Inhibitor Of Endothelium-derived Relaxing Factor Research Articles

A paradigm shift for local blood flow regulation

a remarkable example of physiological regulation is the coordination between metabolic rate and local blood flow in microscopic volumes of an organ. In skeletal muscle an increase in oxygen consumption (Vo2) over a wide range evokes a proportional (∼5 to 6 × Vo2) increase in blood flow ([29][

Vasoactive intestinal peptide (VIP)-induced pulmonary vasodilation mediated by EDRF/NO in isolated perfused rat lungs

We studied the effects of Vasoactive Intestinal Peptide (VIP) on the pulmonary circulation in isolated perfused rat lungs. VIP caused pulmonary vasodilation in a dose-dependent manner. This effect was inhibited by pretreatment with L-N omega-nitro-arginine (L-NNA), a competitive inhibitor of endothelium-derived relaxing factor (EDRF/NO), but not by meclofenamate, a cyclooxygenase inhibitor. Addition of L-arginine, a substrate of EDRF/NO, after treatment with L-NNA reversed VIP-induced pulmonary vasodilation. These results indicate that VIP causes pulmonary vasodilation, and they suggest a role for EDRF/NO in VIP-induced pulmonary vasodilation in isolated rat lungs.

Flow-induced release of endothelium-derived relaxing factor during pulsatile bypass: Experimental study in the fetal lamb

Objective: This study was initiated to test the hypothesis that fetal hemodynamic changes observed under pulsatile flow bypass might be related to the release of endothelium-derived relaxing factor through oscillating shear stress. Methods: Normothermic bypass was instituted in utero in 21 preterm fetal lambs for a 1-hour period through the right atrium and main pulmonary artery. Ultrasonic flowmeters were positioned around the descending aorta and the umbilical artery. The circuit consisted of an oxygenator and a pump set to either continuous flow ( n = 7) or pulsatile flow ( n = 7) and adjusted to maintain a fetal main arterial pressure of 50 mm Hg. In seven other animals, endothelium-derived relaxing factor was blocked by a continuous infusion of N ω-nitro- l-arginine after 30 minutes of pulsatile flow. Results: During the first 30 minutes of bypass, pump flows were significantly lower in the continuous-flow group than in the pulsatile-flow or blocked-flow groups (respectively, 612 ± 144, 907 ± 153 and 987 ± 228 ml/min), with similar changes in aortic and umbilical flows. Systemic vascular resistances were significantly lower in the pulsatile-flow and blocked-flow groups than in the continuous-flow group (550 ± 106 vs 821 ± 212 dynes/sec/cm −5). However, after blockade of endothelium-derived relaxing factor, resistances increased gradually in the blocked-flow group to reach the level of that of the continuous-flow group at the end of bypass (943 ± 77 vs 556 ± 143 dynes/sec/cm −5 in the pulsatile-flow group). Conclusions: Blockade of endothelium-derived relaxing factor after 30 minutes of pulsatile flow returns fetal hemodynamics to continuous flow conditions. The specific inhibitor of endothelium-derived relaxing factor used in this experiment suggests that nitric oxide may be released by fetal endothelium during pulsatile bypass.(J Thorac Cardiovasc Surg 1997;114:738-45)

EDRF and norepinephrine-induced vasodilation in the canine coronary circulation

Experiments were performed to characterize the role of endothelium-derived relaxing factor (EDRF) in coronary vasodilation caused by norepinephrine. The circumflex or left anterior descending coronary artery was cannulated and pump perfused with constant pressure or constant flow in closed-chest anesthetized dogs. Prostaglandin synthesis was blocked with ibuprofen. During constant-pressure perfusion, EDRF inhibition with intracoronary N omega-nitro-L-arginine (L-NNA) did not affect the vasodilation due to nitroglycerin (an endothelium-independent process). However, L-NNA did significantly inhibit (P < 0.001) the vasodilation due to acetylcholine (an endothelium-dependent process). In response to bolus injections of norepinephrine, EDRF inhibition with L-NNA significantly reduced coronary vasodilation (P < 0.001). This inhibition was partially reversed with L-arginine infusion. To determine whether an increase in shear stress due to an increase in flow was the stimulus for EDRF release, experiments were performed during constant-flow conditions. Vasodilation caused by nitroglycerin was not inhibited by L-NNA, but EDRF inhibition did blunt acetylcholine-induced vasodilation significantly (P < 0.001) during constant-flow perfusion. During EDRF inhibition with L-NNA, vasodilation due to norepinephrine was not significantly altered when coronary flow was held constant (P = 0.19). In conclusion, EDRF plays a role in norepinephrine-induced coronary vasodilation that is largely flow dependent.

Endothelium-derived relaxing factor is important in mediating the high output state in chronic severe anemia

We evaluated the endothelial and vascular smooth muscle function in patients with chronic severe anemia to determine whether increased basal nitric oxide levels contribute to the systemic vasodilation and high cardiac output seen in these patients. Patients with chronic severe anemia have a high output state due to a low systemic vascular resistance. However, the cause of the low vascular resistance is unclear. Because hemoglobin is a potent inhibitor of endothelium-derived relaxing factor, we postulated that in chronic severe anemia, low circulating hemoglobin results in reduced inhibition of endothelium-derived relaxing factor. The basal endothelium-derived relaxing factor activity therefore increases, and this contributes significantly to the low systemic vascular resistance and the hyperdynamic state seen in this condition. Hemodynamic variables and forearm blood flow (using plethysmography) were measured in eight patients with chronic severe anemia before (hematocrit 16 +/- 2% [mean +/- SD]) and within 24 h of red blood cell transfusion (n = 6, hematocrit 30 +/- 1%) and in six control subjects. The effect on baseline blood flow of blocking endothelium-derived relaxing factor activity with NG-monomethyl-L-arginine was investigated. In addition, the effects of both endothelium-dependent and endothelium-independent vasodilators on forearm blood flow were tested. Baseline forearm blood flow was markedly increased in untreated patients (6.5 +/- 1.2 ml/min per 100 ml) compared with that in control subjects (2.8 +/- 0.7 ml/min per 100 ml, p < 0.0001, 95% confidence interval [CI] for difference -5 to -2.5). Red blood cell transfusion significantly reduced blood flow in the anemic patients to 3.5 +/- 1.1 ml/min per 100 ml (p < 0.001, 95% CI for difference -4.9 to -1.9), which was not significantly different from that in control subjects; increased systemic vascular resistance (796 +/- 141 to 1,230 +/- 151 dynes.s.cm-5, p < 0.001); and decreased cardiac output (4.9 +/- 0.6 to 3.5 +/- 0.5 liters/min per m2, p < 0.001). NG-monomethyl-L-arginine (16 mumol/min), a specific inhibitor of endothelium-derived relaxing factor, reduced forearm blood flow by an equal amount (p = 0.9, 95% CI for difference -0.7 to 0.8) in control subjects (0.98 +/- 0.39 ml/min) and treated patients (1.03 +/- 0.8 ml/min) but caused a threefold greater decrease in flow (2.9 +/- 0.9 ml/min) in untreated patients (p = 0.0003, 95% CI for difference between untreated patients and control subjects 1.1 to 2.7). These findings suggest increased basal endothelium-derived relaxing factor activity in patients with anemia. Stimulated forearm blood flows (both endothelium dependent and endothelium independent) were similar in all groups, confirming normal endothelial and smooth-muscle function. These findings support the hypothesis that enhanced basal endothelium-derived relaxing factor activity makes an important contribution to the low systemic vascular resistance in chronic severe anemia.

Role of EDRF in the cardiopulmonary dysfunction produced by massive sympathetic activation.

This study was undertaken to determine whether endothelium-derived relaxing factor (EDRF) modulates the pulmonary and systemic hemodynamic responses to massive sympathetic nervous system (SNS) activation and, in so doing, also modulates the degree of SNS-induced left ventricular (LV) dysfunction and the likelihood for pulmonary edema formation. The SNS of 13 anesthetized untreated rabbits and 14 anesthetized rabbits pretreated with the EDRF inhibitor, N omega-nitro-L-arginine (L-NNA, 20 mg/kg), was massively activated with an intracisternal injection of veratrine. Pulmonary and systemic arterial pressures increased to the same extent in both groups, but LV end-diastolic pressure was significantly lower in untreated rabbits. During this time, cardiac output decreased by 37% in L-NNA pretreated rabbits compared with 8% in untreated animals. Peak systemic and pulmonary vascular resistances increased significantly in L-NNA rabbits, whereas only systemic vascular resistance increased significantly in untreated rabbits. However, this increase in systemic vascular resistance was threefold less than that observed for L-NNA-treated animals. Although the degree of LV dysfunction was greater in the L-NNA rabbits, pulmonary edema developed less frequently in this group. We suggest that when EDRF release is inhibited during massive SNS activity, pulmonary vascular resistance increases markedly, which causes the right ventricle to fail. We further suggest that the reduced right ventricular output maintains pulmonary microvascular pressure below levels required for edema development.

Effects of Volatile Anesthetics on Acetylcholine-induced Relaxation in the Rabbit Mesenteric Resistance Artery

Vascular endothelium plays an important role in the regulation of vascular tone. Volatile anesthetics have been shown to attenuate endothelium-mediated relaxation in conductance arteries, such as aorta. However, significant differences in volatile anesthetic pharmacology between these large vessels and the small vessels that regulate systemic vascular resistance and blood flow have been documented, yet little is known about volatile anesthetic action on endothelial function in resistance arteries. Furthermore, endothelium-dependent relaxation mediated by factors other than endothelium-derived relaxing factor (EDRF) has recently been recognized, and there is no information available regarding volatile anesthetic action on non-EDRF-mediated endothelium-dependent relaxation. Employing isometric tension recording and microelectrode methods, the authors first characterized the endothelium-dependent relaxing and hyperpolarizing actions of acetylcholine (ACh) in rabbit small mesenteric arteries, and tested the sensitivities of these actions to EDRF pathway inhibitors and K+ channel blockers. They then examined the effects of the volatile anesthetics isoflurane, enflurane, and sevoflurane on ACh-induced endothelium-dependent relaxation that was sensitive to EDRF inhibitors and that which was resistant to the EDRF inhibitors but sensitive to blockers of ACh-induced hyperpolarization. The effects of the volatile anesthetics on endothelium-independent sodium nitroprusside (SNP)-induced relaxation were also studied. Acetylcholine concentration-dependently caused both endothelium-dependent relaxation and hyperpolarization of vascular smooth muscle. The relaxation elicited by low concentrations of ACh (< or = 0.1 microM) was almost completely abolished by the EDRF inhibitors NG-nitro-L-arginine (LNNA), oxyhemoglobin (HbO2), and methylene blue (MB). The relaxation elicited by higher concentrations of ACh (> or = 0.3 microM) was only attenuated by the EDRF inhibitors. The remaining relaxation, as well as the ACh-induced hyperpolarization that was also resistant to EDRF inhibitors, were both specifically blocked by tetraethylammonium (TEA > or = 10 mM). Sodium nitroprusside, a NO donor, produced dose-dependent relaxation, but not hyperpolarization, in the endothelium-denuded (E[-]) strips, and the relaxation was inhibited by MB and HbO2, but not TEA (> or = 10 mM). One MAC isoflurane, enflurane, and sevoflurane inhibited both ACh relaxation that was sensitive to the EDRF inhibitors and the ACh relaxation resistant to the EDRF inhibitors and sensitive to TEA, but not SNP relaxation (in the E[-] strips). An additional finding was that the anesthetics all significantly inhibited norepinephrine (NE) contractions in the presence and absence of the endothelium or after exposure to the EDRF inhibitors. The results confirm that ACh has a hyperpolarizing action in rabbit small mesenteric resistance arteries that is independent of EDRF inhibitors but blocked by the K+ channel blocker TEA. The ACh relaxation in these resistance arteries thus appears to consist of distinct EDRF-mediated and hyperpolarization-mediated components. Isoflurane, enflurane, and sevoflurane inhibited both components of the ACh-induced relaxation in these small arteries, indicating a more global depression of endothelial function or ACh signaling in endothelial cells, rather than a specific effect on the EDRF pathway. All these anesthetics exerted vasodilating action in the presence of NE, the primary neurotransmitter of the sympathetic nervous system, which plays a major role in maintaining vasomotor tone in vivo. This strongly indicates that the vasodilating action of these anesthetics probably dominates over their inhibitory action on the EDRF pathway and, presumably, contributes to their known hypotensive effects in vivo. Finally, the vasodilating action of these anesthetics is, at least in part, independent from endothelium.

Corelease of nitric oxide and prostaglandins mediates flow-dependent dilation of rat gracilis muscle arterioles

We have studied the mechanisms responsible for the mediation of flow (shear stress)-induced dilation of isolated arterioles of rat gracilis muscle. Active diameter of arterioles at a constant perfusion pressure (PP, 80 mmHg) was approximately 92 microns, while their passive diameter (Ca(2+)-free solution) was approximately 165 microns. At a constant PP the stepwise increase in flow of the perfusion solution (PS, 0-60 microliters/min in 10-microliters/min steps) elicited a gradual increase in diameter up to approximately 140 microns. Flow-induced dilations were eliminated by the removal of the endothelium of arterioles (by air). Dilations were significantly reduced by the cyclooxygenase blocker, indomethacin (Indo, 10(-5) M), by the nitric oxide synthase blocker, N omega-nitro-L-arginine (L-NNA, 10(-4) M), or by the endothelium-derived relaxing factor inhibitor, oxyhemoglobin (Hb, 10(-5) M), as indicated by the significant changes in the slope of the regression lines of the flow-diameter curves. For example, during administration of the inhibitors, dilation to 60 microliters/min perfusate flow was reduced by 41.1, 54.3, and 39.3%, respectively. Combined application of Indo and L-NNA almost completely eliminated flow-induced dilation. Arteriolar dilation maintained calculated wall shear stress close to control values (approximately 30 dyn/cm2 at 60 microliters/min) despite increases in flow, but when the dilation was inhibited by removal of the endothelium or by the combined administration of Indo and L-NNA, wall shear stress was greatly increased as a function of increases in flow of the PS (approximately 125 dyn/cm2).(ABSTRACT TRUNCATED AT 250 WORDS)

"Vasoactivators"--a new concept for naturally secreted vasoconstrictor substances.

Many vasoconstrictor substances have been demonstrated as being vasodilators through the mechanism of endothelium-derived relaxing factor (EDRF) release. The authors have hypothesized that all naturally secreted vasoconstrictor substances may potentially be vasodilators as well in order to maintain an adequate physiological vascular tone. To test this hypothesis, they studied the effect of thromboxane A2--a naturally secreted vasoconstrictor substance released from platelets, which has not been demonstrated as an EDRF stimulus, using its stable analog U46619 in the porcine coronary artery (CA). The results have shown that U46619 is a stimulus for EDRF biosynthesis/release. This effect may be blocked by the specific inhibitor of EDRF biosynthesis/release NG-nitro-L-arginine (L-NNA). Therefore, this study supports the aforementioned hypothesis. The authors suggest that naturally secreted vasoconstrictor substances may be more accurately termed "vasoactivator" substances. These vasoactivators, according to the balance between their constrictive or relaxing effects, may be classified as (1) Type I: vasoconstriction-predominant type such as acetylcholine (ACh), and U46619 in porcine which mainly cause contraction in endothelium-intact arteries; (2) Type II: balanced type such as norepinephrine (NE) and 5-hydroxytryptamine (5-HT) in porcine CA, which cause little contraction in endothelium-intact arteries but cause a great contraction when the endothelium is denuded. Usually, the receptor subtypes located in the endothelium and smooth muscle stimulated by any one of these vasoactivator substances are different. All these characteristics of vasoconstrictor substances may play an important role in maintaining an adequate vascular tone.

Platelets Increase the Tone of Quiescent Rat Aortic Rings by Release of Serotonin and Potentiate the Subsequent Contractile Response to Norepinephrine

To examine the effect of platelets on the tone of quiescent vessels, rat aortic rings with intact endothelium and others with denuded endothelium were exposed to platelets in an organ bath. Suspension of washed platelets induced a mild but consistent contraction of rings with intact endothelium. The subsequent contractile response of these rings to norepinephrine (NE) was also potentiated. Platelets caused a marked contraction of quiescent rings without intact endothelium and potentiated the subsequent contractile effects of NE. The procontractile effect of platelets in rat aortic rings with and without intact endothelium was not modulated by the selective thromboxane A2 (TXA2)/endoperoxide receptor antagonist SQ29,548 or the cyclooxygenase inhibitor indomethacin, but was completely blocked by serotonin (S2) receptor antagonist LY53,857, suggesting that release of serotonin from aggregating platelets is perhaps more important than release of TXA2 in the procontractile effects of platelets on quiescent rat aortic rings. To examine the role of vascular endothelium in the procontractile effect of platelets further, we treated aortic rings with intact endothelium with the inhibitor of endothelium-derived relaxing factor (EDRF) NG-monomethyl-L-arginine (L-NMMA) or the adenine nucleotide scavenger apyrase. In rings treated with L-NMMA or apyrase, the contractile effect of platelets was enhanced (p < 0.01) and mimicked that in rings without intact endothelium. These observations indicate that although basal release of EDRF modulates the contractile effect of platelets, the dominant effect of platelets on quiescent rat aortic rings with intact endothelium is procontractile and this effect is mediated exclusively by release of serotonin.

Hypoxia-Induced Arteriolar Dilation and Endothelium-Derived Relaxing Factor in Normotensive and Hypertensive Rats

Systemic hypoxia (inspired oxygen of 10%) constricts large arterioles (A1) and dilates small arterioles (A3) in striated muscle. The possible role of endothelium-derived relaxing factor (EDRF) or prostaglandins in mediating these responses was examined, as were the arteriolar response to systemic hypoxia in 1K1C (1-kidney, 1-clip) hypertensive rats and EDRF involvement in this response. In the cremaster muscles of anesthetized rats, hypoxia-induced A1 constriction was similar in normotensive and hypertensive animals and was not affected by the blockade of prostaglandin synthesis with mefenamic acid or by EDRF inhibition with hydroquinone (an EDRF inactivator) or L-Nω-nitro-arginine (L-NNA) (an EDRF synthesis inhibitor). A3 dilation in normotensive animals was unaffected by mefenamic acid, but was significantly (p < 0.05) attenuated by hydroquinone and L-NNA. Arteriolar dilation during systemic hypoxia was significantly less (p < 0.05) in the hypertensive compared with the normotensive groups and not alter...

EDRF(NO)-Mediated Modulation of Collagen-Induced Platelet Accumulation in Rat Pulmonary Microcirculation.

Endothelium-derived relaxing factor (EDRF) or nitric oxide (NO) biosynthesis from L-arginine occurs in the endothelium and platelets and may modulate platelet function and contribute to thromboresistance in the vessel wall. A rat model was used to evaluate selective accumulation of (III)In-labeled platelets in the pulmonary microcirculation following the administration of collagen, adenosine 5'-diphosphate (ADP) or thrombin. Platelet aggregation was monitored continuously over the thorax using a microcomputer-based system. Sodium nitroprusside, a stimulator of soluble guanylate cyclase and zaprinast, a phosphodiesterase V inhibitor, both known to cause accumulation of cyclic guanosine monophosphate, exhibited moderate inhibitory activity, which was shared by L-arginine. N(G)-monomethyl-L-arginine (L-NMMA; 1 mg/kg/min), an inhibitor of EDRF(NO), potentiated the aggregatory response to collagen at an intravenous dose of 100 &mgr;g/kg but not at one of 30 &mgr;g/kg. D-NMMA had no such effect. The augmenting effect of L-NMMA was abolished by L-arginine. N(G)-nitro-L-arginine methyl ester (L-NAME; 0.1 mg/kg/min) also markedly augmented the collagen-induced platelet response, and, at higher doses, all treated animals died upon collagen challenge. Both L-NMMA and L-NAME did not affect the responses to ADP and thrombin. The results suggest that in the intact vascular system, basal releae of EDRF(NO) is not critically involved in modulation of platelet function but becomes a significant factor when platelets are exposed to great amounts of collagen fibrils. Copyright 1994 S. Karger AG, Basel

Endothelium-dependent and -independent responses to severe hypoxia in rat pulmonary artery.

The changes in pulmonary artery (PA) tone that develop during hypoxia are complex and appear to involve multiple vasoregulatory pathways. This study was designed to identify the phases of the response to severe hypoxia in isolated segments of rat PA and to evaluate the mechanisms involved. Exposure of preconstricted PA segments to severe hypoxia (i.e., PO2 < 3 Torr) resulted in transient relaxation followed by vigorous hypoxic vasoconstriction (HVC), which averaged 0.26 +/- 0.02 g (or 43.9 +/- 3.7% of the maximal force developed in response to phenylephrine). HVC was followed by sustained and nearly complete inhibition of existing tone. The first two phases of the response were endothelium dependent and could be blocked by nonspecific inhibitors of endothelium-derived relaxing factor (EDRF) activity and by 500 microM N omega-nitro-L-arginine, a specific inhibitor of nitric oxide (NO) formation. The transient early relaxation could also be inhibited by superoxide dismutase (50 U/ml). In contrast, the late relaxation phase was neither endothelium dependent nor could it be blocked by inhibitors of EDRF/NO activity. Rat PA segments were relaxed by pinacidil and lemakalim, both of which activate ATP-sensitive potassium channels. Preincubation with glibenclamide, a selective inhibitor of these channels, blocked this response and also reduced late hypoxic vasodilation by nearly 40% (P < 0.001). These findings demonstrate that the response of rat PA to hypoxia involves both endothelium-dependent and endothelium-independent components. The initial relaxation results from enhanced EDRF/NO activity, which is likely due to diminished breakdown by superoxide radicals.(ABSTRACT TRUNCATED AT 250 WORDS)

The reactions of 3,5-dibromo-4-nitrosobenzenesulfonate and its biological applications.

Recently, 5,5-dibromo-4-nitrosobenzenesulfonate (DBNBS) has been applied to detect biological free radicals. However, DBNBS has various non-specific reactions which lead to preplexing results. Thus, we investigated some basic reactions of DBNBS in combination of other nitroso spin traps to assign DBNBS spin adducts derived from human platelets which presumably related to the endothelium-derived relaxing factor (EDRF). The collagen activated platelets yielded four spin adducts (ST, LT, SS, and LS) in the presence of DBNBS (40 mM). The broad triplet due to ST was also observed by bubbling NO gas into a DBNBS solution. To identify ST, nitrosobenzene (NB) in dry dioxane was mixed with NO-saturated dioxane. The NB-NO spin adduct was observed but decomposed into diphenyl aminoxyl by the addition of H2O indicating that the primary adduct formed by the reaction of NO and DBNBS is unstable and turns into a dimerization product. Although ST could be eliminated by the inhibitor of EDRF, ST was shown to be produced by non-specific reactions. Another triplet was assigned to an S-centered radical because thiyl radicals which were generated from either the decomposition of S-nitrosothiol, or glutathione oxidation exhibited almost identical triplet signals. The other two sextets were assigned to C-centered radical adducts. Thus, DBNBS detected NQ-related, S-centered, and two C-centered radicals derived from human platelets. Special cautions are necessary for the identification of DBNBS spin adducts in a biological system to exclude artifactual radicals.

Receptor Mechanism of Thrombin-Induced Endothelium-Dependent and Endothelium-Independent Coronary Vascular Effects in Dogs

We previously reported that thrombin produces endothelium-dependent relaxation and endothelium-independent constrictions in canine coronary arteries. To determine whether these opposing vascular effects of thrombin are mediated by the same receptor mechanism, but at different cell types, we investigated the effects of thrombin receptor agonist peptide (TRAP) on isolated canine coronary arteries with and without intact endothelium. In coronary arteries with intact endothelium, addition of 0.01-3.0 microM TRAP, a 14-amino acid residue peptide (SFLLRNPNDKYEPF) homologous to the newly exposed N-terminus after cleavage of the cloned human thrombin receptor, produced rapid, dose-dependent relaxation (Emax = -89.6 +/- 2.3%, n = 26). Threshold concentration was 0.03 microM, and IC50 value was 0.3 microM. Mechanical disruption of the endothelium completely abolished the TRAP-induced relaxation; instead a dose-dependent contraction was observed. Expressed as a percentage of the maximum 70 mM KCl-induced contraction, the maximum contraction observed with 3 microM TRAP was 62.0 +/- 4.1% (n = 32). Pretreatment of endothelium-intact coronary arteries with either 3 microM hemoglobin or 0.25 mM NG-monomethyl-L-arginine (L-NMMA), specific inhibitors of endothelium-derived relaxing factor or nitric oxide (EDRF/NO), also inhibited the relaxation and unmasked the constrictor effect. The pharmacokinetic characteristics of the opposing coronary vascular effects of TRAP are similar to those observed with thrombin, but specific thrombin inhibitors, such as hirudin and D-phenylalanyl-prolyl-L-arginine chloromethyl ketone (PPACK), which inhibit both thrombin relaxant and constrictor effects, had no effect on TRAP-induced responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic EDRF inhibition and hypoxia: effects on pulmonary circulation and systemic blood pressure.

It has been suggested that chronic hypoxic pulmonary hypertension results from chronic hypoxic inhibition of endothelium-derived relaxing factor (EDRF) synthesis. We tested this hypothesis by studying whether chronic EDRF inhibition by N omega-nitro-L-arginine methyl ester (L-NAME) would induce pulmonary hypertension similar to that found in chronic hypoxia. L-NAME (1.85 mM) was given for 3 wk in drinking water to rats living in normoxia or hypoxia. Unlike chronic hypoxia, chronic L-NAME treatment did not increase pulmonary arterial pressure. Cardiac output was reduced and mean systemic arterial pressure was increased by chronic L-NAME treatment. The vascular pressure-flow relationship in isolated lungs was shifted toward higher pressures by chronic hypoxia and, to a lesser degree, by L-NAME intake. In isolated lungs, vasoconstriction in response to angiotensin II and acute hypoxia and vasodilation in response to sodium nitroprusside were increased by chronic L-NAME treatment in normoxia and chronic hypoxia. Chronic hypoxia, but not L-NAME, induced hypertensive pulmonary vascular remodeling. Chronic supplementation with the EDRF precursor L-arginine did not have any significant effect on chronic hypoxic pulmonary hypertension. We conclude that the chronic EDRF deficiency state, induced by L-NAME, does not mimic chronic hypoxic pulmonary hypertension in our model. In addition, EDRF proved to be less important for basal tone regulation in the pulmonary than in the systemic circulation.

Endothelium-derived nitric oxide reduces baseline venous tone in awake instrumented rats

To determine whether nitric oxide, which is likely endothelium-derived relaxing factor (EDRF), modulates baseline venous tone, the effects of intravenous NG-monomethyl-L-arginine (L-NMMA) (3-25 mg/kg), an EDRF inhibitor, on mean circulatory filling pressure (MCFP) were determined in 10 awake instrumented rats. MCFP, the equilibrated systemic pressure occurring when the circulation is arrested by transient inflation of a balloon in the right atrium, is a measure of total venous capacitance. L-NMMA caused a dose-dependent increase in mean arterial pressure and a dose-dependent decrease in heart rate. MCFP rose from 6.6 +/- 0.2 to 7.6 +/- 0.2 mmHg at the highest L-NMMA dose. The effects of L-NMMA on MCFP were reversed with L-arginine. In an additional four rats, in which hexamethonium was administered to induce ganglionic blockade, L-NMMA (25 mg/kg) caused a similar increase in MCFP (4.1 +/- 0.6 to 5.0 +/- 0.7 mmHg, P = 0.22) during the ganglionic blocked state as during the control unblocked state. These findings suggest that nitric oxide, which is likely EDRF, reduces baseline venous tone.

Evidence for the participation of vascular endothelium originated humoral factors to cerebral vasospasm.

Vascular endothelium synthesizes mediators which either cause vasorelaxation or vasoconstriction. The purpose of this study is to investigate the role of these mediators in the pathogenesis of cerebral vasospasm. An inhibitor of endothelium-derived relaxing factor is haemoglobin; endothelium removal by triton X-100 and endothelin-1 caused narrowing in the diameter of basilar artery of the rabbit. The pathological, ultrastructural investigations and computerized tomography images are in accordance with this finding. These results indicate that not only an increase in vasoconstrictor mediator level but also the lack of endothelium-derived relaxing mediators are responsible of the spasm of the basilar artery.

Pulmonary and systemic vascular responsiveness to TNF-alpha in conscious rats.

Endotoxin decreases pulmonary vascular reactivity. Because tumor necrosis factor-alpha (TNF-alpha) is a primary mediator of endotoxemia, we tested whether TNF-alpha altered pulmonary vascular reactivity in conscious adult female rats. Osmotic pumps were implanted intraperitoneally, and low-dose TNF-alpha (62 micrograms, TNF62; n = 7), high-dose TNF-alpha (> or = 250 micrograms, TNF250; n = 5), or saline (n = 5) was administered for 2 wk. Pulmonary pressor responses to 14% O2 and angiotensin II (ANG II, 0.0206 micrograms/min for 10 min) were measured without (day 13) or after (day 14) administration of nitro-L-arginine (4.4 mg/kg iv), an inhibitor of endothelium-derived relaxing factor (EDRF). TNF-alpha administration slightly decreased (P < or = 0.08) baseline pulmonary arterial pressure in TNF250 rats and decreased (P < or = 0.05) hypoxia- and ANG II-induced constrictions in TNF62 and TNF250 rats. Whereas nitro-L-arginine potentiated (P < or = 0.05) pressure responses in control rats, it had no effect on hypoxic responses in TNF-alpha-treated rats. Nitro-L-arginine increased (P < or = 0.05) ANG II-induced vasoconstriction in TNF-alpha-treated rats, but the pulmonary arterial pressure response was still lower (P < or = 0.05) in TNF250 than in control and TNF62 rats. These results suggest that chronic TNF-alpha decreases 1) pulmonary vascular reactivity in the intact rat, 2) hypoxic pulmonary vasoconstriction by a mechanism that is independent of EDRF, and 3) ANG II-induced constriction by a mechanism that is partly EDRF dependent.

Triphasic response of rat intracerebral arterioles to increasing concentrations of vasopressin in vitro.

To determine how vasopressin affects the vascular tone of the smaller cerebral arterioles, we carried out an in vitro study of isolated and cannulated intracerebral arterioles of rats. We found that increasing concentrations of vasopressin induced a triphasic response of vasodilation (10(-12)-10(-11) M), vasoconstriction (10(-10)-10(-8) M), and vasodilation stabilizing to control diameter (10(-7)-10(-6) M) and that the maximum constriction was twice the maximum dilation in these smaller arterioles [21.2 +/- 13.1% (mean +/- SD) decrease in diameter vs. 11.2 +/- 5.7% increased]. Pretreatment of the arterioles with NG-monomethyl-L-arginine (10(-4) M), a specific inhibitor of endothelium-derived relaxing factor, abolished the vasopressin-induced vasodilation and significantly increased the vasoconstriction. These results suggest that these arterioles were maintained in a dilated state by an endothelium-derived relaxing factor activated by vasopressin. Both vasodilation and vasoconstriction were found to be mediated through vasopressin V1 receptors in a study of arterioles pretreated with d(CH2)5Tyr(Me)arginine vasopressin (10(-6) M), a vasopressin V1 receptor antagonist. These results support the hypothesis that vasopressin may constrict smaller cerebral arterioles while simultaneously dilating larger cerebral arteries. Our results also suggest that vasopressin may aggravate cerebral ischemia in pathological conditions, such as subarachnoid hemorrhage, when the arteriolar response to vasopressin shifts from vasodilation to vasoconstriction due to increased vasopressin levels in plasma and CSF and impaired endothelium-derived relaxation.