mg kg-1) 1 h prior to administration of endotoxin significantly attenuated the response to L-NMMA. 4. The response to sodium nitroprusside did not differ significantly between veins from control and endotoxin-treated animals. Endothelial denudation did not alter the sensitivity of the veins to sodium nitroprusside. Acetylcholine produced endothelium-dependent relaxations which were similar in veins from control and endotoxin-treated animals. 5. The results of this study demonstrate that intravenous administration of endotoxin induces hyporesponsiveness to U-46619 in jugular veins. This effect is mediated, at least in part, by the induction of NO synthesis in smooth muscle. The induction is prevented by prior treatment with dexamethasone.

Endothelium-derived relaxing factor contributes to the regulation of endothelial permeability

To determine whether endothelium-derived relaxing factor (EDRF) contributes to the regulation of endothelial permeability, the transendothelial flux of 14C-sucrose, a marker for the paracellular pathway across endothelial monolayers (Oliver, J. Cell. Physiol. 145:536-548, 1990), was examined in monolayers of bovine aortic endothelial cells grown on collagen-coated filters. The permeability coefficient of 14C-sucrose was significantly decreased by 10(-3) M 8-Bromoguanosine 3',5'-cyclic monophosphate or by 5 x 10(-6) M glyceryl trinitrate, an activator of soluble guanylate cyclase. Depletion of L-arginine from endothelial monolayers increased 14C-sucrose permeability from 3.21 +/- 0.59 to 3.88 +/- 0.50 x 10(-5) cm.sec-1 (mean +/- SEM; n = 6; P < 0.05). The acute administration of 5 x 10(-4) M L-arginine to monolayers depleted of this amino acid decreased 14C-sucrose permeability from 2.91 +/- 0.27 to 2.52 +/- 0.26 x 10(-5) cm.sec-1 (n = 11; P < 0.05). 14C-sucrose permeability was increased by 10(-7) M bradykinin and this effect was enhanced by the presence of each one of the following compounds: 10(-5) M methylene blue, 4 x 10(-6) M oxyhemoglobin, 5 x 10(-4) M NG-methyl-L-arginine or 5 x 10(-4) M N omega-nitro-L-arginine. These results suggest that EDRF contributes to the sealing of the endothelial monolayer and that EDRF released by bradykinin acts as a feedback inhibitor attenuating the increase in endothelial permeability induced by this peptide. Because endothelial cells have the ability to contract and relax and possess guanylate cyclase responsive to nitric oxide, our results suggest that EDRF decreases 14C-sucrose permeability by relaxing endothelial cells, thereby narrowing the width of endothelial junctions.

Nitric oxide (NO) is involved in increased cerebral cortical blood flow following stimulation of the nucleus basalis of Meynert in anesthetized rats

The effects of i.v. administration of a nitric oxide (NO) synthase inhibitor, L-NG-nitroarginine (L-NOArg), on the increase in cerebral cortical blood flow (cortical BF), following either electrical stimulation of the nucleus basalis of Meynert (NBM), whose cholinergic fibers project to the cortex, or hypercapnia with 10% CO2 inhalation, were studied in anesthetized rats. Cortical BF was measured using laser Doppler flowmetry. The threshold intensity of electrical stimulation of the NBM (0.5 ms, 50 Hz for 10 s) that induced an increase in regional cortical BF was defined as 1T. The cortical BF was increased on a stimulus intensity dependent manner at 1T-5T intensities tested. L-NOArg was administered cumulatively i.v. starting from 0.3 mg/kg, then 3 mg/kg, and 30 mg/kg. Time interval between each cumulative administration of L-NOArg was approximately 40 min. Three and 30 mg/kg of L-NOArg significant reduced the NBM stimulation-induced increase of cortical BF at intensities of 2T and 3T. The response at an intensity of 5T was reduced only by 30 mg/kg of L-NOArg to about half the control response. The reduced responses at 2T, 3T, and 5T were reversed following the i.v. administration of a physiological precursor of NO, L-arg (300 mg/kg). Inhalation of 10% CO2 for 15 s induced an increase in cortical BF which was not influenced by L-NOArg and L-Arg. These results suggest that NO is a necessary factor in the vasodilation of the cortical BF that is brought about by cholinergic fibers originating in the NBM.

Endothelial control of long bone vascular resistance

This in vitro study investigates whether intraosseous endothelial cells can regulate long bone blood flow by secretion of vasodilator prostaglandin and EDRF (endothelium-derived relaxing factor). Canine tibia were perfused through the nutrient artery at a constant flow rate, and the increases in perfusion pressure caused by standard doses of norepinephrine were recorded first under control conditions and then during acetylcholine infusion. Acetylcholine attenuated the norepinephrine pressure responses (-62 +/- 3%). This attenuating effect of acetylcholine was partially abolished by inhibition of prostaglandin synthesis (-20 +/- 6%) and completely abolished by inhibition of EDRF synthesis (+73 +/- 43%) or combined inhibition of prostaglandin and EDRF synthesis (+134 +/- 30%). These results are statistically significant (p less than 0.0001) and suggest that both EDRF and vasodilator prostaglandin are synthesized by intraosseous endothelial cells, and can modify long bone vascular resistance. Thus, as in other organs, intraosseous endothelial cells may provide bone with an autoregulatory control mechanism and enable it to respond to a diverse group of vasodilator stimuli.

Nitric oxide inactivates endothelium-derived contracting factor in the rat aorta

Acetylcholine evokes the simultaneous release of endothelium-derived relaxing and contracting factors in aortas from spontaneously hypertensive rats. Only relaxing factors are released in aortas from normotensive controls. Experiments were designed to determine whether inhibitors of endothelium-dependent relaxations modify endothelium-dependent contractions. Rings of thoracic aortas of normotensive and spontaneously hypertensive rats, with and without endothelium, were suspended in organ chambers for isometric tension recording. Oxyhemoglobin (a scavenger of endothelium-derived relaxing factor) and NG-monomethyl L-arginine (an inhibitor of nitric oxide formation) augmented the contractions to acetylcholine. Methylene blue (an inhibitor of soluble guanylate cyclase) and superoxide dismutase (a scavenger of superoxide anions) did not modify these contractions. The contractions in the presence of oxyhemoglobin or NG-monomethyl L-arginine, like those in untreated rings, were endothelium-dependent; they only occurred in aortas from spontaneously hypertensive rats and were abolished by indomethacin. The contractions to acetylcholine in the presence of oxyhemoglobin were not affected by superoxide dismutase or deferoxamine. These data suggest that endothelium-derived relaxing factor inhibits endothelium-dependent contractions to acetylcholine in the spontaneously hypertensive rat aorta, probably by chemical inactivation of the endothelium-derived contracting factor rather than by stimulation of guanylate cyclase or scavenging of oxygen-derived free radicals.

Inhibition of endothelium-derived relaxing factor enhances endothelin-mediated vasoconstriction

BACKGROUND

The endothelium possesses the ability to modulate vascular tone by the release of vasodilators and vasoconstrictors, among them endothelium-derived relaxing factor (EDRF) and endothelin (ET). Abnormalities in EDRF generation have been demonstrated in various cardiovascular pathophysiological states. Moreover, a twofold increase in plasma ET concentration was reported in these disease states. Recent in vitro studies have suggested the interaction between these two endothelium-derived substances, suggesting that imbalance between the two may contribute to alternation in vascular tone characteristic of these disease states. Thus, the hypothesis of this study was that inhibition of endogenous EDRF will enhance the vasoconstrictor response to a twofold increase in plasma ET concentrations.

METHODS AND RESULTS

Experiments were conducted in three groups of anesthetized dogs. In group 1, ET-1 was infused intravenously to double circulating ET concentrations. Group 2 received both ET and NG-monomethyl L-arginine (L-NMMA), a competitive inhibitor of EDRF generation, and group 3 received a continuous infusion of L-NMMA alone. Twofold increase in plasma ET concentrations was characterized by an increase in systemic and renal vasoconstriction. The inhibition of EDRF markedly enhanced the vasoconstriction to ET specifically involving the systemic, pulmonary, coronary, and renal arterial circulations.

CONCLUSIONS

The present study demonstrates that inhibition of endogenous EDRF augments the vasoconstrictor property of ET and supports a functional role for the balance between endothelium-derived vasodilating and vasoconstricting factors in the regulation of vascular tone.

Evaluation of endothelium-derived nitric oxide mediated vasodilation utilizing ex vivo perfusion of an intact vessel

The traditional evaluation of the endothelium-derived nitric oxide (EDNO) pathway involves isolated aortic rings with attached strain gauges. This model is nonphysiologic and does not permit studies lasting longer than several hours. Our objective was to overcome the limitations of these "traditional" methods utilizing a physiologic, whole vessel model as a reproducible assay of EDNO. Canine carotid arteries (n = 4) were removed (maintaining in vivo arterial geometry), mounted in a specially designed, continuous-flow circuit, and perfused at 100 ml/min, 80 mm Hg with Medium-199/10% canine serum. Physiologic pH, pCO2, pO2, and temperature were precisely regulated. A non-contacting, helium-neon laser micrometer was interfaced with the current system to provide continuous measurement of vessel external diameter and to quantitate changes in vessel wall geometry in response to epinephrine (EPI; 2 x 10(-5) to 2 x 10(-3) mg/ml) and acetylcholine (ACh; 0.1 to 100 microM) challenge. Further characterization of the perfusion system included the use of a competitive inhibitor to EDNO production, NG-monomethyl-L-arginine (L-NMMA), and the effect of this compound on ACh-induced vasodilation. The reversibility of this blockade was verified via the sequential addition of L-arginine (L-ARG; 0 to 3 mM). Data are expressed as the ratio of steady-state vessel cross-sectional area (CSA) following administration of vasoactive substance to the CSA prior to vasoactive challenge. Our results indicate that EPI and ACh produced significant dose-dependent vasoconstrictive and vasodilatory responses, respectively (P less than 0.001, ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)

Baroreceptor reflexes and vascular reactivity during inhibition of nitric oxide synthesis in conscious rabbits

The effect of the nitric oxide (NO) synthase inhibitor N-nitro-L-arginine (NOLA) on vascular reactivity and the baroreceptor heart rate reflex was examined in chronically instrumented conscious rabbits. NOLA (15 mg/kg i.v.) significantly increased mean arterial pressure and hindlimb vascular resistance and decreased heart rate. Increases and decreases in arterial pressure were produced by the intravenous injection of phenylephrine and sodium nitroprusside respectively and the values obtained relating mean arterial blood pressure to heart rate were fitted to a sigmoid curve. NOLA significantly reduced the lower plateau of the arterial pressure--heart rate curve but did not significantly affect baroreceptor sensitivity. Depressor and hindlimb vasodilator responses to acetylcholine were significantly impaired by NOLA whereas responses to sodium nitroprusside were significantly enhanced. The pressor and hindlimb vasoconstrictor responses to phenylephrine were significantly enhanced in the presence of NOLA. We conclude that the bradycardia produced by NOLA does not result from a change in baroreceptor sensitivity. The continuous generation of NO appears to be important in regulating basal vascular resistance and in modulating vascular reactivity to both vasodilator and vasoconstrictor agents.

Decreased endothelium-dependent hyperpolarization to acetylcholine in smooth muscle of the mesenteric artery of spontaneously hypertensive rats

The endothelium-dependent vascular relaxation to acetylcholine (ACh) in spontaneously hypertensive rats (SHR) may be impaired because of an imbalance of endothelium-derived relaxing factor and contracting factor. However, the role of the endothelium-dependent hyperpolarization remains undetermined. We examined the ACh-induced hyperpolarization and its contribution to relaxation in arteries of SHR. Membrane potentials were recorded from the mesenteric artery trunk of 6-8-month-old male SHR and also Wistar-Kyoto (WKY) rats. Endothelium-dependent hyperpolarization to ACh was unaffected by NG-nitro-L-arginine, indomethacin, or glibenclamide; was reduced by tetraethylammonium or high K+ solution; and was enhanced by low K+ solution or methylene blue, thereby indicating that hyperpolarization is not mediated by nitric oxide (endothelium-derived relaxing factor) but is presumably mediated by a hyperpolarizing factor and is due to an opening of K+ channels that probably differ from the ATP-sensitive ones. Hyperpolarizations to ACh were markedly reduced in SHR compared with findings in WKY rats (maximum, 8 +/- 1 versus 17 +/- 1 mV). In addition, under conditions of depolarization with norepinephrine (10(-5) M), the ACh-induced hyperpolarization was even less and transient in SHR, while it was large and sustained in WKY rats (6 +/- 1 versus 29 +/- 2 mV). Endothelium-dependent relaxations to ACh in arterial rings precontracted with 10(-5) M norepinephrine were far less in SHR than in WKY rats, even in the presence of indomethacin. Furthermore, high K+ solution showed smaller inhibitory effects on the relaxations in SHR than in WKY rats. Endothelium-independent hyperpolarizations and relaxations to cromakalim, a K+ channel opener, were similar between SHR and WKY rats. It would thus appear that the endothelium-dependent hyperpolarization to ACh is reduced in SHR and this would, in part, account for the impaired relaxation to ACh in SHR mesenteric arteries.

Intravascular filarial parasites inhibit platelet aggregation. Role of parasite-derived prostanoids

The nematode parasites that cause human lymphatic filariasis survive for long periods in their vascular habitats despite continual exposure to host cells. Platelets do not adhere to blood-borne microfilariae, and thrombo-occlusive phenomena are not observed in patients with circulating microfilariae. We studied the ability of microfilariae to inhibit human platelet aggregation in vitro. Brugia malayi microfilariae incubated with human platelets caused dose-dependent inhibition of agonist-induced platelet aggregation, thromboxane generation, and serotonin release. As few as one microfilaria per 10(4) platelets completely inhibited aggregation of platelets induced by thrombin, collagen, arachidonic acid, or ionophore A23187. Microfilariae also inhibited aggregation of platelets in platelet-rich plasma stimulated by ADP, compound U46619, or platelet-activating factor. The inhibition required intimate proximity but not direct contact between parasites and platelets, and was mediated by parasite-derived soluble factors of low (less than 1,000 Mr) molecular weight that were labile in aqueous media and caused an elevation of platelet cAMP. Prior treatment of microfilariae with pharmacologic inhibitors of cyclooxygenase decreased both parasite release of prostacyclin and PGE2 and microfilarial inhibition of platelet aggregation. These results indicate that microfilariae inhibit platelet aggregation, via mechanisms that may include the elaboration of anti-aggregatory eicosanoids.

Pressure natriuresis in rats during blockade of the L-arginine/nitric oxide pathway

The natriuretic response was studied in anesthetized rats during the intravenous infusion of L-arginine analogues to inhibit the production of endothelium-derived nitric oxide. In an initial experimental series, rats were administered saline vehicle or vehicle containing 300 mumol/kg body wt N omega-monomethyl-L-arginine, N omega-nitro-L-arginine methyl ester, N omega-monomethyl-D-arginine, or L-arginine. Infusion of the competitive inhibitors N omega-monomethyl-L-arginine and N omega-nitro-L-arginine methyl ester significantly increased mean arterial pressure to 155 +/- 3 and 145 +/- 5 mm Hg, respectively, compared with a mean arterial pressure of 118 +/- 3 mm Hg determined in the vehicle control group. Sodium excretion averaged 3.27 +/- 1.08 and 2.52 +/- 0.78 mu eq/min in the N omega-monomethyl-L-arginine- and N omega-nitro-L-arginine methyl ester-treated rats, respectively, and each was significantly higher than the basal sodium excretion of 0.20 +/- 0.05 mu eq/min in the vehicle-treated control animals. Plasma renin activity was significantly lower in the N omega-monomethyl-L-arginine- and N omega-nitro-L-arginine methyl ester-treated groups than in the vehicle-treated group. Neither L-arginine nor N omega-monomethyl-D-arginine administration significantly altered any of the measured variables compared with vehicle alone. In a second experimental series, an adjustable snare was placed around the suprarenal aorta for the purpose of controlling renal perfusion pressure independently of increases in the systemic mean arterial pressure initiated by infusion of N omega-nitro-L-arginine methyl ester (75 mumol/kg i.v.).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of DL-propranolol on nitric oxide production in perfused rat hindquarters

The effect of DL-propranolol on NO release in perfused rat hindquarters was studied by using oxyhemoglobin as a capture system to allow the quantitative assay of NO production. In some experiments the stable prostacyclin metabolite 6-keto-PGF1 alpha (6-keto) was simultaneously assayed. We observed that: (1) DL-propranolol induced an increase in NO and 6-keto release. The dextro isomer was inactive; (2) DL-propranolol-induced NO release was only slightly reduced by acetylsalicylic acid in a concentration that inhibits prostacyclin synthesis, and was abolished by the chemical removal of the endothelium with 3-3 cholamidopropyl dimethylammonium 1-propane sulphonate (CHAPS) applied before or during stimulation; (3) NG-nitro-L-arginine blocked DL-propranolol-induced NO production, an effect that was antagonized by L-arginine but not by its dextro isomer.

Identification of guanidino succinate as a putative endogenous source of the endothelium derived relaxing factor

Using a specific HPLC analysis for guanidines, we find that rat aorta contains guanidino succinate (GS), guanidino acetate (GA), guanidino propionate (GP), guanidino butyrate (GB), methyl guanidine (MG) and guanidine. The concentration of L-arginine (0.05 nmol/mg tissue) is significantly lower than the other guanidines. GS is found to be the most potent vasodilator-guanidine in the rat aorta preparation and this vasodilation depends predominantly on the presence of the endothelium. This effect of GS is antagonized by NG-monomethyl L-arginine (L-NMMA), NW-nitro L-arginine benzyl ester (L-NABA), hemoglobin and by methylene blue, all of which are known to block or attenuate endothelium dependent relaxation. Further, the relaxation mediated by GS is accompanied by the formation of cGMP in the rat aorta. From these results we suggest that GS may be a major endogenous source of EDRF.