Selective inhibition of basal but not agonist-stimulated activity of nitric oxide in rat aorta by NG-monomethyl-L-arginine

Compromised endothelium-dependent hyperpolarization-mediated dilations can be rescued by NS309 in obese Zucker rats

OBJECTIVE

NO and a non-NO/prostacyclin EDH mechanism are major contributors of vascular tone and cerebral blood flow. However, the effect of metabolic syndrome on EDH-mediated responses in cerebral vessels remains unknown and may offer another avenue for therapeutic targeting. The purpose of this study was to investigate EDH-dependent responses in cerebral arteries during metabolic syndrome.

METHODS

EDH-dependent dilations were assessed in MCAs isolated from nondiabetic obese and lean Zucker rats in the presence and absence of NS309, an activator of SKCa and IKCa channels. IKCa channel expression and activity were assessed by western blotting and pressure myography, respectively.

RESULTS

EDH-mediated dilations were significantly attenuated in the obese compared to the lean Zucker rat MCA. Luminal delivery of 1 μM NS309 enhanced EDH-mediated responses in lean and obese Zucker cerebral vessels. Both dose-dependent dilations to luminal NS309 and IKCa protein expression in pooled cerebral arteries were comparable between the two groups.

CONCLUSIONS

Our results suggest that pharmacological targeting of IKCa channels can rescue EDH-mediated dilations in obese Zucker rat MCAs. Compromised EDH-mediated dilations in obesity are not due to impaired IKCa channel expression or activity.

Basal release of nitric oxide in the mesenteric artery in portal hypertension and cirrhosis: role of dimethylarginine dimethylaminohydrolase

BACKGROUND AND AIM

Increased basal release of nitric oxide (NO) in the splanchnic circulation contributes to elevated plasma levels of NO observed in decompensated cirrhosis. We evaluated in rat mesenteric arteries whether the differences in basal release of NO, revealed by asymmetric dimethylarginine (ADMA)- and N(G) -nitro-L-arginine methyl ester (L-NAME)-induced contractions, were associated with changes in messenger RNA (mRNA) expression of endothelial NO synthase (eNOS) and dimethylarginine dimethylaminohydrolases (DDAHs).

METHODS

Rat small mesenteric arteries from 14 Sham-control, from 14 with partial portal vein ligation (PPVL), and from 14 with bile duct excision (BDE)-induced cirrhosis were precontracted under isometric conditions with norepinephrine, and additional contractions were induced with ADMA and L-NAME. mRNA expression of eNOS, DDAH-1, and DDAH-2 in mesenteric arteries were evaluated by real-time polymerase chain reaction.

RESULTS

ADMA and L-NAME caused concentration- and endothelium-dependent contractions. pD2 values to L-NAME were similar in all groups. In contrast, pD2 values to ADMA were similar in PPVL and BDE but were significantly lower than those of the L-NAME and the Sham groups. Relaxation to acetylcholine was not modified by ADMA or L-NAME but was abolished by charybdotoxin plus apamin. There was an increased mRNA expression of eNOS, DDAH-1, and DDAH-2 in mesenteric arteries from PPVL and BDE compared with the Sham group.

CONCLUSION

Basal release of NO is increased in mesenteric arteries of PPVL and BDE rats. The rise in expression of DDAHs indicates a higher degradation of ADMA. This would result in an increased generation of endothelial NO and mesenteric vasodilation.

Characteristics of ACh-induced hyperpolarization and relaxation in rabbit jugular vein

BACKGROUND AND PURPOSE

The roles played by endothelium-derived NO and prostacyclin and by endothelial cell hyperpolarization in ACh-induced relaxation have been well characterized in arteries. However, the mechanisms underlying ACh-induced relaxation in veins remain to be fully clarified.

EXPERIMENTAL APPROACH

ACh-induced smooth muscle cell (SMC) hyperpolarization and relaxation were measured in endothelium-intact and -denuded preparations of rabbit jugular vein.

KEY RESULTS

In endothelium-intact preparations, ACh (≤ 10⁻⁸ M) marginally increased the intracellular concentration of Ca²⁺ ([Ca²⁺](i)) in endothelial cells but did not alter the SMC membrane potential. However, ACh (10⁻¹⁰ -10⁻⁸ M) induced a concentration-dependent relaxation during the contraction induced by PGF(2α) and this relaxation was blocked by the NO synthase inhibitor N(ω) -nitro-l-arginine. ACh (10⁻⁸ -10⁻⁶ M) concentration-dependently increased endothelial [Ca²⁺](i) and induced SMC hyperpolarization and relaxation. These SMC responses were blocked in the combined presence of apamin [blocker of small-conductance Ca²⁺-activated K⁺ (SK(Ca) , K(Ca) 2.3) channel], TRAM 34 [blocker of intermediate-conductance Ca²⁺ -activated K⁺ (IK(Ca) , K(Ca) 3.1) channel] and margatoxin [blocker of subfamily of voltage-gated K⁺ (K(V) ) channel, K(V) 1].

CONCLUSIONS AND IMPLICATIONS

In rabbit jugular vein, NO plays a primary role in endothelium-dependent relaxation at very low concentrations of ACh (10⁻¹⁰ -10⁻⁸ M). At higher concentrations, ACh (10⁻⁸ -3 × 10⁻⁶ M) induces SMC hyperpolarization through activation of endothelial IK(Ca) , K(V) 1 and (possibly) SK(Ca) channels and produces relaxation. These results imply that ACh regulates rabbit jugular vein tonus through activation of two endothelium-dependent regulatory mechanisms.

Stimulus-specific blockade of nitric oxide-mediated dilatation by asymmetric dimethylarginine (ADMA) and monomethylarginine (L-NMMA) in rat aorta and carotid artery

Previous work on female rat aorta has shown that although monomethylarginine (L-NMMA) and asymmetric dimethylarginine (ADMA) each enhance submaximal phenylephrine-induced tone, consistent with blockade of basal nitric oxide activity, neither agent has any major effect on acetylcholine-induced relaxation. The aim of this study was to adopt a variety of different experimental approaches to test the hypothesis that these methylarginines block basal but not agonist-stimulated activity of nitric oxide. Basal activity of nitric oxide was assessed by observing the rise in submaximal phenylephrine-induced tone produced by nitric oxide synthase (NOS) inhibitors in male and female aorta and female carotid artery, and by monitoring the vasodilator actions of superoxide dismutase (SOD) or the PDE 5 inhibitor, T-0156. Agonist-stimulated activity of nitric oxide was assessed by observing the relaxant actions of acetylcholine or calcium ionophore A23187. L-NMMA, ADMA and L-NAME (100 μM) each enhanced submaximal phenylephrine-induced tone and inhibited SOD- or T-0156-induced relaxation, consistent with each NOS inhibitor blocking basal nitric oxide activity. In contrast, L-NMMA and ADMA had little effect on acetylcholine- or A23187-induced relaxation, while L-NAME produced powerful blockade. These observations provide support for the hypothesis that L-NMMA and ADMA selectively block basal over agonist-stimulated activity of nitric oxide in rat vessels.

Differential sensitivity of basal and acetylcholine-induced activity of nitric oxide to blockade by asymmetric dimethylarginine in the rat aorta

BACKGROUND AND PURPOSE

Previous work has shown that N(G)-monomethyl-l-arginine (l-NMMA) paradoxically inhibits basal, but not ACh-stimulated activity of nitric oxide in rat aorta. The aim of this study was to determine if the endogenously produced agent, asymmetric N(G), N(G)-dimethyl-l-arginine (ADMA), also exhibits this unusual selective blocking action.

EXPERIMENTAL APPROACH

The effect of ADMA on basal nitric oxide activity was assessed by examining its ability to enhance phenylephrine (PE)-induced tone in endothelium-containing rings. Its effect on ACh-induced relaxation was assessed both in conditions where ADMA greatly enhanced PE tone and where tone was carefully matched with control tissues at a range of different levels.

KEY RESULTS

ADMA (100 microM) potentiated PE-induced contraction, consistent with inhibition of basal nitric oxide activity. Higher concentrations (300-1000 microM) had no greater effect. Although ADMA (100 microM) also appeared to block ACh-induced relaxation when it enhanced PE tone to maximal levels, virtually no block was seen at intermediate levels of tone in the presence of ADMA. Even ADMA at 1000 microM had no effect on the maximal relaxation to ACh, although it produced a small (two- to threefold) reduction in sensitivity. ADMA and l-NMMA, like l-arginine (all at 1000 microM), protected ACh-induced relaxation against blockade by l-NAME (30 microM).

CONCLUSIONS AND IMPLICATIONS

In the rat aorta, ADMA, like l-NMMA, blocks basal activity of nitric oxide, but has little effect on that stimulated by ACh. Further studies are required to explain these seemingly anomalous actions of ADMA and l-NMMA.

Roles of nitric oxide synthase isoforms in cutaneous vasodilation induced by local warming of the skin and whole body heat stress in humans

Nitric oxide (NO) participates in the cutaneous vasodilation caused by increased local skin temperature (Tloc) and whole body heat stress in humans. In forearm skin, endothelial NO synthase (eNOS) participates in vasodilation due to elevated Tloc and neuronal NO synthase (nNOS) participates in vasodilation due to heat stress. To explore the relative roles and interactions of these isoforms, we examined the effects of a relatively specific eNOS inhibitor, N(omega)-amino-l-arginine (LNAA), and a specific nNOS inhibitor, N(omega)-propyl-l-arginine (NPLA), both separately and in combination, on skin blood flow (SkBF) responses to increased Tloc and heat stress in two protocols. In each protocol, SkBF was monitored by laser-Doppler flowmetry (LDF) and mean arterial pressure (MAP) by Finapres. Cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP). Intradermal microdialysis was used to treat one site with 5 mM LNAA, another with 5 mM NPLA, a third with combined 5 mM LNAA and 5 mM NPLA (Mix), and a fourth site with Ringer only. In protocol 1, Tloc was controlled with combined LDF/local heating units. Tloc was increased from 34 degrees C to 41.5 degrees C to cause local vasodilation. In protocol 2, after a period of normothermia, whole body heat stress was induced (water-perfused suits). At the end of each protocol, all sites were perfused with 58 mM nitroprusside to effect maximal vasodilation for data normalization. In protocol 1, at Tloc = 34 degrees C, CVC did not differ between sites (P > 0.05). LNAA and Mix attenuated CVC increases at Tloc = 41.5 degrees C to similar extents (P < 0.05, LNAA or Mix vs. untreated or NPLA). In protocol 2, in normothermia, CVC did not differ between sites (P > 0.05). During heat stress, NPLA and Mix attenuated CVC increases to similar extents, but no significant attenuation occurred with LNAA (P < 0.05, NPLA or Mix vs. untreated or LNAA). In forearm skin, eNOS mediates the vasodilator response to increased Tloc and nNOS mediates the vasodilator response to heat stress. The two isoforms do not appear to interact during either response.

Differential effects of nitric oxide synthase inhibitors on endothelium-dependent and nitrergic nerve-mediated vasodilatation in the bovine ciliary artery

BACKGROUND AND PURPOSE

We have previously demonstrated that L-NMMA (NG-monomethyl-L-arginine) selectively inhibits vasodilatation produced by endothelium-derived nitric oxide but not nitrergic nerves in the bovine penile artery. The present study investigated whether L-NMMA had a similar selective action in the bovine ciliary artery. We also investigated whether two recently introduced inhibitors of neuronal nitric oxide synthase (nNOS), AAAN (N-(4S)-4-amino-5-[aminoethyl]aminopentyl-N'-nitroguanidine) and L-NPA (NG-propyl-L-arginine), produced selective blockade of vasodilatation induced by nitrergic nerves but not endothelium-derived nitric oxide.

EXPERIMENTAL APPROACH

Rings of bovine ciliary artery were suspended in a wire myograph for tension recording. Neurogenic (nitrergic) vasodilatation was elicited by electrical field stimulation, and endothelium-dependent, nitric oxide-mediated dilatation was evoked using bradykinin.

KEY RESULTS

L-NMMA inhibited vasodilatation induced by endothelium-derived nitric oxide but not the nitrergic nerves. In fact, L-NMMA, acted like L-arginine in protecting nitrergic vasodilatation against inhibition by L-NAME (NG-nitro-L-arginine methyl ester). AAAN had no effect on vasodilatation induced by either nitrergic nerves or endothelium-derived nitric oxide, but L-NPA inhibited both with equal potency.

CONCLUSIONS AND IMPLICATIONS

In the bovine ciliary artery, L-NMMA acts as a selective inhibitor of the vasodilatation induced via endothelial NOS, without affecting that operating via nNOS. Furthermore, the putative nNOS inhibitors, AAAN and L-NPA failed to produce the expected selective inhibition of nitrergic vasodilatation in this artery.

Defective RhoA/Rho-kinase signaling contributes to vascular hypocontractility and vasodilation in cirrhotic rats

BACKGROUND & AIMS

Portal hypertension is associated with arterial hypotension and vascular hypocontractility, which persists despite elevated plasma levels of vasoconstrictors. We investigated the role of the RhoA/Rho-kinase pathway in vascular smooth muscle hypocontractility of rats with secondary biliary cirrhosis.

METHODS

Aortic expressions of RhoA and Rho-kinase were analyzed in sham-operated and BDL rats by reverse-transcription polymerase chain reaction (RT-PCR) and immunoblots. Activation of aortic RhoA was examined by pull down of guanosine triphosphate (GTP)-RhoA and membrane translocation of RhoA. Rho-kinase activity was assessed as phosphorylation of its substrate, moesin. Contractility of isolated aortic rings was determined myographically. The hemodynamic effect of the Rho-kinase inhibitor (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632) was determined in vivo by measuring changes in mean arterial pressure and systemic vascular resistance (SVR) (microspheres).

RESULTS

Contraction of aortic rings from BDL rats was impaired in response to the alpha(1)-adrenergic receptor agonist methoxamine but not to high molar KCl. Aortic expression of RhoA was unchanged in cirrhotic rats, whereas Rho-kinase was down-regulated posttranscriptionally. Methoxamine-induced activation of RhoA as well as basal and methoxamine-induced phosphorylation of moesin were strongly reduced in aortas from cirrhotic rats. Aortic rings from cirrhotic rats precontracted with methoxamine showed an increased sensitivity to relaxation with Y-27632. The drop in SVR induced by Y-27632 was larger in cirrhotic rats than in sham-operated rats.

CONCLUSIONS

An impaired vascular activation of RhoA and a down-regulation of Rho-kinase might contribute to vasodilation and vascular hypocontractility in BDL-induced cirrhosis.

The effect of nitric oxide synthase inhibition on histamine induced headache and arterial dilatation in migraineurs

We have previously proposed that histamine causes migraine via increased NO production. To test this hypothesis, we here examined if the NOS inhibitor, L-NG methylarginine hydrochloride (L-NMMA:546C88), could block or attenuate histamine induced migraine attacks and responses of the middle cerebral, temporal and radial arteries. In a double blind crossover design 12 patients were randomized to receive pretreatment with L-NMMA (6 mg/kg) or placebo i.v. over 15 min followed on both study days by histamine (0.5 microg/kg/min) i.v. for 20 min. Headache scores, mean maximal blood velocity (Vmean) in the middle cerebral artery (MCA) (transcranial doppler) and diameters of temporal and radial arteries (high resolution ultrasound) were repeatedly measured. Pre-treatment with L-NMMA, had no effect on histamine induced headache or migraine, but also had no effect on the magnitude of histamine induced-decrease in MCA blood velocity, or dilatation of neither the temporal nor the radial artery. L-NMMA constricted the temporal artery by 8% before histamine infusion, whereas the radial artery was unaffected. The temporal artery dilated 4-5 times more than the radial artery during histamine infusion. In conclusion the use of a NOS inhibitor in the highest possible dose did not block the histamine-induced headache response or arterial dilatation. Either the concentration of L-NMMA reaching the smooth muscle cell was insufficient or, histamine dilates arteries and causes headache via NO independent mechanisms. Our results showed for the first time a craniospecificity for the vasodilating effect of histamine and for the arterial effects of NOS inhibition.

Two new nitric oxide synthase inhibitors: pyridoxal aminoguanidine and 8-quinolinecarboxylic hydrazide selectively inhibit basal but not agonist-stimulated release of nitric oxide in rat aorta

Structural modification at one of the guanidine nitrogens of L-arginine has led to the development of a number of compounds N(G)-monomethyl-L-arginine (L-NMMA), N(G)-nitro-L-arginine (L-NOARG), N(G)-nitro-L-arginine methyl ester (L-NAME) that competitively inhibit nitric oxide synthase (NOS). It was reported that another chemically related compound known as a glycation inhibitor, aminoguanidine also inhibits NOS. Recently, two new glycation inhibitors, structurally related to aminoguanidine (AG), pyridoxal aminoguanidine (PLAG) and 8-quinoline carboxylic hydrazide (8Q) were synthesized. In this study, the effects of these two inhibitors on responses mediated by constitutive nitric oxide (NO) were investigated in vitro. For this purpose, in the present study vascular responses to phenylephrine and acetylcholine in isolated aortas were evaluated. Incubation (15 min) with PLAG and 8Q (10(-4)M for each) induced potentiation of phenylephrine-induced contraction in endothelium intact but not in endothelium denuded rings of rat aorta. The ability of PLAG or 8Q to augment phenylephrine-induced tone in endothelium containing rings was completely prevented by preincubation with L-arginine (1mM), but not with D-arginine. Both compounds (PLAG, 8Q) did not affect acetylcholine-induced relaxation. These results suggest that both of the new compounds produced a selective inhibition of basal but not agonist stimulated production of nitric oxide in rat aorta.

Chylomicron remnants potentiate phenylephrine-induced contractions of rat aorta by an endothelium-dependent mechanism

The effects of chylomicron remnants on endothelium-dependent contraction of rat aorta were studied in vitro. Chylomicron remnant particles were prepared in vivo from male Wistar rats and were incubated with aortic rings for 45 min before concentration contraction response curves were constructed to phenylephrine. Both native and oxidised chylomicron remnants significantly increased vessel sensitivity to this agonist. Oxidised chylomicron remnants also significantly increased the maximum response. This potentiation was abolished by endothelial removal, but was still evident in the presence of Nomega-nitro-L-arginine, with or without cyclo (D-alpha-aspartyl-L-prolyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), indomethacin or superoxide dismutase. The study demonstrates, for the first time, that lipoprotein particles of dietary origin potentiate vascular contractions. This effect is endothelium-dependent, but is not due to inhibition of basal nitric oxide production or to stimulation of endothelin, superoxide or a cyclo-oxygenase-derived product.

Butanolic fraction from Cuphea carthagenensis Jacq McBride relaxes rat thoracic aorta through endothelium-dependent and endothelium-independent mechanisms

This study evaluated the vasorelaxant properties of the crude hydroalcoholic extract (CE) of Cuphea carthagenensis, as well as its butanolic (BF) and ethyl acetate (EA) fractions, in rings of rat thoracic aorta. In endothelium-intact rings contracted with phenylephrine (30-100 nM), cumulative additions of increasing concentrations of CE, BF, and EA of C. carthagenensis (0.1 microg/ml-3 mg/ml) caused graded relaxations, with BF displaying the lowest median inhibitory concentration (IC5; mean, 6.8 microg/ml; 95% confidence limits, 3.3-14.2). BF-induced relaxations of endothelium-intact rings were virtually abolished by prior incubation with the NO-synthase inhibitor N(omega)-nitro-L-arginine (L-NOARG; 10 or 30 microM), and were markedly reduced after guanylate cyclase inhibition with either methylene blue (10 microM) or ODQ (1 microM; 1H[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one). The inhibition of BF-induced relaxation by L-NOARG was prevented to a large extent by simultaneous incubation with L-arginine (1 mM). In endothelium-denuded rings contracted with phenylephrine, CE and BF caused graded relaxations only at doses >100 microg/ml, whereas the NO-donors SNAP (S-nitroso-N-acetyl-penicillamine) and SIN-1 (3-morpholino-sydnonimine) induced full relaxation at 1 microM. BF (100 microg/ml), which caused little relaxation per se of endothelium-denuded rings, potentiated the relaxant effects of SNAP and even more so of SIN-1 (which, unlike SNAP, also releases superoxide anion O2- in addition to NO), in a manner qualitatively similar to that seen with SOD (superoxide dismutase) against SIN-1. These data indicate that the BF of C. carthagenensis induces relaxation of the rat thoracic aorta by two mechanisms: (a) an endothelium-dependent component of action, which clearly depends on the NO/cyclic guanosine monophosphate (cGMP) pathway and can be attributed, at least in part, to free radical-scavenging properties; and (b) an endothelium-independent component of action, which becomes evident at higher doses (> or = 100 microg/ml) and remains to be further characterized. These results suggest that this native South American plant might be beneficial in cardiovascular disease.

Effects of oral propylthiouracil treatment on nitric oxide production in rat aorta

The effects of oral propylthiouracil (PTU) treatment on vascular nitric oxide (NO) production were studied in the rat aorta. Rats were fed a standard low fat diet with or without 0.1% PTU, for 2 or 4 weeks, or for 2 weeks with additional thyroxine injections. Concentration response curves were then constructed to phenylephrine (PE) in both endothelium-intact and denuded aortic rings from these animals and after incubation with 0.1 mM L-N(G)nitroarginine (L-NOARG). In addition, expression of nitric oxide synthase (NOS) was analysed in sections of aorta from PTU-treated and control rats using rabbit polyclonal antibodies to both inducible NOS (iNOS) and endothelial NOS (eNOS). Oral PTU treatment resulted in a significant reduction in both the maximum response (control, 0.53+/-0.02; 2 week PTU, 0.20+/-0.07; 4 week PTU, 0.07+/-0.02 g mg(-1)) and vessel sensitivity (EC50 values: control, 9.10x10(-8)+/-0.67; 2 week PTU, 7.45x10(-7)+/-1.15; 4 week PTU, 9.73x10(-7)+/-0.45 M) to PE in endothelium-intact vessel rings, as compared to controls (P<0.05). Both endothelial removal and incubation with L-NOARG restored the maximum response after 2, but not 4 weeks, although, in general, vessel sensitivity was not altered by either treatment. Vessels from PTU-treated rats given thyroxine injections showed no significant differences between any of the dose response curve parameters. Immunohistochemical analysis suggested that labelling for eNOS may be increased after PTU treatment as compared to control animals, whereas iNOS antibody immunoreactivity was not different between the two groups. These results suggest that the hyporesponsiveness to PE observed after oral PTU treatment is, in part, due to enhanced nitric oxide (NO) production by the endothelium, and demonstrate for the first time that thyroid hormones may play a role in the regulation of eNOS activity in the rat aorta.

Endothelial-mediated dilations following severe controlled cortical impact injury in the rat middle cerebral artery

The mechanisms associated with dysfunction of the cerebral vasculature following head trauma have not yet been fully elucidated. In an attempt to shed more light on the matter, we investigated the endothelial-mediated dilations in the rat middle cerebral artery (MCA) following severe traumatic brain injury (TBI). Rats were subjected to severe controlled cortical impact injury (CCI; 5 m/s, 130 ms duration, 3 mm deformation) over the right parietal cortex. At 24 h postinjury, ipsilateral segments of MCA and corresponding contralateral segments were isolated, mounted in a vessel chamber, and pressurized. The responses to 2 methylthio-ATP (2MeSATP), a selective agonist for the P2Y1 purinoceptors, N(omega)-nitro-L-arginine (L-NAME), an NO synthase inhibitor, and S-nitroso-N-acetylpenicillamine (SNAP), an exogenous NO donor, were determined. 2MeSATP elicited concentration dependent dilations in all MCAs studied. Ipsilateral MCAs harvested following TBI or sham-TBI, showed similar maximum dilations to 2MeSATP [70 +/- 4% (n = 17) and 72 +/- 6% (n = 13), respectively]. However, TBI reduced the concentration of 2MeSATP necessary to elicit one-half of the maximum dilation (EC50) from 15 to 9 nM (p < 0.05). Inhibition of NO synthase with 10(-5) M L-NAME abolished the dilation to 2MeSATP in both TBI and sham-TBI MCAs. The constriction to L-NAME was significantly reduced in TBI MCAs compared to sham vessels. Dilations to SNAP, an NO donor, were not altered by TBI indicating that the mechanisms of dilation involving NO in the vascular smooth muscle were not affected. Unlike other pathological conditions which often diminish endothelial-mediated responses, severe TBI enhanced the sensitivity to 2MeSATP without altering the maximum response.

The influence of chylomicron remnants on endothelial cell function in the isolated perfused rat aorta

A system for the perfusion of the isolated rat aorta which allowed the study of both the uptake of chylomicron remnants by the artery wall and their effects on endothelial function was developed. Perfusion for 2 h with 125I-labelled native or oxidised (by treatment with copper sulphate) chylomicron remnants showed that small amounts became associated with the artery wall (0.111 +/- 0.034 and 0.216 +/- 0.082 ng protein/mg tissue, respectively). Tests on endothelial function were carried out in vessel rings prepared after perfusion of the aortas in the presence or absence of chylomicron remnants for 2 h. After perfusion of the vessels with oxidised chylomicron remnants, the maximum response to phenylephrine (PE) was significantly increased (from 0.34 +/- 0.06 to 0.51 +/- 0.04 g/mg tissue; P < 0.05), while the maximum % relaxation to carbachol (CCh) was significantly decreased (from 91.6 +/- 2.4 to 71.5 +/- 7.2; P < 0.05) and the response to S-nitroso-N-acetylpenicillimine (SNAP) was unaffected. Perfusion with native chylomicron remnants showed a tendency to induce similar effects, although the changes observed did not reach statistical significance. As the lipoproteins were not present in the solution bathing the vessel rings during these tests, these effects can be attributed to perfusion of the aortas with chylomicron remnants, despite only small quantities being associated with the artery wall. The results suggest that oxidised chylomicron remnants influence vascular endothelial function by interfering with the L-arginine-nitric oxide (NO) pathway. The observed potentiation of contraction to PE may be due to inhibition of the basal release of NO or to the release of contractile factors. These findings support a role for dietary lipoproteins in the modulation of endothelial cell function which occurs in the pathogenesis of atherosclerosis.

Pharmacological characterization of endothelial cell nitric oxide synthase inhibitors in isolated rabbit aorta

Different receptors mediating the release of endothelium-derived nitric oxide (EDNO) have been identified at endothelial level. In the present study we aimed to characterise, on rabbit aorta by means of pharmacological tools, the generation of EDNO by receptors located on endothelial cell membrane (M3, P2u, P2y) and by direct activation of Ca2+ entry into the endothelial cell. Four vasodilating drugs were tested (acetylcholine, UTP, A23187 and 2-methyl-thio-ATP); they were active only if the endothelial layer was intact, suggesting that they act through endothelial receptors. The effect of different nitric oxide synthase (NOS) inhibitors (0.1 mM: L- and D-NAME, L-NMMA, L-NIO and 7-NI) was investigated on NO-mediated relaxation induced by the relaxants in vessels with intact endothelium. NOS inhibitors differently affected relaxation mediated by the vasoactive drugs in isolated rabbit aorta. Reversibility of the inhibition by using a fixed concentration of L-arginine (0.1 mM) was different depending on the relaxing drug and NOS-inhibitor. The data obtained support the coexistence in aortic vessel of more than one endothelial cell NOS isoform, each provided with different receptor coupling.

Investigation of the contributions of nitric oxide and prostaglandins to the actions of endothelins and sarafotoxin 6c in rat isolated perfused lungs

1. The aims of the study were to assess the contribution of prostaglandins and nitric oxide (NO) to the effects of endothelin (ETs) and sarafotoxin 6c (SX6c) in perfused rat lungs. This was carried out by using indomethacin, a cyclo-oxygenase inhibitor and NG-nitro-L-arginine (L-NOARG), a NO synthase inhibitor. Responses were studied under basal perfusion conditions and in other experiments after the elevation of vascular tone with the thromboxane-mimetic, U46619. The sub-types of ET receptors involved were characterized by use of ET receptor antagonists and cross-tachyphylaxis. 2. Pulmonary perfusion pressure (PPP), lung weight and pulmonary inflation pressure (PIP), were continuously recorded. Although L-NOARG (100 microM) did not alter basal parameters it markedly augmented the vasoconstriction and lung weight increases induced by ET-1 (50-400 pmol) or SX6C (25-200 pmol) while vasoconstrictor responses to phenylephrine were not affected by L-NOARG. 3. L-NOARG markedly potentiated the bronchoconstriction induced by ET-1 or SX6C whereas it had no effect on responses to carbachol. 4. When vascular tone was elevated, low doses (1.25-40 pmol) of ET-1, ET-3 and SX6C produced falls in PPP. The vasodilator potencies were SX6C > ET-1 = ET-3. The ETA receptor antagonist, BQ123, did not affect these depressor responses whereas the mixed ETA/ETB antagonist, bosentan, blocked them. 5. Indomethacin (10 microM) partially inhibited vasodilator response to ET-1, whereas it had no effect on SX6C-induced vasodilation. 6. L-NOARG plus indomethacin completely blocked ET-1 induced vasodilation, whereas responses to SX6C were blocked by L-NOARG alone. 7. Repeated injections of submaximal doses of ET-1 or SX6C caused tachyphylaxis to vasodilator responses. Subsequent injections of SX6C or ET-1 did not elicit depressor responses showing cross tachyphylaxis had occurred. 8. These findings indicate that under basal conditions the pulmonary vasoconstrictor, lung weight and bronchoconstrictor responses to ET-1 and SX6C are attenuated by evoked release of nitric oxide (NO). When vascular tone was elevated, lower doses of ETs and SX6C produced vasodilatation. These vasodilator responses are indirect, those to SX6C being mediated via NO production, whereas those to ET-1 involve both NO and prostanoid(s). Tachyphylaxis and ET antagonist experiments indicate that the same receptor subtype is involved in mediating the vasodilatation and that this is of the ETB type located on the endothelium. However the post-receptor vasodilator events triggered by ET-1 or SX6C appear to be different.

Selective pharmacological inhibition of distinct nitric oxide synthase isoforms

Nitric oxide (NO) is produced in physiological and pathophysiological conditions by three distinct isoforms of NO synthase (NOS): endothelial NOS (ecNOS), inducible NOS (iNOS), and brain NOS (bNOS). Selective inhibition of iNOS may be beneficial in various forms of shock and inflammation, whereas inhibition of bNOS may protect against neuroinjury. This article surveys the enzymatic mechanism of NO production, lists the strategies and pharmacological tools for selective inhibition of distinct NOS isoforms, and considers the side-effects of the various approaches. Selective inhibition of NOS isoforms is achieved by: (a) targeting the differential co-factor (calmodulin or tetrahydrobiopterin) requirement of various NOS isoforms, and NOS; (b) targeting the differential substrate requirements of cells expressing various isoforms of NOS (L-arginine uptake blockers or arginase); (c) the use of pharmacological agents that are selectively taken up by cells expressing various isoforms of NOS (7-nitroindazole); or (d) developing pharmacological NOS inhibitors with isoform specificity. The amino acid-based NOS inhibitor, NG-nitro-L-arginine, shows a preference for ecNOS and bNOS over iNOS, whereas L-N6-(1-iminoethyl)lysine is selective for iNOS over bNOS. Certain non-amino acid-based small molecules, such as aminoguanidine and certain S-alkylated isothioureas, also express selectivity towards iNOS and have anti-inflammatory and anti-shock properties. 7-nitroindazole, a bNOS-selective inhibitor, protects in central nervous system injury. Clearly, there are a number of distinct approaches that are worthy of further research efforts in order to achieve even more selective targeting of various NOS isoforms

Nitric oxide synthase in cerebral ischemia. Possible contribution of nitric oxide synthase activation in brain microvessels to cerebral ischemic injury

The results of our continuing studies on the role of nitric oxide (NO) in cellular mechanisms of ischemic brain damage as well as related reports from other laboratories are summarized in this paper. Repetitive ip administration of NG-nitro-L-arginine (L-NNA), a NO synthase (NOS) inhibitor, protected against neuronal necrosis in the gerbil hippocampal CA1 field after transient forebrain ischemia with a bell-shaped response curve, the optimal dose being 3 mg/kg. Repeated ip administration of L-NNA also mitigated rat brain edema or infarction following permanent and transient middle cerebral artery (MCA) occlusion with a U-shaped response. The significantly ameliorative dose-range and optimal dose were 0.01-1 mg/kg and 0.03 mg/kg, respectively. Studies using a NO-sensitive microelectrode revealed that NO concentration in the affected hemisphere was remarkably increased by 15-45 min and subsequently by 1.5-4 h after MCA occlusion. Restoration of blood flow after 2 h-MCA occlusion resulted in enhanced NO production by 1-2 h after reperfusion. Administration of L-NNA (1 mg/kg, ip) diminished the increments in NO production during ischemia and reperfusion, leading to a remarkable reduction in infarct volume. In brain microvessels obtained from the affected hemisphere, Ca(2+)-dependent constitutive NOS (cNOS) was activated significantly at 15 min, and Ca(2+)-independent inducible NOS (iNOS) was activated invariably at 4 h and 24 h after MCA occlusion. Two hour reperfusion following 2 h-MCA occlusion caused more than fivefold increases in cNOS activity with no apparent alterations in iNOS activity. Thus, we report here based on available evidence that there is good reason to think that NOS activation in brain microvessels may play a role in the cellular mechanisms underlying ischemic brain injury.

Differential sensitivity of basal and acetylcholine-stimulated activity of nitric oxide to destruction by superoxide anion in rat aorta

1. In this study we compared the ability of superoxide anion to destroy the relaxant activity of basal and acetylcholine (ACh)-stimulated activity of NO in isolated rings of rat aorta. 2. Superoxide dismutase (SOD, 1-300 u ml-1) induced a concentration-dependent relaxation of phenylephrine (PE)-induced tone in endothelium-containing rings which was blocked by NG-nitro-L-arginine (L-NOARG, 30 microM), but had no effect on endothelium-denuded rings. It was likely therefore that the relaxant action of SOD resulted from protection of basally produced NO from destruction by superoxide anion, generated either within the tissue or in the oxygenated Krebs solution. 3. In contrast, a concentration of SOD (50 u ml-1) which produced almost maximal enhancement of basal NO activity, had no effect on ACh (10 nM-3 microM)-induced relaxation. 4. In the presence of catalase (3000 u ml-1) to prevent the actions of hydrogen peroxide, superoxide anion generation using hypoxanthine (HX, 0.1 mM)/xanthine oxidase (XO, 16 mu ml-1) produced an augmentation of PE-induced tone in endothelium-containing but not endothelium-denuded rings. This was likely to have resulted from removal of the tonic vasodilator action of basally-produced NO by superoxide anion, since it was blocked in tissues treated with SOD (250 u ml-1), NG-monomethyl-L-arginine (L-NMMA, 30 microM) or L-NOARG (30 microM). Pyrogallol (0.1 mM) had a similar action to HX/XO, but produced an additional augmentation of tone by an endothelium-independent mechanism. 5. In contrast to their ability to destroy almost completely the basal activity of NO, HX (0.1 mM)/XO(16 mu ml-1) and pyrogallol (0.1 mM) had no effect on ACh-induced relaxation at any concentration. An increase in the concentration of HX to 1 mM or pyrogallol to 0.3 mM did, however, lead to a profound decrease in the magnitude and time course of ACh-induced relaxation at all concentrations.6. Treatment with diethyldithiocarbamate (DETCA, 0.1 mM, 1 h) to inhibit endogenous Cu-Zn SOD,augmented PE-induced tone in endothelium-containing rings and abolished the ability of HX (0.1 mM)/XO (16 mu ml-1) and L-NMMA (30 microM) to augment tone. It was likely that DETCA had led to the destruction of basal NO activity by increasing superoxide anion levels since its actions were reversed by exogenous SOD (10-300 upsilon ml-1).7. In contrast to its ability to destroy basal activity of NO completely, DETCA (0.1 mM) produced only a slight blockade of ACh-induced relaxation. However, if these tissues were subsequently treated with concentrations of HX (0.1 mM)/XO (16 mu ml-1) or pyrogallol (0.1 mM), which had no effect by themselves on ACh-induced relaxation, a profound blockade was seen and this was reversed completely with SOD (250 u ml-1).8. The data suggest that basal activity of NO is more sensitive to inactivation by superoxide anion than ACh-stimulated activity and this probably results from differential protection by endogenous Cu-ZnSOD. It is possible therefore that endogenous SOD lowers superoxide anion levels to such an extent that only small amounts of NO, such as those produced under basal conditions, are destroyed. Following generation of superoxide anion with HX/XO or pyrogallol, or inhibition of Cu-Zn SOD with DETCA,levels of the free radical will increase such that greater amounts of NO, such as those produced following stimulation with ACh, will then be destroyed.

Characterization of the effects of two new arginine/citrulline analogues on constitutive and inducible nitric oxide synthases in rat aorta

1. New potent inhibitors of nitric oxide synthase (NOS) may be useful in the treatment of septic shock, a disorder characterized by a vascular hyporeactivity to catecholamines caused by an overproduction of nitric oxide (NO-). We examined the effects of L-thiocitrulline (L-TC) and S-methyl-L-thiocitrulline (L-SMTC), novel NOS inhibitors, on the constitutive and inducible NOS in rat aorta and compared those effects with inhibition due to NG-methyl-L-arginine (L-NMA). 2. Phenylephrine evoked similar concentration-contraction curves in the control rings and in the rings treated with these different NOS inhibitors (10 microM), whereas 100 microM of L-NMA, L-TC or L-SMTC increased significantly, and to a similar extent, contractions evoked by phenylephrine in aortic rings with endothelium without significantly affecting the maximal responses. 3. Relaxations evoked by acetylcholine, adenosine triphosphate, or calcium ionophore were significantly inhibited in a dose-dependent manner by L-NMA, L-SMTC, or L-TC (10-100 microM). The potencies of these inhibitors in reducing the relaxations of these vasodilators were not significantly different. 4. In endotoxin-treated preparations with endothelium, the three L-arginine analogues (10 microM) significantly potentiated contractile responses to phenylephrine (pEC50: 6.73 +/- 0.12 and 7.3 +/- 0.12, 7.34 +/- 0.13, or 7.22 +/- 0.14; in the absence and the presence of L-NMA, L-TC, or L-SMTC respectively) and increased maximal contractions from 1.53 +/- 0.15 g to 1.95 +/- 0.13 g, 2.08 +/- 0.12 g, and 2.03 +/- 0.13 g with L-NMA, L-TC, and L-SMTC respectively. A higher concentration of these NOS inhibitors (100 microM)further increased contractions evoked by this alpha1-agonist without further enhancing the maximal contractions; however, contractions evoked by 10 nM phenylephrine were significantly greater in the presence of L-SMTC or L-TC than in the presence of L-NMA (100 microM) (L-NMA: 0.4 +/- 0.11 g; L-TC:0.78 +/- 0.14 g and L-SMTC: 0.82+/-0.17 g). The effects of these inhibitors on NO- synthesis induced by endotoxin were significantly reversed by addition of L-arginine (1 mM) but not by L-citrulline (1 mM). InLPS-treated rings with endothelium, all three NOS inhibitors (100 microM) shifted the concentration contraction curves evoked by phenylephrine significantly to the left (pEC5o: 7.19 +/- 0.03 and 7.79 +/- 0.08,8.01 +/- 0.07, or 8.02 +_ 0.07, in the absence and the presence of L-NMA, L-TC, or L-SMTC, respectively)and increased significantly maximal contractions from 2.05 +/- 0.05 g to 2.38 +/- 0.14 g, 2.5 +/- 0.12 g, and 2.4 +_ 0.21 g with L-NMA, L-TC, and L-SMTC, respectively. L-TC and L-SMTC were significantly more potent than L-NMA in potentiating contractions evoked by 10 nM and 30 nM phenylephrine.5. L-TC and L-SMTC produced dose-dependent increases in tone in LPS-treated aortic rings with and without endothelium. In LPS-treated rings with endothelium, L-NMA induced contractions but in preparations without endothelium low concentrations of L-NMA induced small contractions while high concentrations of this inhibitor evoked relaxations. In both preparations L-TC and L-SMTC were significantly more potent than L-NMA in increasing vascular tone.6. These results suggest that L-SMTC, L-TC and L-NMA were equipotent on basal and agonist stimulated NO- synthesis produced by the constitutive isoform of NOS, whereas the two new L-arginine analogues were more potent than L-NMA in inhibiting the production of NO- induced by endotoxin in rat aorta.

Effect of swimming on vascular reactivity to phenylephrine and KC1 in male rats

1. The present study aimed to examine whether there is any change in vascular responsiveness to phenylephrine and KC1 during exercise, and whether the vascular endothelium plays a role in these changes. 2. Adult male rats were subjected to a swimming schedule every day for 5-6 weeks. Studies were performed in vitro on thoracic aortae. 3. Maximum contractile response to phenylephrine of endothelium-intact thoracic aortic rings (passive tension 1.0 g) obtained from swimming rats (1.2 +/- 0.2 g, n = 8) was lower than of sedentary control rats (2.1 +/- 0.2 g, n = 8). When the endothelium was removed, however, the dose-response curves of both groups of rats were shifted to the left with an increase in maximum responses and they were no longer significantly different (max. tension, swimming rats: 3.2 +/- 0.3 g, n = 6, control rats: 3.4 +/- 0.4 g, n = 5). 4. Indomethacin did not significantly alter the dose-response curves. A similar effect to that obtained by removal of the endothelium was observed when methylene blue and indomethacin were both added. 5. Passive tension in the range of 2.5-3.0 g, caused a significant increase in active tension developed to phenylephrine (1 microM for endothelium-intact and 0.1 microM for endothelium-denuded) of thoracic aortic rings of both swimming and sedentary control rats compared to their corresponding groups when using passive tension of 1.0-1.5 g. 6. The reduction in responses to phenylephrine of endothelium-intact thoracic aortic rings of swimming rats persisted with the use of a passive tension of 3.0 g. The presence of 300 microM N0-nitro-L-arginine (LNOARG)caused a significant leftward shift of the curve with an increase in maximum responses when a passive tension of either 1.0 or 3.0 g was applied to the rings. However, for the rings with a passive tension of 1.0 g, L-NOARG caused a smaller increase in maximal contractile responses to phenylephrine of the rings of sedentary controls than those of swimming rats.7. There was no difference in the dose-response curves to depolarizing concentrations of KCl (20, 40, 80 and 120 mM) of endothelium-intact thoracic aortic rings from swimming and sedentary control rats.When the endothelium was removed, however, the dose-response curves of both groups of animals were shifted to the left with an increase in maximum responses. Moreover, the responses to KCl of endothelium-denuded thoracic aortic rings of swimming rats were greater than those of sedentary control rats.8. These results suggest that there were changes in vascular responsiveness to phenylephrine and KCl during exercise. The fall in sensitivity to phenylephrine with no change in KCl responses, and the increase in maximum responses to phenylephrine in the presence of L-NOARG in endothelium-intact aortae (passive tension 1.0 g) from swimming rats, were due to an increase in spontaneous release and upregulation of phenylephrine-stimulated release of EDRF/NO, and may not be a consequence of an increase in prostaglandins or a decrease in the production of endothelial constrictors by vascular endothelium. EDRF/NO may play an important role in modulating local vasodilatation.

Nitric oxide-based possibilities for pharmacotherapy

The goal of nitric oxide (NO) based pharmacotherapy is to reach proper homeostasis of NO metabolism in the target tissue where endogenous production of NO is either too weak or excessively increased. In addition to the classic NO-based therapy of cardiovascular conditions with nitrates, a variety of new therapeutic possibilities have emerged including sexual disorders, gastrointestinal system, immunology, tumour growth regulation and respiratory disorders. NO levels of target tissues can be affected directly by NO donors, or indirectly by increasing the level of L-arginine, a substrate of nitric oxide synthase (NOS). While increased production of NO by induceable NO (iNOS) by, for example, cytokines does not at present seem therapeutically meaningful, increased NO production by constitutive NOS (cNOS) may be involved in the beneficial effects of ACE inhibitors or oestrogens. NO production may be pharmacologically decreased by inhibition of expression of iNOS by glucocorticoids while both cNOS and iNOS derived NO production is inhibited by administration of false substrates, for example L-NAME. Additionally, the respiratory system and related vessels can be reached directly and more selectively by inhalation of pure NO gas. Possible problems in administering NO and perhaps some NO-donors include the toxic nature of the compound itself whereby vital enzyme systems may be inhibited and tissue damaging radicals formed. Future prospects of NO-based pharmacotherapy may feature selective ligands to different NOS isoforms and tissue selective donors that release NO in a controlled fashion.

Reduction by NG-nitro-L-arginine of H2O2-induced endothelial cell injury

1. The effects of three analogues of NG-nitro-L-arginine (L-NOARG) and NG-monomethyl-L-arginine (L-NMMA), inhibitors of nitric oxide (NO) synthase, on hydrogen peroxide (H2O2)-induced endothelial cell injury were studied. 2. Endothelial cell injury was assessed by measuring the release of intracellular lactate dehydrogenase (LDH) and 51Cr. 3. Addition of H2O2 (250-1,000 microM) to endothelial cells induced the release of LDH dose-dependently. The release of LDH was reduced by pretreatment with NG-nitro-L-arginine methyl ester (L-NAME, 10(-4)-4 x 10(-3) M), L-NOARG (10(-4)-4 x 10(-3) M) and NG-nitro-L-arginine benzyl ester (L-NABE, 10(-4)-4 x 10(-3) M), inhibitors of NO synthase. 4. L-NOARG analogues also reduced H2O2-induced 51Cr release from endothelial cells, while L-NMMA had no effect. 5. The protective effect of L-NAME was not reversed by addition of L-arginine (L-Arg, 1-10 mM). 6. Both L-NAME and L-NMMA completely inhibited L-Arg metabolism to L-citrulline coupled with NO synthesis. 7. These findings suggest that L-NOARG analogues but not L-NMMA reduced H2O2-induced endothelial cell injury, and that these effects may not be related to inhibition of NO production.