Yong‐Xiang Wang

Increase by NG-nitro-L-arginine methyl ester (L-NAME) of resistance to venous return in rats

1 The effects of the nitric oxide (NO) synthase inhibitor, NG‐nitro‐L‐arginine methyl ester (L‐NAME), on mean circulatory filling pressure (MCFP), total peripheral resistance (TPR), cardiac output (CO) and resistance to venous return (Rv) were studied in rats.

POSSIBLE DEPENDENCE OF PRESSOR AND HEART RATE EFFECTS OF NG-NITRO-L-ARGININE ON AUTONOMIC NERVE ACTIVITY

1 The effects of NG‐nitro‐l‐arginine (l‐NNA) on mean arterial pressure (MAP) and heart rate (HR) were investigated in conscious rats. 2 Intravenous bolus cumulative doses of l‐NNA (1–32 mg kg−1) dose‐dependently increased MAP. Both mecamylamine and phentolamine increased MAP responses to l‐NNA, angiotensin II and methoxamine. Propranolol, reserpine, atropine and captopril did not affect MAP response to l‐NNA. 3 A significant negative correlation of HR and MAP responses to l‐NNA was obtained in control rats but not in rats pretreated with reserpine or mecamylamine. Significant negative correlations also occurred in the presence of atropine, propranolol, phentolamine or captopril. 4 A single i.v. bolus dose of l‐NNA (32 mg kg−1) raised MAP to a peak value of 53 ± 3 mmHg and the effect lasted more than 2 h; the rise and recovery of MAP were accompanied by significant decrease and increase in HR, respectively. While both phentolamine and mecamylamine increased peak MAP response to l‐NNA, mecamylamine abolished the biphasic HR response and phentolamine potentiated the bradycardic component of HR. 5 Blockade of the autonomic nervous and renin‐angiotensin systems did not attenuate the pressor effects of l‐NNA. However, the biphasic HR response to l‐NNA is mediated via modulation of autonomic nerve activities.

Inhibitory actions of diphenyleneiodonium on endothelium‐dependent vasodilatations in vitro and in vivo

1 This study examined the in vitro and in vivo inhibitory effects of diphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synthase, on endothelium‐dependent vasodilatations. 2 DPI (3 × 10−8–3 × 10−6 m) concentration‐dependently inhibited acetylcholine (ACh)‐induced relaxation in preconstricted rat thoracic aortic rings, with an IC50 of 1.8 × 10−7 m and a maximal inhibition of nearly 100%. DPI (3 × 10−6 m) also completely inhibited the relaxation induced by the calcium ionophore, A23187 but not by sodium nitroprusside (SNP). The inhibitory effect of DPI (3 × 10−7 m) on ACh‐induced relaxation was prevented by pretreatment with NADPH (5 × 10−3 m) and FAD (5 × 10−4 m) but not l‐arginine (l‐Arg, 2 × 10–3 m). Pretreatment with NADPH did not alter the inhibitory effect of NG‐nitro‐l‐arginine on ACh‐induced relaxation. 3 The inhibitory effect of DPI on ACh‐induced relaxation in the aortae lasted >4 h after washout. In contrast to pretreatment, post‐treatment (1 h later) with NADPH (5 × 10−3 m) reversed only slightly the inhibitory effect of DPI. 4 In conscious rats, DPI (10−5 mol kg−1) inhibited the depressor response to i.v. infused ACh, but not SNP. However, it caused only a transient pressor response which was previously shown to be due completely to sympathetic activation. 5 Thus, DPI is an efficacious and ‘irreversible’ inhibitor of endothelium‐dependent vasodilatation in vivo and in vitro. The mechanism of the inhibition may involve antagonism of the effects of FAD and NADPH, co‐factors of NO synthase. However, unlike the NG‐substituted arginine analogues (another class of NO synthase inhibitors), DPI‐induced suppression of endothelium‐dependent vasodilatation in vivo does not lead to a sustained rise in blood pressure.

Endothelium-derived nitric oxide partially mediates salbutamol-induced vasodilatations

This study examined the ability of salbutamol (selective beta 2-adrenoceptor agonist) to cause endothelium-dependent relaxation in rat aortic rings and depressor response in conscious rats. Salbutamol (0.01-100 microM) concentration dependently relaxed preconstricted aortic rings. The relaxant response was partially attenuated by either mechanical removal of the endothelium or treatment with NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). In conscious rats, either i.v. infused phenylephrine (5 micrograms/kg per min) or i.v. bolus injected L-NAME (12.8 mg/kg), but not the vehicle, caused similar sustained increases in mean arterial pressure (MAP). I.v. infused salbutamol (2-128 micrograms/kg per min, each dose for 5 min) dose dependently decreased MAP in vehicle-treated rats; the depressor responses were potentiated by hypertension induced by phenylephrine. In contrast, the magnitudes of the depressor response to salbutamol in L-NAME-treated rats were less than those in rats pretreated with phenylephrine or the vehicle. I.v. bolus injections of salbutamol (0.25-16 micrograms/kg) also caused dose-dependent and transient decreases in MAP in vehicle-treated rats. The magnitude but not the duration of the depressor response to salbutamol was less in rats treated with L-NAME, compared to those in rats given phenylephrine or the vehicle. These results suggest that endothelium-derived nitric oxide is partially involved in beta 2-adrenoceptor-mediated vasodilatation.

Vasodilator effects of organotransition-metal nitrosyl complexes, novel nitric oxide donors.

Nitrovasodilators cause endothelium-independent relaxation of blood vessels by generating nitric oxide (NO). We examined the relaxation and depressor effects of two organotransition-metal nitrosyl complexes, CpCr(NO)2Cl and CpMo(NO)2Cl, relative to those of the prototypal nitrovasodilators, nitroglycerin, and sodium nitroprusside (SNP), in phenylephrine-preconstricted aortic rings and conscious, unrestrained rats. CpCr(NO)2Cl, CpMo(NO)2Cl, nitroglycerin and SNP caused dose-dependent relaxation of aortic rings at maximal responses (Emax) of -118+/-4, -113+/-4, -104+/-1, and -128+/-5% and EC50 of 0.14+/-0.04, 22+/-4, 1.23+/-0.65, and 0.063+/-0.013 microM, respectively. The dose-response curve of CpCr(NO)2Cl was displaced to the right by hemoglobin, as well as methylene blue, showing involvement of the NO/cGMP pathway. Unlike nitroglycerin, preexposure for 1 h to CpCr(NO)2Cl did not alter subsequent relaxation response to the compound. Intravenous bolus injections of CpCr(NO)2Cl, CpMo(NO)2Cl, nitroglycerin, and SNP caused dose-dependent decreases in MAP with Emax of -42+/-2, -51+/-8, -56+/-6, and -58+/-2 mm Hg and EC50 of 0.041+/-0.010, 13+/-4, 1.6+/-0.4, and 0.037+/-0.004 micromol/kg, respectively. These results show that CpCr(NO)2Cl and CpMo(NO)2Cl are efficacious nitrovasodilators in vitro and in vivo.

Vascular pharmacology of methylene blue in vitro and in vivo: a comparison with NG‐nitro‐l‐arginine and diphenyleneiodonium

1 The vascular effects of the soluble guanylyl cyclase inhibitor, methylene blue as well as the nitric oxide (NO) synthase inhibitors, NG‐nitro‐l‐arginine (l‐NOARG) and diphenyleneiodonium (DPI) were studied in rat isolated aortic rings and conscious, unrestrained rats. 2 Acetylcholine (ACh) and sodium nitroprusside (SNP) caused concentration‐dependent relaxation of preconstricted aortic rings. Both methylene blue (1 × 10−5m) and l‐NOARG (3 × 10−5m) abolished ACh‐induced relaxation; however, methylene blue but not l‐NOARG shifted the concentration‐response curve of SNP to the right. 3 In conscious rats, i.v. infusion of methylene blue (1.1 × 10−5mol kg−1 min−1), at a concentration which reduced the aortic tissue level of cyclic GMP by 50%, did not significantly alter mean arterial pressure (MAP) and heart rate (HR). In contrast, i.v. bolus injection of l‐NOARG (1.5 × 10−4 mol kg−1) markedly increased MAP and decreased HR. 4 Both ACh and SNP dose‐dependently decreased MAP in conscious rats. Methylene blue did not alter the magnitude or duration of ACh‐ or SNP‐induced depressor responses. l‐NOARG, on the other hand, significantly though incompletely, reduced the magnitude and duration of the depressor response to ACh but not SNP. The depressor response to ACh or SNP was not altered by pretreatment with indomethacin (1.4 × 10−5 mol kg−1) or capsaicin (3.3 × 10−4 mol kg−1). 5 NG‐nitro‐l‐arginine methyl ester (l‐NAME) also caused dose‐dependent increases in MAP in conscious rats. Both methylene blue and DPI (1 × 10−5 mol kg−1) selectively shifted the dose‐pressor response curve of l‐NAME to the right. 6 These results suggest that: (1) the inhibition of endogenous NO biosynthesis does not necessarily lead to pressor response in vivo, (2) l‐NOARG may not produce pressor response solely via the inhibition of endogenous endothelial NO biosynthesis, and (3) the depressor responses to ACh and SNP may not involve the release of NO or prostanoids or afferent nerve transmitters.

Probing the role of AMPAR endocytosis and long-term depression in behavioural sensitization: relevance to treatment of brain disorders, including drug addiction.

Modifying the function of postsynaptic α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid subtype glutamate receptors (AMPARs) is one of the most important mechanisms by which the efficacy of synaptic transmission at excitatory glutamatergic synapses in the mammalian brain is regulated. Traditionally these types of modifications have been thought to be achieved mainly by altering the channel gating properties or conductance of the receptors. A large body of evidence accumulated from recent studies strongly suggests that AMPARs, like most integral plasma membrane proteins, are continuously recycled between the plasma membrane and the intracellular compartments via vesicle‐mediated plasma membrane insertion and clathrin‐dependent endocytosis. Regulation of either receptor insertion or endocytosis results in a rapid change in the number of these receptors expressed on the plasma membrane surface and in the receptor‐mediated responses, thereby playing an important role in mediating certain forms of synaptic plasticity, such as long‐term potentiation (LTP) and depression (LTD). These studies have significantly advanced our understanding of the molecular mechanisms underlying LTP and LTD, and their potential contributions to learning and memory‐related behaviours. Here I provide a brief summary of the current state of knowledge concerning clathrin‐mediated AMPAR endocytosis and its relationship to the expression of certain forms of LTD in several brain areas. The potential impact of recent advancements on our efforts to probe the roles of synaptic plasticity in learning and memory‐related behaviours, and their relevance to some brain disorders, particularly drug addiction, are also discussed.

Halothane inhibits the pressor effect of diphenyleneiodonium.

1 We have recently found that diphenyleneiodonium (DPI), a novel inhibitor of nitric oxide (NO) synthase, causes pressor and tachycardic responses in pentobarbitone‐ but not halothane‐anaesthetized rats. The present study investigated the mechanism by which halothane suppresses the pressor response of DPI. The effects of halothane on the pressor response of DPI were also compared with those of other anaesthetic agents. 2 In conscious rats, i.v. bolus injections of DPI (0.025– 1.6 mg kg−1) caused dose‐dependent increases in mean arterial pressure (MAP), with ED50 of 0.07 ± 0.01 mg kg−1 and maximal rise of MAP (Emax) of 59 ± 2 mmHg. While ketamine potentiated Emax without altering the ED50 and pentobarbitone increased the ED50 without changing Emax of the pressor response to DPI, chloralose, urethane and ethanol displaced the curve to the right and potentiated Emax. In contrast, halothane (0.5–1.25%) dose‐dependently and non‐competitively reduced the pressor responses to DPI. 3 Intravenous bolus injection of a single dose of DPI (1.6 mg kg−1) caused immediate and large increases in plasma noradrenaline and adrenaline, as well as MAP in conscious rats. Halothane (1.25%) almost completely inhibited these increases. 4 The results suggest that DPI causes a pressor response in conscious rats by activating the sympathetic nervous system and halothane abolishes this pressor response by inhibiting activities of the sympathetic nervous system. The results also show that influences of anaesthetics must be taken into consideration when evaluating pressor response of vasoactive agents.

Biological Implications of Oxidation and Unidirectional Chiral Inversion of D-amino Acids

Recent progress in chiral separation of D- and L-amino acids by chromatography ascertained the presence of several free Damino acids in a variety of mammals including humans. Unidirectional chiral inversion of many D-amino acid analogs such as exogenous NG-nitro-D-arginine (D-NNA), endogenous D-leucine, D-phenylanine and D-methionine have been shown to take place with inversion rates of 4-90%, probably dependent on various species D-amino acid oxidase (DAAO) enzymatic activities. DAAO is known to catalyze the oxidative deamination of neutral and basic D-amino acids to their corresponding α-keto acids, hydrogen peroxide and ammonia, and is responsible for the chiral inversion. This review provides an overview of recent research in this area: 1) oxidation and chiral inversion of several D-amino acid analogs in the body; 2) the indispensable but insufficient role of DAAO particularly in the kidneys and brain for the oxidation and chiral inversion of D-amino acids analogs; and 3) unidentified transaminase(s) responsible for the second step of chiral inversion. The review also discusses the physiological significance of oxidation and chiral inversion of D-amino acids, which is still a subject of dispute.

NG-nitro-L-arginine contracts vascular smooth muscle by an endothelium-independent mechanism.

We characterized the contractile effect of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NNA) in endothelium-denuded rat aortic rings. Incubation with L-NNA (4 x 10(-6)-6.4 x 10(-5) M) for 5 h dose-dependently contracted endothelium-denuded aortic rings. In contrast, incubation with NG-nitro-D-arginine (D-NNA 6 x 10(-6)-4 x 10(-4) M), diphenyleneiodonium (DPI, NO synthase inhibitor, 3.2 x 10(-6) M) or dexamethasone (10(-7) M, inhibitor of expression of inducible NO synthase) did not contract the denuded rings. The L-NNA-induced contraction was not significantly altered by the presence of the endothelium or by pretreatment with L-arginine (L-Arg 2 x 10(-3) M) or lipopolysaccharide (100 ng/ml). These results suggest that L-NNA causes slow contraction of endothelium-denuded vascular smooth muscle (VSM) by a mechanism independent of the inhibition of constitutive or inducible NO biosynthesis.