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Division of Cardiovascular Surgery, Research Institute of Angiocardiology, Faculty of Medicine, Kyushu University, Fukuoka 812-8582, Japan
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The effects of pulsatility in blood flow on endothelium-derived nitric oxide (EDNO) release in the peripheral vasculature were investigated. The basal and flow-stimulated EDNO release were compared between pulsatile and nonpulsatile systemic flows before and after the administration of NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). Peripheral vascular resistance (PVR) was significantly lower in pulsatile flow than in nonpulsatile flow, but this difference disappeared after L-NMMA. The percent increase in PVR by L-NMMA was significantly larger in pulsatile flow. In reactive hyperemia in the hindlimb, the peak flow did not differ; however, both the repayment flow and the duration were significantly larger in pulsatile flow. Percent changes of these parameters by L-NMMA were significantly larger in pulsatile flow. These data indicated that pulsatility significantly enhances the basal and flow-stimulated EDNO release in the peripheral vasculature under in vivo conditions. We also studied the involvement of the Ca2+-dependent and Ca2+-independent pathways in flow-induced vasodilation using calmodulin inhibitor calmidazolium and tyrosine kinase inhibitor erbstatin A. PVR was significantly elevated by erbstatin A but not by calmidazolium, suggesting that flow-induced vasodilation was largely caused by tyrosine kinase inhibitor-sensitive activation of NO synthase.
nitric oxide synthase; reactive hyperemia; calmodulin; tyrosine kinase; NG-monomethyl-L-arginine
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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THE NECESSITY OF PULSATILITY in the systemic blood flow remains controversial. Earlier studies revealed the advantageous effects of pulsatile systemic blood flow over nonpulsatile blood flow on the peripheral circulation, organ function, or metabolism (6, 15, 26); however, these mechanisms have yet to be clearly elucidated. Endothelial cells have traditionally been recognized to produce and release vasoactive substances to control the vascular tone and peripheral circulation by several mechanical and chemical stimuli (2). Nitric oxide (NO) is one of the endothelium-derived relaxing factors, and a lot of in vitro studies using cultured endothelial cells have shown that pulsatility is one of the mechanical stimuli that enhance the production of endothelium-derived NO (EDNO) (23) and have shown that several mechanotransduction pathways exist to activate EDNO production. In contrast, in vivo studies have yet to adequately demonstrate the beneficial effects of pulsatility. Therefore, the present study was undertaken with the following objectives using the in vivo canine model: 1) to determine whether the increased basal release of EDNO accounts for the decreased vascular resistance in the pulsatile blood flow, 2) to determine whether flow-stimulated EDNO release, one of the regulatory functions of endothelial cells, is enhanced in pulsatile blood flow, and 3) to determine what activation pathway is responsible for flow-induced EDNO-mediated vasodilation.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Surgical Preparation
Eighteen adult mongrel dogs weighing 16.4-26.7 kg (21.9 ± 3.1 kg) were used in this study. Anesthesia was induced with an intravenous thiamylal sodium (25 mg/kg) injection. After endotracheal intubation, mechanical ventilation with a mixture of room air and 100% oxygen was performed with an artificial respirator, and 10 µg/kg of fentanyl was slowly injected. The arterial blood gases and blood pH were maintained within the physiological ranges by adjusting the respiratory rate and tidal volume and also by administering sodium bicarbonate. The arterial pH, PCO2, and PO2 were kept at the range of 7.35-7.45, 35-45 mmHg, and 90-120 mmHg, respectively. The dogs were then placed in the right lateral decubitus position, and catheters were inserted into the descending aorta and inferior vena cava through the left femoral artery and vein for pressure monitoring, blood sampling, and drug administration. Anesthesia was then maintained with a continuous intravenous infusion of fentanyl (10 µg · kg
1 · h1),
midazolam (0.5 mg · kg1 · h1),
and vecuronium bromide (0.2 mg · kg1 · h1).
The proximal site of the right femoral artery was carefully dissected,
and an ultrasonic flow probe was placed around the artery. Two branches
of the artery were cannulated for local pressure monitoring and drug
infusion. A branch of the femoral vein was also cannulated to monitor
the local venous pressure. These pressure values and heart rate were
monitored continuously on a multichannel oscillograph (Polygraph 360 system, NEC Sanei Kogyo, Tokyo, Japan), and all hemodynamic parameters
were recorded on an analog-to-digital converter (MacLab System, AD
Instruments, Dunedin North, New Zealand) simultaneously. A left
thoracotomy was then performed through the fifth intercostal space.
After 300 U/kg of heparin was given, two cannulas (28-Fr.; Polystan
A/S, Varlose, Denmark) were inserted into the left atrium through the
appendage and into the left ventricle through the apex. An
infusion cannula (5.2 mm Sarns; 3M Health Care, Ann Arbor, MI) was
inserted into the thoracic descending aorta. The pulsatile flow was
produced by an air-driven, diaphragm-type blood pump (TCT 20; Toyobo,
Osaka, Japan) while a nonpulsatile flow was generated with a
centrifugal pump (Biopump; Bio-Medicus, Eden Prairie, MN). These pumps
were serially connected. The centrifugal pump flow rate was increased,
and the circulatory blood volume, if necessary, was modulated to
introduce all blood from left side of the heart into the extracorporeal
circuit. The pulsatile pump was driven by nonelectrocardiographic
synchronization, the pulse rate was set at 120 pulses/min, and the
pulse pressure was set at 45-50 mmHg. A clamp-on type ultrasonic
flow probe was placed on the line. The right femoral artery flow rate
and pump flow rate were measured continuously (two channel ultrasonic
blood flowmeter T-208; Transonic Systems, Ithaca, NY). A blood
reservoir was placed within the circuit to control the circulatory
blood volume to maintain a stable systemic circulatory flow and to keep the same hematocrit level. A heat exchanger was also placed within the
circuit to control the blood temperature. The esophageal temperature and blood temperature were carefully kept at the range of
37-38°C, respectively. The hematocrit and
electrolytes were maintained at the same values throughout the
experiment within each case, and the average hematocrit value was
31.6%. Indomethacin (3 mg/kg) and hexamethonium bromide (5 mg/kg) were
administered 30 min before the initiation of the extracorporeal
circulation. Hexamethonium bromide (5 mg · kg1 · h1)
was also continuously infused throughout the experiment.
Humane Animal Care
All animals received humane care in compliance with the Principles of Laboratory Animal Care formulated by the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals prepared by the Institute of Laboratory Animal Resources and published by the National Institute of Health (NIH Publication No. 86-23, Revised 1985). This experiment was reviewed by the Committee of the Ethics on Animal Experiments in the Faculty of Medicine, Kyushu University and the Law (No. 105) and Notification (No. 6) of the Japanese government.Study Protocol
Effects of pulsatility on basal hemodynamics. In eight dogs, after 30 min of stabilization with pulsatile (or nonpulsatile) systemic perfusion, the systemic blood pressure (BP), central venous pressure (CVP), and pump flow rate were measured. The pump flow rates were kept at the same level in both flow conditions. The femoral artery pressure (FAP), femoral vein pressure (FVP), and femoral artery flow rate were also measured. Total systemic vascular resistance (TSVR) and peripheral vascular resistance (PVR) were calculated by the following formula: TSVR = [(mean BP
mean CVP)/pump flow rate] × 80 (dyn · s · cm5),
PVR = [(mean FAP mean FVP)/femoral artery flow
rate] × 80 (dyn · s · cm5).
After the perfusion mode was switched, the hemodynamics were stabilized
for 20 min and the same parameters were then measured. The order of
pulsatile and nonpulsatile perfusion was randomly decided.
Effects of pulsatility on plasma nitrite/nitrate, catecholamine,
and angiotensin II levels. Blood samples were taken from the
central venous catheter and centrifuged by 3,000 rpm for 40 min, and
the plasma was immediately frozen at 80°C until
the measurements were performed. The stable metabolites of NO, nitrite and nitrate, were measured by a chemiluminescence NO analyzer (Sievers). Briefly, the plasma was incubated with Aspergillus nitrate reductase to reduce nitrate into nitrite and then convert nitrite into NO by the addition of hydrochloric acid. In the analyzer the amount of NO was determined by measuring the luminescence generated
in the presence of ozone (31). The plasma epinephrine and
norepinephrine levels were measured by high-performance liquid chromatography, and the plasma angiotensin II level was measured by the
radioimmunoassay double-antibody method.
Effects of pulsatility on reactive hyperemia in the hindlimb.
After we recorded the basal hemodynamic values for 5 min, the right
femoral artery flow rate was adjusted to the same value by modulating
the pump flow rate. The right femoral artery was then clamped for 20 s.
After the release of the clamp, the flow-time response curve and
pressure in the right femoral artery were then continuously recorded
until the femoral artery flow rate returned to the basal level. In this
reactive hyperemia, the peak flow rate, the duration of reactive
hyperemia, and the repayment flow were determined. The duration of
reactive hyperemia was represented by t1/2, which
was defined as the time between the peak flow and the point in which
the flow returned to 50% of the peak (16). The repayment flow during
reactive hyperemia was calculated as follows: repayment flow = total
flow during reactive hyperemia (ml · min1 · s1) [basal flow rate (ml/min) × duration of reactive
hyperemia (s)].
Effects of L-NMMA on basal hemodynamics and
reactive hyperemia in hindlimb. Ten minutes after the end of the
local administration of L-NMMA (7.5 µmol/ml, 1 ml/min × 8 min; Research Biochemicals International, Natick, MA), an
L-arginine analog, which has been shown to specifically
inhibit the formation of NO from the L-arginine, was
administered through a branch of the femoral artery, and the basal
hemodynamics and reactive hyperemia were repeatedly measured in the two
flow conditions. The percent changes [defined as
(hemodynamic values in post-L-NMMA those in
pre-L-NMMA)/those in pre-L-NMMA] of PVR,
the femoral artery flow rate, t1/2, and the
repayment flow of reactive hyperemia were calculated to investigate the effects of pulsatility on the basal and flow-stimulated EDNO release in
the peripheral vascular bed.
Effects of calmodulin inhibitor and tyrosine kinase inhibitor on PVR. In five dogs, after the recording of basal hemodynamics, calmidazolium, a calmodulin inhibitor, (120 µg/ml, 5 ml/min × 10 min; Sigma Chemical, St. Louis, MO), was infused into the femoral artery. After 10 min of stabilization, basal hemodynamics were recorded. L-NMMA was then administered, and the same parameters were recorded. Calculated PVR was compared in these three conditions between pulsatile flow and nonpulsatile flow. In another five dogs, erbstatin A , a tyrosine kinase inhibitor (100 µg/ml, 2 ml/ min × 10 min; Tocris Cookson, Ballwin, MO), was infused into the femoral artery, and the same parameters were recorded and compared.
Statistics
All values are expressed as means ± SE. Comparisons of the variables between pulsatile and nonpulsatile blood flow as well as between before and after drug administration were performed with the paired t-test. Differences were considered to be significant when the P value was <0.05.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Effects of Pulsatility on Basal Hemodynamics
The pulse pressure and maximum rate of pressure development in pulsatile blood flow were 47.6 ± 3.7 mmHg and 544.7 ± 29.5 mmHg/s. The systemic hemodynamic variables in pulsatile and nonpulsatile blood flow are summarized in Table 1. The blood pressure and TSVR were lower in pulsatile than in nonpulsatile blood flow (P < 0.05) at the same systemic blood flow rate. The local hemodynamic variables in the hindlimb are shown in Fig. 1. The femoral artery flow rate was higher in pulsatile than in nonpulsatile blood flow (38.3 ± 3.2 vs. 33.9 ± 2.7 ml/min; P < 0.05), and the calculated PVR was lower in pulsatile blood flow than in nonpulsatile blood flow (194.3 ± 24.2 vs. 247.4 ± 31.3 dyn · s · cm5;
P < 0.01). Thus vascular tone decreased significantly in
pulsatile blood flow even in the absence of sympathetic nerve activity
and endothelium-derived prostacyclin.
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Effects of Pulsatility on Plasma Nitrite/Nitrate, Catecholamine, and Angiotensin II Levels
The plasma nitrite plus nitrate concentration (n = 6) is shown in Fig. 2. It was 16.2 ± 2.1 mmol/l in pulsatile blood flow, whereas it was 13.9 ± 2.1 mmol/l in nonpulsatile blood flow (P < 0.01) at the same systemic flow rate. The plasma catecholamine and angiotensin II concentrations (n = 6) were not different between the two flow conditions (Fig. 3; epinephrine: 21.3 ± 4.5 pg/ml in pulsatile vs. 21.7 ± 6.3 pg/ml in nonpulsatile blood flow, norepinephrine: 35.8 ± 10.6 pg/ml in pulsatile vs. 38.7 ± 12.0 pg/ml in nonpulsatile blood flow, and angiotensin II: 37.4 ± 18.3 pg/ml in pulsatile blood flow and 39.9 ± 22.7 pg/ml in nonpulsatile blood flow, respectively).
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Effects of Pulsatility on Reactive Hyperemia in Hindlimb
The hemodynamic values and calculated data in reactive hyperemia in the hindlimb are summarized in Table 2. The peak flow rates of reactive hyperemia were not different between the two flow conditions, whereas the t1/2 and the repayment flow significantly decreased in nonpulsatile blood flow (P < 0.01) compared with those in pulsatile blood flow. There were no marked changes in the femoral artery pressure during reactive hyperemia. The representative flow-time response curves with the femoral pressure during reactive hyperemia are shown in Fig. 4.
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Effects of L-NMMA on Basal Hemodynamics and Reactive Hyperemia in Hindlimb
The local administration of L-NMMA into the femoral artery significantly decreased the femoral artery flow in both flow conditions from 38.3 ± 3.2 to 28.9 ± 3.8 ml/min (P < 0.05) in pulsatile blood flow and from 33.9 ± 2.7 to 26.8 ± 4.0 ml/min (P < 0.05) in nonpulsatile blood flow (Fig. 1B). The femoral artery pressure increased after L-NMMA to 126.1 ± 10.9 mmHg in pulsatile blood flow and 119.6 ± 10.4 mmHg in nonpulsatile blood flow (Fig. 1A), and the percent increase by L-NMMA was significantly larger in pulsatile blood flow (44.1 ± 7.9% vs. 23.8 ± 6.7%, P < 0.05). The calculated PVR increased from 194.3 ± 24.2 to 417.5 ± 88.0 dyn · s · cm5
(P < 0.01) in pulsatile blood flow and from 247.8 ± 31.3 to
427.0 ± 77.5 dyn · s · cm5
(P < 0.05) in nonpulsatile blood flow (Fig. 1C). In
addition, the percent increase of PVR by L-NMMA was
significantly larger in pulsatile blood flow (113.2 ± 28.7% vs. 67.7 ± 15.2%, P < 0.05). In reactive hyperemia, the peak flow
rate was not influenced by L-NMMA, whereas the
t1/2 and the repayment flow significantly decreased
in pulsatile perfusion, and no difference was seen in the
t1/2 and the repayment flow between the two flow
conditions after administration of L-NMMA (Table 2). The
percent decrease by L-NMMA in the t1/2
and the repayment flow were larger in pulsatile blood flow
(t1/2: 36.1 ± 5.8% vs. 16.3 ± 8.8%, P = 0.031; repayment flow: 23.4 ± 7.1% vs. 11.4 ± 8.6%, P = 0.07).
Effects of Calmodulin Inhibitor and Tyrosine Kinase Inhibitor on PVR
The local administration of calmidazolium into the femoral artery showed slight increase in PVR in both pulsatile flow (from 208.1 ± 29.2 to 223.2 ± 18.1 dyn · s · cm5,
not significant) and nonpulsatile flow (from 228.4 ± 32.9 to 249.8 ± 31.9 dyn · s · cm5,
not significant). PVR in nonpulsatile flow remained significantly higher than that in pulsatile flow (Fig.
5A). In contrast, erbstatin A
significantly increased PVR in both pulsatile flow (from 198.1 ± 27.2 to 286.7 ± 31.6 dyn · s1 · cm5;
P < 0.05) and nonpulsatile flow (from 218.4 ± 32.9 to 298.7 ± 37.6 dyn · s1 · cm5;
P < 0.05), and there was no difference in PVR between the two flow conditions (Fig. 5B). Percent increase in PVR after
L-NMMA was smaller in the erbstatin A group than in the
calmidazolium group (27.5 ± 5.9 vs. 62.6 ± 5.2% in pulsatile flow,
P < 0.01; 19.9 ± 6.9 vs. 43.2 ± 6.5% in nonpulsatile
flow, P < 0.01).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Some investigators have demonstrated an increased systemic vascular resistance in nonpulsatile systemic blood flow (6, 15, 28); however, the factors that increase the vascular tone have yet to be clearly elucidated. Recently, Fukae et al. (10) demonstrated, by the direct measurement of renal sympathetic nerve activity, that an increase in the efferent sympathetic nerve activity occurred after depulsation, which was also significantly related to an increase in the systemic vascular resistance. However, Mendelbaum and Burns (21) suggested the presence of other factors than the sympathetic nerve activity, which contribute to the increase in vascular resistance in nonpulsatile blood flow.
Endothelial cells have been recognized to play an important role in both modulating the vascular tone and coordinating tissue perfusion. Endothelial cells sense the blood flow through their mechanical stimuli, or fluid shear stress, on the membrane surface to produce and release various relaxation and contraction factors continuously (2). The changes in shear stress regulate the synthesis of such endothelium-derived relaxing factors as NO and prostacyclin (12, 23). Noris et al. (23) documented that the flow-induced synthesis of EDNO was dependent on shear stress magnitude and that EDNO release was greater in periodic flow than in steady laminar flow at the same mean degree of the shear stress in cultured endothelial cells. In this study, the sympathetic nerve activity was blocked by hexamethonium bromide to avoid the effects of neurogenic vasoconstriction, and endothelium-derived prostacyclin was also blocked by indomethacin to focus on the effects of pulsatility on EDNO. Whereas shear stress is influenced by blood viscosity, and blood viscosity is influenced by temperature, both the hematocrit level and blood temperature were carefully kept at the same degree throughout the experiment in all dogs.
The first finding in this study was that the basal release of EDNO is enhanced by pulsatility and it may thus contribute, at least partly, to lower PVR in pulsatile blood flow. At a constant systemic blood flow rate, depulsation leads to a significant increase in the mean blood pressure and systemic vascular resistance without any changes in the serum catecholamine and angiotensin II concentrations. An increased PVR in the hindlimb followed by a decrease in the femoral artery flow was also observed in nonpulsatile blood flow. These findings thus indicate the presence of some factors other than the sympathetic nerve activity, endothelium-derived prostacyclin, and neurohormonal substances, which cause an elevated PVR in nonpulsatile blood flow. After the local administration of L-NMMA into the femoral artery, the vascular resistance of the hindlimb increased in both flow conditions to the same level, and the percent increase in the PVR by L-NMMA was larger in pulsatile blood flow. A large response to L-NMMA has been interpreted to reflect an increase in NO synthase activity (3, 24), therefore, these findings suggest that the basal release of EDNO is enhanced by pulsatile flow, which contributes, at least partly, to the decrease in the PVR. The fact that the plasma nitrite/nitrate level, which has been reported to represent an indirect measurement of EDNO production (31), was higher in pulsatile blood flow also supports this explanation.
To further study the effects of pulsatility in blood flow on EDNO release, reactive hyperemia of the hindlimb was induced to assess flow-stimulated, EDNO-mediated vasodilation. Reactive hyperemia has been used to assess the endothelial cell function of the coronary arteries and peripheral arteries both clinically and experimentally (18, 27). According to changes in blood flow, endothelial cells control the vascular diameter by modulating the production of such endothelium-derived vasoactive substances to keep a certain physiological range of fluid shear stress on their membrane surface. In these vessels EDNO is recognized to play a significant role in the duration of reactive hyperemia but not in the peak flow. The rapid increase in flow is considered to cause an acute increase in shear stress, which enhances the EDNO production, and thus results in vasodilation in human forearm vessels (27), the coronary arteries in animal (18), and the hindlimb of dogs (30).
The second finding in this study was that the flow-stimulated release of EDNO is also enhanced in pulsatile blood flow under in vivo conditions. The peak flow rate of reactive hyperemia in the hindlimb was not different between pulsatile and nonpulsatile blood flow, therefore, the vascular reaction to the same degree of an acute increase in flow could be compared under the two flow conditions. The repayment flow and the duration of reactive hyperemia (represented by t1/2) were significantly larger in pulsatile blood flow. L-NMMA had no effect on the peak flow rate; however, it decreased the repayment flow and t1/2 in both flow conditions to the same level. The percent decrease in the two parameters by L-NMMA were larger in pulsatile flow. These findings thus suggest that the flow-stimulated release of EDNO is also enhanced in pulsatile blood flow.
Several investigations have been made regarding the effects of transluminal pressure on EDNO production. Using a cascade bioassay system, Rubanyi (25) and Hutcheson and Griffith (14) showed that rises in the transluminal pressure or pulse pressure caused a decrease in NO release at a constant flow rate. However, they could not exclude the possibility that pressure-induced, endothelium-independent vascular distention could lead to a fall in shear stress on the endothelial cells, which thus would cause a reduction in the NO production rather than the direct effect of transluminal pressure on EDNO reduction. The results of in vitro studies are controversial. Hashikawa et al. (13) demonstrated that by using cultured human umbilical vein endothelial cells and by measuring nitrite/nitrate concentration, the histamine-induced NO production is inhibited by transmural pressure due to some unknown mechanism, and they also showed a significant decrease in NO production under a pressure >40 mmHg. (In our study, the difference in the mean blood pressure between pulsatile and nonpulsatile perfusion is only 10.5 mmHg.) On the other hand, Kelm et al. (17) showed that using cultured porcine aortic endothelial cells and by a specific difference spectrophotometric assay, the basal release of EDNO is not modulated by perfusion pressure. In clinical studies, Celermajer et al. (4) showed by a multiple regression analysis that a higher blood pressure did not independently cause a reduced flow-mediated vasodilation. In addition, in the human brachial artery, Laurent et al. (20) showed that the vasodilation following reactive hyperemia did not differ between normotensive and hypertensive subjects. Furthermore, in our study, we compared the hemodynamic values in pulsatile and nonpulsatile perfusion in the same subject in each experiment, and we consider the elevated blood pressure in nonpulsatile perfusion to thus be the result of reduced EDNO production because no difference was seen in the blood pressure between pulsatile and nonpulsatile perfusion after L-NMMA administration. Taken together, it is unlikely that the higher blood pressure depressed the EDNO production in nonpulsatile perfusion in this study.
Several mechanisms responsible for the increased basal and flow-stimulated EDNO release in pulsatile blood flow can be proposed. One possible mechanism, and the most likely one based on our findings, is the enhanced activation of constitutive NO synthase in endothelial cells (eNOS) in pulsatile blood flow. The NO production in endothelial cells requires the activation of eNOS, and the amount of eNOS seems to remain unchanged during the short period in this study because the change of the eNOS mRNA level requires several hours after a change in the flow condition (23, 29). As a result, the amount of NO production is thus directly related to the degree of activation of eNOS. The eNOS activity is generally thought to be regulated by intracellular Ca2+ through its interaction with calmodulin (9); however, a lot of studies have demonstrated that the Ca2+-independent eNOS activation pathway is also involved in fluid shear stress-induced NO production (1, 7, 8). In this study, we observed a small effect of calmodulin inhibitor on PVR in pulsatile and nonpulsatile flow but a significant increase in PVR by tyrosine kinase inhibitor. Percent change in PVR by L-NMMA was significantly smaller in the erbstatin A group than in the calmidazolium group, which suggests that basal EDNO production was more inhibited by tyrosine kinase inhibitor than by calmodulin inhibitor. The result of this study is consistent with that of Corson et al. (5), who demonstrated, using cultured endothelial cells, that EDNO production and eNOS phosphorylation increase without an increase in intracellular Ca2+ concentration when exposed to an acute increase in fluid shear stress. Furthermore, other sensing and signaling mechanisms in the flow-mediated stimulation of NO release, including G protein activation (19) and the integrine-involved pathway (23), may be involved in the different EDNO production between pulsatile and nonpulsatile flow.
Another possible mechanism in the vasodilator effects of pulsatility in blood flow is the direct effect on vascular smooth muscle cells. Goto et al. (11) reported that the periodic pulsatile pressure dilates the coronary artery, even after the removal of endothelial cells by some unknown mechanism. However, its effects on the peripheral artery in dogs seem to be small because no significant differences were observed between pulsatile and nonpulsatile blood flow after the blockade of EDNO by L-NMMA. It is thus conceivable that the increased EDNO may be the main cause of the decreased vascular tone in pulsatile blood flow in this study.
In summary, pulsatility in blood flow significantly enhances the basal and flow-stimulated release of EDNO in the peripheral vasculature under in vivo conditions, and EDNO production by blood flow is suggested to be mainly stimulated by Ca2+-independent, tyrosine kinase inhibitor-sensitive pathway. These findings may contribute to our understanding of the physiological role of pulsatility in blood flow; however, the present study only evaluated the acute effects of pulsatility, and a future study will thus be required to investigate the change in the vascular responses to chronic nonpulsatile blood flow.
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ACKNOWLEDGEMENTS |
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The authors thank Tomoko Moriyama for valuable technical assistance. We are also grateful to Yumi Murakami and Kikuko Iwaki for help in performing the biochemical analysis.
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FOOTNOTES |
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This study was supported by a grant-in-aid for Scientific Research (07457296) from the Ministry of Education, Science and Culture of Japan.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: R. Tominaga, Division of Cardiovascular Surgery, Research Institute of Angiocardiology, Faculty of Medicine, Kyushu Univ., 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan (E-mail: tomina{at}heart.med.kyushu-u.ac.jp).
Received 5 August 1998; accepted in final form 17 November 1999.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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P < 0.01 vs. before
L-NMMA, 
P < 0.05 vs. before
L-NMMA.
