Shear stress-induced release of nitric oxide from endothelial cells grown on beads

Nitric oxide regulates cerebral arteriolar tone in rats

BACKGROUND AND PURPOSE

Although cerebral penetrating arterioles are main regulators of the brain microcirculation, little is known about the effect of endothelium-derived relaxation factor on these vessels. This study examined the effects of nitric oxide synthase inhibitors on the spontaneous tone of isolated rat cerebral arterioles.

METHODS

Intraparenchymal penetrating arterioles (53 to 102 microns in passive diameter) isolated from Sprague-Dawley rats were cannulated with glass pipettes and subjected to 60 mm Hg of intraluminal pressure. The diameter response to intraluminal and extraluminal treatments was observed with an inverted microscope.

RESULTS

Extraluminal application of Nw-nitro-L-arginine (10(-5) mol/L) contracted the arterioles to 63.9 +/- 2.8% (P < .05) of the control diameter. This contracting effect was stereospecific and easily reversed by L-arginine dose dependently (10(-3), 10(-2) mol/L) but not by D-arginine. Intraluminally applied Nw-nitro-L-arginine also induced a similar degree of contraction. Another nitric oxide synthase inhibitor, NG-monomethyl L-arginine (10(-5), 10(-4) mol/L), applied extraluminally induced a dose-dependent contraction to 77.5 +/- 6.6% and 68.6 +/- 5.4% of the control (P < .05), which was also reversed by L-arginine. L-Arginine alone did not significantly affect vessel diameter, however. Treatment with indomethacin, a cyclooxygenase inhibitor, dilated the vessel to 115.2 +/- 7% (P < .05) but did not change the constricting effect of Nw-nitro-L-arginine.

CONCLUSIONS

Nw-Nitro-L-arginine and NG-monomethyl L-arginine produce substantial contraction in isolated brain arterioles, suggesting that nitric oxide of brain arterioles is continuously produced within the vessel wall. The dilatory effect of indomethacin appears to be independent of the vasoconstriction induced by nitric oxide synthase inhibitor. In these vessels, the effect of nitric oxide synthase inhibitors is not mediated by an indomethacin-sensitive mechanism. A balance probably exists between factors tending to constrict these arterioles and the elaboration of nitric oxide from endothelial cells, which tends to dilate them. The production of nitric oxide from isolated vessels indicates that parenchymal and vessel wall sources of nitric oxide are probably important in brain microcirculatory regulation.

Ca(2+)-dependent and Ca(2+)-independent exhaled nitric oxide, presence in germ-free animals, and inhibition by arginine analogues

Nitric oxide (NO) was detected by chemiluminescence in exhaled air from awake humans, anaesthetized rabbits, guinea pigs, germ-free rats and conventional rats. Rabbits exhibited the highest concentrations, followed by guinea pigs, humans and rats. There was no significant difference between germ-free rats and control rats. The authenticity of NO was confirmed in cold-trap experiments. Intravenous administration of inhibitors of NO synthase (0.01-300 mg kg-1) to guinea pigs dose dependently reduced NO concentrations in exhaled air with the following potency order: L-N omega-nitro-arginine-methylester > asymmetric NG,NG-dimethyl-L-arginine-dihydrochloride = L-NG-mono-methyl -arginine = L-N5- (1-iminoethyl)-ornithine = aminoguanidine > L-canavanine. The effect of the NO synthase inhibitors was partly or fully reversed by L-arginine (1 g kg-1 i.v.), and L-arginine per se induced a significant increment of NO in exhaled air. In rats, L-N omega-nitro-arginine-methylester was considerably less potent than in guinea pigs. The concentration of NO in exhaled air increased 3-fold when changing from in situ blood auto-perfusion of rabbit lungs to in situ perfusion with saline medium. Addition of L-N omega-nitro-arginine-methylester to the saline perfusion medium evoked a reduction of NO concentrations in the air from the ventilated perfused lungs. Perfusion of lungs with Ca(2+)-free medium induced significant decrements in NO concentrations in exhaled air, an effect partly reversed upon reintroducing Ca2+ into the medium. In conclusion, NO was detected in exhaled air from humans and animals by chemiluminescence.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in flow-dependent vasomotor tone in spontaneously hypertensive rats

We studied the effect of endothelium on the flow-induced response of conductance arteries and the resistance arteriolar network in an in situ model of perfused mesenteric artery in normotensive Wistar-Kyoto and spontaneously hypertensive rats. The mesenteric network was perfused with a Tyrode's albumin solution. The diameter of a conductance mesenteric artery was measured using a video camera system, and mesenteric pressure was recorded in a collateral artery. The preparation was perfused at 0.2, 2, and 4 mL/min, and flow-diameter-pressure relations were established (1) under control conditions, (2) during local inhibition of nitric oxide synthesis by topical application of N omega-nitro-L-arginine methyl ester (L-NAME) (1 mmol/L), and (3) after endothelium removal (CO2 drying). In normotensive rats, L-NAME decreased conductance artery diameter by 12 +/- 2% (P < .01) at 0.2 mL/min and 3.3 +/- 1.9% (P < .05) at 2 mL/min. In hypertensive rats, L-NAME did not modify mesenteric diameter. Endothelium removal markedly increased arterial resistance in both strains and decreased conductance artery diameter in normotensive rats (10.3 +/- 3%, P < .05 at 0.2 mL/min and 4.2 +/- 2%, P < .05 at 2 mL/min) but not in hypertensive rats. The present study suggests that the endothelium plays a similar role in the control of mesenteric resistance in both strains and that there is a significant diameter-flow dependency affected by both endothelium removal and inhibition of nitric oxide synthesis in conductance mesenteric arteries from normotensive but not from hypertensive rats.

Role of Ca2+ and protein kinase C in shear stress-induced actin depolymerization and endothelin 1 gene expression

Vascular endothelial cells adapt to changes in blood flow by altering the cell architecture and by producing various substances. We have previously reported that low shear stress induces endothelin 1 (ET-1) expression in endothelial cells and that this induction is mediated by depolymerization of actin fiber. In the present study, we examined the role of Ca2+ and protein kinase C (PKC) in shear stress-induced actin depolymerization and subsequent ET-1 gene expression. Exposure of cultured porcine aortic endothelial cells to low shear stress (5 dyne/cm2) for 3 hours increased the ratio of G-actin to total actin from 54 +/- 0.8% to 80 +/- 1.0%. This shear stress-induced actin depolymerization was completely blocked by chelation of extracellular Ca2+ with EGTA and partially inhibited by intracellular Ca2+ chelation with the tetraacetoxymethyl ester of BAPTA (BAPTA/AM). Pretreatment with staurosporine, a PKC inhibitor, or desensitization of PKC by treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) for 24 hours also resulted in partial inhibition of shear stress-induced actin depolymerization. Although PKC activation by TPA mildly increased G-actin content, the effect of TPA and shear stress on actin depolymerization was not additive. Moreover, shear stress-induced ET-1 gene expression was inhibited by EGTA, BAPTA/AM, and staurosporine to a degree similar to the inhibition of actin depolymerization. In contrast, ET-1 gene expression induced by cytochalasin B, an actin-disrupting agent, was not affected by staurosporine.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic nitric oxide inhibition in utero produces persistent pulmonary hypertension in newborn lambs

Persistent pulmonary hypertension of the newborn (PPHN) is associated with chronic intrauterine events. Acute nitric oxide (NO) inhibition attenuates the normal increase in pulmonary blood flow at birth. We investigated whether chronic NO inhibition in utero causes persistent pulmonary hypertension. 11 fetal lambs received either a continuous infusion of N omega-nitro-L-arginine (an NO synthesis inhibitor) or 0.9% saline. Before infusion, acetylcholine (dependent upon endogenous NO production) and sodium nitroprusside (which releases its own NO) produced potent pulmonary vasodilation. After 10.5 +/- 1.5 d of infusion, acetylcholine did not produce pulmonary vasodilation in N omega-nitric-L-arginine-treated fetal lambs, but did in saline-treated fetal lambs; sodium nitroprusside produced pulmonary vasodilation in both groups. Immediately after birth, at 140 d of gestation, during the 3-h study period, mean pulmonary arterial pressure did not decrease in N omega-nitro-L-arginine-treated lambs; the increase in pulmonary blood flow and decrease in pulmonary vascular resistance were markedly attenuated compared to saline-treated lambs. These hemodynamic derangements were reversed by L-arginine. There were no anatomic abnormalities in the pulmonary circulation. Chronic NO inhibition in utero reproduces many of the physiologic derangements of PPHN. Intrauterine events which result in endothelial dysfunction and inhibition of NO may produce the physiologic derrangements of PPHN.

Pressure promotes DNA synthesis in rat cultured vascular smooth muscle cells

High blood pressure is one of the major risk factors for atherosclerosis. In this study, we examined the effects of pressure on cell proliferation and DNA synthesis in cultured rat vascular smooth muscle cells. Pressure without shear stress and stretch promotes cell proliferation and DNA synthesis in a pressure-dependent manner. Pressure-induced DNA synthesis was inhibited significantly by the phospholipase C (PLC) inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate, the protein kinase C inhibitor H-7, 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine, staurosporine, and the tyrosine kinase inhibitor ([3,4,5-trihydroxyphenyl]methylene)propanedinitrile. To clarify whether activation of PLC and calcium mobilization are involved in pressure-induced DNA synthesis, production of 1,4,5-inositol trisphosphate (IP3) and intracellular Ca2+ was measured. Pure pressure increased IP3 and intracellular Ca2+ in a pressure-dependent manner. The increases in both IP3 and intracellular Ca2+ were inhibited significantly by 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate. This study demonstrates a novel cellular mechanism whereby pressure regulates DNA synthesis in vascular smooth muscle cells, possibly via activation of PLC and protein kinase C.

Role of calcium and calmodulin in flow-induced nitric oxide production in endothelial cells

These experiments demonstrate that exposure of cultured endothelial cells (EC) to well-defined laminar fluid flow results in an elevated rate of NO production. NO production was monitored by release of NOx (NO2- + NO3(2-) and by cellular guanosine 3',5'-cyclic monophosphate (cGMP) concentration. NO synthase (NOS) inhibitor blocked the flow-mediated stimulation of both NOx and cGMP, indicating that both measurements reflect NO production. Exposure to laminar flow increased NO release in a biphasic manner, with an initial rapid production consequent to the onset of flow followed by a less rapid, sustained production. A similar rapid increase in NO production resulted from an increase in flow above a preexisting level. The rapid initial production of NO was not dependent on shear stress within a physiological range (6-25 dyn/cm2) but may be dependent on the rate of change in shear stress. The sustained release of NO was dependent on physiological levels of shear stress. The calcium (Ca2+) or calmodulin (CaM) dependence of the initial and sustained production of NO was compared with bradykinin (BK)-mediated NO production. Both BK and the initial production were inhibited by Ca2+ and CaM antagonists. In contrast, the sustained shear stress-mediated NO production was not affected, despite the continued functional presence of the antagonists. Dexamethasone had no effect on either the initial or the sustained shear stress-mediated NO production. An inducible NOS does not, therefore, explain the apparent Ca2+/CaM independence of the sustained shear stress-mediated NO production.(ABSTRACT TRUNCATED AT 250 WORDS)

Shear-dependence of endothelial functions

Endothelial cells are subjected to shear forces which influence important cell functions. Shear stress induces cell elongation and formation of stress fibers, increases permeability, pinocytosis and lipoprotein internalization, is involved in the formation of atherosclerotic lesions, increases the production of tissue plasminogen activator, and enhances von Willebrand factor release and hence platelet aggregation. It decreases adherence of erythrocytes and leukocytes, and increases the release of prostacyclin, endothelium derived relaxing factor, histamine and other compounds, but decreases erythropoietin secretion. The mechanism of signal transduction to the endothelial cell is not known exactly; shear-sensitive ion channels seem to be involved. It is concluded that a better understanding of shear-dependent endothelial functions will influence pathophysiologic concepts and therapeutic interventions.

Exercise-induced pulmonary vasoconstriction during combined blockade of nitric oxide synthase and beta adrenergic receptors

We studied the effects of inhibition of nitric oxide (NO) (endothelium-derived relaxation factor) synthase in combination with alpha and beta adrenergic receptor blockade on pulmonary vascular tone during exercise. In paired studies, we exercised sheep on a treadmill at a speed of 4 mph, and measured blood flow and pressures across the pulmonary circulation with and without inhibition of NO synthase (N omega-nitro-L-arginine 20 mg/kg intravenous [i.v.]), alpha receptor blockade (phentolamine 5 mg i.v.), beta receptor blockade (propranolol 1 mg i.v.), and combined alpha and beta receptor blockade. Activation of both types of adrenergic receptors occurs with exercise, and because increased release in NO is hypothesized to occur during exercise, these studies were designed to determine the magnitude of effect and interactions of these competing dilator and constrictor influences. We found that inhibition of NO synthase raised pulmonary vascular resistance (PVR) at rest and that, although a reduction in PVR occurred with exercise from this new baseline, vasoconstriction persisted. Combined beta blockade and NO synthase inhibition unmasked unopposed alpha vasoconstriction; PVR rose at rest and continued to rise with exercise; and mean pulmonary arterial pressures approached very high levels, 43.8 +/- 4.4 cmH2O. Using a distal wedged pulmonary artery catheter technique, most of the vasoconstriction was found to be in vessels upstream from small pulmonary veins. During exercise in sheep there appears to be a high degree of alpha and beta adrenergic-mediated tone in the pulmonary circulation. Endogenous production of NO actively dilates pulmonary vessels at rest and opposes potent alpha-mediated pulmonary vasoconstriction during exercise.

Modulation by the endothelium of sympathetic vasoconstriction in an in vitro preparation of the rat tail artery

1. The influence of the endothelium on transmural electrical stimulation was investigated in isolated and perfused segments of the rat tail artery. Noradrenaline release (NA, quantified by h.p.l.c.-electrochemical detection) and changes in perfusion pressure (PP, measured at constant flow rate) were simultaneously recorded in unstimulated and stimulated arterial segments, in the absence and in the presence of drugs. The ratio PP/NA release (mmHg pg-1) was taken as an index of the noradrenergic effectiveness. 2. Removal of the endothelium produced an increase in NA release and PP, in unstimulated and stimulated arteries. This can be taken as evidence of an endothelium-derived inhibitory factor (EDIF) acting at the prejunctional level, inhibiting NA release. Furthermore, in unstimulated arteries, the ratio PP/NA release decreased suggesting the existence of an endothelium-derived contracting factor (EDCF). 3. Perfusion of arteries with N omega-nitro-L-arginine methyl ester (L-NAME, 10 microM) or methylene blue (MeB, 0.5 microM) had no effect on PP or NA release in unstimulated arteries. In stimulated arteries, both drugs potentiated the increase in PP without changing NA release and therefore, led to an increase in noradrenergic effectiveness. After removal of the endothelium, neither L-NAME nor MeB affected the increases in PP and NA release following electrical stimulation. 4. Carbachol (1 microM) attenuated both NA release and the increase in PP during electrical stimulation, and increased the ratio PP/NA release. L-NAME and MeB did not modify the inhibitory effect of carbachol on NA release, or the facilitatory effect of carbachol on the noradrenergic effectiveness. 5. Angiotensin II (All, 0.1 MicroM) potentiated the increase in PP, without modifying NA release following electrical stimulation, and facilitated the vasoconstriction induced by perfusion of NA. In the absence of endothelium, All potentiated both the increase in PP and NA release in arteries stimulated electrically but had no effect on the vasoconstriction induced by perfusion of NA. This suggests an endothelium dependent activity of All in this preparation.6. These findings suggest that, in the rat tail artery, sympathetic vasoconstriction is modulated by three endothelial factors: (1) nitric oxide (NO), the release ot which seems NA-dependent; (2) EDCF,predominant in the unstimulated state, the release of which; can be stimulated by All; and (3) EDIF,unmasked by removal of the endothelial layer, the release of which can be stimulated by All.

Increased ambient pressure stimulates proliferation and morphologic changes in cultured endothelial cells

Very little is known about the effects of pressure within the vascular system on EC phenotype. To study this, bovine aortic EC were seeded on rat type I collagen plates (2,000/cm2) and allowed to attach for 24 hours. The cells were exposed to either atmospheric, 40, 80, or 120 mm Hg pressure by placing them in a plexiglass pressure chamber loaded with 5% CO2/air and maintained at 37 degrees C inside an incubator. Chamber pressure was continuously monitored with an amplified voltage transducer connected to a digital monitor. EC were maintained in DMEM supplemented with 10% calf serum and substrates for up to 9 days. The results indicate that EC proliferation is influenced by their ambient pressure. EC subjected in vitro to pressures comparable to mean systemic blood pressures had a significant increase in cell number compared to EC exposed to atmospheric pressures. EC were elongated and appeared to align randomly. We hypothesize that the systemic pressure which the endothelium is exposed to in vivo may have a significant regulatory influence on the ability of the endothelium to proliferate which may affect the endothelial cell response to injury.

Indomethacin does not modify the role of nitric oxide on blood pressure regulation of SHR

1. The endothelium-dependent relaxation is impaired in spontaneously hypertensive rats (SHR) by the release of a vasoconstrictor prostanoid. We evaluated whether such a vasoconstrictor prostanoid is masking the vasodilatation induced by nitric oxide (NO). 2. For this we observed, in SHR, whether indomethacin (INDO) modified both the pressor response to the inhibition of NO biosynthesis with L-nitro-arginine methyl ester (L-NAME) and the acute hypotensive response to acetylcholine. 3. INDO did not modify basal mean arterial pressure (MAP), either the pressor response to L-NAME, or the depressor response to acetylcholine. 4. It shows that, in awake SHR, a vasoconstrictor prostanoid, did not seem to affect the acute regulatory function of NO on MAP.

Negative feedback regulation of endothelial cell function by nitric oxide

The objective of this study was to determine whether nitric oxide (NO) could function as a negative feedback modulator of endothelial cell function by inhibiting NO synthase in vascular endothelial cells. The rationale for this approach was a previous study from this laboratory, which revealed that NO inhibits neuronal NO synthase from rat cerebellum. In the present study, NO and NO-donor agents noncompetitively inhibited NO synthase derived from bovine aortic endothelial cells. Oxyhemoglobin blocked the inhibitory action of NO and by itself increased NO synthase activity. This finding suggests that NO acts as a negative feedback modulator of NO synthase. In intact aortic endothelial cells grown on microcarrier beads and perfused in a bioassay cascade system, pretreatment of cells with NO-donor agents caused a marked inhibition of endothelial NO biosynthesis in response to bradykinin and increased fluid shear or flow. When isolated bovine pulmonary arterial rings precontracted by phenylephrine were used, pretreatment of arterial rings with NO-donor agents diminished endothelium-dependent arterial relaxation involving the L-arginine-NO pathway without altering endothelium-independent relaxation to NO itself. On the basis of these studies, NO is suggested to play an important negative feedback regulatory role on endothelial NO synthase and, therefore, vascular endothelial cell function.

Disruption of cytoskeletal structures mediates shear stress-induced endothelin-1 gene expression in cultured porcine aortic endothelial cells

Hemodynamic shear stress alters the architecture and functions of vascular endothelial cells. We have previously shown that the synthesis of endothelin-1 (ET-1) in endothelial cells is increased by exposure to shear stress. Here we examined whether shear stress-induced alterations in cytoskeletal structures are responsible for increases in ET-1 synthesis in cultured porcine aortic endothelial cells. Exposure of endothelial cells to 5 dyn/cm2 of low shear stress rapidly increased monomeric G-actin contents within 5 min without changing total actin contents. The ratio of G- to total actin, 54 +/- 0.8% in quiescent endothelial cells, increased to 87 +/- 4.2% at 6 h and then decreased. Following the disruption of filamentous (F)-actin into G-actin, ET-1 mRNA levels in endothelial cells also increased within 30 min and reached a peak at 6 h. The F-actin stabilizer, phalloidin, abolished shear stress-induced increases in ET-1 mRNA; however, it failed to inhibit increases in ET-1 mRNA secondary to other stimulants. This indicates that shear stress-induced increases in ET-1 mRNA levels may be mediated by the disruption of actin fibers. Furthermore, increases in ET-1 gene expression can be induced by actin-disrupting agents, cytochalasin B and D. Another cytoskeleton-disrupting agent, colchicine, which inhibits dimerization of tubulin, did not affect the basal level of ET-1 mRNA. However, colchicine completely inhibited shear stress- and cytochalasin B-induced increases in ET-1 mRNA levels. These results suggest that shear stress-induced ET-1 gene expression in endothelial cells is mediated by the disruption of actin cytoskeleton and this induction is dependent on the integrity of microtubules.

The dysfunctional endothelium in heart failure

Through release of paracrine and autocrine substances, the vascular endothelium exerts a profound influence on the contractile and growth state of underlying vascular smooth muscle. In heart failure, there is now compelling evidence that endothelium-mediated relaxation is attenuated in response to muscarinic stimulation in both the coronary and peripheral circulation of humans. This impaired response is present regardless of the etiology of heart failure. The mechanisms, significance and etiologic importance of this endothelial defect are not yet understood. Possibilities include 1) alterations in endothelial cell surface receptors or abnormalities of postreceptor signal transduction; 2) abnormalities of endothelium-derived relaxing factor production or release; 3) rapid inactivation of endothelium-derived relaxing factor; and 4) an increase in endothelium-derived contracting factor production and activity in heart failure. We currently have little understanding of the mechanisms accounting for this dysfunctional state of the endothelium. Future research efforts should be directed toward an understanding of these mechanisms.

Nitric oxide is a major mediator in reactive hyperaemia evoked by a brief coronary occlusion in the guinea pig heart

In the isolated perfused guinea pig heart coronary occlusion lasting 1-5 s is followed by reactive hyperaemia which is inhibited by 91-100% after pretreatment with a NO synthase inhibitor, NG-nitro-L-arginine (100 microM). Neither indomethacin (5 microM) nor 8-phenyltheophylline (10 microM) influences these responses. Therefore, the brief cardiac reactive hyperaemia seems to be predominantly if not entirely mediated by nitric oxide (NO).

Nitric oxide mediates renal vasodilation during erythropoietin-induced polycythemia

The renal blood flow (RBF) of patients with polycythemia rubra vera is increased despite the high hematocrit (Hct) which elevates the whole blood viscosity. Since blood viscosity determines the shear force on the endothelium which is a major stimulus to nitric oxide (NO) release, we investigated the hypothesis that renal vasodilation during erythropoietin-induced erythrocytosis is mediated by the L-arginine-NO pathway. Groups of Sprague-Dawley rats received thrice weekly injections of erythropoietin (E) for two to five weeks; responses were contrasted with normal rats (N) which received sham injections. The first group was studied after five weeks of erythropoietin injections which led to sharp increases in Hct (E: 72 +/- 3 vs. N: 44 +/- 1%) and mean arterial pressure (MAP: 126 +/- 3 vs. 107 +/- 3 mm Hg). These rats had an elevated basal RBF whether measured by the clearance and renal extraction of PAH or by a transit-time renal blood flow meter. Subsequent groups were studied after two to three weeks of erythropoietin which raised the Hct more modestly to 59 +/- 2%. In this group, the basal MAP was similar in E and N rats. Graded doses of the NO synthase inhibitor, N omega-monomethyl-L-arginine (L-NMA) led to a steeper rise in MAP in E than N; at the highest doses, the MAP had increased by 36 +/- 2 in E and 23 +/- 3 mm Hg in N (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Shear stress elevates endothelial cGMP. Role of a potassium channel and G protein coupling

BACKGROUND

The endothelium acts as the sensor of shear stress and as the mediator of flow-induced changes in vessel tone and structure. The purpose of this study was to delineate the signal transduction pathway of flow-induced release of endothelium-derived relaxing factor (EDRF).

METHODS AND RESULTS

We used a shear stress apparatus (a modified cone-plate viscometer) to expose cultured endothelial cells to a well-defined laminar fluid flow. Confluent bovine aortic endothelial cells (BAECs) were subjected to varying levels of shear stress, and intracellular cyclic GMP (cGMP) in the BAECs was measured by radioimmunoassay. After 60 seconds of laminar fluid flow, BAEC cGMP increased by 300% from basal levels (from 0.54 to 1.70 pmol/mg protein, P < 0.05). The elevation in intracellular cGMP was proportional to the intensity of shear stress within a physiological range up to 40 dynes/cm2. This increase in cGMP was abrogated by L-N-methyl-arginine (the competitive antagonist of nitric oxide [NO] synthase), indicating that the flow-induced activation of soluble guanylate cyclase was mediated by autocrine NO production. Furthermore, a potassium channel antagonist, tetraethylammonium ion (TEA [3 mmol/L]) and a G(i) or G(o) protein inhibitor, pertussis toxin (100 ng/mL) also blocked the flow-induced increase in cGMP. By contrast, calcium ionophore or atrial natriuretic peptide caused elevations of cGMP that were not affected by TEA or pertussis toxin.

CONCLUSIONS

These findings indicate that shear stress elevates endothelial cGMP via an NO-dependent mechanism. The effect of shear stress is mediated by a unique signal transduction pathway that is coupled to a pertussis toxin-sensitive G protein and that requires the activity of an endothelial potassium channel.

Role of shear stress and endothelial prostaglandins in flow- and viscosity-induced dilation of arterioles in vitro

We have studied the effect of changes in shear stress on diameter of isolated arterioles of rat cremaster muscle. The steady-state active diameter of arterioles at a constant perfusion pressure (60 mm Hg) was 80 +/- 1.2 microns. The vessels' passive diameter (Ca(2+)-free solution) was 156 +/- 1.8 microns. Changes in shear stress were induced either by an increase in flow (velocity) or by an increase in viscosity of the perfusion solution. At a constant perfusion pressure, the stepwise increase in perfusion flow (0-80 microliters/min in 10-microliters/min steps) elicited, with a delay of approximately 20 seconds, a gradual increase in diameter up to 46%. At a constant 20-microliters/min flow rate, increases in viscosity of the perfusate (2%, 4%, and 6% dextran [molecular weight, 77,800]) caused a gradual vasodilation up to 22%. Varying flow and viscosity of the perfusate simultaneously resulted in an upward shift of the flow-diameter curve. Both flow- and viscosity-induced dilations were eliminated by the removal of the endothelium of arterioles (by air) or were inhibited by indomethacin (10(-5) M). The efficacy and specificity of these inhibitory treatments were assessed with vasoactive agents whose action, with regard to endothelial mediation, has been determined previously. The arteriolar dilation maintained calculated wall shear stress close to control values during increases in flow and/or viscosity of the perfusate, but when the dilation was inhibited by removal of the endothelium or by indomethacin, wall shear stress increased significantly in a cumulative manner.(ABSTRACT TRUNCATED AT 250 WORDS)

A new approach in the treatment of hypotension in human septic shock by NG-monomethyl-L-arginine, an inhibitor of the nitric oxide synthetase

NG-monomethyl-L-arginine (L-NMMA) is an inhibitor of the enzyme nitric-oxide-synthetase. Nitric oxide (NO), produced by endothelial and vascular cells regulates physiological vascular tone, blood pressure and tissue perfusion via guanylate-cyclase and cGMP. In an advanced stage of therapy resistant septic shock in response to inflammatory mediators, NO is overproduced. This leads to vasodilatation, a fall in systemic blood pressure and an attenuated vasoconstriction-response to sympathetic-stimuli. Two episodes of severe and prolonged hypotension in a patient with sepsis were successfully treated twice by bolus therapy of L-NMMA within 4 weeks. On both occasions blood pressure was reversed to normal and the continuous use of high doses of catecholamines were stopped. In contrast to the immediate response of blood pressure, heart rate and central venous pressure remained stable. Cardiac output dropped to 68% and PaO2 increased. These findings indicate that NO-synthetase-inhibitors may be of value in the therapy of human septic shock.

Mutation of N-myristoylation site converts endothelial cell nitric oxide synthase from a membrane to a cytosolic protein

Endothelial cell nitric oxide synthase (ECNOS) is a membrane-associated enzyme that generates endothelium-derived relaxing factor/nitric oxide (EDRF/NO) from L-arginine. We have suggested, from the cloning of the bovine ECNOS cDNA, that the presence of an N-myristoylation consensus sequence may impart its membrane localization since cytosolic forms of NOS do not contain such domains. To test the hypothesis that N-myristoylation is necessary for particulate ECNOS, we performed site-directed mutagenesis of the myristic acid acceptor site, Gly-2, and changed the glycine codon to alanine by a single nucleotide substitution. Expression of wild-type ECNOS in COS cells resulted in greater than 95% of the enzymatic activity in crude membrane fractions (as measured by the conversion of [3H]L-arginine to [3H]L-citrulline). In contrast, expression of the Gly-2 to Ala-2 mutant (G2A) demonstrated 8% ECNOS activity in membranes and 92% in the cytosol. The back mutation (from Ala-2 to Gly-2, A2G) restored ECNOS activity to the particulate fraction as seen with the wild type. Both wild-type membrane ECNOS and cytosolic G2A ECNOS activities were dependent on NADPH and calcium and were inhibited to the same extent by NG-monomethyl L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME). Moreover, kinetic analysis of these enzymes revealed similar Kms for L-arginine (2-4 microM, n = 3), demonstrating that the mutation did not affect ECNOS function. Thus, N-myristoylation is necessary for the membrane localization of ECNOS and may be of special significance for the basal or flow-induced production of NO by the endothelium.

Cerebral autoregulation during moderate hypothermia in rats

BACKGROUND AND PURPOSE

Little is known about the effects of hypothermia on cerebral autoregulation. The present study was designed to examine cerebral blood flow responses to controlled hemorrhagic hypotension in normothermic and hypothermic rats.

METHODS

Cortical blood flow was measured with a laser-Doppler flowmeter in halothane-anesthetized rats assigned to one of three groups: normothermic group 1 (n = 8) with a pericranial temperature of approximately 36.5 degrees C or hypothermic group 2 (n = 8) or group 3 (n = 8) with a pericranial temperature of approximately 30.5 degrees C. In group 2, a PaCO2 of approximately 40 mm Hg was maintained without correction for body temperature. To evaluate the role of PaCO2, in group 3 animals PaCO2 was kept at approximately 40 mm Hg as corrected for body temperature. In all animals, the mean arterial blood pressure was reduced by hemorrhage in increments of 10 mm Hg every 2 minutes.

RESULTS

In group 1 animals, a typical autoregulatory curve was observed with cerebral blood flow first falling at or below 75% of baseline at a mean arterial pressure of 57 +/- 15 mm Hg (mean +/- SD). Absolute normotensive cerebral blood flow in group 2 fell to < or = 75% of baseline at a mean arterial pressure of 73 +/- 21 mm Hg. In group 3, no evidence of autoregulation was seen. Cerebral blood flow reached values < or = 75% of baseline at a mean arterial pressure of 82 +/- 14 mm Hg, whereas calculated cerebrovascular resistance failed to show any compensatory vasodilation as the mean arterial pressure decreased.

CONCLUSIONS

Different PaCO2 management schemes used during hypothermia may have profound effects on cerebral blood flow and on autoregulation. If PaCO2 is maintained at 40 mm Hg after correction for temperature, autoregulation is abolished. If uncorrected PaCO2 is maintained at approximately 40 mm Hg, some degree of autoregulation is preserved, albeit with a right-shifted "knee."

Selective inhibition of agonist-induced but not shear stress-dependent release of endothelial autacoids by thapsigargin

1. The effects of the Ca(2+)-ATPase inhibitor, thapsigargin, on the shear stress-dependent and on the agonist-stimulated release of endothelium-derived relaxing factor, i.e. nitric oxide (NO), and prostacyclin (PGI2) were studied in bovine and human cultured endothelial cells as well as in endothelium-intact arterial segments of the rabbit. 2. Preincubation with thapsigargin (1 microM for 10 min) had no effect on the shear stress-dependent release of NO from bovine aortic endothelial cells grown on beads, but abolished the release of NO induced by ADP, bradykinin, ionomycin or poly-L-lysine. Similarly, thapsigargin completely abrogated the agonist-stimulated PGI2 release from these cells, but had no effect on the shear stress-dependent release of PGI2. 3. The acetylcholine-induced release of NO from the luminally perfused thoracic aorta and femoral artery of the rabbit was suppressed by pretreatment with thapsigargin (1 microM). In contrast, thapsigargin did not affect the shear stress-dependent release of NO from the femoral artery. 4. Administration of thapsigargin to these vascular preparations or to cultured endothelial cells alone produced a substantial release of both NO and PGI2. This release declined towards previous values after washout of thapsigargin. 5. In human and bovine cultured endothelial cells, thapsigargin (1-1000 nM) caused a dose-dependent sustained rise in [Ca2+]i, an effect that was abolished in the absence of extracellular Ca2+. Stimulation of these cells with bradykinin, histamine, ADP or ionomycin after previous exposure to thapsigargin (30-1000 nM) no longer caused an increase in [Ca2+]i. of the release of these endothelial autacoids caused by shear stress or receptor-dependent and independent agonists.

Attenuation of vasoconstriction by endogenous nitric oxide in rat caudal artery

1. The effects of NG-nitro-L-arginine (L-NNA), NG-nitro-L-arginine methyl ester (L-NAME), haemoglobin and methylene blue have been examined on vascular reactivity in the rat isolated caudal artery. The effects of L-NNA and sodium nitroprusside were also investigated on the stimulation-induced (S-I) efflux of noradrenaline in the rat caudal artery. 2. L-NNA (10 microM) and L-NAME (10 microM) significantly attenuated the vasodilator responses to acetylcholine (1 nM-1 microM), but had no effect on vasodilator responses to papaverine (1-100 microM). 3. Vasoconstrictor responses to sympathetic nerve stimulation (3 Hz, 10 s), noradrenaline (0.01-1 microM), methoxamine (1-10 microM), 5-hydroxytryptamine (0.01-0.3 microM), phenylephrine (0.1-10 microM), endothelin-1 (10 nM) and KCl (40 mM) were significantly enhanced by 10 microM L-NNA. L-NAME (10 microM) caused a significant enhancement of vasoconstrictor responses to noradrenaline and sympathetic nerve stimulation in endothelium-intact, but not in endothelium-denuded tissues. 4. Haemoglobin and methylene blue (both 10 microM) enhanced the vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline. The enhancements were absent in endothelium-denuded arterial segments. 5. In endothelium-denuded arterial segments precontracted with phenylephrine, the vasodilator responses to the nitric oxide donor, sodium nitroprusside (0.1-300 nM) were decreased by increasing the level of precontraction. 6. L-NNA (10 microM) had no effect on the S-I efflux of radioactivity from arteries in which transmitter stores had been labelled with [3H]-noradrenaline. 7. These results suggest that endothelial nitric oxide attenuates vasoconstrictor responses in the rat caudal artery through activation of soluble guanylate cyclase to decrease smooth muscle contractility. Therefore, the findings provide evidence that nitric oxide acts as a functional antagonist to oppose vasoconstriction.

Estrogen-induced uterine vasodilatation is antagonized by L-nitroarginine methyl ester, an inhibitor of nitric oxide synthesis

OBJECTIVES

Our study was designed to determine whether nitric oxide mediates estrogen-induced increases in uterine blood flow.

STUDY DESIGN

Six nonpregnant oophorectomized ewes were instrumented with uterine artery flow probes and catheters. Ewes received estradiol-17 beta 1 microgram/kg, which maximally increased uterine blood flow by 120 minutes. Each animal then received local bolus injections of the nitric oxide synthetase inhibitor L-nitroarginine methyl ester.

RESULTS

Estradiol-17 beta increased uterine blood flow from 16 +/- 6 to 139 +/- 32 ml/min by 120 minutes. Local uterine artery administration of L-nitroarginine methyl ester (1 to 30 mg) caused a dose-related decrease in uterine blood flow, which reached a maximum of 59% +/- 6% inhibition. Higher doses of L-nitroarginine methyl ester less than or equal to 10 mg/kg (330 to 460 mg) given locally led to a maximum inhibition of 79% +/- 3% but showed systemic responses.

CONCLUSION

Estradiol-17 beta-induced increases in uterine blood flow are mediated mainly by nitric oxide; the observed vasodilation can be antagonized by the intraaterial administration of nitric oxide synthetase inhibitor L-nitroarginine methyl ester.

Nitric oxide mediates hyporeactivity to vasopressors in mesenteric vessels of portal hypertensive rats

Increased levels of circulating vasodilators have been claimed to be the causative factor in the hyporesponsiveness to endogenous vasopressors in portal hypertension. To investigate whether this hyporeactivity to vasopressors is also present in an in vitro system perfused with a synthetic medium, the responsiveness to graded concentrations of norepinephrine, arginine-vasopressin, and potassium chloride was tested in perfused superior mesenteric arterial beds of normal rats and rats with portal hypertension induced by partial portal vein ligation (PVL). The same vasopressors were tested after incubation of vessel preparations with the stereo-specific nitric oxide formation inhibitor N omega-nitro-L-arginine (NNA, 10(-4) mol/L). Vessel preparations of PVL compared with normal rats (n = 8 per group and vasopressor) expressed a significant (P less than 0.05) hyporeactivity to norepinephrine, arginine-vasopressin, and potassium chloride over a wide range of concentrations. This hyporesponsiveness was overcome by preincubating vessel preparations with NNA. In summary, portal hypertension is accompanied by a significant in vitro hyporeactivity of splanchnic vessels to norepinephrine, arginine-vasopressin, and potassium chloride, and secretion of nitric oxide in this preparation seems responsible for this blunted response.

Sympathetic modulation of endothelium-derived relaxing factor

To determine whether the release of endothelium-derived relaxing factor (EDRF) is sympathetically mediated, we studied the effects of beta-blockade by propranolol, ganglionic blockade with hexamethonium, or mechanical pithing on the blood pressure response to EDRF inhibition in anesthetized rats. We inhibited EDRF with 10 mg/kg of either NG-monomethyl-L-arginine (L-NMMA) or N omega-nitro-L-arginine-methyl ester (L-NAME). In controls, L-NMMA and L-NAME increased blood pressure by 14 +/- 1 (p less than 0.01) and 22 +/- 2 mm Hg (p less than 0.01), respectively. Propranolol lowered blood pressure from 98 +/- 3 to 72 +/- 4 mm Hg without altering the response to L-NAME (delta 26 +/- 3). This response correlated with the resting blood pressure (r = 0.87; p less than 0.001). Hexamethonium (25 mg/kg) lowered blood pressure from 118 +/- 6 to 85 +/- 4 mm Hg but did not change the response to L-NMMA (delta 15 +/- 1). In pithed rats, blood pressure was lowered, but the pressor response to L-NAME was unchanged. When blood pressure was returned to normotensive levels by angiotensin II, norepinephrine, or phenylephrine, L-NAME increased blood pressure by 50 +/- 2, 68 +/- 8, and 109 +/- 7 mm Hg, respectively (p less than 0.001). We conclude that an intact autonomic nervous system is not needed for the pressor response to EDRF inhibition. The enhanced response in pithed rats treated with vasoconstrictors may be due to removal of the buffering effect of the baroreceptors and the absence of EDRF, which would oppose vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide modulates coronary autoregulation in the guinea pig

A guinea pig heart Langendorff preparation was used in the present study to test the hypothesis that the coronary endothelium modulates coronary autoregulation through the production of nitric oxide (NO). Pacing at 250 beats per minute and venting the left ventricle to ensure that the hearts did no external work were performed in an attempt to reduce the metabolic stimulus to coronary vasomotion and keep it constant. We measured the responses of coronary flow and oxygen metabolism to stepwise changes of the perfusion pressure over the range between 18 and 85 mm Hg. The hearts exhibited autoregulation between 25 and 55 mm Hg and active vasodilation at perfusion pressures above that range. Perfusion with 100 microM NG-nitro-L-arginine (NNLA), an inhibitor of NO synthase, decreased coronary flow over the entire range of perfusion pressures and abolished active vasodilation over 65 mm Hg, thus widening the autoregulatory range. The administration of 200 microM L-arginine, but not D-arginine, reversed the action of NNLA. Inhibition of the cyclooxygenase pathway by 10 microM indomethacin did not affect autoregulation. Perfusion with 1 nM arginine vasopressin, a direct smooth muscle constrictor, lowered coronary flow rate to the same extent as NNLA at 55 mm Hg but did not prevent the pressure-dependent increase in flow above that pressure. These observations suggest that 1) the coronary endothelium actively modulates coronary autoregulation through the production of NO but not prostanoids, 2) mechanical stress (shear stress and/or stretching secondary to vasodilation) may be the stimulus to NO production, especially above the autoregulatory range, and 3) autoregulatory tone is likely to be myogenic in origin rather than mediated by extrinsic vasoconstrictors.

Transmural pressure inhibits nitric oxide release from human endothelial cells

We examined the effect of transmural pressure on histamine-stimulated nitric oxide release from cultured endothelial cells prepared from human umbilical cord veins. PO2 and pH were kept constant throughout the experiments. Various levels of transmural pressure and atmospheric pressure (40, 80, 120 and 160 mm Hg) were applied. Nitric oxide release was inhibited in a pressure-dependent manner. The inhibitory effects were reversible, and nitric oxide had no effect on the morphology of the cells. Our results suggest that transmural pressure-mediated inhibition of nitric oxide release contributes to pressure-induced vasoconstriction and reduced endothelium-dependent relaxation in patients with hypertension.

Nitric oxide as a mediator of relaxation of the corpus cavernosum in response to nonadrenergic, noncholinergic neurotransmission

BACKGROUND

Nitric oxide has been identified as an endothelium-derived relaxing factor in blood vessels. We tried to determine whether it is involved in the relaxation of the corpus cavernosum that allows penile erection. The relaxation of this smooth muscle is known to occur in response to stimulation by nonadrenergic, noncholinergic neurons.

METHODS

We studied strips of corpus cavernosum tissue obtained from 21 men in whom penile prostheses were inserted because of impotence. The mounted smooth-muscle specimens were pretreated with guanethidine and atropine and submaximally contracted with phenylephrine. We then studied the smooth-muscle relaxant responses to stimulation by an electrical field and to nitric oxide.

RESULTS

Electrical-field stimulation caused a marked, transient, frequency-dependent relaxation of the corpus cavernosum that was inhibited in the presence of N-nitro-L-arginine and N-amino-L-arginine, which selectively inhibit the biosynthesis of nitric oxide from L-arginine. The addition of excess L-arginine, but not D-arginine, largely reversed these inhibitory effects. The specific liberation of nitric oxide (by S-nitroso-N-acetylpenicillamine) caused rapid, complete, and concentration-dependent relaxation of the corpus cavernosum. The relaxation caused by either electrical stimulation or nitric oxide was enhanced by a selective inhibitor of cyclic guanosine monophosphate (GMP) phosphodiesterase (M&B 22,948). Relaxation was inhibited by methylene blue, which inhibits cyclic GMP synthesis.

CONCLUSIONS

Our findings support the hypothesis that nitric oxide is involved in the nonadrenergic, noncholinergic neurotransmission that leads to the smooth-muscle relaxation in the corpus cavernosum that permits penile erection. Defects in this pathway may cause some forms of impotence.

Effect of shear stress on cytosolic Ca2+ of calf pulmonary artery endothelial cells

The purpose of the present study was to determine if hemodynamic shear stress increases free cytosolic Ca2+ concentration ([Ca2+]i) of cultured pulmonary artery endothelial cells exposed to steady laminar fluid flow in a parallel plate chamber. Average [Ca2+]i was estimated by measuring cell-associated fura-2 fluorescence using microfluorimetric analysis. To determine [Ca2+]i close to the membrane surface, 86Rb+ efflux via Ca(2+)-dependent K+ channels was measured. Upon initiation of flow or upon step increases in flow, no change in [Ca2+]i was observed using fura-2. However, increases in shear stress produced a large, transient increase in 86Rb+ efflux. The shear stress-dependent increase in 86Rb+ efflux was not blocked by either tetrabutylammonium ions (20 mM) or by charybdotoxin (10 nM), two specific inhibitors of the Ca(2+)-dependent K+ channel of vascular endothelial cells. These results demonstrate that shear stress per se has little effect on either the average cytosolic [Ca2+]i as measured by fura-2 or on [Ca2+]i close to the cytoplasmic surface of the plasmalemma as measured by the activity of Ca(2+)-dependent K+ channels.

Increased blood flow inhibits neointimal hyperplasia in endothelialized vascular grafts

Intimal hyperplasia is a primary cause of failure after vascular reconstruction and may be affected by blood flow. We have studied the effects of increased blood flow on intimal hyperplasia in porous polytetrafluoroethylene grafts implanted in baboons. These grafts develop an endothelial lining by 2 weeks and neointimal thickening due to proliferation of underlying smooth muscle cells by 1 month. Creation of a distal arteriovenous fistula increased flow (from 230 +/- 35 to 785 +/- 101 ml/min, p less than 0.001) and mean shear (from 26 +/- 4 to 78 +/- 10 dynes/cm2, p less than 0.001) without causing a drop in pressure across the grafts. Fistula flow did not alter the pattern of endothelial coverage but did cause a marked reduction in the cross-sectional area of the neointima (from 2.60 +/- 0.52 to 0.42 +/- 0.07 mm2 at 3 months, p less than 0.01). Detailed morphometric analysis revealed an equivalent percentage decrease in smooth muscle cells and matrix content, suggesting that the primary effect of increased flow was to reduce smooth muscle cell number without affecting the amount of matrix produced by individual cells. The neointima remained sensitive to changes in flow at late times; ligation of the fistula after 2 months resulted in a rapid increase in neointimal thickness (from 0.60 +/- 0.03 mm2 after 2 months of fistula flow to 3.88 +/- 0.55 mm2 1 month after ligation of fistula, p less than 0.01). These results support the hypothesis that changes in blood flow affect the structure of diseased as well as normal vessels.