Biosynthesis of nitric oxide from L-arginine. A pathway for the regulation of cell function and communication

Does carbon monoxide have a physiological function?

Recently endothelium-derived relaxing factor (EDRF) has been identified as nitric oxide. The source of the nitric oxide is L-arginine, and the L-arginine-nitric oxide pathway has been proposed to function as a widespread transduction mechanism for the regulation of cell function and communication. Gerald Marks and colleagues suggest that carbon monoxide, which is formed endogenously from heme catabolism and which shares some of the chemical and biological properties of nitric oxide, may play a similar role. This would be achieved by carbon monoxide binding to the iron atom of the heme moiety of soluble guanylyl cyclase and to the iron-sulfur centers of macrophage enzymes.

The effect of inhibitors of the L-arginine/nitric oxide pathway on endotoxin-induced loss of vascular responsiveness in anaesthetized rats

1. The effects on blood pressure and on pressor responses to noradrenaline (NA), of NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), inhibitors of the L-arginine/nitric oxide pathway, were investigated in anaesthetized rats receiving an infusion of bacterial endotoxin (E. coli lipopolysaccharide, LPS). 2. Infusion of LPS (10 mg kg-1 h-1) for 50 min had no effect on mean arterial blood pressure (MABP) but induced a reduction in responsiveness to noradrenaline (100 ng-1 micrograms kg-1). L-NMMA (30 mg kg-1), but not D-NMMA, caused an increase in MABP of approximately 30 mmHg and restored responses to NA. This effect was reversed by L- but not D-arginine (100 mg kg-1). 3. In LPS-treated rats, blood pressure responses to NA were only marginally increased by the cyclooxygenase inhibitor, indomethacin (5 mg kg-1). L-NAME (1 mg kg-1) caused a similar increase in MABP and restored pressor responses to NA both in the presence and absence of indomethacin. 4. Co-infusion of vasopressin (100 ng kg-1, for 10 min) with LPS (10 mg kg-1 h-1) in order to reproduce the hypertensive effect of L-NMMA and L-NAME increased pressor responsiveness to 100 and 300 ng kg-1 NA but not to 1 microgram kg-1 NA. 5. Infusion of sodium nitroprusside (30 micrograms kg-1 min-1) decreased responsiveness to NA even when the hypotension was corrected by co-infusion of vasopressin (50 ng kg-1 min-1). 6. These results demonstrate that the restoration of vascular responsiveness to NA in LPS-treated anaesthetized rats by inhibitors of the L-arginine/nitric oxide pathway is stereospecific and reversible. Furthermore, the experiments involving indomethacin suggest that although cyclo-oxygenase products of arachidonic acid may contribute to the development of LPS-induced hyporeactivity, the effect of L-NAME is unlikely to involve inhibition of the cyclo-oxygenase pathway. Comparison of NA responsiveness during vasopressin and L-NMMA/L-NAME-induced hypertension shows that increasing the blood pressure may modify LPS-induced hyporeactivity, but cannot account for the complete restoration of responses to NA by L-NMMA and L-NAME. These observations suggest that activation of nitric oxide formation from L-arginine makes a direct contribution to the production of vascular hyporeactivity by LPS in vivo.

The role of nitric oxide in inhibitory non-adrenergic non-cholinergic neurotransmission in the canine lower oesophageal sphincter

1. The role of nitric oxide (NO) in non-adrenergic non-cholinergic (NANC) neurotransmission was studied on circular muscle strips of the canine lower oesophageal sphincter (LOS). Electrical field stimulation evoked frequency-dependent relaxations, which were resistant to adrenergic and cholinergic blockade and abolished by tetrodotoxin. 2. Exogenous administration of NO induced concentration-dependent and tetrodotoxin-resistant relaxations which mimicked those in response to electrical stimulation. 3. NG-nitro-L-arginine (L-NNA), a stereospecific inhibitor of NO-biosynthesis, inhibited the relaxations induced by electrical stimulation but not those by exogenous NO or vasoactive intestinal polypeptide (VIP). 4. The effect of L-NNA was prevented by L-arginine, the precursor of the NO biosynthesis but not by its enantiomer D-arginine. 5. Haemoglobin abolished the NO-induced responses and reduced those evoked by electrical stimulation. 6. Cumulative administration of VIP induced concentration-dependent relaxations, which were slow in onset and sustained. A complete relaxation to VIP was not achieved and the relaxations were not affected by L-NNA. 7. In conclusion, our results provide evidence that NANC relaxations are mediated by NO, suggesting NO or a NO releasing substance as the final inhibitory NANC neurotransmitter in the canine LOS.

Haemodynamic responses to N-nitro-L-arginine in conscious rabbits

1. The effect of N-nitro-L-arginine (NOLA) on mean arterial pressure (AP), hindlimb vascular resistance (HVR) and heart rate (HR) was examined in conscious rabbits. 2. NOLA (15 mg kg, i.v.) increased AP (delta AP = 14 +/- 3 mmHg) and HVR (delta HVR = 0.8 +/- 0.3 U) and decreased HR (delta HR = -66 +/- 8 beats/min). AP remained elevated for at least 2 h following NOLA infusion but had returned to control levels after 24 h. In contrast, the hindlimb vaso-constriction and bradycardia were sustained for at least 48 h but had returned to control levels after 72 h. 3. In the presence of total autonomic blockade (hexamethonium 30 mg/kg; propranolol 1 mg/kg and atropine 0.1 mg/kg) NOLA continued to have a pressor (delta AP = 33 +/- 9 mm Hg) and hindlimb vasoconstrictor action (delta HVR = 0.4 +/- 0.1 U) but did not affect HR (delta HR = -1 +/- 3 beats/min). 4. NOLA has a prolonged pressor and vasoconstrictor action which is independent of any action in the central nervous system and which results in a marked reflex bradycardia. These results suggest that the peripheral biosynthesis of nitric oxide is important in regulation vascular tone and arterial pressure.

Bioassay of nitric oxide released upon stimulation of non-adrenergic non-cholinergic nerves in the canine ileocolonic junction

1. The release and the nature of the inhibitory non-adrenergic non-cholinergic (NANC) neurotransmitter was studied in the canine ileocolonic junction. A circular muscle strip of the canine ileocolonic junction served as donor tissue in a superfusion bioassay in which rings of rabbit aorta with the endothelium removed served as detector tissue. 2. The ileocolonic junction released a labile factor with vasodilator activity upon stimulation of non-adrenergic non-cholinergic (NANC) nerves in response to electrical impulses and the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP). This release was respectively frequency- and concentration-dependent. 3. The release was reduced by the blocker of neuronal conductance, tetrodotoxin, and by the inhibitor of the nitric oxide (NO) biosynthesis NG-nitro-L-arginine. The biological activity was enhanced by superoxide dismutase and eliminated by haemoglobin. Hexamethonium abolished only the release in response to DMPP. 4. Injection of adenosine 5'-triphosphate (ATP) or vasoactive intestinal polypeptide (VIP) onto the cascade induced relaxations of the rabbit aorta but they were different from those induced by NO or the transferable factor. 5. Based on organ bath experiments in which the reactivity of different parts of the circular smooth muscle layer of the ileocolonic junction was investigated, a muscle strip of superficial circular muscle with submucosa was chosen as the detector strip in the bioassay cascade. 6. The ileocolonic junction dose-dependently relaxed in response to nitroglycerin and NO. NO was much more potent in the rabbit aorta than in the canine ileocolonic junction. 7. In conclusion, our results demonstrate the release of a transferable vasorelaxant factor in response to NANC nerve stimulation which behaves pharmacologically like NO but not like ATP or VIP. Therefore, we suggest that NO or a NO releasing substance is the inhibitory NANC neurotransmitter in the canine ileocolonic junction.

Nitric oxide mediates the neurogenic vasodilation of bovine cerebral arteries

Nitric oxide (NO) is a mediator of the vasodilation induced by a variety of physiological and pharmacological stimuli. The possible role of NO in the relaxation elicited in cerebral arteries by perivascular nerve stimulation has been investigated. Electrical field stimulation of precontracted bovine cerebral arteries induced a relaxation that was blocked by tetrodotoxin, but not by adrenergic or muscarinic receptor antagonists, suggesting the existence of noradrenergic, noncholinergic dilator nerves, as has been shown in other species. The relaxation was significantly reduced by the inhibitors of NO synthesis, NG-monomethyl-L-arginine and nitro-L-arginine methyl ester, but not by the enantiomer, NG-monomethyl-D-arginine. Such a reduction was reversed by L-arginine. In addition, transmural nerve stimulation (TNS)-induced relaxation was potentiated by superoxide dismutase. No response to TNS was observed in arteries without endothelium. These results suggested that neurogenic relaxation of bovine cerebral arteries is mediated by endothelium-derived NO.

Effect of recombinant human interleukin-6 on nitrite production of mouse myeloid leukemia cells

The effect of recombinant human interleukin-6 (rhIL-6) on induction of nitrite (NO(-2)) production was investigated in a mouse myeloid leukemia cell line (M1) and a subclone (Mm1). NO(-2) was induced by rhIL-6 (greater than 50 U/ml) in these cell lines. Pretreatment with rhIL-6 (100 U/ml) for 1 day synergistically enhanced the production of NO(-2) in Mm1 by LPS. Furthermore, pretreatment with IL-6 (100 U/ml) shortened the lag time of induction. These results indicate that IL-6 is involved in the regulation of the NO(-2) production in macrophage-like cells.

Evidence that an L-arginine/nitric oxide dependent elevation of tissue cyclic GMP content is involved in depression of vascular reactivity by endotoxin

1. The aim of this investigation was to study the relationship between contractile responsiveness, activation of the L-arginine pathway and tissue levels of guanosine 3':5'cyclic monophosphate (cylic GMP) in aortic rings removed from rats 4 h after intraperitoneal administration of bacterial endotoxin (E. coli. lipopolysaccharide, LPS, 20 mg kg-1). 2. LPS-treatment resulted in a reduction of the sensitivity and maximal contractile response to noradrenaline (NA). 3. Depression of the maximal contractile response was restored to control by 6-anilo-5,8-quinolinedione (LY 83583, 10 microM), which prevents activation of soluble guanylate cyclase. 4. Cyclic GMP levels in tissue from LPS-treated rats were 2 fold greater than cyclic GMP levels detected in tissue from control (saline-treated) rats. The LPS-induced increase in cyclic GMP content was observed both in the presence and absence of functional endothelium. 5. Addition of L-arginine 1 mM) to maximally contracted aortic rings produced significantly relaxation of rings from LPS-treated rats but not rings from control animals. In the LPS-treated group, addition of L-arginine was also associated with a significant increase in cyclic GMP content. L-Arginine had no effect on the cyclic GMP content of control rings. D-Arginine (1 mM) was without effect. 6. In rings from LPS-treated rats, NG-nitro-L-arginine methyl ester (L-NAME, 300 microM), an inhibitor of nitric oxide (NO) production, increased the contractile response to NA and prevented the LPS-induced increase in cyclic GMP content. In control rings, L-NAME increased the NA sensitivity only when the endothelium remained intact and reduced the cyclic GMP content of these rings to that of control endothelium-denuded rings. 7. These results demonstrate that LPS-induced hyporeactivity to NA occurs secondarily to activation of the L-arginine pathway and subsequent activation of soluble guanylate cyclase in vascular tissue. In addition they suggest that LPS induces the production of an NO-like relaxing factor in non-endothelial cells.

Norepinephrine increases cyclic GMP in astrocytes by a mechanism dependent on nitric oxide synthesis

Norepinephrine induces a rapid and concentration-dependent rise (EC50 = 1.21 +/- 0.33 microM) in cyclic GMP levels in astrocyte-enriched cultures from rat brain. The response is partially mediated by alpha 1-adrenoceptors since a marked inhibition is observed in the presence of prazosin while the beta-antagonist propranolol shows a smaller effect and the alpha 2-antagonist yohimbine is ineffective. L-NG-methylarginine, an inhibitor of nitric oxide synthesis from L-arginine, blocks the norepinephrine-induced cyclic GMP accumulation. This effect is reversed by L-arginine.

Haemodynamic responses to NG-monomethyl-L-arginine in spontaneously hypertensive and normotensive Wistar-Kyoto rats

1. Nitric oxide (NO) is a major component of endothelium-derived relaxing factor (EDRF) the synthesis of which from L-arginine can be inhibited by NG-monomethyl-L-arginine (L-NMMA). To assess whether basal NO tone is different in experimental hypertension, the haemodynamic effects of L-NMMA have been compared in anaesthetized spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats in which autonomic reflexes were blocked by ganglion blockade. 2. Bolus intravenous injections of L-NMMA, 1-30 mg kg-1, but not D-NMMA, 1-30 mg kg-1, induced dose-related increases in mean arterial pressure and decreases in conductances in the renal, carotid, hindquarters and mesenteric vascular beds in both SH and WKY rats. Although the different vascular beds varied in their maximum responses to L-NMMA, there were neither qualitative nor quantitative differences between the two rat strains in this respect. 3. The effects of L-NMMA, 30 mg kg-1, i.v. on all parameters were rapidly and completely reversed by L-arginine, 30 mg kg-1, i.v., in both SH and WKY rats. 4. The results indicate that NO derived from L-arginine exerts a powerful vasodilator tone in both anaesthetized, ganglion-blocked SH and WKY rats. Although NO appears to contribute differentially to tone in the different vascular beds, there were no major differences between the two rat strains in this respect. Hence a reduced NO tone to the vasculature is unlikely to be a major factor contributing to the elevated blood pressure in the adult SH rat.

Differential inhibition and potentiation of chemoattractant-induced superoxide formation in human neutrophils by the cell-permeant analogue of cyclic GMP, N2,2'-O-dibutyryl guanosine 3':5'-cyclic monophosphate

Human neutrophils possess a superoxide (O2-)-forming NADPH oxidase which is activated by the chemoattractants, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe), complement C5a, platelet-activating factor and leukotriene B4. We studied the roles of cAMP and cGMP in the regulation of O2- formation using the cell-permeant analogues of cyclic nucleotides, N6,2'-O-dibutyryl adenosine 3':5'-cyclic monophosphate (Bt2cAMP) and N2,2'-O-dibutyryl guanosine 3':5'-cyclic monophosphate (Bt2cGMP). Bt2cAMP inhibited O2- formation induced by these chemoattractants to similar extents. Bt2cGMP as low as 10 mumol/l significantly inhibited O2- formation induced by fMet-Leu-Phe at a submaximally effective concentration (50 nmol/l), and Bt2cGMP was more effective in diminishing O2- formation than Bt2cAMP. In contrast, Bt2cGMP did not affect O2- formation induced by fMet-Leu-Phe at a maximally effective concentration (1 mumol/l). Bt2cGMP (0.1 and 1 mmol/l) enhanced O2- formation induced by 0.1 mumol/1 C5a by 23% and 49%, respectively, and Bt2cGMP antagonized inhibition of O2- formation caused by Bt2cAMP. Bt2cGMP inhibited platelet-activating factor-induced O2- formation to a lesser extent than Bt2cAMP and had no effect on that induced by leukotriene B4. Bt2cAMP and Bt2cGMP had no effect on O2- formation induced by NAF, gamma-hexachlorocyclohexane, phorbol myristate acetate, A 23187 and arachidonic acid. Our data suggest that: 1. Bt2cAMP generally inhibits chemoattractant-stimulated O2- formation. 2. Bt2cGMP inhibits fMet-Leu-Phe- and platelet-activating factor-stimulated O2- formation but potentiates C5a-induced O2- formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation of sheep urethral muscle induced by electrical stimulation of nerves: involvement of nitric oxide

Isolated smooth muscle preparations from the sheep urethra responded to electrical field stimulation with contraction when basal tension was low (5-6 mN), but with relaxation when the preparations were contracted with noradrenaline (NA), clonidine, or prostaglandin F2a. No relaxant response could be elicited in high K+ (124 mM) contracted preparations. Electrically induced relaxations had a threshold of less than 1 Hz and a maximum at 8 Hz. Both contractant and relaxant responses were abolished by tetrodotoxin, indicating that they were caused by transmitters released from nerves. The amplitude of the relaxant responses showed a highly significant correlation to the tension induced by noradrenaline. A coefficient (R/T) was calculated relating relaxation to noradrenaline-induced tension. In this way it is possible to separate the effect of drugs on muscle tension (non-specific effect) from their action on the electrically induced relaxation (specific effect). Chemical sympathectomy with 6-OHDA did not significantly modify the relaxant response to 6 Hz in noradrenaline contracted strips, as evaluated by the R/T coefficient. The electrically induced relaxation was not affected by hexamethonium, propranolol, phentolamine, muscarinic receptor blockade, cocaine, indomethacin, or methysergide. Both nifedipine and Bay K 8644 inhibited significantly the response induced by electrical stimulation, decreasing its maximum. Nifedipine, but not Bay K 8644, significantly reduced the level of tension induced by noradrenaline, and its effect, evaluated by the R/T coefficient, was an increase in the electrically induced relaxation, whereas Bay K 8644 had a significant inhibitory effect.(ABSTRACT TRUNCATED AT 250 WORDS)

The release of a non-prostanoid inhibitory factor from rabbit bronchus detected by co-axial bioassay

1. Methacholine relaxed phenylephrine-contracted aorta of the rat with the endothelium intact. This effect was inhibited by haemoglobin, methylene blue, gossypol, phenidone and L-NG-nitroarginine methyl ester (L-NAME). Rat aorta denuded of endothelium failed to relax in response to methacholine, histamine and the peptidoleukotrienes C4, D4 and E4. 2. Methacholine and histamine but not leukotrienes C4, D4 and E4 relaxed phenylephrine-contracted rat aorta without endothelium when surrounded by rabbit epithelium-intact bronchus. The muscarinic antagonist atropine antagonized the methacholine-induced relaxation. 3. Removal of the epithelium either mechanically or chemically, abolished methacholine-induced relaxation of rat aorta in the co-axial bioassay. These data indicate that the epithelium is responsible for the observed relaxant effect to methacholine and histamine. 4. The cyclo-oxygenase inhibitor, indomethacin, the phospholipase A2 inhibitor, mepacrine and the lipoxygenase inhibitor, nordihydroguaiaretic acid (NDGA), failed to inhibit methacholine-induced relaxation of rat aorta in the co-axial bioassay. This indicates that the epithelium-derived inhibitory factor (EpDIF) is not a product of the cyclo-oxygenase or lipoxygenase pathway or a product derived from activation of phospholipase A2. 5. Haemoglobin, methylene blue, phenidone, gossypol and L-NAME failed to inhibit the relaxation of rat aorta in the co-axial bioassay. These results demonstrate that EpDIF detected in the co-axial bioassay is not endothelium-derived relaxing factor (EDRF) or nitric oxide. Similarly, catalase was without effect. 6. EpDIF is unlikely to be a peptide since papain and alpha-chymotrypsin failed to alter the methacholine-induced relaxation of rat aorta in the co-axial bioassay. Furthermore, thiorphan, captopril and aprotinin were also without effect, suggesting that EpDIF is not a substrate for airway peptidases. 7. The results presented in this paper demonstrate the release of a vasoactive epithelium-derived inhibitory factor (EpDIF) from rabbit intrapulmonary bronchi by use of a co-axial bioassay preparation.

Cyclic nucleotide content of the rat anococcygeus during relaxations induced by drugs or by non-adrenergic, non-cholinergic field stimulation

Low concentrations of sodium nitroprusside (0.2 and 1 microM) relaxed carbachol-induced tone of the rat anococcygeus but did not affect the content of either cGMP or cAMP; higher concentrations (10,100 and 1000 microM) produced greater relaxation (greater than 60%) and a rise in cGMP but not cAMP. In the presence of the cGMP-phosphodiesterase inhibitor M&B 22948 (10 microM), 1 microM sodium nitroprusside produced greater relaxation and a selective increase in cGMP. Forskolin (0.5-250 microM) caused relaxation and a selective increase in cAMP; the concentration-response relationships of the two effects were similar. Non-adrenergic, non-cholinergic (NANC) field stimulation (10 Hz; 20 s trains) reduced tone by 52% but had no effect on cyclic nucleotide content; in the presence of 10 microM M&B 22948 or 1 microM sodium nitroprusside, NANC stimulation produced a greater degree of relaxation and increased cGMP but not cAMP content. The results show that NANC stimulation acts like sodium nitroprusside, causing a selective increase in cGMP, and this supports the proposal that NANC transmission in the rat anococcygeus involves an endogenous nitrate; the possibility that multiple pools of cGMP exist in the anococcygeus is discussed.

Sensitivity of bacteria to NaNO2 and to L-arginine-dependent system in murine macrophages

Sensitivity of bacteria to NO2-/NO and to L-arginine-dependent system in murine macrophages was tested. The growth of all bacteria tested, Salmonella typhimurium, Pseudomonas aeruginosa and Staphylococcus epidermidis, was significantly inhibited by NaNO2 at the concentration of 5 to 10 mM. However, L-arginine-dependent effector mechanism of LPS-activated murine macrophages was ineffective in killing these bacteria. It seems that the concentration of NO2-/NO in phagolysosome in the activated macrophages is insufficient to kill the bacteria in situ. We concluded that L-arginine-dependent effector mechanism can hardly work on bacteria killing in activated macrophages.

L-arginine induces relaxation of rat aorta possibly through non-endothelial nitric oxide formation

1. The relaxation of rings of rat thoracic aorta induced by L-arginine and its derivatives was investigated. 2. L-Arginine (0.3-100 microM), but not D-arginine, induced relaxation of the arteries, which was detectable after 2 h and maximal after 4-6 h on its repeated application; it was endothelium-independent. 3. L-Arginine methyl ester, N alpha-benzoyl L-arginine and L-homo-arginine had essentially similar effects to those of L-arginine. 4. NG-nitro L-arginine methyl ester (L-NAME, 3 microM), NG-nitro L-arginine (L-NNA, 1 microM) and NG-monomethyl L-arginine (L-NMMA, 10 microM), inhibitors of nitric oxide (NO) formation from L-arginine, inhibited or reversed the L-arginine-induced relaxation, irrespective of the presence or absence of the endothelium. In contrast, NG-nitro D-arginine was without effect. 5. Haemoglobin (Hb, 10 nM) and methylene blue (MB, 0.3 microM) inhibited or reversed the L-arginine-induced relaxation. 6. L-Arginine (1-100 microM), but not D-arginine, increased guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in the tissues that relaxed in response to L-arginine. This effect of L-arginine was suppressed by Hb (3 microM), MB (1 microM) and L-NAME (100 microM). Removal of the endothelium did not significantly alter the L-arginine-induced cyclic GMP production. 7. These results suggest that L-arginine itself caused a slowly developing relaxation of rat aorta, possibly via formation of NO by an endothelium-independent mechanism.

Hyperdynamic circulation in cirrhosis: a role for nitric oxide?

Hypotension, low systemic vascular resistance, and a reduced sensitivity to vasoconstrictors are features of cirrhosis. These cardiovascular changes might be the result of increased synthesis of a vasodilator. Nitric oxide (NO), a potent vasodilator, is synthesised in and released from peripheral blood-vessels in man. Studies in animals indicate that bacterial endotoxin and cytokines induce NO synthase expression in vessel walls, with sustained NO release and consequent hypotension. Endotoxaemia is a common feature of cirrhosis; persistent induction of NO synthase may account for the associated haemodynamic changes.

Stimulation of lymphocyte natural cytotoxicity by L-arginine

In vitro L-arginine enhanced natural-killer (NK) and lymphokine-activated-killer (LAK) cell activity; this cytotoxicity was mediated by CD56+ cells. In vivo arginine supplements (30 g/day for 3 days) increased the number of circulating CD56+ cells by a median of 32% in eight volunteers (p less than 0.01); this increase was associated with a mean rise of 91% in NK cell activity (p = 0.003) and of 58% in LAK cell activity (p = 0.001) in thirteen volunteers. These findings have potentially important implications for the modulation of natural cytotoxicity in a wide range of disease states.

Nonspecific defence mechanism: the role of nitric oxide

There is a marked contrast between the extraordinary complexity and specificity of the adaptive immune response and the limited number of effector mechanisms that it can direct. Recently, a great deal of interest has focused on the possible role of nitric oxide (NO) in one of these mechanisms. Here F.Y. Liew and Frank Cox examine the evidence supporting a role for NO in parasitic disease and suggest possible mechanism of NO-mediated parasite damage.

Role of nitric oxide and cyclic GMP as mediators of endothelium-independent neurogenic relaxation in bovine mesenteric artery

Electrical field stimulation (EFS) of phenylephrine-contracted bovine mesenteric arteries pretreated with guanethidine elicited a relaxation that amounted to roughly 40%. This relaxation was sensitive to tetrodotoxin pretreatment, suggesting a neurogenic origin. The EFS-induced relaxation was correlated to an increase in cGMP level, from 14.2 +/- 2.5 pmol/g wet wt in nonstimulated arteries to 31.6 +/- 3.4 pmol/g wet wt after 1 minute of EFS. cAMP values were not affected by EFS. Methylene blue (5 microM) and the compound LY 83583 (10 microM), inhibitors of soluble guanylate cyclase, inhibited the EFS-induced relaxation by 60% and 50%, respectively. Zaprinast (1 microM), a selective inhibitor of cGMP degradation, significantly (p = 0.005) potentiated the EFS-induced relaxation. The relaxation induced by EFS in bovine mesenteric arteries exhibits characteristics similar to the relaxations evoked by organic nitroesters and endothelium-dependent vasodilators, both of which are suggested to be mediated by cGMP and probably with nitric oxide as the common activator of the cGMP system. The possible involvement of nitric oxide as a mediator of EFS-induced relaxations was investigated with the use of known modulators of endogenous nitric oxide production. Preincubation of the arteries with 1 mM arginine or 1 mM N-alpha-benzoyl-L-arginine, both reported to potentiate endogenous nitric oxide production, or 5 mM L-canavanine, 0.25 mM NG-monomethyl-L-arginine, or 0.1 mM NG-nitro-L-arginine, alleged inhibitors of endogenous nitric oxide production, were without effect on the relaxation induced by EFS. However, pyrogallol, a generator of superoxide anions, was a potent inhibitor of relaxations induced by EFS in bovine mesenteric arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

High-performance liquid chromatographic analysis of the unusual pathway of oxidation of L-arginine to citrulline and nitric oxide in mammalian cells

A very unusual pathway of the oxidation of L-arginine to citrulline and nitric oxide has been discovered recently in cytotoxic macrophages. In an attempt to detect molecules generated through this metabolic pathway, a fast radio high-performance liquid chromatographic method was developed to analyse the whole set of radiolabelled L-arginine-derived metabolites produced by mammalian cells after appropriate induction. A new intermediate which might be NG-hydroxy-L-arginine was found.

NMDA receptor activation in rat hippocampus induces cyclic GMP formation through the L-arginine-nitric oxide pathway

When slices of young rat hippocampus were exposed briefly (2 min) to N-methyl-D-aspartate (NMDA), a rise in the levels of cyclic GMP took place. This response was dependent on NMDA concentration (EC50 approximately 30 microM) and the maximal elevations exceeded the unstimulated levels by 25-fold. The response to 100 microM NMDA was inhibited by two competitive antagonists of the conversion of arginine to nitric oxide, L-NG-methylarginine and L-NG-nitroarginine (IC50 approximately 6 microM and 100 nM respectively). The inhibitions produced by both antagonists were reduced or abolished when the incubation medium was supplemented with L-arginine (100-300 microM). Slices of adult hippocampus produced smaller increases (5-fold) in cyclic GMP levels in response to 100 microM NMDA than those found in the immature tissue, but the response could similarly be inhibited by NG-methylarginine. The results indicate that NMDA receptor activation in the hippocampus induces the generation of nitric oxide from arginine and that this novel intercellular messenger mediates the increases in cyclic GMP levels.

Role of nitric oxide synthesis in macrophage antimicrobial activity

Research over the past 5 years has demonstrated that immunologic activation of mouse macrophages induces the activity of nitric oxide synthase, which oxidizes a guanidino nitrogen of L-arginine, yielding citrulline and the reactive radical, nitric oxide. A review of the biochemistry and immunologic regulation of this pathway in macrophages provides a backdrop against which to evaluate its effector functions. Reports published in the past 2 years suggest that synthesis of NO mediates much of the antimicrobial activity of mouse macrophages against some fungal, helminthic, protozoal and bacterial pathogens.

Glutamate, nitric oxide and cell-cell signalling in the nervous system

Nitric oxide (NO) is a recently discovered and highly unorthodox messenger molecule. Current evidence indicates that, in the CNS, NO is produced enzymatically in postsynaptic structures in response to activation of excitatory amino acid receptors. It then diffuses out to act on neighbouring cellular elements, probably presynaptic nerve endings and astrocyte processes. In several peripheral nerves, and quite possibly in parts of the CNS as well, NO might be formed presynaptically and thus act as a neurotransmitter. In both cases, a major action of NO is to activate soluble guanylate cyclase and so raise cGMP levels in target cells.

Endothelium-derived relaxing factor: basic review and clinical implications

EDRF is a potent, endogenous vasodilator that is produced and released from endothelial cells and subsequently causes the relaxation of VSM through the activation of soluble guanylate cyclase and an increase in VSM cyclic GMP. Structurally, EDRF is likely to be NO or a related nitrogen oxide-containing compound. It is synthesized in endothelial and other cell types from L-arginine by a calcium-calmodulin and NADPH-dependent enzyme. Its action is very similar to the nitrovasodilators that act directly on VSM. EDRF is present in all vascular beds, large and small vessels, and in a wide range of species. Its role in human vascular physiology and pathophysiology is just beginning to be understood. EDRF is a potent endogenous vasodilator and inhibitor of platelet aggregation and adhesion. Its activity is impaired in hypertension and atherosclerosis, and its absence due to endothelial damage may play a role in cerebral and coronary vasospasm. It is a mediator of flow-dependent vasodilation, and its inhibition by hypoxia may contribute to the hypoxic pulmonary vasoconstrictor response. Endothelial cell damage and impairment of EDRF production may also contribute to acute and chronic pulmonary hypertension. A further understanding of the chemical nature and synthetic pathways of EDRF should lead to the production of analogs and antagonists, which may play an important role in future treatments for atherosclerosis, myocardial infarction, angina, hypertension, and other vascular diseases. The recent realization that EDRF serves as the second messenger for guanylate cyclase activation and cyclic GMP production in a variety of cell types outside of the cardiovascular system, including renal and respiratory epithelium, cerebellar neurons, macrophages, and adrenocytes, suggests even broader implications. The importance of EDRF to the anesthesiologist may go beyond an understanding of its role in cardiovascular physiological and pathophysiological states. Initial studies have shown that the endothelium may play a role in mediating the vascular actions of anesthetics, and that anesthetics can inhibit the production, release, or action of EDRF. How are these interactions mediated? Are there significant differences between anesthetics with regard to their effects on EDRF? Is there a clinically significant effect of anesthetics on basal activity of EDRF, or only in response to exogenous stimulation? Conversely, it is important to determine if alterations in endothelial cell function by various disease states such as hypertension, atherosclerosis, adult respiratory distress syndrome, cerebral vasospasm, and others cause changes in the vascular actions of anesthetics. The potential interactions of anesthetics with EDRF production and action in cell types other than the endothelium have not yet been explored.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitric oxide synthesis inhibitor prevents vasodilation by insulin-like growth factor I

The effect of insulin-like growth factor I (IGF-I) on renal blood flow was measured in the anaesthetized rat using an electromagnetic flow probe. Renal vasodilation induced by IGF-I (50 micrograms/kg) was completely inhibited by NG-nitro-l-arginine methyl ester (50 mg/kg), an inhibitor of nitric oxide biosynthesis, but only partially reduced by indomethacin (10 mg/kg). The involvement of nitric oxide synthesis in the renal actions of IGF-I is proposed.

Evidence for nitric oxide as mediator of non-adrenergic non-cholinergic relaxations induced by ATP and GABA in the canine gut

1 The effects of haemoglobin, and the nitric oxide (NO) biosynthesis-inhibitors NG-monomethyl-L-arginine (L-NMMA), its enantiomer D-NMMA, and NG-nitro-L-arginine (L-NNA) were investigated on nonadrenergic non-cholinergic (NANC)-mediated relaxation of circular muscle strips of the canine terminal ileum and ileocolonic junction induced by electrical stimulation, adenosine 5'-triphosphate (ATP), gamma-aminobutyric acid (GABA) and NO. 2 Tetrodotoxin, L-NMMA and L-NNA, but not D-NMMA, inhibited the relaxations induced by electrical stimulation, ATP and GABA, but not those in response to NO. 3 The inhibitory effect of L-NMMA and L-NNA was prevented by L-arginine, but not by D-arginine. L-Arginine did not potentiate any of the NANC relaxations. 4 Haemoglobin reduced the relaxation induced by electrical stimulation, ATP and GABA, and abolished those in response to NO. 5 Our results demonstrate that the ATP- and GABA-induced relaxations resulting from stimulation of intramural NANC neurones, in addition to those induced by electrical impulses, are mediated by NO or a NO releasing substance and thus provide further evidence in support of the proposal that NO is the final inhibitory NANC neurotransmitter in the canine terminal ileum and ileocolonic junction.

Metabolic and secretory response of parotid cells to cationic amino acids. Uptake and catabolism of L-arginine and L-ornithine

L-Arginine and L-ornithine, which stimulate amylase release, are taken up by rat parotid cells. L-Arginine is converted, in an NADPH-dependent manner and to a limited extent to L-citrulline in parotid cell homogenates, despite the absence of ornithine transcarbamylase activity. L-Arginine is largely converted to urea and L-ornithine. The generation of putrescine and polyamines from L-ornithine occurs at a very low rate, relative to the cell content in performed amines. The major fate of exogenous or arginine-derived ornithine consists in its conversion to L-glutamate, which is then further metabolized. These findings raise several hypotheses for the secretory response of the parotid cells to cationic amino acids, including their accumulation as positively charged molecules inside the cell and the generation of either NO, amines, substrates for a transglutaminase-catalyzed reaction, or ATP through oxidative catabolism. However, each of these hypotheses meets with objections, the modality for the stimulation of amylase release by cationic amino acids being eventually considered as an unsettled matter.

Endogenous nitric oxide release required for long-term synaptic depression in the cerebellum

Conjunctive stimulation of climbing and parallel fibres in the cerebellum evokes a long-term depression of parallel-fibre Purkinje-cell transmission, a phenomenon implicated as the cellular mechanism for cerebellar motor learning. It is suspected that the increase in cyclic GMP concentration that occurs after activation of climbing fibres is required to evoke long-term depression. Excitatory amino acids are known to cause the release of nitric oxide (NO), resulting in elevation of the cGMP level in the cerebellum. Here we report that endogenous NO is released after stimulation of climbing fibres, that long-term depression evoked by conjunctive stimulation of parallel and climbing fibres is blocked by haemoglobin (which strongly binds NO) or L-NG-monomethyl-arginine (an inhibitor of NO synthase), and that exogenous NO or cGMP can substitute for the stimulation of climbing fibres to cause long-term depression in rat cerebellar slices. These results demonstrate that the release of endogenous NO is essential for the induction of synaptic plasticity in the cerebellum.

Effects of NG-nitro-L-arginine methyl ester or indomethacin on differential regional and cardiac haemodynamic actions of arginine vasopressin and lysine vasopressin in conscious rats

1. Measurements of changes in renal, mesenteric and hindquarters haemodynamics or cardiac haemodynamics in response to i.v. bolus doses of arginine vasopressin (AVP) or lysine vasopressin (LVP, 0.7 and 7.0 pmol) were made in conscious, chronically-instrumented Long Evans rats. 2. In some experiments AVP and LVP were administered during an infusion of NG-nitro-L-arginine methyl ester (L-NAME; 1.0 or 0.3 mg kg-1 h-1) to determine whether or not inhibition of nitric oxide production influenced the cardiovascular effects of the peptides. In other experiments, indomethacin (bolus dose of 5 mg kg-1 followed by infusion at 5 mg kg-1 h-1) was given to determine the possible involvement of cyclo-oxygenase products in the responses to AVP and LVP. 3. Under control conditions, the lower dose of LVP had significantly greater effects than AVP on heart rate, mean arterial blood pressure, renal, mesenteric and hindquarters conductances, total peripheral conductance, cardiac index, peak aortic flow and +dF/dtmax. The higher dose of LVP had significantly greater effects than AVP on all variables (i.e. including stroke index and central venous pressure). 4. In the presence of L-NAME (1 mg kg-1 h-1) there was a sustained increase in mean arterial blood pressure (+23 +/- 3 mmHg) and reductions in mesenteric (-38 +/- 4%) and hindquarters (-30 +/- 6%) vascular conductances. Under these conditions the difference in the pressor effects of AVP and LVP was abolished, but their differential effects on regional and cardiac haemodynamics persisted. This dose of L-NAME did not change cardiac baroreflex sensitivity. 5. During infusion of L-NAME at a lower rate (0.3mgkg-th-1), baseline cardiovascular status was unchanged and regional haemodynamic effects of AVP and LVP were enhanced, but the differences in the regional vasoconstrictor responses to the two peptides persisted. 6. Indomethacin (5 mg kg-1 bolus, then 5 mg kg- 'h-1 infusion) augmented the renal vasoconstrictor responses to AVP and LVP, but abolished the difference in the hindquarters vasoconstrictor responses to the two peptides. However, the differences in the pressor and the renal and mesenteric vasoconstrictor effects of AVP and LVP still occurred in the presence of indomethacin. 7. The results indicate that AVP normally has lesser cardiovascular effects than LVP but this difference does not seem to be due to more effective stimulation of nitric oxide-mediated or cyclo-oxygenase-dependent vasodilator mechanisms by AVP than LVP.

Activated neutrophils inhibit cerebrovascular endothelium-dependent relaxations in vitro

The effect of formyl-Met-Leu-Phe- (fMLP-) activated feline neutrophil granulocytes on endothelium-dependent and independent relaxations was studied in the middle cerebral artery of the cat in vitro. Endothelium-dependent relaxations caused by acetylcholine and ATP were markedly inhibited after 30 minutes of incubation of the vessels with neutrophils (5000 cells/microliter) in the presence of 5 microM fMLP, followed by a replacement of the bath solution in order to remove the neutrophils from the medium. Direct vasorelaxations in response to the nitric oxide donor compound SIN-1, however, remained unchanged. Both neutrophils and fMLP caused transient contractions during the incubation period. The present study provides direct evidence for the ability of activated neutrophils to cause an inhibition of vascular endothelium-dependent responses in vitro.

L-arginine evokes both endothelium-dependent and -independent relaxations in L-arginine-depleted aortas of the rat

This study was designed to investigate the effects of L-arginine (the substrate for the formation of endothelium-derived nitric oxide) in vascular tissues. Rat aortic rings, with or without endothelium, were suspended in organ chambers for the measurement of isometric tension; they were contracted with phenylephrine (10(-6) M). After a short incubation period (0.5 hour) in physiological salt solution, L-arginine induced minor changes in both types of rings. In contrast, when the incubation time was increased (2, 4, 6, and 8 hours), L-arginine evoked concentration- and time-dependent relaxations in aortic rings both with and without endothelium. The relaxations were larger in rings with endothelium. The presence of L-arginine (10(-3) M) in the incubation medium inhibited subsequent relaxations evoked by the amino acid. The concentration-relaxation curves associated with acetylcholine in rings with endothelium and the curves associated with Sin-1, a spontaneous donor of nitric oxide, in rings with or without endothelium were slightly but significantly shifted to the right after a 6-hour incubation. Nitro-L-arginine (3 x 10(-5) M) and methylene blue (3 x 10(-7) M) attenuated the relaxations evoked by L-arginine in rings both with and without endothelium. Other basic amino acids (D-arginine, L-homoarginine, L-citrulline, L-lysine, and L-ornithine; all tested at 10(-3) M) either had no effect or induced small relaxations and did not affect the response to L-arginine. These observations suggest that L-arginine specifically and stereoselectively relaxes aortic rings with and without endothelium, probably by restoring the endogenous pool of the amino acid, which is likely depleted by prolonged incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bradykinin antagonists, NG-monomethyl-L-arginine and L-NG-nitro arginine on phospholipase A2 induced oedema in rat paw

1. The rat paw oedema produced by a local injection of phospholipase A2 from Naja mocambique mocambique has been shown to be mainly driven by the liberation of serotonin and kinins. 2. Using specific bradykinin receptor antagonists we have shown that kinins are acting through B2 receptors. 3. Using endothelium-derived relaxing factor (EDRF) synthesis inhibitors NG-monomethyl-L-arginine and L-NG-nitro arginine we have tested the possible envolvement of EDRF as mediator. 4. Our work supports the view that extracellular phospholipases A2 are involved in inflammation, and suggests a role for EDRF as mediator of extravasation in this model of inflammation.

MDP and LPS act synergistically to induce arginine-dependent cytostatic activity in rat alveolar macrophages

Rat alveolar macrophages can be activated in vitro for cytostatic activity against tumor cells by MDP and LPS acting in synergy. MDP can be substituted for by analogs active as adjuvants. Macrophage activation correlates with an increased production of nitrite and citrulline. NG-monomethyl-L-arginine, a specific inhibitor of the L-arginine metabolism having nitrite and citrulline as end products, abolishes the cytostatic activity. We therefore conclude that, in this model, the main effector mechanism of the cytostatic activity is mediated by molecules derived from L-arginine through the newly described NOo-generating pathway.

Cyclophosphamide modifies the induction kinetics but not cell types and cytotoxic mechanisms of antitumor cells elicited with OK-432 plus attenuated tumor cells

The present study was designed to examine whether the antitumor cells induced by treatment with mitomycin-C-treated EL4 cells (EL4MMC) plus OK-432 plus cyclophosphamide differed from those induced by treatment with EL4MMC plus OK-432 in terms of their cell types and antitumor mechanisms. Antitumor activity of peritoneal exudate cells (PEC) from mice receiving either treatment was nonspecific, and inhibition of their target cell growth increased for up to 24 h. Macrophage toxin, silica and trypan blue abrogated the activity in vitro and in vivo, respectively. The activity of the PEC was inhibited with inhibitors of the arachidonic acid cascade, such as dexamethasone, 4-bromophenacyl bromide and nordihydroguaiaretic acid but not esculetin, ibuprofen, indomethacin and BW755C. NG-monomethyl-L-arginine, a specific competitive inhibitor of the L-arginine-dependent nitric oxide synthesis, also inhibited the activity. These results and morphological observations indicated that antitumor cells in the PEC from mice receiving either treatment were macrophages, and that their activity was closely related to the arachidonic acid cascade and to nitric oxide. Antitumor activity of the PEC spontaneously decayed in vitro and this decay was inhibited by the addition of OK-432 or lipopolysaccharide. On the other hand, cyclophosphamide sustained the appearance of antitumor cells in mice treated with EL4MMC plus OK-432. Therefore, cyclophosphamide treatment did not modify cell types and cytotoxic mechanisms of antitumor cells elicited with EL4MMC plus OK-432, but did modify the induction kinetics of such antitumor macrophages.

Nitric oxide synthesis in the CNS endothelium and macrophages differs in its sensitivity to inhibition by arginine analogues

Inhibition of nitric oxide production by arginine analogues was examined in three cell systems; macrophages, CNS tissue and endothelial cells. Nitric oxide production was assessed indirectly using in vitro assays measuring nitrite production (macrophages), cGMP elevation (CNS) and acetylcholine-induced relaxation of aortic ring segments (endothelium). NG-monomethyl-L-arginine and NG-amino-L-arginine possessed similar inhibitory activity in all three assays, while NG-nitro-L-arginine displayed a striking selectivity for inhibition of brain and endothelial cell nitric oxide synthesis, with IC50 values of 0.05 microM in the CNS versus 200 microM in macrophages. These results suggest that distinct enzymes are responsible for nitric oxide synthesis in different cell types, and indicate that it may be possible to selectively modulate nitric oxide production in vivo.

L-NG-nitro arginine methyl ester exhibits antinociceptive activity in the mouse

1. L-NG-nitro arginine methyl ester (L-NAME, 1-75 mg kg-1) administered intraperitoneally (i.p.) elicits dose-related antinociception in the mouse assessed by the formalin-induced paw licking procedure. Antinociceptive activity is still present 24 h after injection. L-NAME (75 mg kg-1, i.p.) is also antinociceptive in the acetic acid-induced abdominal constriction and hot plate procedures. 2. L-NAME additionally produces a dose-related inhibition of formalin-induced paw licking following intracerebroventricular (i.c.v., 0.1-100 microgram per mouse) and oral (p.o., 75-150 mg kg-1) administration. 3. L-Arginine (600 mg kg-1, i.p.) but not D-arginine (600 mg kg-1) or naloxone (5 mg kg-1) reverses the antinociceptive effect of L-NAME in the formalin test. 4. High doses of L-NAME (37.5-600 mg kg-1) but not D-NAME (75 mg kg-1) administered i.p. produce dose-related increases in blood pressure of the urethane-anaesthetized mouse whilst i.c.v. injected L-NAME (0.1 and 100 microgram per mouse) in inactive. 5. L-NAME (75 mg kg-1, i.p.) did not inhibit oedema formation in the formalin-injected mouse hindpaw. 6. L-NAME (75 mg kg-1) did not produce any overt behavioural changes in treated mice and failed to influence locomotor activity or the incidence of dipping, crossing, rearing or circling behaviour assessed by a modified 'head-dipping' board procedure. A high dose of L-NAME (600 mg kg-1) reduced dipping behaviour and locomotor activity suggesting a possible sedative effect. D-NAME (600mgkg 1) was inactive. 7. These results suggest that L-NAME produces an opioid-independent and long-lasting antinociception in the mouse most probably by a direct effect within the central nervous system.

Endothelium and blood flow mediated vasomotion in the conscious dog

Numerous in vitro studies have demonstrated the important role of the vascular endothelium on the vasoactivity of vascular smooth muscle. Experimentation, particularly in conscious animals, is required to study the integrated role of endothelium in the regulation of vascular tone. This article reviews some of the evidence demonstrating endothelium mediated vasodilation and inhibition of vasoconstriction by the endothelium in the chronically instrumented conscious animal. Furthermore, a role for endothelial cells has been shown in the mechanism of blood flow-mediated vasodilation. Finally, the endothelium, through elaboration of constricting factors, e.g., endothelin, can also induce potent vasoconstriction. In the conscious animal endothelin elicits markedly differing degrees of vasoconstriction among the various regional vascular beds.

Endothelium-mediated regulation of coronary tone

To what extent endothelial autacoids like endothelium-derived relaxant factor/nitric oxide (EDRF/NO), in addition to neural-humoral factors, are involved in the regulation of myocardial perfusion, is presently not known. Therefore, we investigated in conscious, chronically instrumented dogs the effect of stereospecific inhibitors (NG-monomethyl-L-arginine (L-NMMA), NG-nitro-L-arginine (L-NNA), NG-monomethylester-L-arginine (L-NAME] of nitric oxide-synthesis and -release on epicardial coronary tone (and coronary diameter) and myocardial perfusion. A hydraulic coronary cuff was used, to produce reactive hyperemia and to keep the myocardial perfusion constant over short periods. 40 mg/kg L-NNA i.v. caused a long-lasting increase in mean arterial blood pressure from 94 +/- 8 to 129 +/- 11 mmHg and a simultaneous decrease in coronary diameter by 2.8 +/- 0.3%. Heart rate dropped from 87 to 58 min-1, but the double product of heart rate and blood pressure dropped by only 8 +/- 2% (p = 0.05). The maximal coronary conductance during peak reactive hyperemia (after 20 s ischemia) indicating complete coronary dilation was diminished by 48% after L-NNA. The severe drop in resting myocardial perfusion and O2-supply, and nearly unchanged rate pressure product and thus myocardial metabolic rate following the inhibition of nitric oxide formation demonstrate a substantial contribution of EDRF/NO to the regulation of myocardial perfusion.

Nitric oxide requirement for vasomotor nerve-induced vasodilatation and modulation of resting blood flow in muscle microcirculation

Intravital microscopy of rabbit tenuissimus muscle was used for studies of endogenous nitric oxide as a microvascular regulator in vivo. Derivatives of arginine were administered in order to modulate the formation of nitric oxide from L-arginine. N omega-nitro-L-arginine methylester (L-NAME) (1-100 mg kg-1 i.v.) dose-dependently reduced microvascular diameters. A concomitant blood pressure increase and a decrease in heart rate was observed. The blood pressure increase induced by L-NAME (30 mg kg-1) was reversed by L-arginine (1 g kg-1) but not D-arginine. Vasodilation in response to topical acetylcholine (0.03-3 microM) was significantly inhibited by L-NAME (30 mg kg-1), whereas vasodilation by sodium nitroprusside (300 nM) was not affected. Vasomotor nerve-induced vasodilatation, induced by stimulation of the tenuissimus nerve after neuromuscular blockade by pancuronium in animals pretreated with guanethidine, was significantly attenuated by L-NAME, an effect also reversed by L-arginine. The vasodilatation in response to active contractions of the muscle induced by motor nerve stimulation as well as the vasodilator response elicited by graded perfusion pressure reductions were unaffected by L-NAME or NG-monomethyl-L-arginine (L-NMMA, 10(-4) M) administered topically. Our results indicate that endogenous nitric oxide formed from L-arginine is a modulator of microvascular tone in vivo. Furthermore, the results suggest that endogenous nitric oxide is required for vasomotor nerve-induced vasodilatation, whereas it does not appear to play a role in myogenic vasodilatation or functional hyperaemia in this tissue.

Synaptic receptors and intracellular signal transduction in the cerebellum

Glutamate receptor subtypes mediating excitatory synaptic neurotransmission in the cerebellar cortex are briefly reviewed from molecular biological, electrophysiological and pharmacological points of view. In particular, molecular biological findings of a novel family of AMPA-selective glutamate receptors are introduced, and the pharmacological and electrophysiological properties and the identity of cerebellar N-methyl-D-aspartate-sensitive receptors probably existing on Purkinje cells are discussed in comparison with well-established cerebral NMDA receptors. As possible intracellular mechanisms of the long-term depression of parallel fiber-Purkinje cell neurotransmission, the perspective of the roles of novel messengers, nitric oxide and arachidonic acid, is particularly commented based on recent information about cerebral long-term events. The specificity and possible independence of cerebellar excitatory amino acid receptors and linked intracellular second messengers are also suggested, taking the highly active guanylate cyclase system in Purkinje cells and other cerebellum-specific proteins into consideration.