NG-methylarginine, an inhibitor of endothelium-derived nitric oxide synthesis, is a potent pressor agent in the guinea pig: does nitric oxide regulate blood pressure in vivo?

[Endothelium-derived factors in hypertensive blood vessels, especially nitric oxide and hypertension]

Endothelium-dependent relaxation (EDR) in the blood vessels of spontaneously hypertensive rats (SHR) and the role of nitric oxide (NO) in the initiation of hypertension are reviewed. EDR was impaired in blood vessels of SHR depending on age and degree of hypertension when compared with those of normotensive rats. The cause of the impairment varied among the type of blood vessels: a decrease in the production of NO and endothelium-derived relaxing factor (EDRF) and an increase in the production of endothelium-derived contracting factor (EDCF) are the main causes of the impairment in large arteries, while a decrease in endothelium-dependent hyperpolarization and increased release of EDCF are the main causes of the impairment in small arteries. Interactions among these endothelium-derived factors and changes in the interactions are also causes of impairment. Superoxide may be involved in the impairment of EDR by destroying NO. The endothelium depresses smooth muscle contraction, including spontaneous tone developed in vascular smooth muscle, and the depressing effect of the endothelium is impaired in the preparations from SHR. The endothelium of blood vessels of SHR are structurally injured as demonstrated by scanning electron microscopy. Antihypertensive treatment prevented these functional and structural changes. Chronic treatment with inhibitors of NO production in normotensive rats impaired EDR and elevated blood pressure. The impairment of EDR is a secondary change due to continued hypertension, and early initiation of antihypertensive therapy is recommended.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

The role of the hypothalamic nitric oxide in the pressor responses elicited by acute environmental stress in awake rats

We quantitatively investigated the change in nitric oxide (NO) in the hypothalamic paraventricular nucleus (PVN) and its effect on cardiovascular regulation during shaker stress (SS) using brain microdialysis in awake rats. Male Wistar rats were fed either N(G)-nitro-L-arginine methyl ester (L-NAME, 0.7 g/L) or tap water for 2 weeks. Two days after implantation of an arterial catheter and guide shaft, a microdialysis probe was placed to perfuse the PVN with degassed Ringer solution at 2 microl/min in awake normotensive Wistar (CONTROL) and chronic L-NAME-treated hypertensive rats. After the rat was placed in a plastic cage set on a shaker, the blood pressure and heart rate was monitored and 10-min SS was loaded at a frequency of 200 cycles/min. Dialysate samples were analyzed by NO analyzer (based on the Griess reaction) every 10 min, and NOx (NO(2)(-) + NO(3)(-)) was measured. Plasma NOx was also measured before and after SS. Pressor responses elicited by SS were significantly greater in L-NAME-treated rats than in the CONTROL. Although NOx in the PVN dialysate were increased by SS in the CONTROL, these responses were attenuated in chronic L-NAME-treated rats. Resting plasma NOx were higher in the CONTROL than in L-NAME-treated rats. SS elicited no difference between two groups in plasma NOx. These results indicated that NO within the PVN, but not in systemic circulation, may play a role on the attenuation of the pressor responses elicited by SS. The dysfunction of NO release within the PVN may, in part, play a role in the exaggerated pressor responses in acute environmental stress.

ecNOS gene polymorphism is associated with endothelium-dependent vasodilation in Type 2 diabetes

The association between endothelial constitutive nitric oxide synthase (ecNOS) gene polymorphism and vascular endothelial function has not been clarified. We investigated the impact of ecNOS gene polymorphism on endothelial function in 95 patients with Type 2 diabetes (ecNOS genotype: 4b/b, n = 62; 4b/a, n = 30; 4a/a, n = 3). Flow-mediated (endothelium dependent, FMD) and nitroglycerin-induced (endothelium independent, NTG) vasodilations of the right brachial artery were studied using a phase-locked echotracking system. There were no significant differences in clinical characteristics among the ecNOS genotypes. The FMD was significantly lower in the patients with ecNOS4a allele than in those without ecNOS4a allele (P < 0.05). Multiple regression analysis showed that ecNOS4a allele and mean blood pressure were significant independent determinants for reduced FMD in all patients (R(2) = 0.122, P = 0.0025). The ecNOS4a allele was an independent determinant for reduced FMD in smokers but not in nonsmokers. These results suggest that ecNOS4a allele is a genetic risk factor for endothelial dysfunction in diabetic patients, especially in smokers.

Medroxyprogesterone acetate does not antagonize estrogen-induced increases in endothelium-dependent vasodilation: potential clinical implications

OBJECTIVE

Determine the effect of combined 17beta-estradiol benzoate (E2) and medroxyprogesterone acetate (MPA) administration on endothelium-dependent vasorelaxation to acetylcholine (ACh).

DESIGN

Prospective, ex vivo study.

SETTING

Academic research laboratory.

ANIMAL(S)

Mature female rats.

INTERVENTION(S)

Ovariectomized rats received one of the following interventions daily for 3 days: [1] corn oil via IM injection, [2] E2 (20 microg/kg IM), or [3] E2 (20 microg/kg IM) and MPA (10 mg/kg IM).

MAIN OUTCOME MEASURE(S)

Basal release of nitric oxide (NO) and endothelium-dependent and endothelium-independent vasodilation from thoracic aortas obtained from each group.

RESULT(S)

Estradiol treatment potentiated the endothelium-dependent relaxation to ACh when compared with the control group. Administration of MPA with E2 did not antagonize the beneficial effect of E2 on endothelium-dependent relaxation. Estradiol treatment alone or in combination with MPA did not affect endothelium-independent vasodilation as compared with the case of the control group. Administration of E2 resulted in increased basal NO release (assessed indirectly by measuring the constrictor response to N(G)-nitro-L-arginine [methyl ester (L-NAME)]) when compared with the case of the control group, and the addition of MPA to E2 did not alter the effect of E2 on basal NO release.

CONCLUSION(S)

Estradiol potentiates endothelium-dependent relaxant responses and increases basal endothelial NO release. Medroxyprogesterone acetate does not antagonize these effects of E2.

Wei Lun Visiting Professorial Lecture: Nitric oxide in the regulation of vascular function: an historical overview

The field of nitric oxide (NO) research has developed in explosive proportions since the discovery of endogenous NO in 1986. The biological importance of NO was first shown by the findings that nitroglycerin causes vasodilation by liberating NO in the smooth muscle, and activating guanylate cyclase to raise smooth muscle levels of cyclic GMP. NO also inhibits platelet aggregation by cyclic GMP mechanisms. NO activates guanylate cyclase by heme dependent mechanisms involving the formation of a nitrosyl-heme complex. The high pharmacological potency of NO was finally understood when NO was shown to be formed endogenously, and to be the same as EDRF. Based on these properties of NO, new drugs can be developed as vasodilators and antiplatelet agents for the treatment of a variety of vascular disorders including impotency. NO elicits many other actions in mammalian systems including inhibition of cell proliferation, airway bronchodilation, antimicrobial effects, other host defense effects, and also modulates learning and memory as well as other central functions. This allows for an extensive opportunity to develop novel drugs for the diagnosis, prevention, and treatment of a number of different diseases, many of which are vascular in origin.

Non-contribution of renin-angiotensin system to pressor response to N(G)-nitro-L-arginine in dogs

Acute systemic blockade of nitric oxide (NO) production by nonselective inhibitors of NO synthase (NOS) isoforms, including N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA), has been shown to produce a long-lasting pressor response in conscious and anaesthetised animals. The present study was undertaken to clarify whether the renin-angiotensin system contributes to the development of this pressor response to L-NNA. Systemic blood pressure and heart rate were continuously monitored in dogs anaesthetised with pentobarbital. Plasma renin activity in the blood obtained from a femoral artery and a renal vein was measured by use of radioimmunoassay. The acute pressor response produced by the intravenous administration of L-NNA was accompanied by reduced renin activity in both systemic and renal vascular beds. Captopril, an angiotensin converting enzyme inhibitor, counteracted the pressor response to L-NNA, whereas candesartan, an angiotensin AT1-receptor antagonist, had no apparent effect on it. The counteraction by captopril of the L-NNA-induced pressor response was likely to be attributable to enhancement by captopril of depressor responses to bradykinin, as HOE-140, a bradykinin B2 receptor antagonist, neutralised the effect of captopril. These results suggest that the pressor response acutely produced by the intravenous injection of a NOS inhibitor is not mediated by the renin-angiotensin system in anaesthetised dogs.

Nitric oxide reduces blood pressure in the nucleus tractus solitarius: a real time electrochemical study

Increasing evidence has demonstrated that nitric oxide (NO) is involved in central cardiovascular regulation. In this study, we directly measured extracellular NO levels, in real-time, in the nucleus tractus solitarius (NTS) of anesthetized cats using Nafion/Porphyrine/o-Phenylenediamine-coated NO sensors. We found that local application of L-arginine (L-Arg) induced NO overflow in NTS and hypotension. These responses were potentiated in the vagotomized animals. Pretreatment with NO synthase (NOS)/guanylate cyclase inhibitor methylene blue, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one or NO scavenger hemoglobin attenuated L-Arg-induced hypotension, suggesting that exogenous supplement of NO suppressed cardiac functions through the NOS/cyclic guanosine monophosphate mechanism. The role of endogenous NO was examined after local application of N(G)-nitro-L-arginine methyl ester (L-NAME). We found that L-NAME suppressed endogenous NO levels in NTS and elicited hypertension and tachycardia. Taken together, our data suggest that NO is tonically released in the NTS to inhibit blood pressure.

Nitric oxide modulates sympathoexcitatory cardiac-cardiovascular reflexes elicited by bradykinin

A number of studies have demonstrated an important role for nitric oxide (NO) in central and peripheral neural modulation of sympathetic activity. To assess the interaction and integrative effects of NO release and sympathetic reflex actions, we investigated the influence of inhibition of NO on cardiac-cardiovascular reflexes. In anesthetized, sinoaortic-denervated and vagotomized cats, transient reflex increases in arterial blood pressure (BP) were induced by application of bradykinin (BK, 0.1-10 microg/ml) to the epicardial surface of the heart. The nonspecific NO synthase (NOS) inhibitor NG-monomethyl-L-arginine (L-NMMA, 10 mg/kg iv) was then administered and stimulation was repeated. L-NMMA increased baseline mean arterial pressure (MAP) from 129 +/- 8 to 152 +/- 9 mmHg and enhanced the change in MAP in response to BK from 32 +/- 3 to 39 +/- 5 mmHg (n = 9, P < 0.05). Pulse pressure was significantly enhanced during the reflex response from 6 +/- 4 to 27 +/- 6 mmHg after L-NMMA injection due to relatively greater potentiation of the rise in systolic BP. Both the increase in baseline BP and the enhanced pressor reflex were reversed by L-arginine (30 mg/kg iv). Because L-NMMA can inhibit both brain and endothelial NOS, the effects of 7-nitroindazole (7-NI, 25 mg/kg ip), a selective brain NOS inhibitor, on the BK-induced cardiac-cardiovascular pressor reflex also were examined. In contrast to L-NMMA, we observed significant reduction of the pressor response to BK from 37 +/- 5 to 18 +/- 3 mmHg 30 min after the administration of 7-NI (n = 9, P < 0.05), an effect that was reversed by L-arginine (300 mg/kg iv, n = 7). In a vehicle control group for 7-NI (10 ml of peanut oil ip), the pressor response to BK remained unchanged (n = 6, P > 0.05). In conclusion, neuronal NOS facilitates, whereas endothelial NOS modulates, the excitatory cardiovascular reflex elicited by chemical stimulation of sympathetic cardiac afferents.

Effects of NG-nitro-L-arginine on the blood pressure of spontaneously hypertensive rats with different degrees of hypertension

The effects of NG-nitro-L-arginine (L-NNA) on blood pressure of various strains of spontaneously hypertensive rats were studied. Blood pressure of the rats was higher in the order of WKY, SHR, SHRSP, M-SHRSP. L-NNA caused an elevation of the blood pressure, which was greatest in SHR and smallest in WKY and M-SHRSP. Endothelium-dependent relaxation of aortae by acetylcholine was greatest in preparations from WKY and it decreased as the blood pressure of rats increased. Phenylephrine (higher than 10(-6) mg/kg) caused an elevation of the blood pressure, which was greatest in SHR and smallest in M-SHRSP. It was suggested that L-NNA elevated blood pressure by inhibiting the basal or flow-induced release of nitric oxide from the endothelium that is causing a reduction in vascular smooth muscle tone. The smaller effect of L-NNA in WKY was due to weak smooth muscle tone, while the smaller effect in SHRSP and M-SHRSP is due to impaired function of endothelium.

Induction of nitric oxide synthase during Japanese encephalitis virus infection: evidence of protective role

The ability of Japanese encephalitis virus (JEV) and JEV-induced macrophage-derived factor (MDF) to modulate nitric oxide synthase (NOS) activity in brain and tumor necrosis factor-alpha (TNF-alpha) and the possible antiviral role of NOS during JEV infection were investigated. NOS activity and particularly that of the inducible form of NOS (iNOS) was significantly enhanced in JEV or JEV-induced MDF-treated mice. Following JEV infection, total NOS activity in brain was gradually increased from Day 3 and reached a peak on Day 6. MDF-induced NOS activity and iNOS activity were dose dependent and maximum activity was observed at 1 h after treatment. The response was sensitive to anti-MDF antibody treatment and N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NOS. Pretreatment of JEV-infected mice with L-NMMA increased the mortality as evident from reduced mean survival time (MST, 11.8 days) compared to placebo treated JEV-infected mice (MST, 17 days). The enhanced level of TNF-alpha observed in the early phase of JEV infection correlated well with the enhanced activity of iNOS. These observations thus provide evidence of the protective role of iNOS during JEV infection and indicate that iNOS may be a key mediator in host innate immune response to infection.

Enhancement of phenylephrine-induced contraction in the isolated rat aorta with endothelium by H2O-extract from an Oriental medicinal plant Leonuri herba

Leonuri herba (I-mu-ts'ao, the Chinese motherwort) is an ancient Chinese traditional herb. Although the pharmacological effects of extracts of Leonuri herba have been shown in platelets and uteri, the effect on the vascular system has not been determined. In the present study, we investigated the effects of extracts of Leonuri herba on the contraction of the isolated rat aorta. Although the H20-extract (0.3-3 mg/ml) by itself showed a limited effect, the extract enhanced phenylephrine-induced contraction of the aorta with endothelium, but not without endothelium. The H20-extract, like N(G)-nitro-L-arginine methyl ester (L-NAME, an inhibitor of nitric oxide synthase), significantly inhibited the relaxation induced by acetylcholine in the aorta with endothelium. The inhibitory effect of H20-extract on the relaxation decreased by co-addition with 1 mM L-arginine. The vasoconstrictive effect of H20-extract was not due to leonurine, which is a constituent in Leonuri herba and shows uterotonic activity. Intravenous injection of the H20-extract (1.5 mg/kg) to rats caused an increase in blood pressure for 5 min, like L-NAME (1.35 mg/kg). These findings suggest that there is a component(s) in Leonuri herba, which shows a vasoconstrictive activity in rat aorta in vitro and in vivo and has similar pharmacological profile to that of L-NAME.

Hypertension and impairment of endothelium-dependent relaxation of arteries from spontaneously hypertensive and L-NAME-treated Wistar rats

Effects of chronic treatment of normotensive Wistar rats with N(omega)-nitro-L-arginine methyl ester (L-NAME) on blood pressure and on endothelium-dependent relaxation of the aorta, carotid and iliac arteries were studied. The endothelium-dependent relaxation was compared in arteries from normotensive Wistar Kyoto rats (WKY) and genetically hypertensive rats (stroke-prone spontaneously hypertensive rats, SHRSP). Chronic treatment of normotensive Wistar rats with L-NAME caused an elevation of blood pressure. The elevated blood pressure at 15 weeks of age was significantly higher in these animals than that of untreated Wistar rats, but lower than that of SHRSP. Endothelium-dependent relaxation of the arteries induced by acetylcholine (ACh) was almost abolished by chronic treatment with L-NAME. The remaining small relaxation in arteries from L-NAME-treated rats was completely inhibited by application of L-NAME (10(-4) M). In such preparations, higher concentrations of ACh induced a contraction, which was abolished by removal of the endothelium or by an application of indomethacin (10(-5) M). Endothelium-independent relaxation induced by sodium nitroprusside was similar between preparations from untreated and L-NAME-treated Wistar rats. Endothelium-dependent relaxation was significantly impaired in preparations from SHRSP, when compared with that in those from WKY. However, the impairment was less prominent in preparations from SHRSP than in those from L-NAME-treated rats. These results suggest that the impairment of endothelium-dependent relaxation in the arteries from L-NAME-treated rats is not due to the elevated blood pressure resulting from the chronic treatment, and that impairment of NO synthesis by the endothelium does not play a major role in the initiation of hypertension in SHRSP.

Therapeutic modification of the L-arginine-eNOS pathway in cardiovascular diseases

L-Arginine is the substrate for nitric oxide production. Endothelium dysfunction could be attributed to L-arginine deficiency or the presence of L-arginine endogenous inhibitors. This hypothesis leads to the assumption that provision of L-arginine could be the key for endothelial function improvement. Many studies have proven that L-arginine has a beneficial effect on endothelium dependent vasoreactivity, as well as on the interaction between vascular wall, platelets and leucocytes. Therefore, individuals with risk factors for atherosclerosis and patients with coronary artery disease or heart failure, could benefit from therapy with L-arginine.

Evidence of nitric oxide and angiotensin II regulation of circulation and cutaneous drinking in Bufo marinus

The nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (l-NAME) increased vascular resistance (VR) 10% above baseline of 3.08+/-0.08 (n=11) mmHg/mL/min at 10 mg/kg and 20% above 3.05+/-0.08 (n=9) at 50 mg/kg in anesthetized toads (Bufo marinus). Blood pressure was unaffected by either dose of L-NAME. Blood flow decreased at the higher dose of L-NAME. L-arginine (300 mg/kg) reversed the effects of L-NAME on VR and blood flow in toads treated with 10 mg/kg but not with 50 mg/kg. Injection of 50 mg/kg L-NAME into empty-bladder toads produced a 10% decrease in water uptake, J(v), resulting in a J(v) of 1,267+/-11 cm(3)/cm(2)/s x 10(-7) (n=9) compared to 1,385+/-12 (n=8) for controls. Injection of 10 microg/kg angiotensin II (ANG II) increased J(v) 15% across the pelvic patch (J(v), cm(3)/cm(2)/s x 10(-7)), resulting in a J(v) of 1,723+/-12 cm(3)/cm(2)/s x 10(-7) (n=8) compared to 1,471+/-12 (n=8) for controls. It is hypothesized that during cutaneous drinking blood flow into the capillary bed of the pelvic patch is regulated by nitric oxide and ANG II.

Novel S-nitrosothiols do not engender vascular tolerance and remain effective in glyceryltrinitrate-tolerant rat femoral arteries

Organic nitrates, such as glyceryltrinitrate, are nitric oxide (NO) donor drugs that engender tolerance with long-term use. Here, we tested the hypothesis that our novel S-nitrosothiols, N-(S-nitroso-N-acetylpenicillamine)-2-amino-2-deoxy-1,3,4,6, tetra-O-acetyl-beta-D-glucopyranose (RIG200) and S-nitroso-N-valeryl-D-penicillamine (D-SNVP), do not induce vascular tolerance ex vivo. Femoral arteries from adult male Wistar rats were preconstricted with phenylephrine and perfused with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Perfusion pressure was measured during 20 h treatment with supramaximal concentrations of NO donor (10 microM). Perfusion with glyceryltrinitrate caused a vasodilatation, which recovered over 2-20 h. In contrast, the S-nitrosothiols caused vasodilatations that were maintained throughout the 20 h perfusion period. Responses to S-nitrosothiols were partially reversed by the NO scavenger ferrohaemoglobin and fully reversed by the soluble guanylate cyclase inhibitor [1H-[1,2,4] oxadiazole [4,3-a]quinoxaline-1-one (ODQ). Glyceryltrinitrate-tolerant vessels were fully responsive to bolus injections of S-nitrosothiols. Resistance to tolerance is an attractive property of our novel compounds, particularly in view of their sustained activity in arteries with damaged endothelium.

Possible involvement of endothelium-derived hyperpolarizing factor (EDHF) in the depressor responses to platelet activating factor (PAF) in rats

1. In anaesthetized rats, platelet activating factor (PAF; 1 microg kg(-1)) decreased mean arterial blood pressure by around 60 mmHg (n=18). This depressor response was completely blocked by the PAF antagonist, CV-6209 (1 mg kg(-1)), indicating the role of PAF-specific receptor in the response. 2. N(G)-nitro-L-arginine methyl ester (L-NAME; 50 mg kg(-1)), an NO synthase inhibitor, profoundly elevated systemic blood pressure (n=19), indicating an important role of NO in the basal blood pressure regulation. The depressor response to PAF (1 microg kg(-1)) normalized against that to sodium nitroprusside (SNP) (10 microg kg(-1)) was not substantially different between rats treated without and with L-NAME (n=4). In contrast, the depressor effect of acetylcholine (0.03 - 1.0 microg kg(-1)) normalized against that of SNP (10 microg kg(-1)) was significantly attenuated by L-NAME (n=5). 3. Charybdotoxin (0.4 mg kg(-1)) plus apamin (0.2 mg kg(-1)) significantly attenuated the depressor response to PAF (1 microg kg(-1)) (n=5) without affecting the blood pressure change due to SNP (1 mg kg(-1)) (n=3). Charybdotoxin (0.4 mg kg(-1)) (n=4) or apamin (0.2 mg kg(-1)) (n=4) alone did not affect the PAF-induced depressor response. 4. These findings suggest that EDHF may make a significant contribution to the depressor response to PAF in rats. Although NO plays the determinant role in the basal blood pressure regulation, its contribution to PAF-produced depressor response seems to be less as compared with that to the depressor response to acetylcholine.

Contractile effects of arginine analogues on human internal thoracic and radial arteries

OBJECTIVES

Plasma levels of endogenous guanidino-substituted analogues of L -arginine are increased in various pathologic conditions. In the present study we determined the effects of some of these compounds on basal and stimulated release of nitric oxide in human internal thoracic and radial arteries.

METHODS

Rings of human internal thoracic and radial arteries were obtained from 16 multiorgan donors. The rings were suspended in organ baths for isometric recording of tension.

RESULTS

N(G)-monomethyl L -arginine (10(-6) to 10(-3) mol/L) and N(G),N(G)-dimethyl L -arginine (10(-6) to 10(-3) mol/L) caused concentration- and endothelium-dependent contractions. Maximal force of contractions for N(G)-monomethyl L -arginine and N(G),N(G)-dimethyl L -arginine in the internal thoracic artery were 18.0% +/- 4.3% and 17.8% +/- 3.8%, respectively, of the contraction to 100 mmol/L KCl, and those found in the radial artery were 9.6% +/- 2.5% and 9.1% +/- 2.4%, respectively. Aminoguanidine (10(-5) to 3 x 10(-3) mol/L) and methylguanidine (10(-5) to 3 x 10(-3) mol/L) produced endothelium-independent contractions. L -Arginine (10(-3) mol/L) prevented the contractions by N(G)-monomethyl L -arginine and N(G),N(G)-dimethyl L -arginine but did not change contractions induced by aminoguanidine and methylguanidine. N(G)-monomethyl L -arginine and N(G),N(G)-dimethyl L -arginine inhibited, in a concentration-dependent manner, the endothelium-dependent relaxation to acetylcholine in the internal thoracic artery and had little attenuating effect in the radial artery; aminoguanidine and methylguanidine were without effect.

CONCLUSIONS

The results suggest that the contractions induced by N(G)-monomethyl L -arginine and N(G),N(G)-dimethyl L -arginine are due to inhibition of both basal and stimulated nitric oxide production, whereas aminoguanidine and methylguanidine do not affect the synthesis of nitric oxide. An increase in the plasma concentration of N(G)-monomethyl L -arginine and N(G),N(G)-dimethyl L -arginine is likely to represent a risk factor for abnormal vasomotor tone in conduit arteries used as coronary grafts.

Chronic nitric oxide inhibition aggravates hypertension in erythropoietin-treated renal failure rats

Alterations in nitric oxide (NO) and endothelin-1 (ET-1) production have recently been reported in erythropoietin (r-HuEPO)-induced hypertension in renal failure rats. The present study was designed to evaluate the effect of NO synthase inhibition with the L-arginine analog NG-nitro-L-arginine methyl ester (L-NAME) on blood pressure (BP) and ET-1 production in control and in uremic rats treated or not treated with r-HuEPO. Renal failure was induced by a two-stage 5/6 nephrectomy. Control and uremic rats were studied separately and subdivided into four groups: vehicle, r-HuEPO, L-NAME + vehicle and L-NAME + r-HuEPO. L-NAME (100 mg/kg/day), r-HuEPO (100 U/kg, subcutaneously, three times per week), the vehicle or both were administered during 4 weeks in control rats and during 2 weeks in uremic rats. Systolic BP was recorded before and after the onset of treatment at weeks 2 and 4 in control rats and at weeks 1 and 2 in uremic rats. Hematocrit, serum creatinine, plasma, blood vessel (thoracic aorta and mesenteric artery bed) and renal cortex immunoreactive (ir) ET-1 concentrations were measured at the end of the protocol. L-NAME enhanced BP in control and uremic rats and the increase was significantly higher in uremic rats under r-HuEPO therapy (222 +/- 7 mmHg vs 198 +/- 6 mmHg, p<0.05). L-NAME induced an increase in thoracic aorta ir-ET-1 concentrations in control and uremic rats. In contrast, ir-ET-1 concentrations were unchanged in the mesenteric arterial bed and the renal cortex of control and uremic animals. R-HuEPO increased thoracic aorta ir-ET-1 contents in L-NAME treated control and uremic rats. These results underline the important role of NO release in opposing the action of vasopressors on blood vessel tone which appears more important in uremic rats treated with r-HuEPO. L-NAME treatment increased large vessel, but not small resistance artery ir-ET-1 concentrations, suggesting differential regulation of ET-1 production in different vascular beds under chronic NO synthase inhibition.

Novel S-nitrosothiols do not engender vascular tolerance and remain effective in glyceryl trinitrate-tolerant rat femoral arteries

Organic nitrates, such as glyceryl trinitrate, are nitric oxide (NO) donor drugs that engender tolerance with long-term use. Here, we tested the hypothesis that our novel S-nitrosothiols, N-(S-nitroso-N-acetylpenicillamine)-2-amino-2-deoxy-1,3,4,6, tetra-O-acetyl-beta-D-glucopyranose (RIG200) and S-nitroso-N-valeryl-D-penicillamine (D-SNVP), do not induce vascular tolerance ex vivo. Femoral arteries from adult male Wistar rats were preconstricted with phenylephrine and perfused with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Perfusion pressure was measured during 20-h treatment with supramaximal concentrations of NO donor (10 microM). Perfusion with glyceryltrinitrate caused a vasodilatation, which recovered over 2-20 h. In contrast, the S-nitrosothiols caused vasodilatations that were maintained throughout the 20-h perfusion period. Responses to S-nitrosothiols were partially reversed by the NO scavenger ferrohaemoglobin and fully reversed by the soluble guanylate cyclase inhibitor [1H-[1,2,4] oxadiazole [4,3-a]quinoxaline-1-one (ODQ). Glyceryltrinitrate-tolerant vessels were fully responsive to bolus injections of S-nitrosothiols. Resistance to tolerance is an attractive property of our novel compounds, particularly in view of their sustained activity in arteries with damaged endothelium.

Improved method for simultaneous determination of L-arginine and its mono- and dimethylated metabolites in biological samples by high-performance liquid chromatography

An improved method has been developed for the determination of L-arginine and its methylated metabolites, N(G)-monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (asymmetric DMA, ADMA) and N(G),N(G)'-dimethyl-L-arginine (symmetric DMA, SDMA) in biological samples. Extraction of these compounds with a strong cation-exchange resin AG50W-X8 with L-homoarginine (2-amino-6-guanidinohexanoic acid) as an internal standard gave a recovery of more than 70% except for SDMA from plasma samples. After extracted samples were converted to fluorescent derivatives with o-phthalaldehyde (OPA) in an alkaline medium, the following high-performance liquid chromatographic separation with a ODS column (wide-pore size, 300 A) was successfully performed with an isocratic mobile phase system. The method permits quantitative determination of L-arginine and its methylated metabolites at concentrations as low as 4 microM and 0.18 microM, respectively. Using this method, the levels of L-arginine, L-NMMA, ADMA and SDMA in human plasma, urine and rat tissue were determined.

Nitric oxide in the pathogenesis of vascular disease

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

Nitric oxide as a signaling molecule in the vascular system: an overview

In retrospect, basic research in the fields of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) during the past two decades appears to have followed a logical course, beginning with the findings that NO and cGMP are vascular smooth muscle relaxants, that nitroglycerin relaxes smooth muscle by metabolism to NO, progressing to the discovery that mammalian cells synthesize NO, and finally the revelation that NO is a neurotransmitter mediating vasodilation in specialized vascular beds. A great deal of basic and clinical research on the physiologic and pathophysiologic roles of NO in cardiovascular function has been conducted since the discovery that endothelium-derived relaxing factor (EDRF) is NO. The new knowledge on NO should enable investigators in this field to develop novel and more effective therapeutic strategies for the prevention, diagnosis, and treatment of numerous cardiovascular disorders. The goal of this review was to highlight the early research that led to our current understanding of the pathophysiologic role of NO in cardiovascular medicine. Furthermore, we discussed the possible mechanism of some drugs interfering with NO signaling cascade.

Vascular effects of aqueous crude extracts of Artemisia verlotorum Lamotte (Compositae): in vivo and in vitro pharmacological studies in rats

Artemisia verlotorum Lamotte (Compositae), growing in almost all the northern hemisphere, is used in folk medicine of some countries of Tuscany, Italy, as a remedy for hypertension. The pharmacological evaluation of the responses evoked by an aqueous dried extract of Artemisia verlotorum on the blood pressure of anaesthetized rats and on in vitro rat isolated aortae showed a marked, but transient, hypotensive activity. This effect was mediated by a strong vasodilator action, closely linked to the release of endothelial nitric oxide and to the nitric oxide-guanosine 3'-5'-cyclic monophosphate (cGMP) pathway, caused by a muscarinic receptor agonism.

Endogenous nitric oxide synthesis: biological functions and pathophysiology

Modern molecular biology has revealed vast numbers of large and complex proteins and genes that regulate body function. By contrast, discoveries over the past ten years indicate that crucial features of neuronal communication, blood vessel modulation and immune response are mediated by a remarkably simple chemical, nitric oxide (NO). Endogenous NO is generated from arginine by a family of three distinct calmodulin- dependent NO synthase (NOS) enzymes. NOS from endothelial cells (eNOS) and neurons (nNOS) are both constitutively expressed enzymes, whose activities are stimulated by increases in intracellular calcium. Immune functions for NO are mediated by a calcium-independent inducible NOS (iNOS). Expression of iNOS protein requires transcriptional activation, which is mediated by specific combinations of cytokines. All three NOS use NADPH as an electron donor and employ five enzyme cofactors to catalyze a five-electron oxidation of arginine to NO with stoichiometric formation of citrulline. The highest levels of NO throughout the body are found in neurons, where NO functions as a unique messenger molecule. In the autonomic nervous system NO functions NO functions as a major non-adrenergic non-cholinergic (NANC) neurotransmitter. This NANC pathway plays a particularly important role in producing relaxation of smooth muscle in the cerebral circulation and the gastrointestinal, urogenital and respiratory tracts. Dysregulation of NOS activity in autonomic nerves plays a major role in diverse pathophysiological conditions including migraine headache, hypertrophic pyloric stenosis and male impotence. In the brain, NO functions as a neuromodulator and appears to mediate aspects of learning and memory. Although endogenous NO was originally appreciated as a mediator of smooth muscle relaxation, NO also plays a major role in skeletal muscle. Physiologically muscle-derived NO regulates skeletal muscle contractility and exercise-induced glucose uptake. nNOS occurs at the plasma membrane of skeletal muscle which facilitates diffusion of NO to the vasculature to regulate muscle perfusion. nNOS protein occurs in the dystrophin complex in skeletal muscle and NO may therefore participate in the pathophysiology of muscular dystrophy. NO signalling in excitable tissues requires rapid and controlled delivery of NO to specific cellular targets. This tight control of NO signalling is largely regulated at the level of NO biosynthesis. Acute control of nNOS activity is mediated by allosteric enzyme regulation, by posttranslational modification and by subcellular targeting of the enzyme. nNOS protein levels are also dynamically regulated by changes in gene transcription, and this affords long-lasting changes in tissue NO levels. While NO normally functions as a physiological neuronal mediator, excess production of NO mediates brain injury. Overactivation of glutamate receptors associated with cerebral ischemia and other excitotoxic processes results in massive release of NO. As a free radical, NO is inherently reactive and mediates cellular toxicity by damaging critical metabolic enzymes and by reacting with superoxide to form an even more potent oxidant, peroxynitrite. Through these mechanisms, NO appears to play a major role in the pathophysiology of stroke, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis.

Modulation of systemic hemodynamics by exogenous L-arginine in normal and bacteremic sheep

OBJECTIVE

To investigate whether exogenous L-arginine, the substrate for nitric oxide synthase, modulates systemic hemodynamics in sepsis.

DESIGN

Prospective, controlled study in a sheep model of sepsis.

SETTING

Animal research facility in a university hospital.

SUBJECTS

Adult sheep weighing between 35 and 55 kg.

INTERVENTIONS

Adult sheep sedated and mechanically ventilated, were monitored with a pulmonary arterial catheter and an ileal tonometer. Four groups of sheep were studied: nonseptic, septic, nonseptic treated with L-arginine, and septic treated with L-arginine. Sepsis was induced by the intravenous administration of Escherichia coli (1.5x10(8) colony-forming units/kg for 30 mins). L-arginine was administered as an intravenous bolus (200 mg/kg for 10 mins) before the septic challenge followed by 200 mg/kg/hr for 300 mins.

MEASUREMENTS AND MAIN RESULTS

Sepsis induced a state of acidosis, hyperlactatemia, hypoxemia, and gastric intramucosal acidosis. During the first 30 mins after the septic challenge, there was a decrease in cardiac index and blood pressure, and an increase in systemic vascular resistance. Thereafter, blood pressure returned to baseline values, and systemic vascular resistance fell. Treatment with L-arginine in nonseptic sheep did not induce any biochemical or hemodynamic effect. In septic sheep, treatment with L-arginine was associated with a greater increase in systemic vascular resistance during the first 30 mins, and a more marked decrease in blood pressure and systemic vascular resistance after 180 mins.

CONCLUSIONS

Exogenous administration of L-arginine does not induce hemodynamic effects in normal animals, exacerbates the acute vasoconstriction associated with the intravenous infusion of E. coli and potentiates the sepsis-induced vasodilation. Our results suggest that a) nitric oxide production is not constitutively modulated by exogenous L-arginine, b) L-arginine probably enhances the sepsis-induced sympathetic discharge, and c) L-arginine becomes rate-limiting for the formation of nitric oxide at approximately 3 hrs after the initiation of the septic challenge.

Effects of some guanidino compounds on human cerebral arteries

BACKGROUND AND PURPOSE

Accumulation of endogenous guanidino-substituted analogues of L-arginine in chronic renal failure might contribute to some of the vascular and neurological disorders of this pathology. We tested the hypothesis that in human cerebral arteries, some guanidino compounds may increase vascular tone, through nitric oxide (NO) synthase inhibition, and impair endothelium-dependent relaxation.

METHODS

Rings of human middle cerebral artery were obtained during autopsy of 26 patients who had died 3 to 12 hours before. The rings were suspended in organ baths for isometric recording of tension. We then studied the responses to N(G)-monomethyl-L-arginine (L-NMMA), N(G),N(G)-dimethyl-L-arginine (asymmetrical dimethylarginine; ADMA), aminoguanidine (AG), and methylguanidine (MG).

RESULTS

L-NMMA (10(-6) to 3x10(-4) mol/L) and ADMA (10(-6) to 3x10(-4) mol/L) caused concentration- and endothelium-dependent contractions (median effective concentrations [EC(50)]=1.1x10(-5) and 1.6x10(-5) mol/L, respectively; E(max)=35. 5+/-7.9% and 43.9+/-5.9% of the response to 100 mmol/L KCl). AG (10(-5) to 3x10(-3) mol/L) and MG (10(-5) to 3x10(-3) mol/L) produced endothelium-independent contractions (E(max)=44.3+/-8.8% and 45.7+/-5.8% of the response to 100 mmol/L KCl, respectively). L-Arginine (10(-3) mol/L) prevented the contractions by L-NMMA and ADMA but did not change contractions induced by AG and MG. L-NMMA and ADMA inhibited endothelium-dependent relaxation induced by acetylcholine in a concentration-dependent manner; AG and MG were without effect.

CONCLUSIONS

The results suggest that the contractions induced by L-NMMA and ADMA are due to inhibition of endothelial NO synthase activity, whereas AG and MG do not affect the synthesis of NO. An increase in the plasma concentration of L-NMMA and ADMA associated with uremia is likely to represent a diminished release or effect of NO, and consequently, an increased cerebrovascular tone in uremic patients is highly conceivable.

L-arginine binding to nitric-oxide synthase. The role of H-bonds to the nonreactive guanidinium nitrogens

Nitric-oxide synthase (NOS) catalyzes the oxidation of L-arginine to nitric oxide and L-citrulline. Because overproduction of nitric oxide causes tissue damage in neurological, inflammatory, and autoimmune disorders, design of NOS inhibitors has received much attention. Most inhibitors described to date include a guanidine-like structural motif and interact with the guanidinium region of the L-arginine-binding site. We report here studies with L-arginine analogs having one or both terminal guanidinium nitrogens replaced by functionalities that preserve some, but not all, of the molecular interactions possible for the -NH(2), =NH, or =NH(2)(+) groups of L-arginine. Replacement groups include -NH-alkyl, -alkyl, =O, and =S. Binding of L-canavanine, an analog unable to form hydrogen bonds involving a N(5)-proton, was also examined. From our results and previous work, we infer the orientation of these compounds in the L-arginine-binding site and use IC(50) or K(i) values and optical difference spectra to quantitate their affinity relative to L-arginine. We find that the non-reactive guanidinium nitrogen of L-arginine binds in a pocket that is relatively intolerant of changes in the size or hydrogen bonding properties of the group bound. The individual H-bonds involved are, however, weaker than expected (<2 versus 3-6 kcal). These findings elucidate substrate binding forces in the NOS active site and identify an important constraint on NOS inhibitor design.

Prolonged sinus node recovery time in humans after the intracoronary administration of a nitric oxide synthase inhibitor

In vitro studies indicate that nitric oxide synthase (NOS) inhibitors alter sinus node automaticity. Moreover, whereas the systemic delivery of N(G)-monomethyl-L-arginine (L-NMMA), a NOS inhibitor, results in sinus bradycardia and arterial hypertension, its intracoronary administration has little effect on sinus heart rate. Therefore whether L-NMMA directly alters sinus node function in humans is not known. By using a crossover design, we evaluated the effect of intracoronary L-NMMA (20 micromol/min x 10 min) on corrected sinus node recovery time (CSNRT), heart rate, mean arterial blood pressure, electrocardiographic intervals, and coronary artery blood flow in nine men and 13 women aged 48+/-12 years. All were in sinus rhythm and had normal baseline CSNRTs. Baseline measurements were made during a dextrose infusion, and then L-NMMA was administered, and these parameters remeasured. In 11 patients, the infusions were near the origin of the sinus node artery (Concordant), whereas in the remaining 11, they were into the opposite coronary circulation (Discordant). After L-NMMA, significant prolongations in CSNRT were seen in Concordant (p < 0.001) and Discordant patients (p < 0.05), but were most pronounced in the Concordant group (p < 0.05). Although a significant reduction in coronary artery blood flow and nonsignificant changes in blood pressure and heart rate were observed after L-NMMA, these changes were not related to changes in CSNRT (r2 < or = 0.2; p > or = 0.2). These data support the notion that NO is a modifier of human sinus node automaticity.

Strain differences of hypertension induced by dietary NG-nitro-L-arginine in normotensive rats

When the potent inhibitor of nitric oxide (NO) synthesis NG-nitro-L-arginine (L-NNA) was incorporated into the diet, hypertension was induced and sustained due to the effects of the long-term inhibition of endothelium-dependent relaxing factor (EDRF)/NO. The effects of L-NNA on normotensive rats of four strains (Donryu, Sprague-Dawley (SD), Wistar, and Wistar-Kyoto (WKY)) were compared relative to control rats. L-NNA administration caused a sharp initial increase in systolic blood pressure (SBP) at 2 weeks in all animals, and this was followed by a gradual and steady increase until 4 weeks. At the end of the experiments (5 weeks), the mean SBP of Donryu and SD rats was decreased. The maximum blood pressure of Donryu and Wistar rats during the experiments exceeded 200 mmHg, but that of SD and WKY rats was below 200 mmHg. Body weight loss and death were observed only in L-NNA-fed Donryu rats. Pathological changes in the kidneys and the morbidity rates for the lesions were determined, and indicated that the Donryu L-NNA group was 100% positive. These results suggest that the Donryu strain is more sensitive to L-NNA than the other strains. That dietary L-NNA-induced hypertension in normotensive rats of the four strains provides a new artificially-induced hypertensive model in which vasoconstriction occurs mainly due to EDRF deficiency.

Inhibitory mechanism of N(G)-nitro-L-arginine on acetylcholine-induced depressor responses in dogs

The significance of the blood pressure elevation caused by N(G)-nitro-L-arginine (L-NNA) to inhibitory mechanism of the drug on depressor responses to acetylcholine in anesthetized dogs was investigated. L-NNA (50 mg kg(-1), i.v.) elevated blood pressure to a plateau of 30-50 mm Hg above baseline level and shifted the dose-response curve for acetylcholine-induced responses to the right by about 70-fold. Prevention by hydralazine (1 mg kg(-1), i.v.) of the blood pressure elevation over baseline level caused by L-NNA attenuated the inhibitory effect of L-NNA on the responses to acetylcholine. Intravenous neostigmine (30 microg kg(-1) bolus followed by 15 microg kg(-1) min(-1)) attenuated the inhibitory effect of L-NNA. The magnitude of the rightward shift in the dose-response curve for carbachol-induced depressor responses was only 3-fold. These results suggest that the accelerated acetylcholine metabolism by blood pressure elevation contributes to a considerable degree to the inhibitory mechanism of L-NNA.

Enhanced nitric oxide synthesis reverses salt-induced alterations in blood flow and cGMP levels

To understand the role of nitric oxide in salt-induced hypertension, we evaluated cardiovascular, hemodynamic and biochemical parameters in Dahl salt-sensitive rats fed low (0.3%) and high (8.0%) sodium diets. Two high salt groups received 1.25 and 2.5 g/L l-arginine in their drinking water. After three weeks of treatment, blood pressure was greater in the high salt groups. l-arginine did not modify salt-induced hypertension. Eicosapentaenoic acid (EPA) caused a smaller depressor response compared to normotensive rats. The increase in blood pressure was associated with decreases in aortic and renal blood flows. In renal artery, the reduction was counteracted by both l-arginine doses; whereas in the aorta, only the higher l-arginine one restored blood flow. The salt-induced reduction in aortic cyclic GMP level was only overcome by the higher l-arginine treatment. These data suggest that at the dose levels tested, nitric oxide reverses the reduction in cGMP and blood flow, but not the blood pressure changes associated with salt-induced hypertension.

A large blood pressure-raising effect of nitric oxide synthase inhibition in humans

In experimental animals, systemic administration of nitric oxide synthase (NOS) inhibitors causes large increases in blood pressure that are in part sympathetically mediated. The aim of this study was to determine the extent to which these conclusions can be extrapolated to humans. In healthy normotensive humans, we measured blood pressure in response to two NOS inhibitors, NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), the latter of which recently became available for use in humans. The major new findings are 3-fold. First, L-NAME produced robust increases in blood pressure that were more than 2 times larger than those previously reported in humans with L-NMMA and approximated those seen in experimental animals. L-NAME (4 mg/kg) raised mean arterial pressure by 24+/-2 mm Hg (n=27, P<0.001), whereas in subjects who received both inhibitors, a 12-fold higher dose of L-NMMA (50 mg/kg) raised mean arterial pressure by 15+/-2 mm Hg (n=4, P<0.05 vs L-NAME). Second, the L-NAME-induced increases in blood pressure were caused specifically by NOS inhibition because they were reversed by L-arginine (200 mg/kg, n=12) but not D-arginine (200 mg/kg, n=6) and because NG-nitro-D-arginine methyl ester (4 mg/kg, n=5) had no effect on blood pressure. Third, in humans, there is an important sympathetic component to the blood pressure-raising effect of NOS inhibition. alpha-Adrenergic blockade with phentolamine (0.2 mg/kg, n=9) attenuated the L-NAME-induced increase in blood pressure by 40% (P<0.05). From these data, we conclude that pharmacological inhibition of NOS causes large increases in blood pressure that are in part sympathetically mediated in humans as well as experimental animals.

Effect of anaemia on haemodynamics in patients with cirrhosis

BACKGROUND

It has been suggested that increased blood haemoglobin attenuates splanchnic vasodilatation in portal-hypertensive rats by nitric oxide inactivation. However, the haemodynamic effect of anaemia in cirrhotic patients of varying severity has been rarely discussed. The aim of this study was to evaluate the influence of anaemia on systemic and splanchnic haemodynamics in cirrhotic patients of differing severity.

METHODS

Two hundred and twenty-five cirrhotic patients were included in this study. All biochemical and haemodynamic results were utilized for analysis. Anaemia was defined as a haemoglobin level below the cut-off value of 12 g/dL, which might best predict low systemic vascular resistance.

RESULTS

Compared with non-anaemic patients, anaemic patients had decreased mean arterial pressure (90 +/- 1 vs 95 +/- 1 mmHg, P = 0.002), and decreased systemic vascular resistance (1022 +/- 25 vs 1227 +/- 30, P < 0.0001), and increased cardiac index (4.3 +/- 0.1 vs 3.8 +/- 0.1 L/min per m2, P < 0.0001) and increased hepatic venous pressure gradient (16.7 +/- 0.5 vs 14.4 +/- 0.6 mmHg, P = 0.006). Haemoglobin concentration exerted an influence on the degree of vasodilatation in cirrhotic patients, with Child-Pugh's A class (but not in Child-Pugh's B and C classes), and in patients without ascites (but not in patients with ascites).

CONCLUSIONS

It was concluded that anaemia has a negative effect on hyperdynamic circulation in patients with early cirrhosis which is not observed in patients with advanced cirrhosis.

N-Substituted analogues of S-nitroso-N-acetyl-D,L-penicillamine: chemical stability and prolonged nitric oxide mediated vasodilatation in isolated rat femoral arteries

Previous studies show that linking acetylated glucosamine to S-nitroso-N-acetyl-D,L-penicillamine (SNAP) stabilizes the molecule and causes it to elicit unusually prolonged vasodilator effects in endothelium-denuded, isolated rat femoral arteries. Here we studied the propanoyl (SNPP; 3 carbon side-chain), valeryl (SNVP; 5C) and heptanoyl (SNHP; 7C) N-substituted analogues of SNAP (2C), to further investigate other molecular characteristics that might influence chemical stability and duration of vascular action of S-nitrosothiols. Spectrophotometric analysis revealed that SNVP was the most stable analogue in solution. Decomposition of all four compounds was accelerated by Cu(II) and cysteine, and neocuproine, a specific Cu(I) chelator, slowed decomposition of SNHP. Generation of NO from the compounds was confirmed by electrochemical detection at 37 degrees C. Bolus injections of SNAP (10 microl; 10(-8)-10(-3) M) into the perfusate of precontracted, isolated rat femoral arteries taken from adult male Wistar rats (400-500 g), caused concentration-dependent, transient vasodilatations irrespective of endothelial integrity. Equivalent vasodilatations induced by SNVP and SNHP were transient in endothelium-intact vessels but failed to recover to pre-injection pressures at moderate and high concentrations (10(-6)-10(-3) M) in those denuded of endothelium. This sustained effect (> 1 h) was most prevalent with SNHP and was largely reversed by the NO scavenger, haemoglobin. We suggest that increased lipophilicity of SNAP analogues with longer sidechains facilitates their retention by endothelium-denuded vessels; subsequent slow decomposition within the tissue generates sufficient NO to cause prolonged vasodilatation. This is a potentially useful characteristic for targeting NO delivery to areas of endothelial damage.

Protective role of nitric oxide in ischemia and reperfusion injury of the liver

BACKGROUND

The suppressed production of nitric oxide (NO), associated with endothelial dysfunction, is thought to be a cause of ischemia and reperfusion injury of the liver. But findings of the salutary effects of NO enhancement on such injury have been conflicting. In this study, we tested our hypothesis that NO enhancement would attenuate ischemic liver injury. For this purpose, an NO precursor, L-arginine, and a novel NO donor, FK409, were applied to a 2-hour total hepatic vascular exclusion model in dogs.

STUDY DESIGN

L-arginine was administered IV at a dose of 100 mg/kg twice (n = 5), while 300 mg/kg twice of FK409 was infused continuously into the portal vein (n = 5). The drugs were given to the animals for 30 and 60 minutes before and after ischemia, respectively. Non-treated animals were used as the control (n = 10). Two-week survival, systemic and hepatic hemodynamics indices, liver function tests, energy metabolism, and histopathology were analyzed.

RESULTS

Both treatments comparably augmented hepatic tissue blood flow, decreased liver enzyme release, and increased high-energy phosphate restoration during the reperfusion period, all of which contributed to rescuing all of the treated animals from the 2-hour total hepatic ischemia. In contrast, ischemia caused 70% mortality in the control group. Histologically, structural abnormality and neutrophil infiltration were markedly attenuated by the treatments. Systemic hypotension was observed in the animals treated with FK409, however.

CONCLUSIONS

Our data demonstrate that NO enhancement alleviates the liver injury caused by ischemia and reperfusion. The supplementation of L-arginine, rather than FK409, is considered more applicable to clinical use because of the absence of systemic adverse effects.

Endothelium-dependent hyperpolarization in isolated arteries taken from animals treated with NO-synthase inhibitors

To study the effects of chronic in vivo inhibition of NO synthase on endothelium-dependent hyperpolarization, cell-membrane potential (in individual vascular smooth-muscle cells) and changes in tension (in isolated rings) were recorded from isolated canine coronary arteries and guinea-pig carotid arteries and aortas. In coronary arteries taken from control dogs and contracted with U46619, acetylcholine- and bradykinin-induced endothelium-dependent relaxations, which were unaffected by short-term in vitro exposure to indomethacin but were inhibited partially by L-nitro-arginine (LNA). In coronary arteries taken from dogs treated over the long term in vivo with LNA (30 mg/kg on the first day and 20 mg/kg the 7 following days, i.v.), the response to acetylcholine and bradykinin was inhibited when compared with arteries from control dogs. Short-term in vitro exposure to LNA or indomethacin or both did not influence the effects of either agonist. In these arteries, the hyperpolarizing response to acetylcholine, observed in the presence of LNA and indomethacin, was enhanced, whereas that to bradykinin was partially inhibited. In the guinea pig isolated aorta, the relaxation to bradykinin was abolished by long-term in vivo treatment with L-nitro-arginine-methyl-ester (L-NAME; 1.5 mg/ml, in the drinking water for > or =4 days). In the isolated guinea pig carotid artery studied in the presence of LNA and indomethacin, acetylcholine induced a hyperpolarization that was not significantly affected by long-term in vivo treatment with L-NAME. These findings indicate that endothelium-dependent hyperpolarizations are maintained during long-term inhibition of NO synthase and probably act as a back-up mechanism to elicit endothelium-dependent relaxations.

Endogenous pulmonary nitric oxide in the regulation of airway microvascular leak

Endogenous nitric oxide (NO) is an important modulator of airway function, but its role in the regulation of airway microvascular leak (AMVL) remains unclear. Thus we assessed the effects of NO synthase (NOS) inhibition on expired NO (ENO) levels and on AMVL measured by the Evans blue dye technique in guinea pigs. In control unsensitized animals, systemic NG-nitro-L-arginine methyl ester (L-NAME) reduced ENO by 70 +/- 8% (P < 0.01) and reduced AMVL by 92 +/- 1 and 44 +/- 17% (P < 0.05 for both) in the extrapulmonary and intrapulmonary airways, respectively. In animals sensitized and challenged with intratracheal antigen, markedly increased levels of AMVL and ENO were similarly attenuated by L-NAME. In contrast, aminoguanidine, a relatively selective type II NOS inhibitor, reduced ENO in both antigen-sensitized and control unsensitized animals by 39 +/- 3% (P < 0.01) but had no effect on AMVL. These data indicate that endogenous pulmonary NO contributes to both basal and antigen-stimulated levels of AMVL in guinea pigs and that this NO-dependent activity does not appear to be derived from type II NOS.

Endogenous asymmetrical dimethylarginine and hypertension associated with puromycin nephrosis in the rat

1. The present experiments were designed to investigate the role of asymmetrical NG,NG-dimethyl-L-arginine (ADMA) in causing hypertension associated with the focal and segmental glomerulosclerosis (FSGS) produced by a single bolus of puromycin aminonucleoside (PAN) and successive injection of protamine for 7 days in rats which had undergone unilateral nephrectomy. 2. After the unilateral nephrectomy, and administering PAN and protamine, histological examinations of the kidney revealed a typical FSGS, that is, evident abnormalities including segmental mesangial proliferation, obliteration of glomerular capillary lumens and adhesions between the glomerulus and Bowman's capsule could be observed. Changes in the glomerular epithelial cells consisted of the swelling with bleb formation. 3. In the FSGS rats, urine volume and urinary protein were significantly (P<0.05 and P<0.005) increased throughout 4-week experimental period, while the creatinine clearance was significantly (P<0.005) and transiently decreased, and recovered 4 weeks later. These changes were associated with the sustained elevation of the systolic blood pressure. 4. ADMA levels in aortic endothelial cells, plasma and urine were significantly (P<0.05 and P<0.005) increased in the FSGS rats, but the level in the kidney remained unchanged. 5. The basal level and net production of cyclic GMP in the aortic vessel wall with endothelium when stimulated by norepinephrine and acetylcholine were significantly (P<0.05 and P<0.01) attenuated in the FSGS rats. 6. There were significant and positive correlations between systolic blood pressure (y) and ADMA levels (x) in endothelial cells (y=4.43x+122.2, r=0.979, P<0.0001), plasma (y=0.10x+71.9, r=0.921, P<0.001) and urine (y=0.48x+126.9, r =0.699, P<0.005), but not significant in the kidney (y=0.06x+102.7, r=0.252, NS). 7. These findings suggest that ADMA as an endogenous inhibitor of NO synthesis may play an important role for the pathogenesis in the hypertension associated with the experimental FSGS in the rat.

Design of isoform-selective inhibitors of nitric oxide synthase

Nitric oxide synthase, the mammalian enzyme catalyzing the oxidation of L-arginine to L-citrulline and nitric oxide, is present in three isoforms that have distinct physiological roles. Overstimulation or overexpression of individual nitric oxide synthase isoforms plays a role in a wide range of disorders including septic shock, arthritis, diabetes, ischemia-reperfusion injury, pain and various neurodegenerative diseases. Animal studies and early clinical trials suggest that nitric oxide synthase inhibitors could be therapeutic in many of these disorders, but preservation of physiologically important nitric oxide synthase functions might require use of isoform-selective inhibitors. Within the past few years both amino acid and nonamino acid nitric oxide synthase inhibitors with pharmacologically useful isoform selectivity have been reported. Selectivity has been achieved on the basis of initial binding affinity and, for mechanism-based inactivators, on the basis of isoform-dependent catalytic activation; particularly interesting are N5-(1-imino-3-butenyl)-L-ornithine, ARL 17477, 1400W and S-(2-aminoethyl)isothiourea.

Acetylcholine-induced systemic vasodilation resistant to NG-nitro-L-arginine in an anaesthetized rats

1. The effects of NG-nitro-L-arginine (L-NNA; 20 mg/kg bodyweight (BW), i.v.) and metyrapone (300 mg/kg BW, s.c.) on acetylcholine (ACh)-induced depressor responses were investigated in anaesthetized rats. 2. Acetylcholine (0.05, 0.5, 5 micrograms/kg BW, i.v.) dose-dependently evoked a sharp fall in mean blood pressure (BP) followed by a slow recovery under control conditions. 3. Basal BP level was elevated when rats were treated with L-NNA, indicating endogenous nitric oxide (NO) participated in BP regulation. However, pretreatment with L-NNA did not attenuate but rather augmented the ACh-induced maximum vasodilation. In contrast, the time for recovery of mean BP to the pre-ACh administration level was shortened by L-NNA. These observations suggested that ACh-induced vasodilation consisted of two phases: a sharp and transient fall (phase 1) that was resistant to L-NNA followed by a longer depressor response (phase 2) that was suppressed by L-NNA. 4. To examine whether augmentation of phase 1 by L-NNA resulted from the elevation of basal BP, an appropriate dose of phenylephrine was infused to obtain similar BP elevation. Phenylephrine infusion augmented the phase 1 in a similar manner to L-NNA pretreatment but showed little effect on phase 2, supporting the selective inhibition of phase 2 by L-NNA. 5. The s.c. pretreatment with metyrapone for 3 days failed to attenuate phase 1. Thus, the involvement of endothelium-derived hyperpolarizing factor that could be formed by a metyrapone-sensitive oxidase in phase 1 was unlikely. 6. These results suggest that some factor(s), which is not inhibitable by L-NNA or metyrapone, may induce the phase 1 depressor response to ACh while NO is responsible for the phase 2 response. The mechanism inducing the phase 1 response remains to be identified.

Maintained vasodilatory response to cromakalim after inhibition of nitric oxide synthesis

Activation of vascular smooth-muscle adenosine triphosphate-sensitive potassium channels (KATP channels) causes membrane hyperpolarization, reduced entry of Ca2+ through L-type voltage-gated Ca2+ channels, and subsequent smooth-muscle relaxation. Conversely, opening of endothelial KATP channels elicits hyperpolarization but may induce Ca2+ influx and stimulation of endothelium-derived nitric oxide (EDNO) because these cells appear not to possess L-type Ca2+ channels. We therefore hypothesized that EDNO contributes to KATP channel-mediated vasodilation. To test this hypothesis, we examined vasodilatory responses to the KATP channel opener cromakalim in conscious rats, perfused rat tail artery segments, and isolated perfused rat lungs in the presence or absence of the EDNO synthesis inhibitor Nomega-nitro-L-arginine (L-NNA). Additionally, we compared the effect of cromakalim with the EDNO-dependent dilator bradykinin on NO production and intracellular Ca2+ in cultured rat pulmonary artery endothelial cells. Vasodilatory profiles to cromakalim were unaffected by L-NNA in conscious rats, tail arteries, and isolated lungs. Consistent with these results, cromakalim had no apparent effect on either NO synthesis or Ca2+ levels in cultured endothelial cells. These data suggest a lack of a role for EDNO in contributing to KATP-channel-mediated vasodilation in the rat.

The therapeutic potential of nitric oxide in lung transplantation

Endogenously produced oxides of nitrogen appear to play important roles in tissue and organ homeostasis. Endogenous production of nitric oxide, which can be altered in response to various stimuli, can modulate vascular tone, oxyradical cascades, cell adhesion, and other aspects of inflammation. Because exogenously administered (inhaled) nitric oxide can mediate pulmonary vasodilatation and improve pulmonary function in some patients with lung injury, treatment of lung allograft recipients with inhaled nitric oxide may ameliorate ischemia-reperfusion injury, thereby improving perioperative pulmonary function and diminishing ventilatory support requirements. This review examines the biology of nitric oxide and present data that support its potential therapeutic effects for lung transplant recipients.

N5-(1-Imino-3-butenyl)-L-ornithine. A neuronal isoform selective mechanism-based inactivator of nitric oxide synthase

Nitric oxide synthase (NOS) catalyzes the NADPH- and O2-dependent conversion of L-arginine to nitric oxide (NO) and citrulline; three isoforms, the neuronal (nNOS), endothelial, and inducible, have been identified. Because overproduction of NO is known to contribute to several pathophysiological conditions, NOS inhibitors are of interest as potential therapeutic agents. Inhibitors that are potent, mechanism-based, and relatively selective for the NOS isoform causing pathology are of particular interest. In the present studies we report that vinyl-L-NIO (N5-(1-imino-3-butenyl)-L-ornithine; L-VNIO) binds to and inhibits nNOS in competition with L-arginine (Ki = 100 nM); binding is accompanied by a type I optical difference spectrum consistent with binding near the heme cofactor without interaction as a sixth axial heme ligand. Such binding is fully reversible. However, in the presence of NADPH and O2, L-VNIO irreversibly inactivates nNOS (kinact = 0.078 min-1; KI = 90 nM); inactivation is Ca2+/calmodulin-dependent. The cytochrome c reduction activity of the enzyme is not affected by such treatment, but the L-arginine-independent NADPH oxidase activity of nNOS is lost in parallel with the overall activity. Spectral analyses establish that the nNOS heme cofactor is lost or modified by L-VNIO-mediated mechanism-based inactivation of the enzyme. The inducible isoform of NOS is not inactivated by L-VNIO, and the endothelial isoform requires 20-fold higher concentrations to attain approximately 75% of the rate of inactivation seen with nNOS. Among the NOS inactivating L-arginine derivatives, L-VNIO is the most potent and nNOS-selective reported to date.

Effect of the NO scavenger carboxy-ptio on endothelium-dependent vasorelaxation of various blood vessels from rabbits

In the present study, we investigated the effect of a nitric oxide (NO) scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide [carboxy-PTIO], on endothelium-dependent relaxation of a series of blood vessels from rabbits, such as thoracic aorta and femoral, renal, mesenteric, and pulmonary arteries, using a functional muscle bath technique. Carboxy-PTIO produced concentration-dependent contractions in various vessels. The contractile responses in renal, mesenteric, and pulmonary arteries were significantly greater than those in the aorta and femoral artery. Similarly, phenylephrine-induced contractions in renal, mesenteric, and pulmonary arteries were markedly enhanced after pretreatment with carboxy-PTIO. Also, carboxy-PTIO inhibited acetylcholine-induced relaxation in various blood vessels. The maximum inhibitions in aorta and femoral artery were significantly greater than those in renal, mesenteric, and pulmonary arteries. The present data demonstrate that carboxy-PTIO reduces basal, phenylephrine-, and acetylcholine-induced release of NO in rabbit blood vessels. However, different degrees of inhibition of endothelium-dependent vasorelaxation were observed in various vessels. Specifically, the thoracic aorta and femoral artery are less susceptible to the action of carboxy-PTIO without acetylcholine than renal, mesenteric, and pulmonary arteries. Conversely, the most potent carboxy-PTIO-induced inhibition of acetylcholine-induced vasorelaxation was observed with aorta and femoral arteries. Thus, it is suggested that the contribution of endogenous NO to vascular tone and regional blood flow may vary among different rabbit blood vessels.

Activity of nitric oxide synthase in the ventilatory muscle vasculature

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