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

The influence of atropine and atenolol on the cardiac haemodynamic effects of NG-nitro-L-arginine methyl ester in conscious, Long Evans rats

1. In the present study, the extent to which baroreflexes contribute to the cardiac effects of NG-nitro-L-arginine methyl ester (L-NAME) was assessed in conscious, Long Evans rats chronically instrumented with thoracic electromagnetic flow probes for the measurement of cardiac haemodynamics. 2. L-NAME (10 mg kg-1, i.v.) was administered in the absence (n = 6) and in the presence (n = 7) of atropine (1 mg kg-1) and atenolol (1 mg kg-1). 3. L-NAME caused a marked increase in mean arterial pressure and marked reductions in total peripheral conductance, cardiac output, heart rate, stroke volume, peak thoracic flow and the maximum rate of rise of aortic flow. 4. Administration of atropine, after the maximal bradycardic effect of L-NAME was established, restored the heart rate to resting levels. Concurrently, there was a reduction in stroke volume, such that cardiac output, although transiently elevated, did not show a sustained increase. No other variables were significantly affected by atropine. Additional administration of atenolol had no effect other than to cause a slight bradycardia, such that in the presence of atropine and atenolol, heart rate was not different from that in animals receiving atropine and atenolol before L-NAME. 5. In the presence of atropine and atenolol, L-NAME had similar pressor, vasoconstrictor and cardiac haemodynamic effects to those in untreated animals, although the bradycardia was significantly attenuated. However, there was still a significant reduction in heart rate following L-NAME in the presence of atropine and atenolol.6. These results indicate that the major component of the bradycardia following L-NAME is indirect and mediated through an increase in vagal efferent activity. However, the substantial reduction in cardiac function caused by L-NAME is not dependent on the autonomic control of the heart but rather, may depend on the increase in afterload and/or a direct effect of L-NAME on the heart and/or its vasculature.

Nitrergic transmission: nitric oxide as a mediator of non-adrenergic, non-cholinergic neuro-effector transmission

1. The possibility that transmission at some non-adrenergic, non-cholinergic (NANC) neuro-effector junctions is mediated by nitric oxide (NO) arose from the discoveries that NO mediated the effects of nitrovasodilator drugs and that endothelium-derived relaxing factor (EDRF) was NO or a NO-yielding substance. 2. NO donated by nitrovasodilator drugs or formed by endothelial cells activates soluble guanylate cyclase in smooth muscle and the consequent increase in cyclic guanosine monophosphate (cGMP) results in relaxation. The relaxations produced by stimulation of some NANC nerves are also due to a rise in cGMP. 3. The biosynthesis of NO by oxidation of a terminal guanidino nitrogen of L-arginine is inhibited by some NG-substituted analogues of L-arginine. These substances block EDRF formation by NO synthase and endothelium-dependent vasodilatation, and the blockade is overcome by L-arginine 4. NANC relaxations in some tissues are blocked by NG-substituted analogues of L-arginine and restored by L-arginine. Other agents that affect endothelium-dependent vasodilator responses produce corresponding changes in responses to stimulation of these NANC nerves. Such observations indicate that transmission is mediated by NO: we have termed this mode of transmission nitrergic. 5. There is evidence for nitrergic innervation of smooth muscle in the gastrointestinal tract, genito-urinary system, trachea and some blood vessels (penile and cerebral arteries). 6. The recognition of a mediator role for NO in neurotransmission calls for reconsideration of previously accepted generalizations about mechanisms of transmission. 7. Studies on nitrergic transmission will provide new insights into physiological control mechanisms and pathophysiological processes and may lead to new therapeutic developments.

Effects of endothelium-derived nitric oxide on cerebral circulation during normoxia and hypoxia in the rat

The aim of this study was to determine the effects of endogenous nitric oxide (NO) on cerebral blood flow (CBF) and cerebrovascular resistance (CVR) under conditions of normoxia and hypoxia. Experiments were performed on anesthetized, mechanically ventilated Wistar rats. CBF was measured using the intracarotid 133Xe injection technique. NO formation was inhibited by NG-monomethyl-L-arginine (L-NMMA). Administration of L-NMMA (100 mg kg-1 i.v.) during normoxia resulted in an increase in mean arterial blood pressure from 113 +/- 4 to 145 +/- 4 mm Hg (p less than 0.001), a decrease in CBF of 21% (from 91 +/- 4 to 75 +/- 5 ml 100 g-1 min-1, p less than 0.001), and an increase in CVR of 53% (from 1.3 +/- 0.1 to 2.0 +/- 0.2 mm Hg ml-1 100 g min, p less than 0.001). These effects were reversed by i.v. administration of 300 mg kg-1 of L-arginine but not D-arginine. Moreover, the administration of L-NMMA abolished the enhancement of CBF and the diminution in CVR observed during intracarotid infusion of acetylcholine (ACh). The increase in CBF and decrease in CVR during hypoxia in the group of rats that received L-NMMA were similar to that in the control group, although CBF and CVR levels attained during hypoxia in both groups were different. The results show that NO is involved in the maintenance of basal CBF and CVR, and is responsible for the ACh-elicited increase in CBF and the decrease in CVR in rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of endothelium-dependent vasorelaxation by arginine analogues: a pharmacological analysis of agonist and tissue dependence

1. Isolated rings of rabbit external jugular vein (RbJV) and rat thoracic aorta (RA) were used to study the effect of the NO synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME) on muscarinic and 5-hydroxytryptamine (5-HT) receptor-stimulated, endothelium-dependent vascular relaxations. 2. In RbJV relaxations produced by the endothelial 5-HT receptor agonist alpha-methyl-5-HT were potently and non-surmountably inhibited by L-NAME (10 microM), whereas acetylcholine relaxations in this tissue were unaffected by this concentration of inhibitor. By contrast, acetylcholine relaxations in RA were virtually abolished by 10 microM L-NAME. In each case an equivalent concentration of D-NAME was without effect on agonist-induced relaxations. 3. The different effect of L-NAME on acetylcholine relaxations in RbJV and RA was not due to muscarinic receptor differences. Affinity estimates for acetylcholine (pKA = 6.12 +/- 0.09; 6.09 +/- 0.08 respectively) and for 4-diphenyl-acetoxy-N-methylpiperidine methobromide (4-DAMP, pKB = 9.01 +2- 0.012; 9.24 +/- 0.16 respectively) indicated that the receptors in both tissues belong to the same M3 class. Tissue differences resulting from the release of a cyclo-oxygenase product or a glibenclamide-sensitive K(+)-channel-linked hyperpolarizing factor were also ruled out by selective inhibition of these pathways. 4. When phenoxybenzamine was used to reduce the efficacy of acetylcholine in RbJV so that it behaved as a partial agonist in this tissue, L-NAME (10 microM) now produced non-surmountable inhibition of relaxation responses. In untreated tissues the same concentration of L-NAME also profoundly inhibited responses produced by butyrylcholine and pilocarpine, both of which behave as partial agonists at the M3 receptor in RbJV. 5. A simple model was developed which describes the theoretical behaviour of receptor-stimulated synthesis and release of NO. The model predicts that competitive inhibition of NO formation results in parallel displacements of the agonist response curve in the case of high efficacy agonist, but right-shift with concomitant depression of the curve maximum in the case of low efficacy agonists. Simulations based on the model showed reasonable agreement with the experimental data. 6. It is concluded that analogues of L-arginine demonstrate tissue- and agonist-dependence in terms of their ability to inhibit receptor-mediated events involving the liberation of NO. This behaviour can reflect differences in agonist efficacy in the receptor systems being studied, a possibility that should be ruled out before apparent resistance to inhibition is taken as evidence for the involvement of heterogeneous endothelium-derived relaxing factors (EDRFs).

Evidence for dual components in the non-adrenergic non-cholinergic relaxation in the rat gastric fundus: role of endogenous nitric oxide and vasoactive intestinal polypeptide

The effects of trypsin and arginine analogues, alone or in combination, on half-maximal non-adrenergic, non-cholinergic (NANC) relaxation elicited by different pulse trains of electrical field stimulation were studied in the rat gastric fundus in order to investigate further the relative contribution of peptides and NO. Trypsin (1 microM) partially inhibited electrically-induced NANC relaxation especially when longer pulse trains were used. L-NOARG, L-NAME and L-NMMA, but not D-NOARG or D-NAME (3-300 microM) produced concentration-dependent inhibition of the electrically induced NANC relaxation. L-Arginine (L-Arg), but not D-Arginine (D-Arg) (3.8 microM-3.8 mM) produced a concentration-dependent reversal of the inhibitory effect of L-NOARG IC50 (38 microM). Neither L-NOARG (38 microM) nor L-Arg (380 microM) influence submaximal relaxation induced by VIP (3 nM), isopropylnoradrenaline (10 nM), ATP (10 microM) or sodium nitroprusside (300 nM). Moreover L-NOARG (100 microM) did not influence neurally-induced VIP release. L-NOARG inhibition of NANC relaxation was significant only when short pulse trains were used, while trypsin showed significant inhibition only of relaxation induced by longer pulse trains. These results suggest that the relaxation induced by the activation of the NANC inhibitory neurotransmission of the rat gastric fundus consists of at least two components, one trypsin-sensitive and the other trypsin-resistant, to which VIP and NO contribute, respectively.

Inhibition of nitric oxide, bradykinin, and prostaglandins in normal rats

We assessed the vasodilator effect of endothelium-derived nitric oxide by inhibiting its formation with NG-monomethyl L-arginine (LNMMA) on systemic and regional hemodynamics in conscious, normotensive rats, using the radioactive microsphere technique. In rats injected with 10 mg/kg LNMMA (n = 8), mean blood pressure increased by 16.2 +/- 2.6 mm Hg, and heart rate decreased by 54.3 +/- 16.7 beats per minute. In comparison with rats injected with 5% dextrose (n = 14), cardiac index was lower by 35.6% (p less than 0.01), and total peripheral vascular resistance was higher by 51.6% (p less than 0.01); regional blood flows were lower and vascular resistance higher in most organs. Changes were significant in the heart, kidney, stomach, large intestine, skin, and adrenals (p less than 0.05). Preinjection of 100 mg/kg L-arginine prevented the pressor response but only partially attenuated the other hemodynamic effects of LNMMA. Combination of LNMMA with the bradykinin antagonist (D-Arg-Arg-Pro-Hyp-Gly-Thi-Ser-D-Phe-Thi-Arg)trifluoroacetic acid (50 micrograms/min for 5 minutes) did not produce systemic or regional effects different from those obtained with LNMMA alone. Combination of LNMMA with indomethacin (10 mg/kg) resulted in additional changes in the cerebral circulation, blood flow decreasing by an additional 44.2% (p less than 0.01) and vascular resistance increasing by 75.3% (p less than 0.01) compared with changes produced by LNMMA alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Myogenic vascular regulation in skeletal muscle in vivo is not dependent of endothelium-derived nitric oxide

The hypothesis, based on in vitro experiments on large conduit arteries, that endothelium-derived nitric oxide is a mediator of vascular myogenic reactivity was tested in cat gastrocnemius muscle in vivo. This was done by comparing, in the absence and presence of effective endothelium-derived nitric oxide blockade by the specific inhibitors NG-monomethyl-L-arginine or NG-nitro-L-arginine methyl ester, myogenic responses in defined consecutive vascular sections to dynamic vascular transmural pressure stimuli, to arterial occlusion (reactive hyperaemia), and to arterial pressure changes (autoregulation of blood flow and capillary pressure). The results demonstrated that the myogenic vascular reactivity to quick ramp transmural pressure stimuli was not attenuated by endothelium-derived nitric oxide blockade, but rather reinforced. The amplitude of the reactive hyperaemia response was unaffected by endothelium-derived nitric oxide blockade, but its duration was shortened because of faster myogenic constriction, especially of large-bore arterial resistance vessels greater than 25 microns, in the recovery phase. Both the improved myogenic responsiveness to transmural pressure stimuli and the shortening of the reactive hyperaemia by endothelium-derived nitric oxide blockade suggested that endothelium-derived nitric oxide released in vivo acts as a 'metabolic' factor which certainly does not improve, but rather depresses myogenic vascular reactivity. Autoregulation of blood flow and capillary pressure were well preserved in the presence of endothelium-derived nitric oxide blockade. It was concluded from the results of these multifaceted tests that myogenic vascular regulation in skeletal muscle in vivo seems independent of endothelium-derived nitric oxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of the L-arginine-nitric oxide pathway in internal anal sphincter relaxation

The purpose of this study was to examine the role of the L-arginine-nitric oxide pathway in neurogenic relaxation of the internal anal sphincter. Muscle strips representing the internal anal sphincter were prepared from 17 adult opossums. The preparations were mounted in organ baths for recording of isometric tension. N omega-nitro-L-arginine, an agent known to inhibit the L-arginine-nitric oxide pathway, concentration-dependently reduced relaxations induced by transmural field stimulation. At the highest concentration of N omega-nitro-L-arginine (10(-4) mol/L), no relaxation was evoked at any frequency tested (0.5-40 Hz). The inhibitory response to exogenous vasoactive intestinal polypeptide was unaffected by N omega-nitro-L-arginine pretreatment, indicating that vasoactive intestinal polypeptide relaxation does not use the L-arginine-nitric oxide pathway. In addition, responses to forskolin and sodium nitroprusside were not influenced by N omega-nitro-L-arginine preincubation, suggesting that the effect observed was not caused by a direct influence on the adenylate or the guanylate cyclases. It is concluded that the nonadrenergic, noncholinergic innervation of the internal anal sphincter involves an inhibitory substance generated from the L-arginine-nitric oxide pathway. Whether this substance is nitric oxide or a related nitroso compound remains to be settled.

Nitric oxide but not prostacyclin is an autocrine endothelial mediator

Using porcine aortic endothelial cells, the present study investigates whether stimulation of prostacyclin (PGI2) and nitric oxide also causes elevation of the respective second messengers cAMP and cGMP in the endothelial generator cells. The calcium ionophore A23187 at 0.3-3 microM increased endothelial cGMP levels up to 27-fold in an L-arginine-dependent manner as assessed through complete inhibition by NG-monomethyl-L-arginine (100 microM). The 36-fold PGI2 stimulation by 3 microM A23187 was not accompanied by an intracellular increase in cAMP or an enhanced cAMP efflux. Correspondingly, the PGI2 mimetic iloprost (10 pM-100 microM) did not change endothelial cAMP levels. However, forskolin (1-100 microM) and prostaglandin E2 (PGE2) (0.1-10 microM) produced concentration-dependent increases in cAMP with a 9-fold and 8-fold stimulation at 100 microM forskolin and 10 microM PGE2, respectively. These results demonstrate that in contrast to NO, PGI2 acts as a strictly paracrine hormone without affecting the respective second messenger cAMP in the endothelial generator cells.

Characterization of acetylcholine-induced membrane hyperpolarization in endothelial cells

The characteristics of the hyperpolarization response to acetylcholine (ACh) in endothelial cells from the guinea pig coronary artery were studied by microelectrode recording technique. ACh (30 nM to 3 microM) induced membrane hyperpolarization in a dose-dependent manner. The sustenance of the response required the presence of external calcium. The hyperpolarization was not affected by nifedipine (1 microM) but was inhibited by the potassium channel blockers charybdotoxin (10 nM), tetraethylammonium (1 mM), and 4-aminopyridine (0.5 mM). Glibenclamide (10 microM) and apamin (1 microM) were not effective. The inhibitors of endothelium-derived relaxing factor/nitric oxide synthesis N omega-nitro L-arginine (50 microM) and NG-monomethyl L-arginine (30 microM) had no effect on the resting membrane potential or the ACh-induced responses. No hyperpolarization was observed with application of sodium nitroprusside (10 microM) or 8-bromo-cGMP (0.1 microM). Ouabain (10 microM) depolarized the membrane significantly by 5 mV, but the ACh hyperpolarization was not affected. Indomethacin (10 microM) was without effect on the resting membrane potential or the hyperpolarization to ACh. These results show that ACh-induced hyperpolarization is dependent on external calcium and can be inhibited by certain potassium channel blockers. The hyperpolarization response is not mediated by endothelium-derived relaxing factor/nitric oxide, cGMP, a cyclooxygenase product, or stimulation of the Na-K pump.

Effect of 3-morpholinosydnonimine (SIN-1), NG-nitro-L-arginine (NNA) and NG-monomethyl-L-arginine (NMMA) on isolated anaphylactic guinea-pig hearts

In isolated perfused ovalbumin-sensitized guinea-pig hearts modulating effects of nitric oxide (NO) on cardiac function and eicosanoid release were investigated. While the NO-donor SIN-1 exhibited a protective effect during cardiac anaphylaxis, inhibition of NO biosynthesis by NNA or NMMA aggravated anaphylactic changes of cardiac functions. Exogenous and endogenous NO seems to functionally antagonize the effects of vasoconstrictor mediators released during the anaphylactic reaction. In addition, inhibition of cysteinyl-leukotriene (cys-LT) release could contribute to the protective effect of SIN-1 observed.

Actions of L- and D-arginine and NG-monomethyl-L-arginine on the blood pressure of pithed normotensive and spontaneously hypertensive rats

We have examined the depressor effects of L- and D-arginine on the diastolic blood pressure of pithed normotensive Wistar (NW), Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats after the administration of a single bolus injection of the nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA). A single bolus intravenous injection of L-NMMA, 30 mg/kg, produced an increase in both the systolic and diastolic blood pressure of pithed rats. Injections of bolus doses, 1-300 mg/kg, of D-arginine did not lead to sustained reductions of the blood pressure in pithed NW rats although slight decreases in the blood pressure of WKY and SH rats were observed, and these transient effects of D-arginine appeared to be more pronounced in the WKY strain. Immediately following the bolus injections of the higher doses of D-arginine a transient decrease in both the systolic and diastolic pressure occurred. In contrast to the actions of D-arginine single bolus injections of L-arginine, 1-300 mg/kg, produced a dose-dependent sustained reduction in both the systolic and diastolic blood pressures of all rats. The threshold for the depressor actions of L-arginine was the same for NW, WKY and SH rats. The final dose of L-arginine (300 mg/kg), produced a significantly greater depressor effect in WKY and SH rats as compared to NW rats. The blood pressure remained elevated after the dose-response curve to D-arginine and, in order to determine whether D-arginine-treated rats are sensitive to the effects of other vasodilators and whether differences in vasoactive actions exist for vasodilators acting other than via nitric oxide synthesis, a dose-response curve to the calcium channel antagonist verapamil was constructed. Injections of verapamil, 0.1-1000 micrograms/kg, produced a dose-dependent reduction in blood pressure with no difference in either threshold or sensitivity to the actions of verapamil among the three strains of rats. Our results suggest that strain differences exist between the depressor actions of L-arginine and that it is possible that these differences may be due to an alteration in the endogenous levels of nitric oxide synthase and/or the activity of guanylate cyclase, however, no relationship to the hypertensive state of the spontaneously hypertensive rats was apparent.

Mechanisms of coronary vasodilatation produced by ATP in guinea-pig isolated perfused heart

1. Isolated hearts of guinea-pigs were perfused in vitro with a physiological salt solution via a retrograde aortic cannulation (Langendorff preparation) at constant perfusion pressure. Bolus intra-arterial injections of various vasodilator drugs were made and the coronary flow responses were measured with an electromagnetic flow probe placed in the arterial inflow circuit. Inhibitory drugs were infused intra-arterially. 2. Nitro-L-arginine (NLA; 500 microM), an NO synthesis inhibitor, decreased coronary baseline flow by 16 +/- 0.8%, converted acetylcholine-induced coronary vasodilatation to vasoconstriction and had no effect on coronary flow responses to adenosine or papaverine. Sodium nitroprusside-induced responses were enhanced during NLA infusion by 46 +/- 11%. 3. Adenosine 5'-triphosphate (ATP) increased coronary flow but coronary flow responses to ATP were not altered by infusion of NLA. 4. ATP-induced coronary dilatation was not significantly attenuated by infusion of the adenosine receptor antagonist XAC, (xanthine amine congener; 2 microM), whereas XAC decreased coronary flow responses to adenosine by 75% +/- 5%. 5. ATP-induced coronary flow responses were reduced by only 31 +/- 4% during indomethacin infusion (2.8 microM) whereas indomethacin completely eliminated the initial vasoconstriction phase and greatly attenuated the peak flow and duration of the later vasodilatation phase seen in response to arachidonic acid (0.75 nmol). Indomethacin had no effect on vasodilatations produced by adenosine or prostaglandin I2. 6. These results indicate that ATP-induced coronary dilatation in the isolated, perfused heart of the guinea-pig is not dependent upon NO production or upon degradation of ATP to adenosine. The coronary dilator action of ATP may be partially dependent (approximately 30%) upon the production of vasodilator prostaglandins.

Endogenous nitric oxide and sensory neuropeptides interact in the modulation of the rat gastric microcirculation

1. The effects of depletion of sensory neuropeptides from primary afferent neurones by capsaicin pretreatment, on the changes in resting gastric mucosal blood flow following administration of inhibitors of nitric oxide biosynthesis have been investigated in the pentobarbitone-anaesthetized rat. 2. Bolus administration of the NO-synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 0.8-12.5 mg kg-1 i.v.), induced a dose-dependent increase in systemic arterial blood pressure (BP) and a reduction in resting mucosal blood flow, as determined by laser Doppler flowmetry. 3. Concurrent administration of L-arginine (300 mg kg-1 i.v.) attenuated the effects of L-NAME (6.25 mg kg-1) on resting mucosal blood flow and BP. The enantiomer, D-NAME (50 mg kg-1 i.v.), which does not inhibit NO biosynthesis, had no effect on either parameter. 4. The fall in mucosal blood flow induced by submaximal doses of L-NAME (0.8-3.2 mg kg-1) was substantially augmented in rats pretreated 2 weeks earlier with capsaicin. 5. The fall in resting mucosal blood flow induced by the less potent NO-synthase inhibitor, NG-monomethyl-L-arginine (L-NMMA; 1.6-25 mg kg-1 i.v.) was likewise significantly augmented in capsaicin-pretreated rats. 6. Pretreatment (15 min) with indomethacin (5 mg kg-1 i.v.) did not augment further the microvascular actions of L-NAME or L-NMMA in capsaicin-pretreated rats, suggesting the lack of interaction of endogenous prostanoids with these other mediators in regulating local blood flow. The effects of L-NAME on BP were not altered by capsaicin and indomethacin administration.7. These findings indicate that endogenous sensory neuropeptides and NO can interact in the regulation of the gastric microcirculation.

Acetylcholine released from guinea-pig submucosal neurones dilates arterioles by releasing nitric oxide from endothelium

1. The role of the endothelium as an effector of the neurogenic cholinergic vasodilatation in submucosal arterioles of the guinea-pig ileum was investigated by measuring changes in arteriolar diameter in response to exogenous application of muscarine or electrical stimulation of the submucosal ganglia. 2. NG-Monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) synthesis, competitively inhibited the vasodilatation produced by muscarine in arterioles which had been preconstricted with the prostaglandin analogue U46619. L-Arginine (10 mM), but not D-arginine (10 mM), prevented the inhibition by L-NMMA. 3. Neither tetrodotoxin (TTX, 1 microM), nor the cyclo-oxygenase inhibitor, indomethacin (10 microM), altered the muscarinic vasodilatation or the inhibitory effect of L-NMMA. 4. Sodium nitroprusside (SNP), an activator of the soluble guanylate cyclase, dilated the arterioles in a concentration-dependent manner. This vasodilatation was unaffected by L-NMMA but was abolished by the guanylate cyclase inhibitor, methylene blue (10 microM). In addition, methylene blue antagonized the muscarinic vasodilatation to a similar degree as did L-NMMA. 5. The vasodilatation produced by ganglionic stimulation (10 Hz, 10 s) was blocked by TTX and the muscarinic receptor antagonist, 4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP, 1 microM). The neurally evoked vasodilatation was inhibited by 70% in the presence of L-NMMA; this inhibition was prevented by L-arginine. Methylene blue inhibited the neurogenic vasodilatation to the same extent as did L-NMMA. 6. These results show that arteriolar vasodilatation by muscarine is mediated mainly through the release of NO formed from L-arginine; the origin of the L-arginine appears to be the endothelium. These results also demonstrate that acetylcholine released from submucosal nerves onto submucosal blood vessels reaches the endothelium to cause the release of NO formed from L-arginine; the endothelial-derived NO dilates the arteriole.

Effects of L-NMMA and L-NNA on the selective ATP-induced enhancement of intratumoral blood flow

We studied the effects of NG-monomethyl-L-arginine (L-NMMA) and N omega-nitro-L-arginine (L-NNA) on the selective ATP and adenosine-induced enhancement of intratumoral blood flow in rats measured by the hydrogen clearance method. Both adenosine and ATP produced a selective enhancement of the intratumoral blood flow. Neither L-NMMA nor L-NNA had a significant effect on either the CBF or the intratumoral blood flow. Adenosine-induced enhancement was not inhibited by L-NMMA or L-NNA. On the other hand, the ATP-induced enhancement was totally inhibited by both L-NMMA and L-NNA. The inhibitory action of L-NMMA against ATP was blocked by L-arginine, but not by D-arginine. It is suggested that the ATP-induced increase of intratumoral blood flow is evoked by nitric oxide synthesized from the endothelium of the intratumoral blood vessels.

Protective effect of NG-nitro-L-arginine (N5-[imino(nitroamino)methyl]-L-ornithine) against cyanide-induced convulsions in mice

The effects of NG-nitro-L-arginine (NNA; an inhibitor of the oxidative L-arginine pathway) on convulsions induced by cyanide were investigated in mice. NNA prevented cyanide-induced convulsions in a dose-dependent manner. Furthermore, the inhibitory effect against convulsions induced by cyanide with NNA was abolished by pretreatment of L-arginine. However, NNA did not change blood cyanide levels in mice 5 min after injection of potassium cyanide. Since NNA prevents against oxidative L-arginine pathway-dependent guanylate cyclase activation in intact cells, it was suggested that the protection against cyanide-induced convulsions with NNA may be due to the inhibition of guanylate cyclase. In support, methylene blue, an inhibitor of guanylate cyclase, elicited a similar inhibition against convulsions induced by cyanide.

Endothelium-dependent relaxation of rabbit middle cerebral artery to a histamine H3-agonist is reduced by inhibitors of nitric oxide and prostacyclin synthesis

1. The possible involvement of prostanoids and endothelium-derived relaxing factor (EDRF) in the vasodilatation induced by a histamine H3-agonist was examined in the rabbit perfused middle cerebral artery preconstricted with K+ (50 mM). 2. The endothelium-dependent relaxation to (R)-alpha-methylhistamine [(R)-alpha-MeHA] was competitively antagonized by thioperamide (an H3-antagonist) with a pA2 of 9.05, but unaffected by propranolol, atropine, L- and D-sulpiride. This effect was stereoselective since the (S)-isomer was 100 times less potent than the (R)-isomer. 3. Two inhibitors of nitric oxide synthesis, NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), inhibited the relaxation induced by (R)-alpha-methylhistamine. The inhibitory effects of 10(-5) M NG-nitro-L-arginine methyl ester and 10(-5) M NG-monomethyl-L-arginine were reversed by equimolar concentrations of L-arginine, but strongly enhanced by 10(-4) M tranylcypromine. Tranylcypromine alone (10(-5) M-10(-4) M) partially reduced the (R)-alpha-methylhistamine-induced relaxation. Both dexamethasone and indomethacin also inhibited this relaxation. 4. The results suggest that the H3-mediated relaxation of the rabbit middle cerebral artery may involve release of both a prostanoid, probably prostacyclin, and endothelium-derived relaxing factor. The relaxant effects of these two endogenous compounds appear to be synergistic.

Possible involvement of nitric oxide in chlordiazepoxide-induced anxiolysis in mice

Mice challenged with the anxiolytic benzodiazepine chlordiazepoxide exhibited significant increases in the percent of total entries into and percent of total time spent on open arms of an elevated plus maze. Systemic pretreatment with the nitric oxide synthase-inhibitor L-NG-nitro arginine (L-NOARG) antagonized these effects of chlordiazepoxide. This inhibitory effect of L-NOARG was stereospecifically and completely reversed by intracerebroventricular administration of L-arginine but not D-arginine. These findings suggest a possible role of nitric oxide in the anxiolytic effect of chlordiazepoxide in the elevated plus maze.

Effects of inhibiting nitric oxide biosynthesis on the systemic and splanchnic circulation of rats with portal hypertension

1. The effects of inhibiting endogenous nitric oxide (NO) synthesis with NG-monomethyl-L-arginine (L-NMMA) on the systemic and splanchnic circulation have been investigated in rats with experimental chronic portal hypertension, anaesthetized with ketamine. 2. Portal hypertension was induced by partial portal vein ligation, 2 weeks prior to study. This procedure induced a reduction in systemic arterial blood pressure (MAP), an increase in cardiac output as measured by radiolabelled microspheres, a reduction in peripheral and splanchnic vascular resistance and an increased portal venous inflow (PVI) and portal pressure, as compared to control non-ligated rats. 3. L-NMAA (6.25 and 50 mg kg-1, i.v.) dose-dependently increased MAP, reduced cardiac output and PVI, and increased peripheral and splanchnic vascular resistance. With L-NMMA (50 mg kg-1), PVI and the vascular resistances returned to values comparable to those determined in control non-ligated anaesthetized rats under resting conditions. 4. Porto-collateral resistance was also increased by these doses of L-NMMA, whereas portal pressure was unchanged. The increase in renal blood flow and decrease in renal vascular resistance also seen in portal-hypertensive rats was reversed by L-NMMA (50 mg kg-1). 5. These effects of L-NMMA (50 mg kg-1) were inhibited by prior administration of L-arginine (300 mg kg-1, i.v.). 6. These findings indicate that the chronic hyperdynamic circulatory characteristics following portal vein stenosis can be attenuated by L-NMMA. Thus, the excessive formation of endogenous NO may be implicated in the pathogenesis of the haemodynamic disturbances and splanchnic vasodilatation associated with chronic portal hypertension.

Possible mechanisms of age-associated reduction of vascular relaxation caused by atrial natriuretic peptide

We investigated the effect of aging on atrial natriuretic peptide (ANP)-induced relaxation and cyclic GMP (cGMP) formation in the rat thoracic aorta. In the aorta from young rats (4 weeks old), removal of the endothelium, and treatment with the nitric oxide synthesis inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), the radical scavenger, hemoglobin (Hb), and the soluble guanylate cyclase inhibitor, methylene blue (MB), attenuated ANP-induced relaxation and considerably reduced ANP-stimulated cGMP formation. With increasing age of the rats, the ANP-induced relaxation and cGMP formation in endothelium-intact aorta decreased, and Hb, L-NAME and MB no longer inhibited the ANP-induced effects, irrespective of whether the endothelium was present or absent. In the arteries without endothelium, the age-associated reduction in ANP-induced relaxation was less than in arteries with endothelium. Aging also decreased the relaxation induced by the soluble guanylate cyclase activator, nitroprusside. Potentiation due to the cGMP-phosphodiesterase (cGMP-PDE) inhibitor, M&B 22948, of the ANP-induced relaxation was greater in aortas from old rats than in those from young rats, suggesting that the degradation of cGMP may be accelerated in old rats. These results suggest that the relaxant action of ANP on the thoracic aorta from young rats is in part modulated by endothelium-derived relaxing factor (EDRF/nitric oxide), which in turn activates soluble guanylate cyclase, thus elevating the cGMP level. Aging may decrease the ANP-induced relaxation and ANP-stimulated increase in cGMP level by decreasing the ability of endothelial cells to produce EDRF, by decreasing guanylate cyclase activity, and by enhancing cGMP-PDE activity.

A requirement for the intercellular messenger nitric oxide in long-term potentiation

Long-term potentiation (LTP) of synaptic transmission is a widely studied model of neuronal plasticity. The induction of LTP is known to require processes in the postsynaptic neuron, while experimental evidence suggests that the expression of LTP may occur in the presynaptic terminal. This has led to speculation that a retrograde messenger travels from the post- to the presynaptic cell during induction of LTP. Extracellular application or postsynaptic injection of two inhibitors of nitric oxide synthase, N-nitro-L-arginine or NG-methyl-L-arginine, blocks LTP. Extracellular application of hemoglobin, which binds nitric oxide, also attenuates LTP. These findings suggest that nitric oxide liberated from postsynaptic neurons may travel back to presynaptic terminals to cause LTP expression.

Pirarubicin-induced endothelium-dependent relaxation in rat isolated aorta

The mechanism of relaxation produced by pirarubicin [(2"R)-4'-O-tetrahydropyranyladriamycin, THP] has been studied in rat isolated aorta. THP (1.5 x 10(-6)-4.5 x 10(-5) M) markedly relaxed contractions induced by noradrenaline (10(-7) M) in the aorta with endothelium, but not in that without endothelium. The relaxation induced by 1.5 x 10(-5) M THP was inhibited by methylene blue (5 x 10(-6) M), hydroquinone (10(-4) M), phenidone (5 x 10(-5) M), haemoglobin (10(-6) M) and p-bromophenacyl bromide (5 x 10(-5) M), but not by indomethacin (2.5 x 10(-5) M). The relaxation induced by THP (1.5 x 10(-7) -4.5 x 10(-5) M) was inhibited by NG-nitro-L-arginine (10(-5) M), but enhanced by superoxide dismutase (10 units mL-1) or by L-arginine (10(-2) M). However, the THP-induced relaxation was not inhibited by various receptor antagonists such as atropine (10(-6) M), cimetidine (10(-5) M), diphenhydramine (3 x 10(-6) M) and [D-Pro4, D-Trp7,9,10]-substance P(4-11) (1.5 x 10(-6) M). In fifteen anthracycline analogues, THP and 13-dihydropirarubicin (both with a tetrahydropyranyl group) produced endothelium-dependent relaxations. These results suggest that the THP-induced relaxation which is probably mediated by endothelium-derived relaxing factor (EDRF) was not produced by an activation of muscarine, histamine H1 or H2, or substance P receptor, and further that the tetrahydropyranyl group must play an important role in the THP-induced relaxation.

The L-arginine-nitric oxide pathway in the canine femoral vascular bed: in vitro and in vivo experiments

Vascular endothelial cells synthesize nitric oxide from L-arginine, and this pathway can be inhibited by various analogues of L-arginine, including NG-nitro L-arginine methyl ester (L-NAME). To investigate the role of this pathway in the regulation of femoral arterial tone, the effect of L-NAME was studied in vitro in isolated canine femoral arteries suspended in organ chambers for isometric tension recording, and in vivo in conscious dogs chronically instrumented for the measurement of iliac blood flow and iliac artery diameter. In vitro, L-NAME induced an endothelium-dependent contraction, inhibited the endothelium-dependent relaxations to acetylcholine or bradykinin, and potentiated the relaxation evoked by the nitric oxide donor SIN-1. In vivo, locally administered L-NAME induced a decrease in iliac artery diameter and an increase in iliac resistance, potentiated the iliac responses to the organic nitrate nitroglycerin, but did not affect the iliac responses to the endothelium dependent vasodilator acetylcholine. Thus, in the canine femoral vascular bed: a) basal release of nitric oxide contributes in vivo to the maintenance of a permanent vasodilator tone at the level of both large conductance and small resistance vessels; b) the endothelium-dependent relaxations to acetylcholine and bradykinin in vitro are mostly mediated through the release of nitric oxide from L-arginine; c) the endothelium-dependent relaxations to acetylcholine in vivo are probably mediated by a relaxing factor distinct from nitric oxide, or by a nitric oxide-like molecule released from endothelial pools; and d) removal of the NO-mediated vasodilator tone by L-NAME leads to a supersensitivity to nitrovasodilators, both in vitro and in vivo.

NG-nitro-L-arginine methyl ester attenuates vasodilator responses to acetylcholine but enhances those to sodium nitroprusside

The effects of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the synthesis of the endothelium-derived relaxing factor nitric oxide, were studied in two isolated perfused vascular beds: the rat mesenteric arterial bed and the hepatic arterial bed of the rabbit liver. The tone of both preparations was raised with noradrenaline (10 and 30 microM for rabbit and rat preparations, respectively). In both preparations, L-NAME (30 microM) significantly attenuated vasodilator responses to the endothelium-dependent vasodilator acetylcholine, but enhanced responses to sodium nitroprusside (a direct smooth muscle dilator). The evidence supports the view, previously established from work carried out in isolated vessels, that in addition to acting as an inhibitor of nitric oxide, L-NAME enhances the responsiveness of smooth muscle to direct relaxation by nitrovasodilators.

NG-monomethyl-L-arginine and NG-nitro-L-arginine inhibit endothelium-dependent relaxations in human isolated omental arteries

The L-arginine analogues NG-monomethyl-L-arginine (L-NMMA, 10(-4) M) and NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), which specifically inhibit the synthesis of nitric oxide from L-arginine, significantly reduced acetylcholine-induced endothelium-dependent relaxations in rings of human omental arteries. The inhibitory potency of L-NMMA and L-NAME was similar. Addition of L-NMMA or L-NAME to the organ bath did not induce any significant changes in the resting tension of the tissues. The effects of L-NMMA were reversed by L-arginine (3 x 10(-4) M). The L-NMMA enantiomer, D-NMMA (10(-4) M), did not influence either the basal tone of the preparation or the relaxing effects of acetylcholine. Arterial relaxations induced by sodium nitroprusside (10(-6) M) were not influenced by incubation with L-NMMA or L-NAME. These results suggest that endothelium-dependent relaxations in human omental arteries are mediated by the endogenous and substrate-specific generation of nitric oxide from L-arginine.

Does superoxide underlie the pathogenesis of hypertension?

Although active oxygen species play important roles in the pathogenesis of various diseases, the molecular mechanism for oxygen toxicity in vascular diseases remains to be elucidated. Since endothelium-derived relaxing factor (EDRF) is inactivated by superoxide radicals in vitro, oxidative stress in and around vascular endothelial cells may affect the circulatory status of animals. To study the role of superoxide radicals and related enzymes, such as superoxide dismutase (SOD), in vascular diseases, we have developed a fusion protein (HB-SOD) consisting of human Cu/Zn-type SOD and a C-terminal basic peptide with high affinity for heparan sulfate on endothelial cells. When injected intravenously, HB-SOD bound to vascular endothelial cells, underwent transcellular transport, and localized within vascular walls by a heparin-inhibitable mechanism. The blood pressure of spontaneously hypertensive rats (SHR) but not normal animals was decreased significantly by HB-SOD. Heparin inhibited the depressor effect of HB-SOD. In contrast, native SOD had no effect on blood pressure of either SHR or normal rats. Neither H2O2-inactivated HB-SOD nor the C-terminal heparin-binding peptide showed such a depressor effect, suggesting that the catalytic function of HB-SOD is responsible for its depressor action. To know the source of superoxide radicals, we determined xanthine oxidase activity in the aorta and uric acid levels in the plasma. Although no appreciable difference in xanthine oxidase activity was found between the two animal groups, uric acid levels were significantly higher in SHR than in normal rats. Oxypurinol, a potent inhibitor of xanthine oxidase, also decreased the blood pressure of SHR but not of normal rats. These findings indicate that superoxide radicals in and around vascular endothelial cells play critical roles in the pathogenesis of hypertension of SHR.

Responses of atherosclerotic human coronary arteries in vivo to the endothelium-dependent vasodilator substance P

BACKGROUND

The effects of the endothelium-dependent vasodilator substance P (SP) on atherosclerotic human coronary arteries was studied.

METHODS AND RESULTS

[125I]-SP binding to luminal cells was shown to be preserved in the atherosclerotic epicardial coronary arteries of four patients. No binding to medial smooth muscle cells was demonstrated. Intracoronary infusions of SP were undertaken in patients with coronary artery disease. SP was infused for 2-minute periods starting at a dose of 2.8 pmol/min rising by doubling increments to 22.4 pmol/min. Analysis of the epicardial coronary artery diameter, using a computerized analysis system (CAAS) of the angiograms, was performed at the end of each infusion. Analysis of seven smooth vessel segments from seven coronary vessels, which were stenosed at more proximal sites, was performed. Significant dose-dependent dilatation was seen (p = 0.04), which was maximal at 5.6 pmol/min SP. No additional dilatation was produced with 2 mg intracoronary isosorbide dinitrate (ISDN). Two of these seven patients showed no response to SP, and only one of these appeared to sustain dilatation with ISDN (2 mg intracoronary). In a second group of six patients with discrete coronary stenoses, analysis at the site of the stenosed segments appeared to reveal dilatation in response to SP in only one instance. One other stenotic segment dilated with isosorbide dinitrate but failed to dilate with SP; the remaining four were fixed. The segment immediately proximal to the stenosis preserved a dose-dependent vasodilator response.

CONCLUSIONS

These findings demonstrate that the endothelium-dependent vasodilator substance P can still produce epicardial vasodilatation in vivo in the presence of coronary atherosclerosis.

Endothelium-independent contractions to NG-monomethyl-L-arginine in canine basilar artery

BACKGROUND AND PURPOSE

NG-substituted analogues of L-arginine are potent and selective inhibitors of nitric oxide synthase(s). The present study was designed to determine the effects of these analogues on the vascular smooth muscle of isolated canine basilar arteries.

METHODS

Basilar artery rings without endothelium were suspended for isometric tension recording in Krebs-Ringer bicarbonate solution bubbled with 94% O2-6% CO2 (temperature = 37 degrees C, pH = 7.4). A radioimmunoassay technique was used to determine the levels of guanosine 3',5'-cyclic monophosphate (cyclic GMP).

RESULTS

NG-Monomethyl-L-arginine (L-NMMA) caused concentration-dependent contractions, whereas the D-enantiomer and NG-nitro-L-arginine did not. Contractions to L-NMMA were reduced in the presence of L-arginine but not in the presence of D-arginine. Superoxide anions generated by xanthine plus xanthine oxidase in the presence of catalase abolished contractions to L-NMMA but did not affect contractions to the prostaglandin H2/thromboxane A2 agonist U46619. Zaprinast, a selective cyclic GMP phosphodiesterase inhibitor, caused concentration-dependent relaxations. L-NMMA selectively inhibited these relaxations. The inhibitory effect of L-NMMA was reversed in the presence of L-arginine. L-NMMA selectively reduced the basal production of cyclic GMP.

CONCLUSIONS

These studies suggest that in cerebral arteries, contractions of smooth muscle cells to L-NMMA are mediated by inhibition of nitric oxide synthase with a resultant decrease in the basal production of nitric oxide.

Basal and acetylcholine-stimulated nitric oxide formation mediates relaxation of rabbit cavernous smooth muscle

Externally applied acetylcholine (ACh) in human corpus cavernosum has been shown to cause endothelium-dependent smooth muscle relaxation. Changes in isometric tension in rabbit cavernous smooth muscle strips mounted in organ bath chambers were monitored in the presence of blocking agents. Nitric oxide (NO) is known as an endothelium-derived relaxation factor (EDRF). Addition of specific inhibitors of nitric oxide synthesis, such as L-n-monomethyl arginine (L-NMMA) at 5 x 10(-4) mol/l.. or L-n-nitro arginine (L-NOARG) at 2 x 10(-4) mol/l. to strips precontracted with phenylephrine (PE) at 3.16 x 10(-6) mol/l. led to significant increases in tension. In the presence of L-NMMA or L-NOARG, relaxing effects of ACh at 10(-8)-3.16 x 10(-5) mol/l. mediated by muscarinic receptors were almost completely abolished. These data indicate that rabbit cavernous smooth muscle is under the control of basal NO release. They constitute strong evidence that cholinergically induced endothelial formation of NO plays a crucial role in relaxing cavernous smooth muscle.

L-arginine abrogates salt-sensitive hypertension in Dahl/Rapp rats

This study examined the contribution of nitric oxide (NO) to the susceptibility or resistance to the hypertensive effects of high sodium chloride (8.0% NaCl) intake in young Dahl/Rapp salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats. Using NG-monomethyl-L-arginine (L-NMMA) as a probe for NO production in vivo, we found that increasing dietary sodium chloride increased NO activity in salt-resistant rats, but not in salt-sensitive rats. Exogenous L-arginine, the substrate for NO synthesis, decreased blood pressure to normotensive levels in salt-sensitive rats made hypertensive for 2 wk from 8.0% NaCl chow. D-arginine had no effect on blood pressure of these rats and L-arginine did not change blood pressure of salt-resistant rats. Intraperitoneal injections of L-arginine and its precursor, L-citrulline, and oral L-arginine, but not D-arginine, prevented the increase in blood pressure in salt-sensitive rats on the high salt chow over 2 wk of observation. In contrast, L-arginine did not alter the development of hypertension in spontaneously hypertensive rats. Mean urinary cGMP levels were higher in salt-sensitive rats on oral L-arginine than salt-sensitive rats on D-arginine. Infusion of L-NMMA acutely decreased, whereas intravenous L-arginine rapidly increased, urinary cGMP in both groups. L-arginine and L-citrulline increased production of NO and prevented salt-sensitive hypertension in Dahl/Rapp rats.

The effect of inhibitors of nitric oxide biosynthesis and cyclic GMP formation on nerve-evoked relaxation of human cavernosal smooth muscle

1. The inhibitory transmission in isolated preparations of cavernosal smooth muscle from human penis has been studied. 2. Electrical field stimulation (EFS; 2-64 pulses/train, 0.8 ms pulse duration, 10 Hz) evoked relaxation of preparations treated with guanethidine (50 microM). The EFS-evoked relaxations were atropine-resistant and tetrodotoxin-sensitive indicating their origin to be non-adrenergic, non-cholinergic (NANC) nerve stimulation. 3. EFS-evoked relaxation was attenuated dose-dependently by the nitric oxide (NO)-synthase inhibitor, L-NG-nitro arginine (L-NOARG; 0.3-100 microM) but not by D-NG-nitro arginine. The inhibitory effect of L-NOARG on transmission was antagonized by L-arginine (100 microM), a NO precursor, but not by D-arginine. 4. Incubation with methylene blue (10-50 microM), a known inhibitor of guanylate cyclase activation by NO, caused a concentration-related inhibition of EFS-evoked relaxation. 5. It is concluded that NANC nerve-evoked relaxation of human cavernosal smooth muscle is mediated by NO or a NO-like substance.

Bradykinin and ATP stimulate L-arginine uptake and nitric oxide release in vascular endothelial cells

The effects of bradykinin and ATP on L-arginine transport and nitric oxide (NO) production were studied in porcine aortic endothelial cells cultured and perfused on microcarriers and deprived of L-arginine for 24 h. Stimulation of cells with bradykinin (100 nM) or ATP (100 microM) resulted in a rapid increase in L-arginine uptake and NO release. In the presence of nitro-L-arginine (100 microM), an inhibitor of NO synthase, the stimulatory effect of bradykinin on L-arginine uptake was partially inhibited while NO release was completely abolished. Nitro-L-arginine alone was not an inhibitor of basal L-arginine transport, suggesting that its inhibitory action was not directly on the L-arginine transporter but a result of the inhibition of NO generation. These data indicate that during agonist-stimulated NO production there is a concomitant increase in the transport of L-arginine into endothelial cells providing a mechanism for the continual generation of NO.

Nitric oxide influences ventrolateral medullary mechanisms of vasomotor control in the cat

In acute experiments on cats changes in the background sympathetic activity in the renal nerve, chosen as a model of vaso-constrictor nerve, together with arterial pressure shifts have been found following injections of nitric oxide (NO) containing drugs into the ventrolateral medulla. This is the first report of evidence that both sodium nitroprusside, which produces NO spontaneously and thus mimics endothelium-derived relaxing factor (EDRF), and L-arginine, being a precursor for NO, as well as L-NG-monomethyl-L-arginine (L-NMMA) which inhibits synthesis of NO, induce remarkable changes in the background activity of the renal nerve and systemic arterial pressure (SAP) level shifts, following unilateral injections of drugs examined into the rostral (RVLM) and caudal (CVLM) ventrolateral medulla. These sites are essential for the maintenance of arterial pressure level and vascular tone control. Injections of NO-containing drugs in the RVLM induce attenuation of the renal nerve sympathetic activity and lower the SAP level, while injections in the CVLM reverse these effects. After previous application of Methylene blue on the ventral medullary surface we failed to induce any of the effects following NO injections in the sites examined. Our results raise the possibility that NO influences mechanisms of the neurogenic vasomotor control, realized by neurons within the RVLM and the CVLM via activation of the guanylate cyclase.

Endothelial cells modulate renin secretion from isolated mouse juxtaglomerular cells

Utilizing cocultures of mouse renal juxtaglomerular cells with bovine microvascular endothelial cells, we have examined whether endothelial cells exert direct influence on renin secretion from renal juxtaglomerular cells. In the presence of endothelial cells both spontaneous and forskolin (10 microM) or isoproterenol (10 microM) stimulated renin release were markedly attenuated. The stimulatory effect of the calmodulin antagonist calmidazolium (10 microM) on renin secretion was not altered by endothelial cells, whereas the stimulatory effect of ethylisopropylamiloride (50 microM) an inhibitor of sodium-proton exchange was enhanced in the presence of endothelial cells. Indomethacin (10 microM) and NG-monomethyl-l-arginine (NMMA) (1 mM) used to inhibit cyclooxygenase activity and production of endothelium-derived relaxing factor (EDRF) decreased spontaneous renin release in the presence of endothelial cells only, but had no effect on forskolin stimulated renin secretion. Endothelin (1 microM) inhibited cAMP stimulated renin release both in the absence and in the presence of endothelial cells. ATP (10 microM) which acts on both endothelial and juxtaglomerular cells via purinergic P2 receptors inhibited cAMP stimulated renin release only in the absence but not in the presence of endothelial cells. This modulatory effect of endothelial cells was no altered by indomethacin nor by NMMA. Taken together, our findings provide first evidence for a local control function of the endothelium on cAMP stimulated renin secretion from renal juxtaglomerular cells, which could in part be mediated by endothelin.

Humoral factor mediates acetylcholine-induced endothelium-dependent relaxation of chicken aorta

A superfusion cascade system was used to determine whether endothelium-dependent vasorelaxation induced by acetylcholine (ACh) is mediated by a humoral factor(s) in the domestic fowl. An abdominal aorta (5 cm in length) with intact endothelium, excised from the donor bird, was mounted in an organ bath and perfused with avian Ringer solution. The vasorelaxing activity of the effluent was determined by an endothelium-denuded aortic ring (bioassay ring) equilibrated and precontracted with phenylephrine (PHE) (10(-6) M). The effluent (containing PHE) from the donor aorta with intact endothelium did not significantly change the PHE-induced tension of the bioassay rings. ACh (10(-6) M), when added to the perfusion medium that runs through the control polyethylene tubing, produced further contraction of the PHE-precontracted endothelium-denuded bioassay rings, whereas ACh caused relaxation of the rings when added to the perfusate that passes through the endothelium-intact donor aorta. The ACh-induced relaxation of the bioassay ring was slightly potentiated by superoxide dismutase (100 U/ml). Hemoglobin (10(-5) M) increased the basal tension of the bioassay rings and completely inhibited the ACh-induced relaxation of the rings. These results suggest that ACh-induced relaxation of fowl aorta is, at least partially, mediated by a humoral factor(s) that resembles endothelium-derived relaxing factor demonstrated in mammals.

NG-monomethyl-L-arginine increases platelet deposition on damaged endothelium in vivo. A scanning electron microscopic study

There is increasing evidence that nitric oxide (NO) synthetized in vascular endothelium and in platelets by NO synthase influences vascular tone, down regulates platelet function and platelet-vessel wall interaction both in vitro and in vivo. We investigated the effect of a NO synthase inhibitor, NG-mono-methyl-L-arginine (L-NMMA, 100 mg/kg iv) on platelet-endothelial cell interaction in rabbit arteries ex vivo using scanning electron microscope (SEM). The effect of L-NMMA was examined on intact endothelium and on that damaged by arterial constriction. The infusion of L-NMMA increased systemic blood pressure and decreased carotid blood flow, however, it did not change the appearance of an intact endothelium and did not result in platelet activation on intact endothelial cells. In contrast, SEM of endothelial areas damaged by constriction showed extensive platelet adhesion and aggregation on subendothelium. These morphological changes were not detected in control animals with intact or damaged by arterial constriction endothelium. These results show that under physiological conditions, the inhibition of NO synthase alone does not result in platelet activation in vivo. However, when combined with endothelial injury it may lead to platelet activation and thrombosis.

Enhanced endothelium-derived relaxing factor activity in pregnant, spontaneously hypertensive rats

Pregnancy is normally associated with vasodilation that, in hypertensive animals such as the spontaneously hypertensive rat, causes a profound decrease in blood pressure. To test the possibility that enhanced basal endothelium-derived relaxing factor activity has a role in the vasodilation of pregnancy, we measured the changes in mean arterial pressure and heart rate induced by NG-monomethyl-L-arginine, a specific inhibitor of endothelium-derived relaxing factor synthesis, in conscious nonpregnant and pregnant (postmating day 20 to 21) normotensive Wistar-Kyoto and spontaneously hypertensive rats. NG-monomethyl-L-arginine caused similar dose-dependent increases in mean arterial pressure in nonpregnant and pregnant Wistar-Kyoto rats, but the accompanying decrease in heart rate was significantly greater in nonpregnant rats than in pregnant ones. In the spontaneously hypertensive rats, NG-monomethyl-L-arginine caused significantly greater dose-dependent increases in mean arterial pressure in pregnant compared with nonpregnant rats; there were no differences in the decreases in heart rate. These pressor responses were partially reversed by excess L-arginine but not D-arginine. Indomethacin had no effect on the pressor response to NG-monomethyl-L-arginine or the depressor response to L-arginine after NG-monomethyl-L-arginine. Therefore basal endothelium-derived relaxing factor plays a role in vascular tone and blood pressure regulation in vivo, and pregnancy may be associated with enhanced basal endothelium-derived relaxing factor activity in the hypertensive spontaneously hypertensive rats.

Endogenous nitric oxide modulates adrenergic neural vasoconstriction in guinea-pig pulmonary artery

1. Electrical field stimulation (EFS) of guinea-pig isolated pulmonary artery induced a frequency-dependent contraction. This was abolished by tetrodotoxin (1 microM) and prevented by phentolamine and prazosin (both 1 microM), indicating a role for alpha 1-adrenoceptors activated by noradrenaline (NA) released from perivascular adrenergic nerves. 2. L-NG-monomethyl arginine (L-NMMA, 0.3-100 microM) caused a concentration-dependent enhancement of the EFS-induced contraction with a 3.4 +/- 0.5 fold increase at 100 microM n = 6). The augmenting effect of 30 microM L-NMMA on the contraction to EFS was completely reversed by 100-300 microM L-arginine, but not by an identical concentration of D-arginine. 3. The contractile response to exogenous NA was similarly enhanced by 30 microM L-NMMA (2.9 +/- 0.6 fold increase, n = 5). 4. The contractile responses to exogenous phenylephrine and prostaglandin F2 alpha while matched the contraction to EFS (4 Hz) were equally augmented by 30 microM L-NMMA. 5. In vessel rings submaximally contracted with the thromboxane analogue U44069 (2 microM), the selective alpha 2-adrenoceptor agonist UK14304 induced concentration-dependent relaxation, which was abolished by removal of endothelium. NA had little relaxant effect on these precontracted vessel rings unless in the presence of prazosin (1 microM). 6. Indomethacin had no significant effect on the contractile response to EFS or NA, indicating that vasodilator cyclo-oxygenase products such as prostacyclin are not involved in modulating these responses. 7. Our results suggest that endogenous nitric oxide inhibits the contractile response to adrenergic nerve stimulation in the guinea-pig pulmonary artery by a postjunctional mechanism, but release of prostacyclin does not modulate these responses. Basal release of nitric oxide from endothelial cells may account for this inhibition.

Endothelium-dependent and independent relaxation of the rat aorta by cyclic nucleotide phosphodiesterase inhibitors

1. The effects of selective inhibitors of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) phosphodiesterases (PDEs) were investigated on PDEs isolated from the rat aorta and on relaxation of noradrenaline (1 microM) precontracted rat aortic rings, with and without functional endothelium. 2. Four PDE forms were isolated by DEAE-sephacel chromatography from endothelium-denuded rat aorta: a calmodulin-activated PDE (PDE I) which hydrolyzed preferentially cyclic GMP, two cyclic AMP PDEs (PDE III and PDE IV) and one cyclic GMP-specific PDE (PDE V). The latter was selectively and potently inhibited by zaprinast. The two cyclic AMP PDEs were discriminated by specific inhibitors: one was inhibited by cyclic GMP (PDE III) and by new cardiotonic agents (milrinone, CI 930, LY 195115 and SK&F 94120); the other was inhibited by denbufylline and rolipram (PDE IV). None of these drugs significantly inhibited PDE I. 3. The PDE III inhibitors caused endothelium-independent relaxations of rat aortic rings with the following EC50 values (microM concentration producing 50% relaxation): LY 195115: 3.4, milrinone: 5.7, CI 930; 7.8, SK&F 94120: 14.7. Neither NG-monomethyl-L-arginine (L-NMMA, 300 microM), an inhibitor of the L-arginine-NO pathway, nor L-arginine (1 mM) modified the effect of PDE III inhibitors. However, methylene blue (10 microM) an inhibitor of soluble guanylate cyclase abolished relaxation induced by PDE III inhibitors except in the case of compound CI 930. 4. The specific PDE IV and PDE V inhibitors both produced endothelium-dependent relaxations which were inhibited by L-NMMA and by methylene blue (10 microM). In the presence of L-NMMA, relaxation was restored by subsequent addition of L-arginine. 5. The relaxant effects of denbufylline and rolipram were studied in the presence of drugs stimulating either adenylate cyclase (forskolin and isoprenaline) or soluble guanylate cyclase (sodium nitroprusside, SNP), or inhibiting PDE III (milrinone). In endothelium-denuded rings, a relaxing effect of both denbufylline and rolipram was found in the presence of milrinone (EC5o values 1.7 and 12 microM, respectively) or SNP (EC50 values 12.3 and 124 microM, respectively), but not in the presence of forskolin or isoprenaline. However in the presence of functional endothelium, relaxations produced by PDE IV inhibitors were significantly potentiated by forskolin, isoprenaline, milrinone and SNP (respective EC50 values for denbufylline: 2, 2, 0.4 and 0.7 microM and for rolipram: 7, 13, 7 and 1.2 microM). 6. These results indicate that the relaxant effects of inhibitors of the cyclic AMP-specific PDE IV are markedly enhanced by cyclic GMP elevating agents and by the PDE III inhibitor milrinone. They support the hypothesis that cyclic GMP enhances cyclic AMP-mediated relaxation, possibly through the inhibition of the cyclic GMP-inhibited PDE III.

NW-nitro L-arginine benzyl ester, a potent irreversible inhibitor of endothelium dependent relaxation

In the rat aorta preparation, we have used the nitro-arginine derivative, NW-nitro L-arginine benzyl ester (NABE), as a probe to investigate the relationship between L-arginine and the endothelium derived relaxing factor (EDRF). We find NABE to be a potent endothelium dependent vasoconstrictor and inhibitor of relaxation. The effect of NABE is irreversible and not removed by subsequent washing. The vasoconstrictor effect of NABE differs from other EDRF inhibitors like NG-monomethyl L-arginine (L-NMMA) in that it is not antagonized by pre-treatment with excess L-arginine. In contrast, like other EDRF inhibitors, at high concentration NABE exhibits vasodilation which is antagonized by methylene blue. We suggest that the previous reports on the antagonism between L-arginine and the putative EDRF inhibitors like L-NMMA are due to their structural similarities rather than to externally added L-arginine acting as a substrate for EDRF synthesis.

Comparison of vasopressor effects of nitro arginine in stroke-prone spontaneously hypertensive rats and Wistar-Kyoto rats

1. NG-nitro-L-arginine (NO2Arg) is a guanidine nitro arginine derivative and an inhibitor of endothelium-dependent vascular relaxation. Significant rise of the systolic blood pressure was observed after 1 week administration of NO2Arg in food (0.023% in weight, about 2.8 mg of NO2Arg/rat per day) in female rats of stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). The rises were not different between SHRSP (21 mmHg) and WKY (23 mmHg). 2. In ring preparations of the thoracic aorta of NO2Arg-administered rats of both strains, relaxation by acetylcholine decreased markedly compared with those of the control rats (to 43-44%). On the contrary, glyceryltrinitrate-induced relaxation was slightly but significantly increased in the aorta of WKY after NO2Arg administration and the same tendency was observed in SHRSP. 3. The rise of blood pressure and the decrease of acetylcholine-induced relaxation suggested that NO2Arg inhibited the endothelium-dependent relaxation not only in WKY but also in SHRSP. The relaxation of the thoracic aorta preparation of SHRSP by acetylcholine was much less (ca 38%) than that of WKY; however, that of SHRSP by glyceryltrinitrate was slightly less (ca 74%), indicating that endothelium-dependent relaxation declined in vascular preparation of SHRSP. 4. The present results suggest that endothelium-dependent relaxation has some contribution on blood pressure regulation in the hypertensive state, although a decline of endothelium-dependent relaxation is evident in vascular preparation of SHRSP compared with WKY.

Regional coronary haemodynamic effects of two inhibitors of nitric oxide synthesis in anaesthetized, open-chest dogs

1. The role of endothelial nitric oxide synthesis from L-arginine in the regulation of coronary vascular tone and myocardial tissue perfusion was evaluated in anaesthetized, open-chest dogs. Coronary blood flow was measured with an electromagnetic flow probe placed around the left circumflex coronary artery. Coronary vascular resistance was calculated from mean arterial blood pressure and mean coronary blood flow, whereas regional myocardial tissue flow was determined by use of the radioactive microspheres technique. 2. NG-monomethyl L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), administered directly into the left circumflex artery, induced a small increase in arterial blood pressure and an increase in coronary vascular resistance. However, myocardial tissue perfusion, assessed by the microspheres technique (whether subendocardial, subepicardial, or transmural), was unaffected by L-NMMA or L-NAME. 3. Acetylcholine, administered intracoronarily, induced an increase in left circumflex coronary blood flow and a decrease in coronary vascular resistance, without affecting systemic haemodynamics. This coronary vasodilator effect of acetylcholine was markedly inhibited by L-NMMA and L-NAME, the latter being a more potent antagonist than the former. 4. These results indicate that the endothelial L-arginine pathway is largely responsible for the coronary vasodilator effect of acetylcholine. However, although basal release of nitric oxide from L-arginine apparently contributes to the regulation of resting coronary vascular tone, blockade of this pathway does not affect myocardial tissue perfusion, possibly because of compensatory mechanisms occurring at the level of small arterioles and/or capillaries.

The role of the L-arginine-nitric oxide pathway in relaxation of the opossum lower oesophageal sphincter

1. The role of the L-arginine-nitric oxide pathway in lower oesophageal sphincter (LOS) relaxation and oesophageal peristalsis was investigated. 2. Twenty four adult opossums were anaesthetized and the right vagus nerve was isolated in the neck and sectioned. Electrical stimulation, applied to the peripheral end of the nerve, resulted in a frequency-dependent relaxation of the LOS, and peristaltic and non-peristaltic contractions in the oesophageal body. 3. N omega-nitro-L-arginine (L-NNA, 10(-8)-10(-5) mol kg-1), an inhibitor of the L-arginine-nitric oxide pathway, inhibited LOS relaxation in a dose-dependent manner, but did not affect resting LOS pressure. At the highest dose of L-NNA no relaxation of the LOS was elicited in response to vagal stimulation. The effect of L-NNA, (10(-5) mol kg-1) was fully reversed by infusion of 10(-4) mol kg-1 L-arginine. Peristaltic velocity and amplitude of contractions in the oesophageal body were unaffected by L-NNA. 4. Infusion of sodium nitroprusside reduced LOS pressure to zero, and the drug was equally potent in control animals (-log ED50:8.1 +/- 0.2 mol kg-1) and in animals pretreated with L-NNA (-log ED50:8.2 +/- 0.3 mol kg-1). This suggests that the effect of L-NNA was not directly on guanylate cyclase. 5. A significant elevation of blood pressure was recorded after administration of L-NNA (10(-5) mol kg-1). 6. It is suggested that the L-arginine-nitric oxide pathway plays an important functional role for relaxation of the LOS, but not for oesophageal peristalsis. Whether the active substance is nitric oxide or a related nitroso-compound remains to be settled.