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

Effect of the vascular endothelium on norepinephrine-induced contractions in uterine radial arteries from the nonpregnant and pregnant human uterus

OBJECTIVE

Our purpose was to investigate the role of the endothelium in the human uterine arterial response to norepinephrine in the nonpregnant and pregnant states.

STUDY DESIGN

Tissue was obtained from six pregnant and six nonpregnant women undergoing cesarean section or hysterectomy. Uterine radial arteries were isolated and subjected to norepinephrine dose-response curves with and without intact endothelium.

RESULTS

Responses were obtained over a dose range of 10(-8) to 10(-4) norepinephrine. Initially there was no difference between vessels from pregnant and nonpregnant patients, but removal of the endothelium significantly increased the response in vessels from pregnant women. Addition of nitro-L-arginine methyl ester when the endothelium was intact did not alter the dose-response curves.

CONCLUSIONS

In pregnancy human uterine radial arteries are more sensitive to norepinephrine than during the nonpregnant state. This increase is countered by an endothelium-derived relaxing factor. The factor is unlikely to be nitric oxide.

Treatment of persistent pulmonary hypertension in the newborn lamb by inhaled nitric oxide

We previously showed that fetal lambs whose ductus arteriosus is ligated prenatally will have persistent pulmonary hypertension at birth. We investigated the effect of inhaled nitric oxide on the pulmonary circulation in this animal model. The ductus arteriosus of six fetal lambs was ligated at 126 days of gestation. The lambs were delivered and studied at 136 days of gestation. Mechanical ventilation was maintained at a fraction of inspired oxygen of 0.80. Nitric oxide gas was administered at five different concentrations (6, 12, 25, 50, and 100 ppm) for 5-minute periods separated by 10-minute periods of ventilation without nitric oxide. Inhaled nitric oxide caused dose-dependent decreases in pulmonary arterial pressure and vascular resistance and dose-dependent increases in pulmonary blood flow without affecting systemic arterial pressure. Thus pulmonary arterial pressure decreased from equal to aortic pressure to less than aortic pressure. At the highest dose, mean pulmonary arterial pressure decreased by 27% +/- 2%, pulmonary blood flow increased by 86% +/- 6%, and pulmonary vascular resistance decreased by 59% +/- 4%. Nitric oxide also caused dose-dependent increases in systemic arterial oxygen tension and in the saturation of hemoglobin with oxygen. Partial pressure of arterial oxygen increased from 43 +/- 16 mm Hg at baseline to 185 +/- 72 mm Hg at the highest dose; saturation increased from 74% +/- 8% to 96% +/- 2%. In our model of persistent pulmonary hypertension of the newborn, inhaled nitric oxide selectively dilates the pulmonary circulation, thereby improving systemic arterial oxygenation. Nitric oxide is a promising new treatment of persistent pulmonary hypertension of the newborn.

Roles of endothelin-1 and nitric oxide in the mechanism for ethanol-induced vasoconstriction in rat liver

This study was designed to investigate the mechanism for ethanol-induced hepatic vasoconstriction in isolated perfused rat liver. Upon initiation of ethanol infusion into the portal vein at concentrations ranging from 25 to 100 mM, portal pressure began to increase in a concentration-dependent manner and reached maximal levels in 2-5 min (initial phase), followed by a gradual decrease over the period of ethanol infusion (escape phenomenon). Endothelin-1 antiserum significantly inhibited this ethanol-induced hepatic vasoconstriction by 45-80%. Cessation of infusion of endothelin-1 antiserum was followed by a subsequent increase in portal pressure. On the other hand, when a nitric oxide synthesis inhibitor, NG-monomethyl-L-arginine (L-NMMA), was infused into the portal vein simultaneously with ethanol, the initial phase of the response of portal pressure to ethanol was not altered and the peak values of portal pressure remained unchanged. However, after the peak increase in portal pressure, the rate of decrease was less than in the absence of L-NMMA. Thus, L-NMMA diminished the escape phenomenon and sustained the vasoconstriction. This study supports the hypothesis that two endothelium-derived vasoactive factors, endothelin-1 and nitric oxide, regulate hepatic vascular tone in the presence of ethanol.

Nitric oxide affects mammalian distal colonic smooth muscle by tonic neural inhibition

1 The role of the L-arginine-nitric oxide (NO) pathway in tonic neural inhibition of spontaneous mechanical activity of distal colonic circular smooth muscle (DCCSM) was investigated in male Wistar rats. 2 Muscle strips were mounted in organ baths and spontaneous contractions recorded with isometric force transducers. They were characterized as low frequency (LFCs) 0.41 +/- 0.03 N cm-2 or high frequency contractions (HFCs) 0.22 +/- 0.04 N cm-2. The latter occurred intermittently to produce summation contractions (SCs) range 0.5-12 N cm-2. 3 Tetrodotoxin (100 nM) increased the forces of LFCs and SCs. Increase in force to tetrodotoxin did not occur after incubation of the muscle with NG-monomethyl-L-arginine (L-NMMA) 500 microM, an inhibitor of NO biosynthesis. 4 L-NMMA but not its enantiomer D-NMMA increased the force of LFCs (EC50: 200 microM) and SCS (EC50:175 microM) in a concentration-dependent manner which was reversed by L-arginine but not by D-arginine. 5 Muscle, precontracted by acetylcholine, relaxed to sodium nitroprusside (EC50:1.8 microM) NO gas (EV50:70 microliters) and NO solutions (EC50:4 microM) in a concentration-dependent manner. Guanosine 3':5'-cyclic monophosphate tissue concentrations (pmol mg-1 protein) were elevated in muscle after relaxation by sodium nitroprusside (500 microM) from 0.32 +/- 0.06 to 1.2 +/- 0.37 and by 1 ml of NO gas from 0.49 +/- 0.05 to 1.54 +/- 0.14. 6 These data suggest that DCCSM is under tonic neural inhibition mediated by NO biosynthesis.

Distribution and metabolism of NG-nitro-L-arginine and NG-nitro-L-arginine methylester in canine blood in vitro

The aim of the present study was to investigate the metabolism of the two NO-synthase inhibitors NG-nitro-L-arginine (L-NA) and NG-nitro-L-arginine methylester (L-NAME) in canine blood in vitro. Blood and plasma samples were incubated with L-NAME or L-NA respectively and the drug levels were determined in blood, plasma and blood cells by means of high performance liquid chromatography. Incubation of blood or plasma with L-NAME revealed that L-NAME is metabolized to L-NA in blood and plasma. After plasma incubation with L-NA, the L-NA levels remain stable over the whole observation period; in agreement with the data in plasma the whole amount of L-NA added to blood was detectable in blood after 4 h of incubation suggesting that L-NA undergoes no further metabolism. Drug concentrations determined in blood cells after 4 h of blood incubation with L-NAME or L-NA revealed that L-NAME easily enters the blood cells, whereas only a small portion of L-NA is found in the blood cells 4 h after blood incubation with L-NA. In conclusion, the present study demonstrates that L-NAME is metabolized to L-NA in canine blood and plasma in vitro. The fact that L-NAME but nearly no L-NA enters the cellular blood compartment led us to the assumption that although L-NA is an active metabolite of L-NAME, NO synthase may be differently inhibited by L-NA and L-NAME due to their different distribution characteristics.

Urinary nitrite excretion in premature infants: effects of transfusion or indomethacin

Urinary nitrite excretion, an index of L-arginine-dependent nitric oxide formation, was quantified daily for two weeks, in very low-birth-weight (< 1500 g) premature infants. A transient 52% reduction in nitrite excretion was noted on the day of transfusions (54 +/- 10 versus 26 +/- 6 mumol/mmol creatinine, before and during transfusion, respectively, n = 24, p < 0.02). Indomethacin administration in six infants was associated with a dramatic increase in nitrite excretion from a basal median value of 3 to 76 mumol/mmol creatinine (p < 0.05). Nitrite excretion returned to baseline on day 3 after indomethacin administration. In two infants who received indomethacin and transfusions on the same day, the stimulatory effect on nitrite excretion by indomethacin overwhelmed any depressive effect of transfusions. These results suggest that L-arginine utilization is influenced by common therapeutic strategies in these high-risk infants.

Beta-adrenoceptor agonist mediated relaxation of rat isolated resistance arteries: a role for the endothelium and nitric oxide

1. Isoprenaline (10(-9)-10(-5) M) relaxed rat isolated mesenteric resistance arteries pre-contracted with K+ (30-60 mM) (p EC50 (M) 8.03 +/- 0.40; maximum relaxation 66.79 +/- 2.43%, n = 7). This relaxation was partially attenuated by the nitric oxide (NO) synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 10(-4) M). 2. The beta 2-adrenoceptor agonist, salbutamol (10(-9)-10(-5) M), produced a modest maximum relaxation (35.93 +/- 2.93%), which was not sensitive to L-NAME. 3. The beta 1-adrenoceptor agonist, dobutamine (10(-9)-10(-5) M), relaxed arteries precontracted with K+. This relaxation was abolished by L-NAME (10(-4) M) and also by propranolol (10(-6) M), but not affected by D-NAME (10(-4) M). The inhibition by L-NAME was partially reversed by L-arginine (10(-3) M). Removal of the endothelium severely attenuated relaxation to dobutamine. 4. Contractile responses to depolarizing K+ solutions were enhanced by the addition of L-NAME, and also by removal of the endothelium. 5. The above findings demonstrate that beta 1-adrenoceptor causes relaxation via NO release from the endothelium of rat mesenteric resistance arteries. In addition, contraction to K+ is modified by release of NO from the endothelium, possibly in response to tension development.

Sustained hypertension induced by orally administered nitro-L-arginine

To study the hemodynamic and metabolic effects of chronic inhibition of endothelium-derived nitric oxide, we treated conscious rats with an oral solution of N omega-nitro-L-arginine (LNA), an inhibitor of nitric oxide production by endothelial cells. After 3 days of treatment with 2.74 mM LNA, rats had higher blood pressures (136 +/- 5 versus 113 +/- 3 mm Hg, p < 0.0005) than did the control animals. This effect was maintained through 7 days of treatment (142 +/- 6 versus 109 +/- 4 mm Hg, p < 0.0005) and in three animals for 35 days (167 +/- 7 mm Hg). The blood pressure rise was dose dependent. The hypertensive effect of oral LNA was not enhanced by the administration of 20 mg intraperitoneal LNA and was prevented by pretreatment with L-arginine, although L-arginine also caused a transient but significant increase in urinary sodium excretion. When LNA treatment was discontinued, blood pressure fell gradually, with an effective biological half-life of 4.2 days. Metabolic balance studies did not identify differences in sodium or potassium balance between treated and control animals. Plasma renin activity was lower in LNA-treated animals, and aldosterone concentrations tended to be lower. In contrast, atrial natriuretic factor levels and serum electrolyte concentrations were unchanged after 7 days of treatment with LNA. These data support the premise that endothelium-derived nitric oxide plays an important role in basal hemodynamic homeostasis. Oral administration of LNA may serve as a model of chronic nitric oxide-deficient hypertension and allow for the future study of endothelium dependence in hypertension.

Inhibition of nitric oxide formation in the nucleus tractus solitarius increases renal sympathetic nerve activity in rabbits

It has been shown that nitric oxide (NO) is synthesized in the central nervous system as well as in vascular endothelial cells. However, the physiological role of NO in cardiovascular regulation by the central nervous system remains unclear. This objective of this study was to examine the possibility that NO plays a role in neural transmission in the nucleus tractus solitarius (NTS) and thus contributes to control of sympathetic nerve activity in rabbits. We examined the effects of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of the formation of NO from L-arginine, microinjected into the NTS on arterial pressure (AP), heart rate (HR), and renal sympathetic nerve activity (RSNA). L-NMMA increased AP and RSNA in rabbits with intact as well as denervated sinoaortic baroreceptors and vagi. L-NMMA increased HR only in rabbits with sinoaortic denervation and vagotomy. Pretreatment with L-arginine microinjected into the NTS, which did not alter baseline AP, HR, and RSNA, prevented the increases in AP and RSNA evoked with subsequent L-NMMA. Pretreatment with D-arginine did not alter the effects of subsequent L-NMMA injections into the NTS. The gain of arterial baroreflex control of RSNA assessed by the slope of the regression line relating changes in AP and those in RSNA caused by intravenous phenylephrine or nitroglycerin did not differ before and after microinjections of L-NMMA. L-NMMA microinjected into the area postrema did not alter AP, HR, or RSNA. These results suggest that in rabbits NO is involved in the mechanism in the NTS that mediates tonic inhibition of RSNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial activation and the kidney: vasomediator modulation and antioxidant strategies

We have come to appreciate that the endothelium plays a major role in regulation of renal hemodynamics and excretory function. In the normal state, the endothelium maintains an intricate balance of interacting relaxing and contracting factors that can influence vasomotor tone and renal sodium handling, but also plays a role in the control of the coagulation system and cellular proliferation. Studies of reactive oxygen species as mediators of endothelial injury have shown that the perturbed endothelium can respond to such a threat, calling on intrinsic protective mechanisms such as induction of heme oxygenase and ferritin synthesis. In vivo studies have demonstrated that these mechanisms may confer protection in experimental models of acute renal injury. However, when endothelial injury or dysfunction does occur, adverse renal hemodynamic consequences, systemic hypertension, enhanced platelet aggregation, and mesangial cell proliferation could all contribute to progressive renal dysfunction. The role of the endothelium in modulation of normal renal function and in the pathogenesis of renal diseases will be the focus of future research efforts.

Coronary venular responses to flow and pressure

In previous studies, we demonstrated that both endothelium-dependent flow-induced vasodilation and endothelium-independent myogenic responses occur in porcine coronary arterioles. However, it was not established whether these responses are present in the coronary venular microcirculation. The aim of this study was to test the hypotheses that 1) coronary venules, like arterioles, exhibit flow-induced dilation and myogenic responsiveness, and 2) venular flow-induced dilation is endothelium-dependent and is mediated by the release of a nitrovasodilator. Experiments were performed in porcine subepicardial coronary venules, 80-120 microns in diameter, by using cannulated isolated vessel techniques to allow intraluminal pressure and flow to be independently controlled. Flow was initiated by simultaneously moving two perfusion reservoirs connected to the cannulating pipettes in equal amounts but in opposite directions. In the absence of flow, i.e., zero pressure gradient (delta P) between the two reservoirs, venules developed spontaneous tone to 75-80% of maximum diameter at 10 cm H2O intraluminal pressure. Venules gradually dilated in response to stepwise increases in flow (i.e., delta P). The threshold for the flow-induced dilation occurred at delta P = 1 cm H2O (flow = 3.5 nl/sec), and the maximal response (dilation to 93 +/- 2% of maximum diameter) occurred when delta P was elevated to > or = 6 cm H2O (flow = 21 nl/sec at delta P = 6 cm H2O). Flow-induced dilation was abolished after the endothelium was damaged by perfusion of an air bolus through the lumen. Vasoconstriction was observed when denuded venules were subjected to relatively high luminal flows (> or = 21 nl/sec).(ABSTRACT TRUNCATED AT 250 WORDS)

Neurogenic and non-neurogenic mechanisms of plasma extravasation in the rat

We describe two distinct mechanisms for the enhancement of plasma extravasation in the knee joint of the rat. One is activated by bradykinin and is neurogenic; the other is activated by platelet-activating factor and is non-neurogenic. Bradykinin-induced synovial plasma extravasation is known to be dependent on the sympathetic postganglionic neuron terminal, and to involve prostaglandins, ATP, adenosine A2 receptor action, and the attraction and activation of neutrophils. In this study we found that bradykinin-induced plasma extravasation also involves endothelium-derived relaxing factor; specifically we found that bradykinin-induced plasma extravasation was antagonized stereospecifically by the inhibitor of endothelium-derived relaxing factor synthesis, NG-monomethyl-L-arginine. Perfused alone, platelet-activating factor produced an increase in synovial plasma extravasation which was markedly reduced by the platelet-activating factor receptor antagonists BN 52021 and WEB 2086 (these antagonists did not affect bradykinin-induced plasma extravasation). Platelet-activating factor-induced plasma extravasation was not affected by NG-monomethyl-L-arginine, indomethacin (a prostaglandin synthesis inhibitor), phenol 3-(5H-thiozolo[2,3b]quinazolin) (an A2 receptor adenosine antagonist), dextran sulfate (an inhibitor of leukocyte rolling), hydroxyurea (a depletor of leukocytes), chronic sympathectomy or the depletion of unmyelinated afferent fibers. Of note, the magnitude of platelet-activating factor-induced plasma extravasation was increased by co-perfusion with prostaglandin E2 and attenuated by co-perfusion with L-arginine; that is, two of the mediators involved in neurogenic bradykinin-induced plasma extravasation exerted an influence on non-neurogenic plasma extravasation. Separate mechanisms for bradykinin and platelet-activating factor plasma extravasation were further demonstrated in the streptozotocin-treated diabetic rat, in which there is a peripheral neuropathy.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhaled nitric oxide reverses pulmonary vasoconstriction in the hypoxic and acidotic newborn lamb

We determined whether inhaling low levels of nitric oxide (NO) gas could selectively reverse hypoxic pulmonary vasoconstriction in the near-term newborn lamb and whether vasodilation would be attenuated by respiratory acidosis. To examine the mechanism of air and NO-induced pulmonary vasodilation soon after birth, we measured plasma and lung cGMP levels in the newly ventilated fetal lamb. Breathing at FIO2 0.10 nearly doubled the pulmonary vascular resistance index in newborn lambs and decreased pulmonary blood flow primarily by reducing left-to-right blood flow through the ductus arteriosus. Inhaling 20 ppm NO at FIO2 0.10 completely reversed hypoxic pulmonary vasoconstriction within minutes. Maximum pulmonary vasodilation occurred during inhalation of > or = 80 ppm NO. Breathing 8% CO2 at FIO2 0.10 elevated the pulmonary vascular resistance index to a level similar to breathing at FIO2 0.10 without added CO2. Respiratory acidosis did not attenuate pulmonary vasodilation by inhaled NO. In none of our studies did inhaling NO produce systemic hypotension or elevate methemoglobin levels. Four minutes after initiating ventilation with air in the fetal lamb lung, cGMP concentration nearly doubled without changing preductal plasma cGMP concentration. Ventilation with 80 ppm NO at FIO2 0.21 increased both lung and preductal plasma cGMP concentration threefold. Our data suggest that inhaled NO gas is a rapid and potent selective vasodilator of the newborn pulmonary circulation with an elevated vascular tone due to hypoxia and respiratory acidosis that acts by increasing lung cGMP concentration.

Endothelium-dependent vasorelaxation evoked by desmopressin and involvement of nitric oxide in rat aorta

It is known that in vivo administration of desmopressin (DDAVP; a selective V2-vasopressin receptor agonist) results in prostacyclin-independent vasodilation. The in vitro effects of DDAVP and its mechanisms were examined using rat aortic strips. DDAVP from a concentration of 1 x 10(-9) M caused a concentration-dependent relaxation of the aorta precontracted with norepinephrine (10(-7) M) with intact endothelium. However, no relaxation was induced in aorta with the endothelium removed. The DDAVP-induced relaxation was not influenced by the presence of indomethacin but was inhibited by L-NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide (NO) synthesis. The inhibition by L-NMMA was reversed by the addition of L-arginine but not D-arginine. Further, the endothelium-dependent relaxation due to DDAVP was potentiated by superoxide dismutase, a scavenger of superoxide anions, and was inhibited by hemoglobin. DDAVP induced an increase in guanosine 3',5'-cyclic monophosphate levels in the aorta with endothelium but not in aorta without endothelium, and this was suppressed by L-NMMA and hemoglobin. The suppression by L-NMMA was also partially reversed by L-arginine but not by D-arginine. Two selective V2-receptor antagonists had no effect on the DDAVP-induced vasorelaxation. Selective V1-receptor antagonists (a peptidic and a nonpeptidic) caused a concentration-dependent but nonparallel shift to the right of the concentration-response curves to DDAVP. However, DDAVP did not affect the tension of the strip with or without endothelium in nonprecontracted aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation to increased dietary salt intake in the rat. Role of endogenous nitric oxide

Previous studies have suggested that nitric oxide (NO) plays a role in regulation of renal vascular tone and sodium handling. We questioned whether the effects of NO synthase inhibition on renal function are direct or due to increased renal perfusion pressure (RPP) and whether stimulation of endogenous NO activity plays a role in adaptation to increased dietary salt intake. Intrarenal arterial infusion of the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) in control rats resulted in decreased glomerular filtration rate, renal vasoconstriction, natriuresis, and proteinuria. When RPP was held at basal levels with suprarenal aortic snare, L-NMMA had similar hemodynamic effects but decreased sodium excretion and did not induce proteinuria. Exposure of rats to high salt intake (1% NaCl drinking water) for 2 wk induced increased serum concentration and urinary excretion of the NO decomposition products, NO2 + NO3. Urinary NO2 + NO3 and sodium excretion were significantly correlated. Compared with controls, chronically salt-loaded rats also demonstrated enhanced renal hemodynamic responses to NO synthase inhibition. We conclude that the endogenous NO system directly modulates renal hemodynamics and sodium handling and participates in the renal adaptation to increased dietary salt intake. Enhanced NO synthesis in response to increased salt intake may facilitate sodium excretion and allow maintenance of normal blood pressure.

Role of NO in vagally-mediated relaxations of guinea-pig stomach

Vagal stimulation of the stomach induces a relaxation mediated via non-adrenergic, non-cholinergic (NANC) nerves. The neurotransmitter which is responsible for this relaxation is still unknown. To determine whether nitric oxide (NO) or a NO related substance mediates this relaxation, an intact guinea-pig stomach was mounted in an organ bath, with electrodes surrounding the vagal nerves. Electrical stimulation of the vagal nerves, in the presence of atropine, induced frequency dependent, tetrodotoxin-(TTX) sensitive relaxations of the stomach quantified as changes in volume. These relaxations were not affected by alpha- or beta-adrenoceptor antagonists or guanethidine. Thus they were evoked by non-adrenergic, non-cholinergic (NANC) inhibitory nerves. The relaxant responses could be inhibited in a concentration-dependent manner by NG-nitro-L-arginine (L-NNA) a substance that inhibits the formation of nitric oxide (NO). Addition of L-arginine, the substrate for NO-synthase, reversed the L-NNA-induced-inhibition of the relaxation. Addition of nitroglycerin (a NO-donor) to a non-stimulated stomach mimicked the relaxations observed after vagal stimulation in a concentration dependent manner. These relaxations were insensitive to TTX. Relaxation of the stomach by vagal stimulation was prevented by an inhibitor of soluble guanylate cyclase, methylene blue, further supporting our conclusions. These data indicate that NO or a substance releasing NO plays an important role in NANC-neurotransmission after vagal stimulation of the guinea-pig stomach.

Effects of NG-nitro-L-arginine on electrical and mechanical responses to stimulation of non-adrenergic, non-cholinergic inhibitory nerves in circular muscle of the rat gastric fundus

The effects of L-nitroarginine (L-NNA), an inhibitor of nitric oxide (NO) synthesis, on non-adrenergic, non-cholinergic inhibitory transmission in the circular smooth muscle of the rat gastric fundus were studied. The relaxation in response to nerve stimulation in the presence of guanethidine and atropine was depressed by the application of L-NNA (10(-4) M), and the depression was reversed by the addition of L-arginine (10(-4) M). The time course of the relaxation was altered from a monophasic to biphasic one, both of which were depressed. Stimulation of non-adrenergic, non-cholinergic nerves initiated inhibitory junctional potentials (i.j.p.). In addition, slow after-hyperpolarization was observed when stimuli were applied repeatedly at frequencies higher than 0.5 Hz. L-NNA reduced the i.j.p. partially and abolished the after-hyperpolarization, indicating the partial involvement of NO in non-adrenergic, non-cholinergic inhibitory transmission. The effects of L-NNA on the nerve-mediated relaxation are thought to be due to changes in the i.j.p. and slow after-hyperpolarization.

Mediation by the same muscarinic receptor subtype of phasic and tonic contractile activities in the rat isolated portal vein

1. The effects of several agonists on the phasic and tonic contractile responses to muscarinic receptor stimulation have been investigated in the rat portal vein in vitro. 2. Neither chemical denervation with 6-hydroxydopamine nor the presence of the alpha 1-adrenoceptor antagonist, prazosin, influenced the spontaneous or the stimulated myogenic activity of the portal vein. 3. Indomethacin and NG-nitro-L-arginine were used to investigate the influence of vasoactive factors in this preparation. They slightly increased the frequency and the amplitude of the spontaneous myogenic activity of the portal vein, respectively. NG-nitro-L-arginine but not indomethacin enhanced the maximal phasic response to carbachol. Both indomethacin and NG-nitro-L-arginine failed to influence the tonic response to carbachol. 4. Muscarinic agonists increased phasic activity according to the rank order of potency: acetylcholine > muscarine > methacholine > carbachol > aceclidine > bethanechol. These effects were superimposed on a sustained contracture at higher concentrations. Oxotremorine was more potent than arecoline in increasing the mechanical phasic activity, without inducing a sustained contracture. Pilocarpine and McN A343 were weak agonists, producing submaximal effects only on phasic activity. 5. The muscarinic antagonists AF-DX116, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), P-fluorohexahydrosiladiphenidol (pFHHSiD) and pirenzepine antagonized the phasic and tonic mechanical responses to carbachol. Although the tonic contracture was slightly more sensitive to all antagonists studied, the rank order of potency: 4-DAMP > pFHHSiD > pirenzepine > AF-DX 116 was the same for both types of responses, which is indicative of a M3-receptor subtype. 6. The tonic contractile response of the rat portal vein to carbachol was more susceptible to partial receptor inactivation with propylbenzilylcholine mustard than the phasic contractile response. The dissociation constants (KA) obtained from an analysis according to Furchgott & Bursztyn (1967) were found to be 4.32 +/- 0.31 1AM for the phasic and 3.56 +/- 0.21 1AM for the tonic type of carbachol-induced response, respectively. Since the EC50-values for both carbachol-induced effects were different (phasic0.232 +/- 0.02 1AM; tonic 2.75 +/- 0.1 1AM) the phasic type of response appears to involve a large receptor reserve.

Neural mechanism of hypertension by nitric oxide synthase inhibitor in dogs

This study aimed to determine the mechanism of hypertension associated with nitric oxide synthase inhibition. Intravenous injections of NG-nitro-L-arginine, a nitric oxide synthase inhibitor, produced a sustained increase in systemic blood pressure and a decrease in heart rate in anesthetized dogs, whereas NG-nitro-D-arginine had no effect. L-Arginine reversed the pressor response. NG-Nitro-L-arginine-induced hypertension was markedly attenuated or abolished by treatment with hexamethonium; this inhibition was still observed when the blood pressure fall caused by the ganglionic blocking agent was compensated by continuous infusion of angiotensin II. In dogs treated with phentolamine in a dose sufficient to lower blood pressure to the level similar to that elicited by hexamethonium and to suppress the pressor response to norepinephrine, the hypertensive effect of NG-nitro-L-arginine was not attenuated. We conclude that hypertension caused by the nitric oxide synthase inhibitor is associated with an elimination of nitroxidergic neural function rather than an impairment of the basal release of nitric oxide from the endothelium.

Substance P increases cyclic GMP levels on coronary postcapillary venular endothelial cells

The vasodilating effect of substance P (SP) at the microvascular level is endothelium-dependent. In the present study we evaluated whether SP activates nitric oxide (NO) production by venular endothelial cell. We evaluated NO activation by measuring cyclic GMP levels in cultured endothelial cells isolated from coronary postcapillary venules of bovine origin (CVEC). Our results indicate that 5 min exposure of CVEC to 10 nM SP doubled basal cyclic GMP levels. Cell treatment with the NO synthase inhibitor L-NMMA reduced the basal levels of cyclic GMP and abolished the effect of SP but did not modify the increase in cyclic GMP in response to exogenous NO. These data indicate that a) microvascular endothelium responds in an autocrine fashion to NO with increased cyclic GMP levels, b) SP activates cyclic GMP pathway through NO production.

Transient cerebral vasodilatory effect of neuropeptide Y mediated by nitric oxide

The effects of intracarotidly injected neuropeptide Y (NPY; 0.1 micrograms/kg) on the local cerebral blood volume (CBV) and blood flow (CBF) in the parieto-temporal cortex were examined by the photoelectric method in 17 anesthetized cats. CBV reflects the cumulative crosssectional area of the cerebral microvascular beds. NPY immediately caused transient but significant increases in CBV and CBF, which lasted for less than 5 min. Thereafter, CBV returned to and remained at the control level, although CBF was decreased by 30-40% for 60 min during the monitoring period. The CBV increases after NPY were prevented by a 15-min preinjection of 0.35 mg/kg/min of NG-monomethyl-L-arginine (L-NMMA), which is a competitive blocker of nitric oxide synthesis. The CBV increases after NPY reappeared following a 15-min administration of 0.25 mg/kg/min of L-arginine, which is a precursor of nitric oxide. We conclude that NPY administered in vivo exerts a previously unreported effect of transient vasodilatation on the cerebral microvessels. This action appears to be mediated by nitric oxide, which is a major candidate as an endothelium-derived relaxing factor (EDRF).

Nitric oxide in gastroprotection by aluminium-containing antacids

Pretreatment with aluminium-containing antacids at their original pH or after acidification is known to protect the gastric mucosa against the damaging action of strong irritants and this protection is accompanied by an increase in gastric blood flow (GBF) but the mechanisms underlying these effects have not been elucidated. We investigated the role of endogenous nitric oxide (NO) and prostaglandins (PS) in the prevention of ethanol-induced gastric damage and the alteration of GBF by Maalox and its active component Al(OH)3. Maalox and Al(OH)3 at their original and acidic pH induced dose-dependent gastroprotection accompanied by attenuation of the reduction in GBF caused by 100% ethanol; similar protective and hyperemic effects were recorded after treatment with nocloprost, a locally active PGE2 analog, and nitroglycerin, a donor of NO. Pretreatment with indomethacin that suppressed mucosal PGE2 by about 90%, failed to affect the protective influence of Maalox or Al(OH)3 at their original or acidic pH. On the contrary, pretreatment with NG-nitro-L-arginine (L-NNA), a potent selective inhibitor of NO synthase, reversed the gastroprotective and hyperemic effects of Maalox or Al(OH)3 at original and acidic pH and this reversal was significantly antagonized by L-arginine but not D-arginine. The gastroprotective and hyperemic effects of nocloprost were not influenced by the pretreatment with L-NNA. We conclude that aluminium-containing antacids activate the NO system, which may contribute to the gastroprotective activity of these drugs through an increase in mucosal microcirculation.

Inhibition and stimulation of nitric oxide synthesis in the human forearm arterial bed of patients with insulin-dependent diabetes

Patients with insulin-dependent diabetes mellitus have an increased mortality and morbidity due to vascular complications. Nitric oxide from the vascular endothelium contributes to the control of normal vascular tone, and endothelial dysfunction has been implicated in the pathogenesis of diabetic vascular disease. In this study we have examined basal and stimulated nitric oxide-mediated vasodilatation in insulin-dependent diabetics and age- and sex-matched healthy controls. Drugs were infused locally into the brachial artery and forearm blood flow measured using venous occlusion plethysmography. Noradrenaline and NG-monomethyl-L-arginine produced similar reductions in resting forearm blood flow in healthy controls. However, in the diabetics, NG-monomethyl-L-arginine was significantly less effective than noradrenaline. Comparing between groups, the response to NG-monomethyl-L-arginine was also significantly less in the diabetics compared with the healthy controls. The response to sodium nitroprusside was significantly less in the diabetics compared with the healthy controls, whereas the responses to both acetylcholine and verapamil were the same in the two groups. The results provide evidence for an abnormality of basal nitric oxide-mediated dilatation in the forearm arterial bed of patients with insulin-dependent diabetes mellitus, and suggest that the vascular smooth muscle is less sensitive to nitric oxide.

Bioassay of EDRF from internal mammary arteries: implications for early and late bypass graft patency

To study the basal, luminal release of endothelium-derived relaxing factor, 35-mm segments of canine internal mammary artery (IMA) were cannulated and perfused at 5 mL/min in vitro with physiological salt solution. Vasoactive properties of the effluent were bioassayed on coronary artery smooth muscle. Effluent from IMAs produced significant vasodilation of the bioassay ring compared with effluent from a prosthetic conduit (n = 24; p < 0.05). The vasodilation by the effluent could be eliminated by mechanically removing the intima of the IMA, or by treating the IMA segments with NG-monomethyl-L-arginine or NG-nitro-L-arginine, two competitive inhibitors of nitric oxide synthesis from L-arginine; vasodilation was not influenced by treatment with indomethacin. In 83% of the superfusion experiments, effluent from the left IMA induced greater relaxation of the bioassay ring than did effluent from the right IMA. In addition, the average vasodilation induced by left IMA effluent was 28% +/- 2.3% versus 17.4% +/- 3.1% for the right (n = 24; p < 0.05). However, in organ chamber experiments, right and left IMAs exhibited comparable endothelium-dependent vasodilation to acetylcholine (n = 6). Because endothelium-derived relaxing factor induces vasodilation and also inhibits platelet adhesion, platelet aggregation, and atherogenesis, luminal release of endothelium-derived relaxing factor by the IMA could contribute to superior results when the artery is used in bypass grafting.

Inhibitors of nitric oxide synthase selectively reduce flow in tumor-associated neovasculature

1. The effects of L-arginine analogues, NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA) and methylene blue on blood flow in a murine adenocarcinoma and melanoma have been investigated. 2. Sponge implants in Balb/c and C57/BL mice were used to host proliferating tumour cells while the washout of 133Xe was employed to assess local blood flow in the implanted sponges. 3. Pharmacological inhibition of nitric oxide (NO) reduced blood flow in both tumours but this effect was reversed by administration of L-arginine. 4. In marked contrast, the effect of these same NO inhibitors on the blood flow in sponge-induced non-neoplastic granulation tissue was negligible. 5. These results strongly suggest that: (a) flow in tumour vessels is modulated by nitric oxide which maintains a dilator tone in neoplastic tissue; (b) the constrictor activity (as monitored by an increase in t1/2 of 133Xe) of NO inhibitors may be attributed to the removal of such dilator tone; (c) many of the abnormalities described in tumour vasculature, such as hyporeactivity or unresponsiveness to vasoactive mediators and maximum vasodilation, may be due to an increase in NO synthesis in cancers.

Modulation of the hyperdynamic circulation of cirrhotic rats by nitric oxide inhibition

The effects of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide (NO) biosynthesis on the splanchnic and systemic circulation, were investigated in rats with cirrhosis induced by carbon tetrachloride. Portal hypertension in these rats was accompanied by decreased arterial blood pressure and peripheral vascular resistance as well as by splanchnic vasodilation with increased portal venous inflow and decreased splanchnic resistance. Intravenous bolus administration of L-NMMA (25 mg/kg) significantly increased systemic blood pressure and decreased cardiac output. L-NMMA also significantly increased systemic and splanchnic vascular resistance; whereas blood flow to the stomach, small intestine, colon, pancreas, mesentery, spleen, and kidney was decreased significantly. L-NMMA did not alter the portal pressure or portosystemic shunting in these cirrhotic rats, yet portal vascular resistance increased, suggesting effects on the intrahepatic and collateral circulation. Pretreatment with L-arginine (300 mg/kg) prevented the hemodynamic changes induced by L-NMMA. These findings support the concept that local excess formation of NO contributes to changes in splanchnic circulation associated with portal hypertension in cirrhosis.

Endothelium-derived relaxing factor modulates platelet aggregation in an in vivo model of recurrent platelet activation

It has been shown that endothelium-derived relaxing factor (EDRF) may inhibit platelet aggregation in vitro through activation of platelet-soluble guanylate cyclase. To assess whether EDRF may also affect platelet function in vivo, intravascular platelet aggregation was initiated by placing an external constrictor around endothelially injured rabbit carotid arteries. Carotid blood flow velocity was measured continuously by a Doppler flow probe placed proximal to the constrictor. After placement of the constrictor, cyclic flow reductions (CFRs), due to recurrent platelet aggregation, developed at the site of the stenosis. After CFRs were observed for 30 minutes, a solution of authentic nitric oxide (NO, n = 10) was infused into the carotid artery via a small catheter placed proximally to the stenosis. Before infusion of NO, CFR frequency averaged 18.3 +/- 2.9 cycles per hour, and CFR severity (lowest carotid blood flow as percentage of baseline values) was 6 +/- 1%. NO completely inhibited CFRs in all animals, as shown by the normal and constant pattern of carotid blood flow (CFR frequency, 0 cycles per hour, p < 0.001; carotid blood flow, 92 +/- 5%, p = NS versus baseline). These effects were transient; CFRs were restored spontaneously within 10 minutes after cessation of NO infusion. After CFRs returned, S-nitroso-cysteine (S-NO-cys), a proposed form of EDRF, was infused into the carotid artery. S-NO-cys also abolished CFRs in all animals but at a significantly lower dose than NO (0.3 +/- 0.1 versus 12 +/- 4 nmol/min). The role of endogenously released EDRF in modulating in vivo platelet function was then tested in additional experiments. In 10 animals, endogenous release of EDRF was stimulated by infusing acetylcholine into the aortic root during CFRs. Infusion of acetylcholine was also associated with a complete inhibition of CFRs, similar to that observed during exogenous infusion of NO or S-NO-cys. These antithrombotic effects of acetylcholine were completely lost when EDRF synthesis was prevented by administration of the L-arginine analogue NG-monomethyl L-arginine (L-NMMA). Furthermore, in six additional rabbits the basal release of EDRF was blocked by L-NMMA after CFRs had been previously abolished with aspirin or the combination of aspirin and ketanserin, a serotonin S2 receptor antagonist. L-NMMA caused restoration of CFRs in all animals, indicating that even the basal release of EDRF is important in modulating platelet reactivity in vivo. Taken together, the data of the present study demonstrate that endogenous EDRF might importantly contribute to the modulation of platelet function in vivo.

Effects of infusion of L-arginine into the left anterior descending coronary artery on acetylcholine-induced vasoconstriction of human atheromatous coronary arteries

Hypercholesterolemia and atherosclerosis are conditions associated with impaired endothelium-dependent relaxation. In hypercholesterolemic animals, intravenous administration of L-arginine, the precursor of nitric oxide, normalizes endothelium-dependent vasodilator activity. In the present study, we questioned whether intracoronary administration of L-arginine in patients with coronary artery disease could improve coronary vascular reactivity to acetylcholine. Thirteen hypercholesterolemic patients with diffuse coronary atherosclerosis but nonstenotic lesions of the left anterior descending (LAD) coronary artery were investigated. Quantitative coronary angiography and subselective intracoronary Doppler flow velocity measurements were performed to determine LAD diameters and coronary blood flow. Intracoronary infusion of acetylcholine was performed during 3 consecutive 3-minute periods at incremental rates adjusted to achieve estimated final concentrations of 5 x 10(-7), 10(-6) and 5 x 10(-6) M. After evaluation of the response to acetylcholine, L-arginine was infused into the LAD at the rate of 25 mg/min (10(-3) M) and the same stepwise 3-minute infusions of acetylcholine were repeated during infusion of L-arginine. Infusion of acetylcholine induced a dose-dependent reduction of distal epicardial LAD diameter reaching -48.5 +/- 17% at 5 x 10(-6) M (p < 0.01 vs control values). L-arginine alone had no effect on the distal LAD diameter but attenuated acetylcholine-induced vasoconstriction to -21 +/- 9% at 5 x 10(-6) M acetylcholine (p < 0.01). Coronary blood flow showed a biphasic response to acetylcholine, increasing by 41 +/- 12% at 5 x 10(-7) M (p < 0.01) and decreasing by 21 +/- 13% at 5 x 10(-6) M (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of N(G)-nitro-L-arginine methyl ester on the hypotensive and hypertensive responses to 5-hydroxytryptamine in pithed rats

The potential ability of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME; 1 to 10 mg/kg), to modulate blood pressure responses to 5-hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) (DOI) was studied in pithed rats. The hypotensive responses to 5-HT in the presence of ketanserin were augmented by L-NAME as well as by phenylephrine infusion, both of which themselves increased blood pressure. L-NAME also tended to prolong the duration of the response of 5-HT. Likewise, the hypotensive responses to 5-CT were potentiated. The magnitude of the hypertensive responses to 5-HT was unaffected by L-NAME or phenylephrine. However, in contrast to phenylephrine, L-NAME prolonged the duration of these responses. The magnitude and duration (middle dose only) of the hypertensive responses to DOI were augmented by L-NAME, but phenylephrine was ineffective. These results suggest that L-NAME increases blood pressure, probably by inhibiting the basal release of NO in animals with a low vascular tone. However, the hypotensive responses to 5-HT and 5-CT seem to be largely independent of NO release by the endothelium, but the hypertensive responses to 5-HT and DOI appear to be limited by the release of NO in the pithed rat.

Endothelium-derived relaxing factor in brain blood vessels is not nitric oxide

BACKGROUND

The endothelium-derived relaxing factor that mediates the actions of acetylcholine is now most frequently identified as nitric oxide. Nitric oxide is believed to have numerous important regulating actions in neurons, blood vessels, and several other biological systems.

SUMMARY OF REVIEW

The literature concerning tissue other than cerebral blood vessels supports the conclusion that the endothelium-derived relaxing factor for acetylcholine is either nitric oxide or a compound formed from and containing nitric oxide (for example, a nitrosothiol). However, papers can be found indicating that this endothelium-derived mediator is not nitric oxide. In brain blood vessels the evidence is strongly against the conclusion that nitric oxide is the endothelium-derived mediator for acetylcholine. If this mediator is formed from nitric oxide, either in brain vessels or in other vessels, no data are available delineating how this synthesis is regulated or whether and where nitric oxide leaves the nitroso compound to initiate dilation. Indeed, cerebrovascular data now cast doubt on the commonly held belief that nitrosovasodilators regulate vascular tone by giving off nitric oxide to vascular smooth muscle.

CONCLUSIONS

In brain blood vessels the chemical identity of the endothelium-derived relaxing factor mediating the action of acetylcholine is unknown, but this relaxing factor does not appear to be nitric oxide. If the mediator contains nitric oxide, as is probably the case, the means by which it activates vascular guanylate cyclase and/or produces dilation is unknown. Since this relaxing factor inhibits platelet adhesion/aggregation in cerebral vessels as well as relaxing these vessels, the chemical identification of this relaxing factor and the elucidation of its mode of action are extremely important to our understanding and control of cerebrovascular phenomena in health and disease.

Role of nitric oxide from the endothelium on the neurogenic contractile responses of rabbit pulmonary artery

The effects of L-NG-nitro arginine (L-NO2Arg), a stereospecific inhibitor of nitric oxide formation, on the responsiveness of rabbit pulmonary artery to transmural electrical stimulation were studied. The contractile response evoked by electrical stimulation at 4 Hz was abolished by tetrodotoxin (10(-7) M) and depressed to approximately 10% by bunazosin (10(-6) M), an alpha 1-antagonist. Pretreatment with L-NO2Arg (10(-5) M) significantly potentiated the response to electrical stimulation without changing the resting tension. D-NO2Arg (10(-5) M) did not show such a potentiating action. In endothelium-denuded arteries, L-NO2Arg did not potentiate the response to electrical stimulation. The effect of L-NO2Arg on endogenous noradrenaline release in response to electrical stimulation was also examined by HPLC with electrochemical detection; L-NO2Arg did not affect noradrenaline release. The contractions induced by exogenous noradrenaline (10(-6)-10(-5) M) were enhanced by L-NO2Arg, but not by D-NO2Arg. These results suggest that the vasoconstriction induced by sympathetic nerve stimulation in the rabbit pulmonary artery is modulated by endogenous nitric oxide or nitric oxide-like substances released from endothelial cells.

Endothelial role in ouabain-induced contractions in guinea pig carotid arteries

The influence of endothelium on the direct contractile effects of ouabain in vascular smooth muscle was analyzed in isolated perfused guinea pig carotid arteries. After blocking the neurogenic component of the glycoside contraction with alpha-adrenergic receptor blocking drugs or treating the animals with reserpine, ouabain-induced contractions were markedly reduced in vessels with intact endothelium. However, removal of the vascular endothelium from reserpinized carotid arteries resulted in ouabain-induced contractions similar to those observed in control arteries. These effects were not mimicked by the inhibitor of nitric oxide NG-monomethyl L-arginine or by the cyclooxygenase blocker indomethacin. Bioassay experiments suggested that these endothelial effects are mediated by diffusible factors. Uptake of 86Rb to measure sodium pump activity was significantly reduced by removal of the endothelium. These results suggest the existence of an inhibitory modulation by the endothelium of contractions induced by ouabain, likely mediated by a diffusible factor (or factors) released from these cells. The nature of this substance is unknown, but it is neither related to prostaglandins nor a nitric oxide-related compound. Its mechanism of action could be the stimulation of vascular sodium pump activity, the antagonism of the pump's inhibition by ouabain, or both.

Role of nitric oxide and guanosine 3',5'-cyclic monophosphate in mediating nonadrenergic, noncholinergic relaxation in guinea-pig pulmonary arteries

1. Nonadrenergic, noncholinergic (NANC) nerves mediate vasodilatation in guinea-pig pulmonary artery (PA) by both endothelium-dependent and endothelium-independent mechanisms. The transmitter(s) involved in the endothelium-independent pathway have not yet been identified. We have therefore investigated the possibility that nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cyclic GMP) may mediate this neural vasodilator response in guinea-pig branch PA rings denuded of endothelium. 2. Electric field stimulation (EFS, 50 V, 0.2 ms) induced a frequency-dependent (1-24 Hz), tetrodotoxin-sensitive relaxation of the U44069-precontracted PA rings in the presence of adrenergic and cholinergic blockade. 3. The NO synthase inhibitors NG-monomethyl L-arginine (L-NMMA, 100 microM) and NG-nitro L-arginine methyl ester (L-NAME, 30 microM), and the guanylyl cyclase inhibitor methylene blue (5 microM) inhibited the EFS (16 Hz)-induced relaxation by 53 +/- 5, 74 +/- 9 and 82 +/- 9% respectively (n = 5-7, P < 0.01, compared with control rings). 4. Excess concentrations of L-, but not D-arginine (300 microM) completely reversed the inhibitory effect of L-NMMA. 5. The EFS-elicited relaxation (4 Hz) was potentiated by 1 microM zaprinast, a type V phosphodiesterase inhibitor which inhibits guanosine 3':5'-cyclic monophosphate (cyclic GMP) degradation, but was unaffected by 0.1 microM zardaverine, a type III/IV phosphodiesterase inhibitor which inhibits cyclic AMP degradation. 6. EFS (50 V, 0.2 ms, 16 Hz) induced a 3 fold increase in tissue cyclic GMP content, an action which was inhibited by L-NMMA (100 microM). 7. Pyrogallol (100microM), a superoxide anion generator, also inhibited the EFS-induced relaxation by 53 +/- 9%, and this effect was prevented by superoxide dismutase.8. Chemical sympathetic denervation with 6-hydroxydopamine had no effect on the relaxant response to EFS in the endothelium-denuded PA rings.9. In endothelium-denuded branch PA rings at resting tone, L-NMMA (100 microM) significantly augmented the adrenergic contractile response, an effect which was completely reversed by L-arginine,but not by D-arginine. In the same groups of vessel rings, L-NMMA had no significant effect on the matched contractile response to exogenous noradrenaline.10. These results suggest that NO may be released from intramural nerve endings other than adrenergic nerves (probably NANC nerves), and this leads to vasodilatation via activation of guanylyl cyclase.

Effect of nitric oxide blockade by NG-nitro-L-arginine on cerebral blood flow response to changes in carbon dioxide tension

The importance of nitric oxide (NO) for CBF variations associated with arterial carbon dioxide changes was investigated in halothane-anesthetized rats by using an inhibitor of nitric oxide synthase, NG-nitro-L-arginine (NOLAG). CBF was measured by intracarotid injection of 133Xe. In normocapnia, intracarotid infusion of 1.5, or 7.5, or 30 mg/kg NOLAG induced a dose-dependent increase of arterial blood pressure and a decrease of normocapnic CBF from 85 +/- 10 to 78 +/- 6, 64 +/- 5, and 52 +/- 5 ml 100 g-1 min-1, respectively. This effect lasted for at least 2 h. Raising PaCO2 from a control level of 40 to 68 mm Hg increased CBF to 230 +/- 27 ml 100 g-1 min-1, corresponding to a percentage CBF response (CO2 reactivity) of 3.7 +/- 0.6%/mm Hg PaCO2 in saline-treated rats. NOLAG attenuated this reactivity by 32, 49, and 51% at the three-dose levels. Hypercapnia combined with angiotensin to raise blood pressure to the same level as the highest dose of NOLAG did not affect the CBF response to hypercapnia. L-Arginine significantly prevented the effect of NOLAG on normocapnic CBF as well as blood pressure and also abolished its inhibitory effect on hypercapnic CBF. D-Arginine had no such effect. Decreasing PaCO2 to 20 mm Hg reduced control CBF to 46 +/- 3 ml 100 g-1 min-1 with no further reduction after NOLAG. Furthermore, NOLAG did not change the percentage CBF response to an extracellular acidosis induced by acetazolamide (50 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of perfusion with oxygenated Krebs-Ringer solution to preserve the eccrine function of the vascular endothelium in bone

An ex vivo canine tibia preparation was perfused at a constant rate with aerated (95% O2-5% CO2) Krebs-Ringer solution for 24 h. Bolus injections of norepinephrine (0.125-0.5 micrograms) were given and then acetylcholine (5 x 10(-5) M) was used to stimulate endothelial production of smooth muscle relaxing factors. Following 1 h of perfusion the addition of acetylcholine resulted in significant attenuation of the response to norepinephrine (p < 0.001). After 4 h perfusion acetylcholine did not attenuate the norepinephrine response, but addition of L-arginine (the precursor of endothelial-derived relaxing factor) resulted in significant attenuation in the presence of acetylcholine (p < 0.005). At 6, 12, and 24 h the acetylcholine did not attenuate the norepinephrine response. It is concluded that normothermic, continuous perfusion with oxygenated Krebs-Ringer solution results in normal endothelial eccrine activity up to 1 h. Following this period there is substrate depletion but endothelial eccrine function can be demonstrated for up to 4 h. At 6 h this function cannot be demonstrated, suggesting degradation of the functional integrity of the endothelium.

Nitric oxide mediates neurogenic relaxation induced in rabbit cavernous smooth muscle by electric field stimulation

We investigated the relaxant effect of electric field stimulation (EFS) on rabbit cavernous smooth muscle strips in vitro precontracted by phenylephrine. Effects of EFS were monitored alone, and following muscarinic receptor blockade, and inhibition of nitric oxide (NO) formation by L-N-monomethylarginine (L-NMMA) or by L-N-nitroarginine (L-NOARG). Atropine only slightly reduced the relaxant effect of EFS to 89.0 +/- 6.1 percent. Additional application of L-NMMA further reduced the relaxant effect to 37.3 +/- 15.3 percent. Substitution of L-NOARG for L-NMMA led to a more pronounced inhibition of relaxant effects to 16.2 +/- 8.7 percent. The results indicate that neurogenically induced relaxation of rabbit cavernous smooth muscle is mediated mainly by NO formation and argue against a substantial role of relaxing peptidergic neurotransmitters, such as vasoactive intestinal polypeptide and calcitonin-gene-related peptide, in penile erection.

Effect of nitro-L-arginine on endothelium-dependent hyperpolarizations and relaxations of pig coronary arteries

1. Endothelium-dependent relaxation is caused by an endothelium-derived relaxing factor (EDRF) identified as nitric oxide (NO). Our objective was to test whether one or several distinct endothelium-dependent relaxing factors exist. 2. In pig coronary arteries, a hyperpolarization accompanied by the relaxation caused by high concentrations of substance P (SP) and bradykinin (BK). 3. To examine the role played by nitric oxide and prostacyclin in the endothelium-dependent relaxations and hyperpolarizations caused by SP and BK on pig coronary arterial strips, the production of NO was inhibited by NG-nitro-L-arginine (L-NNA) and the production of prostacyclin was inhibited by indomethacin, while monitoring smooth muscle membrane potential and isometric tension. 4. Indomethacin had no effect on resting isometric tension nor on SP and BK relaxations of strips precontracted by prostaglandin F2 alpha. 5. L-NNA contracted arterial strips with intact endothelium, without changing the membrane potential of smooth muscles. 6. The inhibitor shifted to the right the concentration-response curve of kinins by 0.2 nM SP and 20 nM BK. It inhibited the maximal relaxations and hyperpolarizations by 30%. 7. The results show that, in pig coronary arteries, EDRF (NO) mainly controls the basal tension, whereas other factor(s) play(s) an important role in hyperpolarizations and relaxations caused by the kinins.

Iron-sulphur cluster nitrosyls, a novel class of nitric oxide generator: mechanism of vasodilator action on rat isolated tail artery

1. Two iron-sulphur cluster nitrosyls have been investigated as potential nitric oxide (NO.) donor drugs (A: tetranitrosyltetra-mu 3-sulphidotetrahedro-tetrairon; and B: heptanitrosyltri-mu 3-thioxotetraferrate(1-)). Both compounds are shown to dilate precontracted, internally-perfused rat tail arteries. 2. Bolus injections (10 microliters) of compound A or B generate two kinds of vasodilator response. Doses below a critical threshold concentration (DT) evoke transient (or T-type) responses, which resemble those seen with conventional nitrovasodilators. Doses > DT produce sustained (or S-type) responses, comprising an initial, rapid drop of pressure, followed by incomplete recovery, resulting in a plateau of reduced tone which can persist for several hours. 3. T- and S-type responses are attenuated by ferrohaemoglobin (Hb) and by methylene blue (MB), but not by inhibitors of endothelial NO. synthase. Addition of either Hb or MB to the internal perfusate can restore agonist-induced tone when administered during the plateau phase of an S-type response. Moreover, subsequent removal of Hb causes the artery to re-dilate fully. 4. We conclude that T- and S-type responses are both mediated by NO.. It is postulated that S-type responses represent the sum of two vasodilator components: a reversible component, superimposed upon a non-recoverable component. The former is attributed to free NO., preformed in solution at the time of injection; and the latter to NO. generated by gradual decomposition of a 'store' of iron-sulphur-nitrosyl complexes within the tissue. This hypothesis is supported by histochemical studies which show that both clusters accumulate in endothelial cells.

Colony-stimulating factors activate human macrophages to inhibit intracellular growth of Histoplasma capsulatum yeasts

Recombinant cytokines and colony-stimulating factors (CSFs) were tested for their abilities to activate human monocytes/macrophages (M phi) to inhibit the intracellular growth of or kill Histoplasma capsulatum yeasts. None of the cytokines or CSFs or combinations of cytokines and CSFs activated M phi fungistatic activity when they were added to M phi monolayers concurrently with yeasts. In contrast, culture of monocytes for 7 days in the presence of interleukin 3, granulocyte-M phi CSF, or M phi CSF stimulated M phi fungistatic (but not fungicidal) activity against H. capsulatum yeasts in a concentration-dependent manner. Optimal activation of M phi by CSFs required 5 days of coculture, and the cultures had to be initiated with freshly isolated peripheral blood monocytes. Culture of monocytes with combinations of CSFs or addition of CSFs during the 24 h of coculture with the yeasts did not further enhance M phi fungistatic activity for H. capsulatum. Addition of gamma interferon or tumor necrosis factor alpha to CSF-activated M phi also did not enhance M phi fungistatic activity. These results suggest that interleukin 3, granulocyte-M phi CSF, and M phi CSF may play a role in the cell-mediated immune response to H. capsulatum by enhancing monocyte/M phi fungistatic activity.

The role of nitric oxide in the vascular hyporesponsiveness to methoxamine in portal hypertensive rats

This study examined whether an increased activity of the endothelium-derived relaxing factor, nitric oxide, may account for the hyporesponsiveness to vasoconstrictors in portal hypertension. We performed dose-response curves to methoxamine, an alpha-adrenoceptor agonist, with and without N omega-nitro-L-arginine, a specific inhibitor of nitric oxide synthesis, in experimental portal hypertension. Partial portal vein-ligated or sham-operated rats were pretreated with a continuous intravenous infusion of either N omega-nitro-L-arginine (50 micrograms.kg-1.min-1) or saline. Thirty minutes after starting the infusion of N omega-nitro-L-arginine or saline an infusion of methoxamine (10, 30 and 100 micrograms.kg-1.min-1) was added. Total peripheral resistance was calculated from mean arterial pressure and cardiac index. Repeated measurements of cardiac index were performed by a thermodilution technique. In portal vein-ligated rats pretreated with saline, the increase in total peripheral resistance after methoxamine infusion was significantly less than that of sham-operated rats (0.2 +/- 0.1 vs. 1.0 +/- 0.3, 0.6 +/- 0.1 vs. 1.6 +/- 0.3 and 3.7 +/- 0.5 vs. 6.1 +/- 0.7 mm Hg.ml-1.min.100 gm, p less than 0.05, methoxamine 10, 30 and 100 micrograms.kg-1.min-1, respectively). In the presence of N omega-nitro-L-arginine, the change in total peripheral resistance after methoxamine infusion was similar in both groups (p greater than 0.05). In conclusion, this study demonstrates that a vascular hyporesponsiveness to methoxamine is present in portal vein-ligated rats and that this hyporesponsiveness is reversed by blockade of nitric oxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnesium sulfate therapy in preeclampsia is associated with increased urinary cyclic guanosine monophosphate excretion

OBJECTIVE

Our objective was to determine if maternal urinary cyclic guanosine monophosphate levels are altered in preeclampsia.

STUDY DESIGN

Aliquots from 24-hour urine samples collected from 57 women with preeclampsia and 14 normotensive pregnant women in the third trimester of pregnancy were assayed for urinary cyclic guanosine monophosphate. Urinary cyclic guanosine monophosphate values were expressed per milligram of urinary creatinine to standardize for renal function.

RESULTS

There was no difference in gestational age at time of urine collection between the two groups. Urinary cyclic guanosine monophosphate levels (mean +/- SD) were similar between normotensive and preeclamptic pregnant women (751 +/- 498 vs 632 +/- 363 pmol/mg urinary creatinine, respectively, p = 0.12). Preeclamptic women receiving magnesium sulfate had significantly higher levels of urinary cyclic guanosine monophosphate than those not receiving magnesium sulfate (786 +/- 360 vs 555 +/- 344 pmol/mg urinary creatinine, respectively, p = 0.02).

CONCLUSIONS

These preliminary results indicated that cyclic guanosine monophosphate excretion increases in patients with preeclampsia during magnesium sulfate infusion. The vascular smooth muscle relaxation effects of magnesium sulfate may be mediated by directly increasing cyclic guanosine monophosphate production or indirectly through endothelium-derived relaxing factor.

Nitric oxide is a mediator of hypoxic coronary vasodilatation. Relation to adenosine and cyclooxygenase-derived metabolites

Hypoxia is a potent coronary-vasodilating signal; its mechanisms are still controversial. We have assessed the possible role of nitric oxide (NO) in hypoxic coronary vasodilatation (HCVD) in isolated guinea pig hearts perfused at constant pressure. HCVD was elicited by a 1-minute 100% N2 exposure; coronary flow doubled within 1 minute of hypoxia (early phase) and returned to baseline within 40 seconds after reoxygenation (late phase). The early phase of HCVD was associated with a rapid approximately eightfold increase in cGMP overflow, an indication of NO release. The specific NO synthase inhibitor N omega-methyl-L-arginine (NMA, 0.1-1 mM) antagonized HCVD and the associated increase in cGMP spillover (maximum inhibition, approximately 65%); excess arginine (1.2 mM) prevented both effects. The late phase of HCVD was associated with an increase in adenosine overflow and was attenuated by the adenosine receptor antagonist BW A1433 (1 microM; maximum inhibition, approximately 45%). Indomethacin (10 microM) inhibited HCVD in spontaneously beating hearts by approximately 35% but had no effect in hearts paced at faster rates. NMA and BW A1433 were more effective in combination than alone (maximum inhibition, approximately 72%). However, irrespective of the concentrations used, there was no synergism among the anti-HCVD effects of NMA, BW A1433, and indomethacin, nor was HCVD completely inhibited by the antagonists, whether alone or in combination. Our findings indicate that NO is an important mediator of the early phase of HCVD, whereas additional mechanisms and/or factors, including adenosine and vasodilatatory prostaglandins, contribute to the late phase.

Effects of nitric oxide synthase inhibitors, L-NG-nitroarginine and L-NG-nitroarginine methyl ester, on responses to vasodilators of the guinea-pig coronary vasculature

1. The effects of L-NG-nitroarginine (L-NOARG) and L-NG-nitroarginine methyl ester (L-NAME) on vasodilatation induced by ATP, substance P, 5-hydroxytryptamine (5-HT), bradykinin and sodium nitroprusside (SNP) were examined in the guinea-pig coronary bed, by use of a Langendorff technique. The effects of these inhibitors of nitric oxide synthesis were assessed on their ability to inhibit both the amplitude and the area of the vasodilator response. 2. The vasodilator responses evoked by low doses of 5-HT (5 x 10(-10)-10(-8) mol) were almost abolished by L-NAME and L-NOARG (both at 10(-5), 3 x 10(-5) and 10(-4) M), although L-NOARG (3 x 10(-5) M) was significantly less potent than L-NAME (3 x 10(-5) M) as an inhibitor of vasodilator responses to 5-HT (5 x 10(-8) mol). 3. The vasodilator responses evoked by substance P (5 x 10(-12)-5 x 10(-9) mol) were reduced in the presence of L-NAME and L-NOARG (both at 10(-5) and 3 x 10(-5) M). The response to substance P was almost abolished by L-NAME and L-NOARG (both at 10(-4) M). 4. The amplitude of the vasodilator responses to ATP (5 x 10(-11) and 5 x 10(-9)-5 x 10(-7) mol) was little affected by either L-NAME or L-NOARG (both at 10(-5), 3 x 10(-5) and 10(-4) M).7. It is concluded that in the guinea-pig coronary vasculature, the vasodilatation evoked by substance P and low doses of 5-HT is mediated almost exclusively via nitric oxide, whereas the vasodilatations evoked by ATP and bradykinin appear to involve other mechanisms in addition to the release of nitric oxide. L-NAME was a more effective agent than L-NOARG in inhibiting the vasodilator actions of 5-HT and ATP in this preparation.

Endothelium-derived relaxing factor inhibition augments vascular angiotensin II reactivity in the pregnant rat hind limb

OBJECTIVE

The purpose of our study was to determine whether endothelium-derived relaxing factor plays a role in the blunting of maternal vascular reactivity in pregnancy.

STUDY DESIGN

We measured the concentration-pressor responses to norepinephrine (10(-8) to 10(-4) mol/L and angiotensin II (10(-10) to 10(-6) mol/L) in isolated, perfused hind limbs of nonpregnant and pregnant (postmating day 20 to 21) normotensive Wistar-Kyoto and spontaneously hypertensive rats. The hind limbs were perfused at 4 ml/min with Krebs-Ringer solution containing indomethacin (10(-5) mol/L to inhibit prostaglandin production and were either infused with N omega-monomethyl-L-arginine (10(-4) mol/L), a specific inhibitor of endothelium-derived relaxing factor synthesis, or 0.9% saline solution (untreated).

RESULTS

Baseline perfusion pressure was similar in the nonpregnant and pregnant hind limbs of both strains, and N omega-monomethyl-L-arginine had no effect on perfusion pressure. Norepinephrine induced similar pressor responses in the nonpregnant and pregnant hind limbs of both strains, and N omega-monomethyl-L-arginine did not alter these responses. Angiotensin II pressor responses were significantly attenuated in the pregnant rat hind limbs compared with the nonpregnant rat hind limbs. N omega-monomethyl-L-arginine enhanced the angiotensin II responses in the pregnant, but not in the nonpregnant, rat hind limbs.

CONCLUSION

The results suggest that rat pregnancy is not associated with generalized refractoriness to all vasoconstrictors and that endothelium-derived relaxing factor plays a role in attenuating vascular reactivity to angiotensin II.