The role of EDRF in flow distribution: a microangiographic study of the rabbit isolated ear

Endothelial caveolin and its scaffolding domain in cancer

Since the initial reports implicating caveolin-1 (CAV1) in neoplasia, the scientific community has made tremendous strides towards understanding how CAV1-dependent signaling and caveolae assembly modulate solid tumor growth. Once a solid neoplastic tumor reaches a certain size, it will increasingly rely on its stroma to meet the metabolic demands of the rapidly proliferating cancer cells, a limitation typically but not exclusively addressed via the formation of new blood vessels. Landmark studies using xenograft tumor models have highlighted the importance of stromal CAV1 during neoplastic blood vessel growth from preexisting vasculature, a process called angiogenesis, and helped identify endothelium-specific signaling events regulated by CAV1, such as vascular endothelial growth factor (VEGF) receptors as well as the endothelial nitric oxide (NO) synthase (eNOS) systems. This chapter provides a glimpse into the signaling events modulated by CAV1 and its scaffolding domain (CSD) during endothelial-specific aspects of neoplastic growth, such as vascular permeability, angiogenesis, and mechanotransduction.

Peripheral circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulations.

Molecular dimensions of Hb-based O(2) carriers determine constriction of resistance arteries and hypertension

The effect of molecular dimension of hemoglobin (Hb)-based O(2) carriers on the diameter of resistance arteries (A(0), 158 +/- 21 microm) and arterial blood pressure were studied in the conscious hamster dorsal skinfold model. Cross-linked Hb (XLHb), polyethylene glycol (PEG)-conjugated Hb, hydroxyethylstarch-conjugated XLHb, polymerized XLHb, and PEG-modified Hb vesicles (PEG-HbV) were synthesized. Their molecular diameters were 7, 22, 47, 68, and 224 nm, respectively. The bolus infusion of 7 ml/kg of XLHb (5 g/dl) caused an immediate hypertension (+34 +/- 13 mmHg at 3 h) with a simultaneous decrease in A(0) diameter (79 +/- 8% of basal value) and a blood flow decrease throughout the microvascular network. The diameter of smaller arterioles did not change significantly. Infusion of larger O(2) carriers resulted in lesser vasoconstriction and hypertension, with PEG-HbV showing the smallest changes. Constriction of resistance arteries was found to be correlated with the level of hypertension, and the responses were proportional to the molecular dimensions of the O(2) carriers. The underlying mechanism is not evident from these experiments; however, it is likely that the effects are related to the diffusion properties of the different Hb molecules.

Constriction of resistance arteries determines l-NAME-induced hypertension in a conscious hamster model

The influence of infusion of a nitric oxide (NO) synthase inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME), on resistance arteries (diameter, 150 +/- 8 microm) and its relationship with hypertension were examined in conscious hamsters fitted with a dorsal skinfold window. After infusing l-NAME (10 and 30 mg/kg), hamsters showed immediate hypertension of +13 +/- 9 and +21 +/- 9 mm Hg, respectively, relative to basal values, and a maximum of +44 +/- 4 mm Hg at 30 min for the high-dose group. There was simultaneous significant vasoconstriction of the resistance arteries (A(0)) which reduced to 60 +/- 5% of baseline diameter at 3 h; however, there was no significant vasoconstriction in large and small arterioles with diameters diameters less than 70 microm. Blood flow rate in all the vessels decreased in consonance with the vasoconstriction of the resistance artery, irrespective of microvessel classification. These results indicate that the resistance artery plays a key role as a regulator and microvascular resistance in determining blood flow distribution and hypertension when a NO synthase inhibitor is infused.

Changes in resistance vessels during hemorrhagic shock and resuscitation in conscious hamster model

The unanesthetized hamster dorsal skinfold preparation was used to monitor diameters and blood flow rates in resistance arteries (small arteries, A0: diameter, 156 +/- 23 micrometers) and capacitance vessels (small veins, V0: 365 +/- 64 micrometers), during 45 min of hemorrhagic shock at 40 mmHg mean arterial pressure (MAP) and resuscitation. A0 and V0 vessels constricted significantly to 52 and 70% of the basal values, respectively, whereas precapillary arterioles (A1-A4, 8-60 micrometers) and collecting venules (VC-VL, 26-80 micrometers) did not change or tended to dilate. Blood flow rates in the microvessels declined to <20% of the basal values. Resuscitation with shed autologous blood (SAB) showed incomplete recovery of A0 and V0 diameters even 2 h after resuscitation (71 +/- 14% and 81 +/- 18%, respectively, of basal value), whereas other vessels did not change significantly. The behavior of A0 diameter coincided with the incomplete recovery of blood flow rates in all the vessels (ca. 50%) according to Poiseuille's law, and the incomplete recovery of functional capillary density (ca. 75%). Resuscitation with 8% human serum albumin in saline (HSA) tended to show higher levels of A0 constriction and A4 dilation and lowered blood flow rates. Resuscitation with SAB restored tissue PO2 27 +/- 10 mmHg after 2 h, which was near control levels (28 +/- 5 mmHg), whereas resuscitation with HSA caused tissue PO2 to remain significantly depressed (6 +/- 2 mmHg), and flow rates were significantly lower than resuscitation with SAB. These results indicate that response of the A0 vessels is the crucial determinant of blood flow in the observed area. The constriction of A0 may help sustain MAP, and constriction of V0 may enhance blood redistribution from the skin to the vital organs under the hypotensive condition.

Endothelial control of lower limb blood flow in chronic heart failure

BACKGROUND

Limitation of the blood supply to skeletal muscle in chronic heart failure may contribute to the symptoms of fatigue and diminished exercise capacity. The pathophysiology underlying this abnormality is not known. The purpose of this study was to assess the effect of endothelium dependent and independent vasodilator agents on blood flow in the leg of patients with heart failure.

METHODS AND RESULTS

Blood flow in the leg was measured in patients with heart failure (n = 20) and compared with that in patients with ischaemic heart disease and normal left ventricular function (n = 16) and patients with chest pain and normal coronary arteries (n = 8). External iliac artery blood flow was measured using intravascular Doppler ultrasound and quantitative angiography. Flow was recorded at rest and in response to bolus doses of the endothelium independent vasodilator, papaverine. Endothelium dependent responses were measured by infusion of acetylcholine and substance P. Mean (SEM) baseline blood flow was reduced at rest (2.9 (0.4) v 4.5 (0.3) ml/s, P < 0.001) and vascular resistance was raised (37.4 (3.6) v 27.1 (3.0) units, P < 0.05) in patients with heart failure compared with that in controls. The peak blood flow response to papaverine (8 mg), acetylcholine (10(-7)-10(-5) mol/l), and substance P (5 pmol/min) was reduced in heart failure, with greater impairment of the response to acetylcholine than substance P. There was a correlation between baseline blood flow in the heart failure group and diuretic dose (r = -0.62, P = 0.003), New York Heart Association classification (r = -0.65, P = 0.002), and left ventricular ejection fraction (r = 0.80, P = 0.0004).

CONCLUSIONS

There is reduced blood flow and raised vascular resistance at rest in the legs of patients with heart failure. The degree of impaired blood flow in the leg correlates with the severity of heart failure. There is impairment of the response to both endothelium dependent and independent vasodilators. Abnormal function of the vascular myocyte in heart failure may explain these results as would structural abnormalities of the resistance vessels.

PGI2 and nitric oxide involvement in the regulation of systemic and pulmonary basal vascular tone in the pig

In anesthetized, ventilated pigs we analyzed the involvement of nitric oxide (NO) and prostaglandins (PGs) in the regulation of systemic and pulmonary basal vascular tone. Endogenous release of NO was blocked by NG-nitro-L-arginine methyl ester (L-NAME) and prostanoid biosynthesis by indomethacin. Blocking NO raised pulmonary and systemic arterial pressure and vascular resistances. These effects show that in the pig there is continuous release of minute amounts of NO. Blocking prostanoid biosynthesis did not affect the vasoconstrictor effect of L-NAME on the pulmonary vascular bed, but significantly strengthened the hypertensive effect of L-NAME on the systemic vascular bed. These data show that different mechanisms regulate pulmonary and systemic vascular tone. Administration of a stable analogue of PGI2 to pigs pretreated with indomethacin reversed the systemic vasoconstrictor effect of L-NAME. In conclusion, our data show that NO especially modulates pulmonary vascular tone, while PGI2 preferentially modulates systemic vascular tone.

Effects of estrogen on nitric oxide biosynthesis and vasorelaxant activity in sheep uterine and renal arteries in vitro

OBJECTIVES

Our purpose was to determine whether estrogen alters the relaxation responses to bradykinin and superoxide dismutase of the uterine and renal arteries and to determine the role of nitric oxide in that response.

STUDY DESIGN

Ten nulliparous, ovariectomized nonpregnant sheep received either estradiol-17beta or vehicle solution. In vitro studies evaluating vasorelaxation were done with either bradykinin or superoxide dismutase. The nitric oxide inhibitor nomega-nitro-L-arginine methyl ester was used to determine the role of nitric oxide in this process. Nitric oxide synthase activity was assessed by measuring citrulline generation.

RESULTS

We found a dose dependency of relaxation to bradykinin and superoxide dismutase. Estrogen enhanced this response in uterine arteries. Estrogen increased citrulline generation in uterine but not renal arteries. Nomega-nitro-L-arginine methyl ester blocked relaxation responses and citrulline generation in both arteries.

CONCLUSION

In nonpregnant sheep we found that nitric oxide release and nitric oxide synthase activity is enhanced by estrogen in the uterine arteries but not in the renal arteries. Increases in nitric oxide synthase activity may be important in the hyperemic response of the uterus during estrus.

Longitudinal gradients for endothelium-dependent and -independent vascular responses in the coronary microcirculation

BACKGROUND

Coronary microvessels (< 300 microns in diameter) have been demonstrated to be important in the regulation of local resistance and flow. Recent studies also suggest that these microvessels are more responsive to physiological and pharmacological stimuli than conduit vessels. However, little is known regarding the relative sensitivity of different microvascular segments in response to flow (shear stress) and agonists. The goal of this study was to test the hypothesis that a longitudinal gradient for shear stress- and agonist-induced dilation exists in the coronary microcirculation.

METHODS AND RESULTS

Experiments were performed in four different sizes of porcine subepicardial coronary arterial microvessels: small arterioles (40 +/- 1-micron ID with resting tone); intermediate arterioles (60 +/- 1 micron); large arterioles (106 +/- 4 micron); and small arteries (179 +/- 9 microns). Vessels were isolated and cannulated to allow luminal pressure and flow to be independently controlled. All vessels developed active tone (to approximately 65% to 75% of maximum diameter) at their control luminal pressures and showed graded dilations to stepwise increases in shear stress (0 to 10 dynes/cm2). For arterioles, the magnitude of the dilations increased as vessel size increased. The highest shear stress produced 21 +/- 3%, 32 +/- 2%, and 52 +/- 5% increases in diameter in small, intermediate, and large arterioles, respectively. Small arteries dilated only 22 +/- 6%. The endothelium-dependent vasodilator substance P (SP) produced dose-dependent dilation of all vessels with a threshold at 10(-16) mol/L. Arterioles were maximally dilated at 10(-9) mol/L SP. However, this dose produced only 80% dilation in small arteries. The ED50 for SP was shifted to the right by two orders of magnitude in small arteries compared with the arterioles. Adenosine preferentially dilated small arterioles, and the dose-response curves shifted to the right for larger vessels. The thresholds for adenosine-induced dilation were 10(-12), 10(-11), and 10(-9) mol/L for small, intermediate, and large arterioles, respectively. The endothelium-independent vasodilator nitroprusside produced identical dose-dependent dilations in all vessel segments.

CONCLUSIONS

The results indicate that the pig coronary circulation exhibits a heterogeneity in physiological and pharmacological responses along the microvascular network. Small arterioles are more sensitive to adenosine, but large arterioles are more responsive to shear-stress stimulation. We speculate that site-specific preferential responses may play a crucial role in coordinating overall vascular function in the coronary microvascular network.

Effect of inhibitors of nitric oxide release and action on vascular tone in isolated lungs of pig, sheep, dog and man

1. The actions of inhibitors of the release or action of nitric oxide (NO) on pulmonary vascular resistance (PVR) were investigated in lungs isolated from pig, sheep, dog and man. 2. In pig, sheep and human lungs perfused with Krebs-dextran solution, both N omega-nitro-L-arginine methyl ester (L-NAME; 10(-5) M) and Methylene Blue (10(-4) M) increased basal PVR. This increase was reversed by sodium nitroprusside (10(-5) M). In pig lungs N omega-monomethyl-L-arginine (10(-4) M) increased PVR by 154%. This increase was partially reversed by L-arginine (10(-3) M). L-NAME had no effect in dog lungs. 3. Pulmonary artery pressure-flow (PPA/Q) relationships were studied over a wide range of flows. In pigs, sheep and human lungs perfused with Krebs-dextran solution, L-NAME increased the PPA/Q slope. This increase was reversed by sodium nitroprusside. In dog lungs L-NAME had no effect. 4. In blood-perfused lungs, the respective responses to L-NAME were similar to those observed with saline. Acute hypoxia in pig and dog lungs increased intercept pressure. Addition of L-NAME during hypoxia increased the PPA/Q slope in both species. 5. In the human, there was no difference in the absolute increase of PVR or PPA/Q slope elicited by L-NAME between hypertensive and control lungs. 6. We conclude that NO is continuously released in the pulmonary vascular bed of pig, sheep and humans under normoxic conditions. In dog lungs inhibition of NO synthesis increases PVR only under hypoxic conditions. In human lungs with pulmonary hypertension, NO is still released under basal conditions.

Intraluminal flow-initiated hyperpolarization and depolarization shift the membrane potential of arterial smooth muscle toward an intermediate level

We examined the effect of intraluminal flow of physiological saline on the membrane potential of vascular smooth muscle cells in isolated rabbit cerebral arteries. Intraluminal flow (20 microL/min) caused a depolarization of 4.8 +/- 0.7 mV in muscle cells with a resting membrane potential of -62.5 +/- 1.2 mV (n = 19). However, when cells were depolarized to -48.7 +/- 1.8 mV using histamine and serotonin, the response to intraluminal flow was the opposite, a hyperpolarization of 5.6 +/- 1.0 mV (n = 9). These opposing effects of flow on membrane potential appear to balance at -57.8 +/- 1.1 mV (n = 31). Our results suggest that intraluminal flow may affect the level of basal tone present in arteries in vivo through modulating the membrane potential of vascular smooth muscle cells by concurrently activated depolarization and hyperpolarization.

On a mathematical theory of embolism

A theory of embolism based on an optimization model of blood flow is proposed and used to explain the topographic distribution of emboli in arterial trees.

Flow-induced relaxation of the rabbit middle cerebral artery is composed of both endothelium-dependent and -independent components

BACKGROUND AND PURPOSE

The flow-induced relaxation of a branch of the rabbit middle cerebral artery was examined to determine if an endothelial-independent as well as -dependent component occurs in pial as well as systemic small arteries and the possible role of products of the cyclooxygenase and the L-arginine nitric oxide synthase pathways.

METHODS

Intraluminal flow was achieved by the infusion of a tissue bath solution into isometrically mounted rabbit pial arteries in a resistance artery myograph through a small pipette.

RESULTS

Intraluminal flow caused relaxation of the artery segment precontracted with 10 microM histamine. Treatment of endothelium-intact vessels with the nitric oxide synthase inhibitors NG-nitro-L-arginine (L-NNA) (100 microM) or NG-nitro-L-arginine methyl ester (L-NAME) (0.3 mM) significantly reduced the relaxation at flow rates of 5-30 microliters/min. This effect was partially reversed by 1 mM L-arginine. These inhibitors had no effect on the flow-induced relaxation of endothelium-denuded vessels. L-NNA did not influence the relaxation to 1 and 3 microM papaverine. Exposure to 10 microM aspirin, 10 microM indomethacin, or 300 nM tetrodotoxin had no effect on the flow-induced relaxation of either endothelium-intact or -denuded vessels (n = 6). Flow-induced relaxation was attenuated, but not abolished, by removal of the cerebrovascular endothelium. This reduction was not statistically significant.

CONCLUSIONS

These results show that intraluminal flow caused relaxation of a branch of the rabbit middle cerebral artery, in part through a mechanism sensitive to inhibitors of nitric oxide synthase, most likely the generation of nitric oxide from the vascular endothelium. The major component of the relaxant response is independent of the endothelium and of nitric oxide synthesis through an L-NNA- or L-NAME-sensitive mechanism. The relaxation does not involve cyclooxygenase products nor neurogenic mediators. These results suggest that pial arteries, like those of the rabbit ear, exhibit a novel mechanism for the flow-induced relaxation of agonist-induced tone that is intrinsic to the tissues of the vascular wall subjacent to the endothelium.

A new mounting technique for perfusion of isolated small arteries: the effects of flow and oxygen on diameter

There is at present no suitable technique available for performing pressure-flow studies in isolated small arteries (i.e., less than 500 microns), in which the effects of flow and pressure on artery dimensions can be studied independently. A new mounting technique is presented in which the ends of a vessel segment are cemented to the inner surface of two cannulae, with a tip diameter slightly larger than the outer diameter of the vessel, using two-component human fibrin glue. By means of this technique the pressure drop over the cannulae can be made small. First the effect of the glue on constrictive properties is studied. The glue used has no significant influence on the norepinephrine dose-response relation or on the relaxation in response to 1.0 microM acetylcholine. Small mesenteric arteries of the rabbit with outer passive diameters (at zero pressure) of 315 microns (+/- 22 microns SEM) are studied with this method. The effects of flow (shear stress) and oxygen are investigated (vessels are preconstricted (30%) with norepinephrine (1-2 microM)). The flow range used resulted in shear stresses between 0 and 290 dyn.cm-2, a range including values found in vivo. There is a significant (P less than 0.001) decrease in diameter when flow is increased, and hypoxia (pO2 less than 30 mm Hg) augmented the preconstriction with norepinephrine (P = 0.002). The flow effect and the oxygen influence are independent of each other. These results are similar to our previous findings in the femoral artery of the rabbit (diameter about 1200 microns).

Dependence of traumatic vasospasm on blood flow in the central artery of the ear in rabbits

The effect of total obstruction of blood flow on traumatically induced vasospasm was studied in the central arteries of rabbits' ears. All side branches were ligated along a 7 cm segment of the artery. Obstruction of blood flow was achieved by ligation of the central artery distal to this segment. The inner diameter of the artery was measured microscopically after transillumination. A standardised injury was induced with a clip applying forceps. The duration, intensity (reduction of diameter) and severity (integrated change in diameter over time) of the resulting spasm were assessed. Spasm induced after cessation of blood flow lasted significantly less time (mean (SEM), 6.0 (2.0) minutes compared with 18.1 (2.5) min) and was less severe than spasm induced when the blood flow was intact (1.6 (0.6) mm/min compared with 6.2 (0.6) mm/min). The results may be explained by local accumulation of vasodilating substances in the injured segment of the vessel in the absence of blood flow.

Differential effects of L-arginine on the inhibition by NG-nitro-L-arginine methyl ester of basal and agonist-stimulated EDRF activity

1. An isolated, buffer-perfused rabbit ear preparation was used to investigate the influence of NG-nitro-L-arginine methyl ester (L-NAME) on endothelium-dependent vasodiltation and modulation of vasoconstrictor responses and vascular conductance. 2. Acetylcholine (0.55 pmol-1.6 nmol) caused dose-related vasodilatation of preparations constricted by the combination of 5-hydroxytryptamine and histamine (both 1 microM), with an ED50 = 31.1 +/- 7.8 pmol and a maximum dilatation of 69.9 +/- 4.3%. In the presence of 10 microM L-NAME the dose-response for vasodilator effects was shifted significantly (P less than 0.001) to the right (ED50 = 3.07 +/- 1.18 nmol) and there was a significant (P less than 0.01) depression of the maximum response (Rmax = 44.3 +/- 4.0%). The higher concentration of 100 microM L-NAME completely abolished vasodilatation to acetylcholine. L-Arginine (10 mM) did not reverse the inhibitory actions of L-NAME at either concentration. 3. L-NAME 100 microM, augmented vascular tone induced by 1 microM 5-hydroxytryptamine and 1 microM histamine, thus altering the characteristics of both pressure/flow and conductance/flow relationships such that conductance was reduced at all flow rates. The augmentation of constrictor tone was reversed in a concentration-dependent manner by L-arginine (10 microM-10 mM) and the effect of L-NAME on the conductance/flow relationships was similarly reversed by 10 mM L-arginine. The augmentation of tone was endothelium-dependent as it did not occur following functional destruction of the endothelium by perfusion of the vascular bed with the detergent CHAPS (0.3%) for 150s. 4. In conclusion, L-NAME is a potent inhibitor of agonist-induced endothelium-dependent vasodilatation. L-NAME reduces vascular conductance in pharmacologically constricted preparations and this emphasizes the important role of EDRF in vascular regulation. The ability of L-arginine to reverse L-NAME-induced inhibition of basal EDRF activity but not L-NAME-induced inhibition of agonistinduced endothelium-dependent relaxations suggests that there is pharmacological heterogeneity in the mechanisms responsible for the conversion of L-arginine to EDRF.

Hypoxic vasodilatation in isolated, perfused guinea-pig heart: an analysis of the underlying mechanisms

1. The mechanisms underlying hypoxic dilatation of coronary arteries were studied in isolated guinea-pig hearts perfused with physiological salt solution at 37 degrees C. The hearts were perfused at a constant rate of 3-10 ml min-1; coronary perfusion pressure (CPP) and isovolumetric left ventricular pressure (LVP) were measured with piezoresistive transducers. 2. Addition of the K+ channel opener cromakalim (500 nM) to the perfusate caused a maximal vasodilatation in beating hearts, i.e. a decrease in CPP of about 50%. Switching from normal perfusate (partial pressure of O2 (PO2), 650-700 mmHg) to hypoxic perfusate (PO2, 9-10 mmHg) caused a similar vasodilatation. Both of these effects were prevented by 2 microM-glibenclamide, a blocker of ATP-sensitive potassium channels. Hypoxic vasodilatation was accompanied by a marked decrease in LVP, which was reduced by 56 +/- 22% (mean +/- S.D.) in the presence of glibenclamide. 3. In hearts arrested by increasing the K+ concentration of the perfusate to 15 mM, the addition of the adenosine-uptake inhibitor dipyridamole evoked a maximal vasodilatation and this was inhibited by 76 +/- 7% in the presence of glibenclamide. 4. The adenosine antagonist 8-phenyltheophylline (8-PT; 5 microM) inhibited the vasodilatation induced by dipyridamole by 88 +/- 10%. In contrast, hypoxic vasodilatation was unaffected by 5 microM 8-PT. This suggests that hypoxic dilatation of coronary arteries is not mediated by release of adenosine from cardiomyocytes. 5. In order to test whether release of endothelium-derived relaxing factor (EDRF) contributed to hypoxic vasodilatation we blocked EDRF synthesis with N omega-nitro-L-arginine (NNA). When applied at a perfusion rate of 10 ml min-1 to arrested hearts, 10 microM-NNA increased CPP by 35% and prolonged the delay between application of hypoxic solution and half-maximal vasodilatation from 52 +/- 9 to 129 +/- 29 s. 6. Under control conditions the relation between perfusion rate and the CPP measured in the steady state was linear. In the presence of 10 microM-NNA coronary resistance was increased more than twofold at low perfusion rates; at perfusion rates between 4 and 10 ml min-1 coronary resistance decreased progressively. This change in the pressure-flow relationship may be responsible for the alterations in the time course of hypoxic vasodilatation induced by NNA. 7. In order to test whether changes in energy metabolism in coronary smooth muscle cells were responsible for hypoxic vasodilatation we blocked glycolysis by replacing the glucose in the perfusate with deoxyglucose (DOG).(ABSTRACT TRUNCATED AT 400 WORDS)

Augmentation of endothelium-independent flow constriction in pial arteries at high intravascular pressures

The effects of an increase in intraluminal pressure and flow on the diameter and active smooth muscle tone of pial arteries was studied in perfused segments. Resistance arteries (approximately 250-300 microns i.d.) were perfused under controlled pressure and flow conditions, and changes in arterial diameter registered with an automated video device. In any particular segment, diameter measurements were normalized to that observed at 5 mm Hg. Changes in active wall force were determined by relating the observed diameter under a particular set of conditions to the diameter at the same intramural pressure when smooth muscle tone was inhibited (calcium-free physiological saline solution) and to the diameter when smooth muscle cells were activated close to maximum (KCl; 89 mM). At 60 mm Hg, the diameter decrease of 21% in the absence of flow represented stretch-induced tone. No additional changes in diameter were encountered with a flow of 20 microliters/min. Diameter decreased a further 7% at 100 microliters/min. When intraluminal pressure was 90 mm Hg, diameter decreased 39% without flow. Additional constriction of 10% and 19% occurred at flows of 20 and 100 microliters/min, respectively. At the higher pressure, the vasoconstriction occasioned by flow was significantly greater than that at the lower pressure. After endothelium inactivation by passing hypo-osmotic Krebs' solution followed by air through the segment, mean diameter was less at each combination of pressure and flow, although this difference did not reach statistical significance. The diameter reductions to increases in pressure from 60 to 90 mm Hg and to flow at 40 microliters/min were not altered by endothelium inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)

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

An in vitro bioassay system was developed to study endothelium-mediated, shear stress-induced, or flow-dependent generation of endothelium-derived relaxing factor (EDRF). Monolayers of aortic endothelial cells were grown on a rigid and large surface area of microcarrier beads and were packed in a small column perfused with Krebs bicarbonate solution. The perfusate was allowed to superfuse three endothelium-denuded target pulmonary arterial strips arranged in a cascade. Fluid shear stress caused a flow-dependent release of EDRF from the endothelial cells. The action of EDRF was abolished by oxyhemoglobin and methylene blue, and the generation of EDRF in response to shear stress was markedly inhibited or abolished by NG-nitro-L-arginine, by NG-amino-L-arginine, by calcium-free extracellular medium, and by depleting endothelial cells of endogenous L-arginine. Addition of L-arginine to arginine-deficient but not arginine-containing endothelial cells rapidly restored the capacity of shear stress and bradykinin to generate EDRF. These observations indicate that fluid shear stress causes the generation of EDRF with properties of nitric oxide from aortic endothelial cells and that the bioassay system described may be useful for studying the mechanism of mechanochemical coupling that leads to nitric oxide generation.

Blood flow and optimal vascular topography: role of the endothelium

We have used x-ray microangiography to investigate the influence of EDRF and endothelin-1 on arterial diameters (70-800 microns) at bifurcations in the isolated rabbit ear and the "optimality" of its branching geometry. The median value of the junction exponent x (which is given by d0x = d1x + d2x, where d0, d1 and d2 are parent and daughter artery diameters respectively) was close to 3 at different flow rates in unconstricted preparations. When x = 3, branching geometry is optimal in that i) power losses and intravascular volume are both minimised, and ii) fractal considerations suggest that the total surface area for metabolic exchange is maximised. Under conditions of vasoconstriction (by 5HT/histamine) the junction exponent deviated from its control value but was restored towards 3, both by basal and by acetylcholine-stimulated EDRF activity. In contrast, endothelin-1 caused a dose-dependent reduction in the junction exponent from its optimal value 3. This suggests that the endothelium helps to optimise microvascular function through EDRF but not endothelin-1 release.

Vascular activities of the endothelins

The endothelins are a family of novel 21 amino-acid peptides and are the most potent vasoconstrictor substances yet discovered. The endothelins not only produce prolonged pressor responses in intact animals but they also constrict large and small arterial and venous vessels studied as isolated vascular preparations, influence autonomic transmission, exert positive inotropic effects on the heart and have been shown to be capable of releasing EDRF, prostanoids and atrial natriuretic factor. Release of endothelins occurs after de novo synthesis which may be stimulated by various agonists, fluid-flow and possibly hypoxia. The endothelins have been implicated in the pathophysiology of a variety of cardiovascular disorders but their precise role remains to be elucidated.

Activities of endothelin-1 in the vascular network of the rabbit ear: a microangiographic study

1. The effects of endothelin-1 on perfusion pressure and on arterial and venous diameters were examined simultaneously in a rabbit isolated ear preparation perfused with physiological buffer. The effects of hypoxia and inhibition of endothelium-derived relaxant factor (EDRF) activity on vascular responses to endothelin-1 were also investigated. 2. Endothelin-1 was potent at increasing perfusion pressure (ED50 = 46.7 +/- 11.0 pmol; Rmax = 85.3 +/- 5.3 mmHg). The potency and maximum reactivity were not significantly affected by hypoxia, inhibition of EDRF activity with 50 microns N-nitro-L-arginine methyl ester (NAME) or a combination of hypoxia and NAME. 3. Endothelin-1 caused equipotent dose-dependent constrictions of the first four generations of arterial branch vessels (G1-G4) but did not influence the diameter of the central ear artery except at high doses of the peptide when paradoxical dilatation' was observed. The peptide was also equipotent at causing constriction of the smaller venous vessels (V1-V4) but did not affect the large veins (V0). 4. Under conditions of hypoxia the potency of endothelin-1 was reduced in G2 and G3, was unaffected in G4 and the peptide did not significantly constrict either G0 or G1. Hypoxia reduced the potency of endothelin-1 in the smaller venous vessels (V1-V4), but conversely unmasked a marked constriction of the large veins (V0), which was not observed under normoxic conditions. 5. NAME 50 micron abolished the vasodilator effects of acetylcholine in this preparation. Inhibition of EDRF activity with NAME under normoxic conditions did not influence the constrictor activity of endothelin-1 on the arterial or venous branch vessels. However, inhibition of EDRF activity under hypoxic conditions prevented the reduction of potency of endothelin-1 as a constrictor of arterial and venous branch vessels which occurred in hypoxia. In the presence of NAME endothelin-1 constricted VO in both normoxia and hypoxia with equipotency but the maximum effect was greatest in hypoxia. 6. In conclusion, endothelin-1 is a powerful vasoconstrictor which acts with greater potency in veins than arteries in the rabbit isolated ear. Although hypoxia does not influence pressor responses it nevertheless alters the spatial pattern of vasoconstriction. In particular hypoxia unmasks constriction of the large veins by endothelin-1. Constriction of these veins was also observed in the absence of EDRF in normoxia, but to a much lesser degree so that the effect of hypoxia may only be partially due to reduced EDRF activity. Hypoxia may therefore directly or indirectly increase the sensitivity of the main veins to endothelin-1.

Quantitative in vivo studies of hyperemia in the course of the tissue response to biomaterial implantation

Hyperemia associated with an inflammatory response has been investigated in rats, by using four different experimental models, i.e., "positive" and "negative" polymer implants from the pharmacopea, operative control, and abscess induced by turpentine oil. 133Xenon clearance, infrared thermography and Laser Doppler Flowmetry (LDF) were used to monitor the subcutaneous local hemodynamic changes from 1 to 40 postoperative days. LDF proved to be a sensitive, reproducible method, able to discriminate positive from negative implants already at the 3rd postoperative day and up to 40 days. This increased local blood flow was also visualized at the site of positive implants at the 14th and 21st postoperative days by means of 133Xe Clearance. Additional information obtained by infrared thermography allowed discrimination between positive implants and control sites but only at the very early stage (1 to 3 days). The significance of the different data collected by the three techniques was correlated with histological events occurring at the different implant sites. LDF may therefore represent a useful technique for noninvasive semiquantitative assessment of tissue response to biomaterials.

Basal EDRF activity helps to keep the geometrical configuration of arterial bifurcations close to the Murray optimum

We have used X-ray microangiography to investigate the hypothesis that the potent endogenous vasodilator endothelium-derived relaxing factor (EDRF) contributes to the maintenance of "optimality" in vascular branching by modulating the diameters of the parent (D0) and daughter (D1 and D2) arteries at bifurcations. Five anatomically different types of bifurcation were studied in buffer-perfused rabbit ear preparations both under resting conditions and after pharmacological constriction by 5-hydroxytryptamine (5HT). A range of flow rates (1-5 ml min-1) was employed as release of EDRF from endothelial cells is stimulated by shear stress. Experimental data obtained in the presence and absence of EDRF activity were compared with theoretical predictions in three ways. (1) Junction exponents (x) were determined at each bifurcation from the equation Dx1 + Dx2 = Dx0, and their frequency distributions constructed. Murray (1926a, Proc. natn. Acad. Sci., U.S.A. 12, 207-214; 1926b, J. gen. Physiol. 9, 835-841.) proposed that x will be exactly 3 if power losses and intravascular volume are minimized simultaneously. In unconstricted preparations, either in the presence or absence of EDRF activity, and in preparations constricted by 0.1 microM 5HT in the presence of EDRF activity, the modes and medians of the frequency distributions of x were found to be close to 3 at all flow rates. In contrast, in 0.1 microM 5HT-constricted preparations in the absence of EDRF activity, no single mode common to all flow rates was apparent and medians were significantly larger at all flow rates. (2) Theoretically "optimal" branching angles were derived from experimental diameter measurements using four mathematical models which minimize respectively the total surface area, total volume, total drag (shear stress) and total power losses at bifurcations (Murray, 1926b). These calculated branching angles were then compared with actual branching angles. EDRF activity was found to be necessary for accurate prediction of branching angles by the minimum volume and power loss models in 5HT-constricted but not in resting preparations. (3) For each model or "minimization principle", there is an optimal mathematical relationship between the junction exponent, x, and the angle between daughter arteries, psi 12, at a bifurcation (Roy & Woldenberg, 1982, Bull. math. Biol. 44, 349-360.) Experimentally determined values of x and psi 12 agreed closely with those predicted both by the minimum volume and the minimum power loss principles, except again in 5HT-constricted preparations in the absence of EDRF activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived relaxing factors. A perspective from in vivo data

We review below published studies of endothelium-dependent vasodilation in vivo. Endothelium-dependent vasodilation has been demonstrated in conduit arteries in vivo and in the cerebral, coronary, mesenteric, and femoral vascular beds as well as in the microcirculation of the brain and the microcirculation of cremaster muscle. The available evidence, although not complete, strongly suggests that the endothelium-derived relaxing factor generated by acetylcholine in the cerebral microcirculation is a nitrosothiol. The endothelium-derived relaxing factor generated by bradykinin in this vascular bed is an oxygen radical generated in association with enhanced arachidonate metabolism via cyclooxygenase. In the microcirculation of skeletal muscle, on the other hand, the vasodilation from bradykinin is mediated partly by prostacyclin and partly by an endothelium-derived relaxing factor similar to that generated by acetylcholine. Basal secretion of endothelium-derived relaxing factor is controversial in vivo but is usually present in vitro. On the other hand, it appears that endothelium-derived relaxing factor mediates flow-dependent vasodilation in both large vessels and in the microcirculation in vivo. The generation and release of endothelium-derived relaxing factor from endothelium may be abnormal in a variety of conditions including acute and chronic hypertension, atherosclerosis, and ischemia followed by reperfusion. Several mechanisms for these abnormalities have been identified. These include inability to generate endothelium-derived relaxing factor or destruction of endothelium-derived relaxing factor by oxidants after its release in the extracellular space. These abnormalities in endothelium-dependent relaxation may contribute to the vascular abnormalities in these conditions.

Regulation of large cerebral arteries and cerebral microvascular pressure

Resistance of large arteries appears to be greater in the cerebral circulation than in other vascular beds. Large arteries contribute importantly to total cerebral vascular resistance and are major determinants of local microvascular pressure. Recent studies have shown that resistance of large arteries and cerebral microvascular pressure are affected by several physiological stimuli, including changes in systemic blood pressure, increases in cerebral metabolism, activity of sympathetic nerves, and humoral stimuli such as circulating vasopressin and angiotensin. Stimuli such as sympathetic stimulation and vasopressin produce selective responses of large arteries and, thereby, regulate microvascular pressure without a significant change in cerebral blood flow. These findings lead to the new hypothesis that the brain may be sensitive to changes in cerebral microvascular pressure, resulting in activation of compensatory neurohumoral mechanisms. Important changes occur in large cerebral arteries under pathophysiological conditions. Chronic hypertension increases resistance of large cerebral arteries, which protects the microcirculation against hypertension. Atherosclerosis potentiates constrictor responses of large cerebral arteries to serotonin and thromboxane, which may contribute to vasospasm and transient ischemic attacks.

EDRF-mediated dilatation in the rat isolated perfused kidney: a microangiographic study

1. X-ray microangiographic techniques were used to study the influence of endothelium-derived relaxing factor (EDRF) on vasomotion in the isolated, intact, buffer-perfused kidney of the rat. The main renal (R0), segmental (R1) and interlobar (R2) arteries (control diameters ca. 600, 400 and 300 microns respectively) were studied quantitatively. 2. Inhibition of basal EDRF activity by haemoglobin (1 microM) did not elevate perfusion pressure or constrict R0, R1 and R2 in control preparations, implying a low level of spontaneous myogenic tone. In preparations preconstricted by 0.3 microM methoxamine, haemoglobin caused a further rise in perfusion pressure and amplified constrictor responses in R1 and R2 while also inducing 'paradoxical' dilatation of R0. 3. A spatially heterogeneous pattern of diameter responses (constriction of R2 and R1 with minimal dilatation of R0) was observed with two concentrations of methoxamine (0.3 microM and 3 microM). The magnitude of these responses was, however, smaller with 3 microM than 0.3 microM methoxamine, even though it increased perfusion pressure to a greater extent (88 mmHg cf. 24 mmHg). This 'paradoxical' behaviour indicates more pronounced constriction of distal arteries (which could not be resolved quantitatively) with 3 microM methoxamine. 4. In contrast to the heterogeneity of constrictor responses induced by methoxamine, the dilator action of acetylcholine was spatially homogeneous: log IC50 values calculated from the diameter changes induced in R0, R1 and R2 were similar and, moreover, equivalent to that calculated from the corresponding alterations in perfusion pressure. The fall in perfusion pressure induced by an approximately median effective concentration of acetylcholine (0.3 microM) was completely reversed by haemoglobin, consistent with the involvement of EDRF, although, reversal of the acetylcholine-induced dilatation of R0, R1 and R2 was not observed. 5. The results are consistent with the idea that constriction of distal vessels can attenuate and even directionally reverse intrinsic constrictor responses in the proximal R0, RI and R2 'feed' arteries by producing an overriding increase in 'upstream' pressure. This effect explains the paradoxical dilatation of Ro induced by haemoglobin in the presence of 0.3 microM methoxamine, the smaller magnitude of the diameter changes induced in R0, RI and R2 by 3 microM as compared to 0.3 microM methoxamine, and the failure of haemoglobin to reverse the acetylcholine-induced dilatation of R0, R1 and R2.