Pressure releases a transferable endothelial contractile factor in cat cerebral arteries

The vascular renin-angiotensin system contributes to blunted vasodilation induced by transient high pressure in human adipose microvessels

Increased intraluminal pressure can reduce endothelial function in resistance arterioles; however, the mechanism of this impairment is unknown. The purpose of this study was to determine the effect of local renin-angiotensin system inhibition on the pressure-induced blunting of endothelium-dependent vasodilation in human adipose arterioles. Arterioles (100-200 μm) were dissected from fresh adipose surgical specimens, cannulated onto glass micropipettes, pressurized to an intraluminal pressure of 60 mmHg, and constricted with endothelin-1. Vasodilation to ACh was assessed at 60 mmHg and again after a 30-min exposure to an intraluminal pressure of 150 mmHg. The vasodilator response to ACh was significantly reduced in vessels exposed to 150 mmHg. Exposure of the vessels to the superoxide scavenger polyethylene glycol-SOD (100 U/ml), the ANG II type 1 receptor antagonist losartan (10(-6) mol/l), or the angiotensin-converting enzyme inhibitor captopril (10(-5) mol/l) prevented the pressure-induced reduction in ACh-dependent vasodilation observed in untreated vessels. High intraluminal pressure had no effect on papaverine-induced vasodilation or ANG II sensitivity. Increased intraluminal pressure increased dihydroethidium fluorescence in cannulated vessels, which could be prevented by polyethylene glycol-SOD or losartan treatment and endothelial denudation. These data indicate that high intraluminal pressure can increase vascular superoxide and reduce nitric oxide-mediated vasodilation via activation of the vascular renin-angiotensin system. This study provides evidence showing that the local renin-angiotensin system in the human microvasculature may be pressure sensitive and contribute to endothelial dysfunction after acute bouts of hypertension.

Protein restriction during pregnancy induces hypertension and impairs endothelium-dependent vascular function in adult female offspring

Intrauterine undernutrition plays a role in the development of adult hypertension. Most studies are done in male offspring to delineate the mechanisms whereby blood pressure may be raised; however, the vascular mechanisms involved in female offspring are unclear. Female offspring of pregnant Sprague-Dawley rats fed either a control (C; 18%) or a low-protein (LP; 6%) diet during pregnancy were used. Birth weight and later growth were markedly lower in LP than in C offspring. LP offspring exhibited impaired estrous cyclicity with increased mean arterial pressure. Hypotensive response to acetylcholine (ACh) and the hypertensive response to phenylephrine (PE) were greater in LP than in C rats. N-nitro-L-arginine methyl ester (L-NAME) induced greater hypertensive responses in C than in LP rats. Endothelium-intact mesenteric arteries from LP offspring exhibited increased contractile responses to PE and reduced vasodilation in response to ACh. In endothelium-denuded arteries, relaxation responses to sodium nitroprusside were similar in both groups. Basal and ACh-induced increase in vascular nitrite/nitrate production was lower in LP than in C offspring. L-NAME or 1H-1,2,4-oxadiazolo-4,3-quinoxalin-1-one inhibited ACh relaxations and enhanced PE contractions in C offspring, but had minimal effect in LP rats. The decreased NO-mediated vascular response might explain the increased vascular contraction and arterial pressure in female offspring with low birth weight.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Elevated pressure selectively blunts flow-evoked vasodilatation in rat mesenteric small arteries

BACKGROUND AND PURPOSE

The present study investigated mechanisms underlying impaired endothelium-dependent vasodilatation elicited by elevating the intraluminal pressure in rat mesenteric small arteries.

EXPERIMENTAL APPROACH

Arterial segments (internal diameter 316+/-2 microm, n=86) were mounted in a pressure myograph. The effect of elevating pressure from 50 to 120 mmHg for 1 h before resetting it to 50 mmHg was studied on endothelium-dependent vasodilatation.

KEY RESULTS

In arteries constricted with U46619 in the presence of indomethacin, shear stress generated by flow, evoked vasodilatation that was abolished by an inhibitor of nitric oxide (NO) synthase, asymmetric dimethylarginine (1 mM), whereas acetylcholine-induced vasodilatation was unchanged. After elevation of intraluminal pressure for 1 h and then resetting it to 50 mmHg, vasodilatation induced by shear stress and the NO donor, S-nitrosopenicillamine was inhibited, while vasodilatation induced by a guanylyl cyclase activator, BAY 412272, and acetylcholine was unaltered. Superoxide levels sensitive to polyethylene glycol superoxide dismutase were increased in segments exposed to elevated pressure. A superoxide scavenger, tempol (300 microM), a general endothelin receptor antagonist, SB 217242 and the selective ET(A) receptor antagonist, BQ 123 preserved shear stress-evoked vasodilatation.

CONCLUSIONS AND IMPLICATIONS

The present study shows that transient exposure to an elevated intraluminal pressure selectively inhibits flow-evoked NO-mediated vasodilatation, probably through activation of endothelin receptors and increased formation of superoxide. In contrast, elevation of pressure did not affect the acetylcholine-evoked endothelium-derived hyperpolarizing factor type vasodilatation in mesenteric small arteries.

Cytochrome P450 enzymes: Central players in cardiovascular health and disease

Cardiovascular disease (CVD) is a human health crisis that remains the leading cause of death worldwide. The cytochrome P450 (CYP) class of enzymes are key metabolizers of both xenobiotics and endobiotics. Many CYP enzyme families have been identified in the heart, endothelium and smooth muscle of blood vessels. Furthermore, mounting evidence points to the role of endogenous CYP metabolites, such as epoxyeicosatrienoic acids (EETs), hydroxyeicosatetraenoic acids (HETEs), prostacyclin (PGI(2)), aldosterone, and sex hormones, in the maintenance of cardiovascular health. Emerging science and the development of genetic screening have provided us with information on the differences in CYP expression among populations and groups of individuals. With this information, a link between CYP expression and activity and CVD, such as hypertension, coronary artery disease (CAD), myocardial infarction, heart failure, stroke, and cardiomyopathy and arrhythmias, has been established. In fact many currently used therapeutic modalities in CVD owe their therapeutic efficacy to their effect on CYP metabolites. Thus, the evidence for the involvement of CYP in CVD is numerous. Concentrating on treatment modalities that target the CYP pathway makes ethical sense for the affected individuals and decreases the socioeconomic burden of this disease. However, more research is needed to allow the integration of this information into a clinical setting.

Carotid compression: investigation of cerebral autoregulative reserve in rats

Easy-to-perform, reversible techniques to analyse cerebral autoregulation are still missing in animal research. The carotid compression technique has been established to investigate dynamic cerebral autoregulation in humans. Adapting the carotid compression technique, we compared data from the new application with that of a classical exsanguination method. Compressing the ipsilateral carotid artery with a non-traumatic clip device for 10s modulated cerebral perfusion pressure. After clip release, the peaking laser-Doppler flow velocity increase over the somatosensory cortex allowed calculation of the transient hyperaemic response ratio (THRR) in relation to baseline. Modulating blood-pressure levels maintenance of cerebral blood-flow velocity was compared with THRR responses. With decreasing blood-pressure levels, the THRR first increased (29+/-16% at 95+/-10 mmHg to 39+/-13% at 75+/-10 mmHg) before it returned to baseline values at 54+/-10 mmHg (27+/-14%). THRR significantly dropped to 11+/-12% at 34+/-11 mmHg when resting cerebral blood-flow velocity levels also started to decline. Based on the close correlation between blood-flow velocity levels and THRR responses, we have concluded that carotid compression is an alternative technique that can be used to assess cerebral autoregulation in rats. The technique allows less invasive and reversible testing of dynamic autoregulation to be performed, and the technique can easily be applied in conjunction with functional tests to potentially allow deeper insights into cerebral vasoregulative mechanisms.

Endothelial factors and autoregulation during pressure changes in isolated newborn piglet cerebral arteries

To analyze newborn cerebrovascular autoregulation, middle cerebral arteries from 3-4-d-old piglets were cannulated, and diameter changes after transmural pressure variation were measured. After an equilibration period at 30 mm Hg, pressure was modified from 10 to 70 mm Hg in 20-mm Hg steps. Segments with endothelium showed vasodilation during pressure decrease and vasoconstriction during pressure increase. In each case the maximum response was about 5% that of the resting diameter. Segments without endothelium responded passively to pressure change. Vasodilation during pressure decrease was reduced by the preferential calcium-activated potassium (KCa) channel blocker, tetraethylammonium (1 mM), and was absent with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methylester (L-NAME, 10 microM). The NO synthase substrate, L-arginine (10 microM), counteracted the dilation blockade caused by L-NAME. The cyclooxygenase inhibitor indomethacin (10 microM) and the endothelin A receptor antagonist BQ-123 (10O microM) eliminated the pressure increase-induced vasoconstriction. The ATP-sensitive potassium channel blocker, glibenclamide (1 microM), and the endothelin B receptor antagonist, BQ-788 (10 nM), did not modify the autoregulatory response. None of these drugs modified the passive changes produced by pressure variations in segments without endothelium. These results suggest that: 1) piglet middle cerebral artery autoregulation is endothelium-dependent; 2) NO and KCa channels are involved in vasodilation during transmural pressure decrease, and 3) endothelin-1, through endothelin A receptors, and prostanoids mediate vasoconstriction during pressure increase.

Steady-state myogenic response of rat coronary microvessels is preserved by isoflurane but not by halothane

The myogenic response of vascular smooth muscle produces vasomotion in response to changes in vessel transmural pressure. While this is an important determinant of coronary blood distribution, the effect of volatile anesthetics on the response has not been previously investigated. In this study, we examined the effect of isoflurane and halothane on this myogenic response. Coronary resistance arteries were isolated from Wistar rats. As the intraluminal pressure was increased from 10 to 120 mm Hg in the presence of either isoflurane (1%, 2%, and 3%), halothane (1% and 2%), or no volatile agent (control), the vessel intraluminal diameter was monitored using a video detection system. Passive changes in vessel diameter were measured after exposure to papaverine 100 microM. Additionally, the myogenic responses of endothelium-intact and endothelium-denuded vessels were compared. Endothelium-intact control vessels demonstrated myogenic constriction above 80 mm Hg of intraluminal pressure. This response was not affected by endothelial denudation. The response was preserved by isoflurane 1%, 2% or 3% but abolished by halothane 1% or 2%. We conclude that, in rat coronary resistance arteries, myogenic constriction can be demonstrated above 80 mm Hg of intraluminal pressure and is endothelium independent. This response is preserved by isoflurane but abolished by halothane. These findings may have implications for the effect of the anesthetics on coronary blood flow distribution.

Electrophysiological properties of the rat middle cerebral artery at different levels of passive wall tension

1. Simultaneous measurements of intracellular membrane potential and myogenic tone of proximal segments of the rat middle cerebral artery, mounted in a small vessel myograph, were made at two levels of passive wall tension. 2. At low levels of passive tension (less than 0.25 mN/mm) vessels had a resting membrane potential of approximately -65 mV. Addition of KCl (5-60 mmol/L), BaCl2 (0.01-3 mmol/L) or tetraethylammonium (TEA; 0.1-3 mmol/L) resulted in a concentration-dependent depolarization, to approximately -40 mV, generally associated with a contractile response. After the application of high levels of passive tension (to approximately 2 mN/mm maximum) the resting membrane potential of the smooth muscle cells was -40 to -45 mV. This more positive membrane potential was generally associated with an increase in myogenic tone of the vessel. Under these conditions, addition of 5-20 mmol/L KCl resulted in a strong hyperpolarization of the cell along with a concomitant decrease in myogenic tone of the artery. The hyperpolarization and vasorelaxation induced by KCl (5-20 mmol/L) were blocked by BaCl2 (0.5-1 mmol/L). 3. While the addition of ryanodine (10 mumol/L) to vessels under low tension had no effect, when added to a vessel under high tension, this agent caused a rhythmic oscillation in membrane potential. This oscillation was augmented by BaCl2 (1 mmol/L) and inhibited by nifedipine (10 nmol/L) and 4-aminopyridine (1 mmol/L). 4. This study suggests that the electrophysiological and mechanical properties of the isolated rat middle cerebral artery depend on the passive resting conditions under which the vessel is studied. The depolarization of membrane potential observed with increased passive tension appears to result from the closure of an inward rectifying K+ channel. These results indicate that the inward rectifying K+ channel plays an important role in regulating vascular reactivity due to its functional dependence on the mechanical status of the blood vessel.

The role of nitric oxide and other endothelium-derived vasoactive substances in vascular disease

Alteration in the release and action of endothelium-derived vasoactive factors is responsible for changes in vascular reactivity early in the course of vascular disease. These factors include nitric oxide, eicosanoids, endothelium-derived hyperpolarizing factor, endothelin, and angiotensin II. Because endothelial dysfunction occurs at early stages of disease, it may reflect physiological changes that, if allowed to become chronic, are responsible for changes in vascular structure and growth and adhesivity to platelets and leukocytes, ultimately leading to atherosclerosis and thrombosis. Each of the major risk factors predisposing to vascular disease are associated with endothelial cell dysfunction, suggesting a direct etiologic link between the effects of the risk factors on the endothelium and their propensity to accelerate vascular disease. Restoration or replacement of endothelium-derived factors such as nitric oxide and prostacyclin, which impede the progression of vascular disease, or preventing the action of mediators such as vasoconstrictor eicosanoids, angiotensin II, or endothelin, which accelerate the progression of vascular disease, has become a useful paradigm in the treatment and prevention of vascular disease. Thus, understanding the physiology of endothelium-derived vasoactive factors is a necessary part of every physician's education.

The vascular endothelium as a regulator of the ocular circulation: a new concept in ophthalmology?

The endothelium influences local vascular tone by releasing endothelium-derived relaxing factors such as nitric oxide, prostacyclin and a putative hyperpolarizing factor. In isolated ophthalmic arteries and the perfused eye, all endothelial factors importantly contribute to vascular regulation. In larger ophthalmic vessels, this is due to their effects on vascular smooth muscle cells; in smaller vessels, pericytes can be influenced as well. Contracting factors formed include peptide endothelin-1 and cyclooxygenase products, such as thromboxane A2 and prostaglandin H2. In the peripheral circulation endothelial dysfunction occurs under pathological conditions, both in conduit arteries and the microcirculation. An imbalance of endothelium-derived relaxing and contracting factors could be important for the development of vascular ophthalmic complications like hypertension, diabetes, arteriolosclerosis and retinal ischemia. Endothelial dysfunction may also contribute to vasospastic events in retinal migraine and some forms of low tension glaucoma associated with Raynaud phenomenon and migraine.

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.

Hemodynamic vascular forces contribute to impaired endothelium-dependent vasodilation in reperfused canine epicardial coronary arteries

OBJECTIVES

We studied canine coronary arterial vasoreactivity after occlusion and reperfusion to examine whether reduced flow or pressure contributed to the abnormalities observed.

BACKGROUND

Ischemia and reperfusion alter endothelial and myocardial function. Causative factors may include altered flow, complement activation or free radical production by endothelial or white blood cells after reoxygenation and neutrophil activation.

METHODS

The coronary arteries of anesthetized, open chest dogs were subjected to 90-min occlusion +/- 2 h of reperfusion. The effect of reperfusion on arterial responses to intracoronary acetylcholine, nitroprusside and phenylephrine was studied using in vivo ultrasound. Arterial segments were also harvested, perfused ex vivo with cell-free buffer and exposed to potassium chloride, nitroprusside, acetylcholine and bradykinin. The effect of ex vivo flow cessation with or without maintained intralumen pressure was also studied.

RESULTS

Results are expressed as mean value +/- SEM. In vivo arterial cross-sectional area increased during infusion with acetylcholine (10(-5) mol/liter [18.5 +/- 9%]) and nitroprusside (10(-5) mol/liter [22.5 +/- 10%]) and decreased with phenylephrine (10(-5) mol/liter [7.6 +/- 7%]). After reperfusion, acetylcholine caused 13.5 +/- 9% vasoconstriction. Nitroprusside and phenylephrine responses were unchanged. Reperfused arterial segments also showed impaired vasodilation in response to 10(-6) mol/liter of acetylcholine (10.6 +/- 5.1% vs. 47.1 +/- 4.9% in control vessels) and 10(-8) mol/liter of bradykinin (4.4 +/- 6.7% vs. 27.9 +/- 8% in control vessels). Ex vivo flow cessation impaired acetylcholine-mediated vasodilation, but this abnormality was prevented when high intralumen pressure was maintained during the no-flow period.

CONCLUSIONS

Reduction in flow and intralumen pressure contribute to the impaired acetylcholine-mediated vasodilation seen after coronary occlusion. This is prevented by maintaining high intralumen pressure during the no-flow period, suggesting that hemodynamic forces may change endothelial function independent of circulating complement or blood cell elements.

In-vivo effects of endothelin-1 and ETA receptor blockade on arterial, venous and capillary functions in skeletal muscle

Results from in vitro studies have indicated that endothelin-1 is a main candidate for endothelium-derived contracting factors. The aim of this in vivo study was to describe in quantitative terms the effects of endothelin-1 (ET-1), and of ETA receptor blockade, on vascular tone (resistance) in large-bore arterial resistance vessels (> 25 microns), small arterioles (< 25 microns) and the veins, as well as on capillary pressure and fluid exchange in cat gastrocnemius muscle. Endothelin-1 (100-1600 ng kg-1 min-1, i.a.) elicited, after an initial transient dilation, a strong dose-dependent constrictor response in all three consecutive vascular sections, yet with a preferential action on the small arterioles and the veins. The vasoconstriction developed very slowly over about 1 h and was also long-lasting after cessation of the infusion. Our main quantitative analysis refers to effects elicited by 20 min long i.a. infusions of ET-1 at a dose of 400 ng kg-1 min-1. At the end of this period, the peptide caused, on average, a three-fold increase in total regional vascular resistance, in turn explained by a 70% increase in large-bore arterial resistance, a 280% increase in arteriolar resistance and a 220% increase in venous resistance. The latter effect was also manifested as a pronounced capacitance response, and as a decrease in the pre- to post-capillary resistance ratio leading regularly to a rise in capillary pressure, net transcapillary fluid filtration and oedema formation which is unusual for a vasoconstrictor. The new specific competitive ETA receptor antagonist FR 139317 was found to be fully effective in vivo, insofar as it abolished the constrictor response to endothelin-1. ETA receptor blockade, or administration of phosphoramidon, an inhibitor of ET-1 production, did not influence the level of basal vascular tone, indicating no significant endogenous release of ET-1 under resting conditions. This contrasts to the established pronounced endogenous release of endothelium-derived nitric oxide. Finally, vascular myogenic regulation was found not to be mediated by ET-1. The results, taken together, suggest a possible role of ET-1 in long-term, rather than short-term, regulation of vascular tone in vivo, perhaps especially during pathophysiological conditions.

Electrophysiology of cerebral blood vessels

In spite of the relatively large amount of in vitro and in vivo data indicating that, in a number of ways, cerebral arteries are pharmacologically different from peripheral arteries, the mechanisms responsible for these differences are far from clear. An understanding of these mechanisms is particularly important for a rational approach to the treatment of disorders of the cerebral circulation including migraine, hypertension and the responses of cerebral vessels to subarachnoid haemorrhage. This review outlines electrophysiological data which are available from cerebrovascular smooth muscle cells, including the possibility that inwardly-rectifying potassium channels, active at potentials close to the resting membrane potential, are intimately involved in the changes in smooth muscle tone which couple blood flow to regional changes in nerve cell activity. The membrane potential changes in response to perivascular nerve stimulation, noradrenaline, 5-hydroxytryptamine and endothelium-derived hyperpolarizing factor are also described, together with the underlying membrane mechanisms and their relationship to smooth muscle contraction and relaxation.

Nitric oxide is an important determinant of coronary flow in the isolated blood perfused rat heart

Many vasoactive substances are involved in the regulation of vasomotor tone and some of them, like nitric oxide (NO), are derived from the endothelium. Nitric oxide is able to relax preconstricted coronary resistance vessels almost completely. However, it is not clear what the contribution of NO is to vasomotor tone in the intact blood perfused heart. The aim of the present study was to evaluate the contribution of NO to coronary pressure-flow relations. We used isovolumically beating, donor supported, blood perfused isolated rat hearts. We measured pressure-flow relations under control conditions, after blocking endothelial NO production with NG-nitro-L-Arginine (LNNA) and after administration of L-Arginine (L-Arg) in order to overrule the blocking effect. Administration of LNNA at a perfusion pressure of 105 mm Hg resulted, after about 40 min, in a significant (Wilcoxon's signed-rank test, (n = 8) p < 0.05) reduction of coronary flow to 47 +/- 5% (mean +/- SEM) of control and a reduction of developed isovolumic left-ventricular pressure to 62 +/- 4% of control. L-Arg returned flow to 60 +/- 7% of control which is a significant increase with respect to LNNA (p < 0.05). L-Arg did not increase the left-ventricular pressure. The entire perfusion pressure-flow relation (pressure range 65-125 mm Hg) was significantly shifted downwards after LNNA with respect to control. Pressure-flow relations after L-Arg were in between those during control and after block of NO production. L-Arg alone was found to have no effect on flow and left-ventricular pressure (n = 2) and both LNNA and L-Arg were found to have no effect on contractility of isolated trabeculae (n = 6), thus, coronary blood flow reduction after LNNA administration is mainly the result of inhibition of endothelial NO production. At a perfusion pressure of 105 mm Hg reactive hyperemia is still present after LNNA and subsequent L-Arg administration, indicating that endothelial NO is not the only factor involved in flow regulation. We conclude that endothelium-derived NO is involved in the control of coronary flow in the blood perfused rat heart.

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.

Mechanisms of pharmacologic intervention at the level of the calcium channel

Calcium channels are large, complex membrane proteins that mediate transmembrane calcium currents. At least 2 kinds of calcium channels are found in heart muscle--the transient type and the long-lasting type. Calcium currents are modulated by diverse endogenous and exogenous factors including hormones, catecholamines, and calcium antagonists. Calcium antagonists act preferentially on vascular smooth muscle and have relatively less effect on the calcium channels of heart muscle. Compared with heart muscle, vascular smooth muscle is relatively depolarized, suggesting that vascular smooth muscle cells have predominantly the long-lasting type of calcium currents. The differential binding to different types of calcium channels underlies the clinical efficacy of the calcium antagonists. A drug such as bepridil, which acts preferentially on the coronary vasculature rather than on the peripheral vasculature, dilates the coronary vessels without depressing cardiac contraction, a putative clinical advantage.

Stretch-activated single-channel and whole cell currents in vascular smooth muscle cells

Mechanosensitive ion channels may play a key role in transducing vascular smooth muscle (VSM) stretch into active force development. To test this hypothesis, we recorded single-channel and macroscopic currents during mechanical stimulation of enzymatically dispersed vascular smooth muscle cells. Patch pipette suction activated a nonselective cation channel that was permeable to K+, Na+, and Ca2+. Whole cell stretch was accomplished using two patch-type micropipettes attached to the cell ends with suction. Stretch elicited a sustained depolarization with a magnitude similar to that observed in pressurized arteries. Under whole cell voltage clamp, stretch activated an inward current with a reversal potential near -15 mV. In another series of experiments, whole cell stretch failed to modify the current-voltage relationship for voltage-gated calcium currents. Thus, in VSM, both single-channel and whole cell data are consistent with activation of a nonselective cation channel by stretch. This mechanism may, in part, account for pressure-induced activation of intact blood vessels.

Endothelium-dependent contractile responses to 5-hydroxytryptamine in the rabbit basilar artery

1 5-Hydroxytryptamine (5-HT) and 5-carboxamidotryptamine (5-CT) stimulated additional, endothelium-dependent contractions in rabbit isolated basilar arteries which had been submaximally contracted with either histamine or potassium chloride. 2 The additional contractions to 5-HT were not altered by the 5-HT2 antagonist, ketanserin (1 microM), but were abolished in the presence of the cyclo-oxygenase inhibitor indomethacin (3 microM). 3 The additional smooth muscle contraction stimulated by 5-HT was increased in the presence of the competitive substrate inhibitor for nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). 4 Neither of the selective 5-HT agonists, 8-hydroxy-dipropylaminotetralin (8-OH DPAT) or alpha-methyl 5-HT stimulated endothelium-dependent contraction, but these agonists did reduce the rate at which histamine-induced tension spontaneously declined. This effect represented a direct action on the smooth muscle cells, as it was independent of the presence of endothelial cells. 5 Smooth muscle relaxation was not obtained in response to 5-HT, whether or not indomethacin was present to block endothelium-dependent contraction. None of the other selective 5-HT agonists, 5-CT, 8-OH DPAT or alpha-methyl 5-HT produced endothelium-dependent smooth muscle relaxation, when applied against a background of contraction. 6 These data show that endothelium-dependent smooth muscle contraction can be produced by stimulating 5-HT receptors in the partially contracted rabbit basilar artery. Similar contraction to 5-CT indicates an involvement by 5-HT1 receptors. The susceptibility of the contractions to indomethacin suggest they are mediated by a metabolite of arachidonic acid.

Regulation of the cerebral circulation by endothelium

Endothelium exerts an important influence on cerebral vascular tone through the production and release of a diverse group of vasoactive factors. Relaxing factors produced by endothelium include nitric oxide (or a nitric oxide-containing compound), a hyperpolarizing factor, and prostacyclin. Endothelium-derived contracting factors include cyclooxygenase products of arachidonic acid and endothelins. Several pathophysiological conditions are associated with increased formation of endothelium-derived contracting factors. Such endothelial dysfunction in the cerebral circulation may shift the balance of vascular tone toward constriction and may potentially contribute to the onset or maintainance of cerebral ischemia and stroke.

Endothelium-derived relaxing and contracting factors

Key discoveries in the past decade revealed that the endothelium can modulate the tone of underlying vascular smooth muscle by the synthesis/release of potent vasorelaxant (endothelium-derived relaxing factors; EDRF) and vasoconstrictor substances (endothelium-derived contracting factors; EDCF). It has become evident that the synthesis and release of these substances contribute to the multitude of physiological functions the vascular endothelium performs. Accumulating evidence suggests that at least one of the EDRFs is identical with nitric oxide (NO) or a labile nitroso compound, which is produced from L-arginine by an NADPH- and Ca(2+)-dependent enzyme, arginine oxidase. The existence of more than one chemically distinct EDRF has been proposed, including an endothelium-derived hyperpolarizing factor (EDHF). The target of EDRF (NO) is soluble guanylate cyclase (increase in cyclic GMP) while EDHF appears to activate a K(+)-channel in vascular smooth muscle. Recent data suggest that muscarinic receptor subtypes selectively mediate the release of EDRF(NO) (M2) and EDHF (M1). EDRF(NO) affects not only the underlying vascular smooth muscle, but also platelets, inhibiting their aggregation and adhesion to the endothelium. The antiaggregatory effect of EDRF is synergistic with prostacyclin, so their combined release may represent a physiological mechanism aimed at preventing thrombus formation. An additional proposed biological function of EDRF(NO) is cytoprotection by virtue of scavenging superoxide radicals. The endothelium can also mediate vasoconstriction by the release of a variety of endothelium-derived contracting factors (EDCF). Other than the unique peptide endothelin, the nature of EDCFs has not yet been firmly established. Autoregulation of cerebral and renal blood flow and hypoxic pulmonary vasoconstriction may represent the physiological role of endothelium-dependent vasoconstriction. Growing evidence indicates that the endothelium can serve as a unique mechanoreceptor, sensing and transducing physical stimuli (e.g., shear forces, pressure) into changes in vascular tone by the release of EDRFs or EDCFs. In physiological states, a delicate balance exists between endothelium-derived vasodilators and vasoconstrictors. Alterations in this balance can result in local (vasospasm) and generalized (hypertension) increase in vascular tone and also in facilitated thrombus formation. Endothelial dysfunction may also contribute to the pathophysiology of angiopathies associated with hypercholesterolemia and atherosclerosis.

Endothelial cells as part of a vascular oxygen-sensing system: hypoxia-induced release of autacoids

Higher developed organisms are equipped with many central and local control mechanisms, which enable an adequate blood and oxygen supply to tissues over a wide range of demands. Global adaptive responses include changes in the circulatory and ventilatory system as well as increases in the oxygen carrying capacity of the blood. At the level of the specialized organs there exist additional control systems for the regulation of local blood flow. Most systems make use of highly specialized cells which are able to sense the oxygen partial pressure of the transport medium, blood, and within the tissues. In the past years, it has been shown that the vascular endothelium lining the entire circulatory system can actively modulate the vascular tone and platelet functions by the release of autacoids, among them prostacyclin and endothelium-derived nitric oxide (EDRF). Recent experiments demonstrate that the release of EDRF is PO2-dependent, which suggests that endothelial cells may act as functional local oxygen sensors within the vascular system.

Role of endothelium-formed nitric oxide on vascular responses

1. Endothelial cells of blood vessels generate factors which can modulate underlying smooth muscle tone, inducing vasorelaxation, (endothelium-derived relaxing factor, EDRF, and endothelium-derived hyperpolarizing factor) and/or vasoconstriction (endothelium-derived contracting factors, EDCFs, including the peptide endothelin). 2. EDRF is nitric oxide (NO) or a RNO compound from which this oxide is released. Its half-life is very short (6-50 sec), and it produces rapid vasodilations and inhibits platelet aggregation. 3. NO is formed from the terminal guanidino of L-arginine, but not of D-arginine. NO effects and NO formation are inhibited by NG-monomethyl-L-arginine (L-NMMA), but not by D-NMMA. These inhibitory effects are blocked by L-arginine. 4. Removal of endothelium or pathological situations that can induce endothelial dysfunction (atherosclerosis, diabetes, hypertension or subarachnoid hemorrhage) cause increases on the vascular contractility elicited by agonists (noradrenaline, serotonin, EDCFs, etc.). These findings suggest that EDRF produces a physiological inhibitory modulation of vascular smooth muscle tone and its alteration produces or facilitates the development of diseases such as hypertension or coronary and cerebral vasospasm.

Endothelium-derived contractile factors

1. Vascular endothelium releases different substances (endothelium-derived contractile factors, EDCFs), which mediate vasoconstrictor responses induced by several agents. 2. Clear differences have been reported in endothelium-dependent contractions, which suggest at least three distinct EDCFs, named EDCF1, EDCF2 and EDCF3, respectively. 3. EDCF1 is a cyclooxygenase metabolite(s) of arachidonic acid. EDCF2 is a polypeptide released from cultured endothelial cells. It has been isolated and identified as a 21-amino acid peptide called endothelin, which is described as the most potent vasoconstrictor agent known to date. EDCF3 is an unidentified contractile factor(s), which is neither EDCF1 nor EDCF2. 4. The physiological role of these endothelial contractile factors is not yet clear. However, they have been implicated in the local mechanisms involved in blood flow regulation, as well as in some pathological conditions, such as hypertension or cerebral vasospasm.