Evidence for a Multifactorial Process Involved in the Impaired Flow Response to Nitric Oxide in Hypertensive Patients With Endothelial Dysfunction
Abstract The assessment of endothelial function in hypertensive patients receiving acetylcholine has revealed conflicting results. Whether an impaired flow response to acetylcholine is explained solely by a diminished endothelial synthesis of nitric oxide (NO) remains unclear as yet. In the present study, we tested the hypothesis that mechanisms other than reduced NO synthesis contribute to the hypertension-associated impairment of endothelium-dependent vasodilation. Therefore, the dilatory response to endogenous and exogenous NO was measured in resistance arteries and cutaneous microvessels in the forearm circulation of 12 normotensive individuals and 17 hypertensive patients. In addition, the overall dilatory capacity was assessed by peak flow during reactive hyperemia after 3 minutes of ischemia. Forearm blood flow was quantified by venous occlusion plethysmography at rest, during application of the NO donor sodium nitroprusside, and during stimulation of endogenous NO synthesis by acetylcholine and bradykinin. Blood flow velocity in the cutaneous microvasculature was measured with laser-Doppler flowmetry in parallel. Resting forearm flow was comparable in both groups (3.1±0.2 and 3.4±0.2 mL·min−1·100 mL−1 tissue), whereas blood pressure and thus peripheral vascular resistance was significantly elevated in hypertensive compared with normotensive subjects. Hyperemic peak flow was significantly blunted in hypertensive patients. Sodium nitroprusside, acetylcholine, and bradykinin increased flow in a dose-dependent manner to a comparable extent in the control group (13.3±0.8, 13.6±1.3, and 14.6±0.7 mL·min−1·100 mL−1 tissue, respectively). In contrast, in hypertensive patients maximum increase in resting flow was significantly reduced (sodium nitroprusside, −36%; acetylcholine, −44%; and bradykinin, −56%). The flow response after stimulation of endogenous NO synthesis by bradykinin was significantly more blunted compared with that of exogenous NO after application of sodium nitroprusside. In the cutaneous microvasculature, bradykinin-induced increases in blood flow velocity were selectively impaired in hypertensive patients, whereas flow response to acetylcholine was preserved. Thus, we conclude that in arterial hypertension endothelium-dependent, NO-mediated dilation of resistance arteries and cutaneous microvessels of the forearm vasculature is heterogeneously impaired, depending on the type of endothelial receptor stimulated. Furthermore, the present data suggest that in hypertensive patients the impairment of NO-dependent dilation of resistance arteries is caused by at least three different mechanisms: (1) a reduced endothelial synthesis of NO due to either a disturbed signal-transduction pathway and/or a reduced activity of NO synthase, (2) an accelerated NO degradation within the vessel wall, and (3) alterations in the vessel architecture resulting in an overall reduced dilatory capacity of resistance arteries.
- vascular diseases
- nitric oxide
- arterial hypertension
Arterial hypertension is associated with structural and functional alterations of the vessel wall.1 2 3 4 Recent evidence suggests that the vascular endothelium may be affected or may itself promote functional changes of the vessel wall in arterial hypertension.5 6 7 Endothelial cells continuously synthesize NO from l-arginine by the constitutive NO synthase. NO activates soluble guanylyl cyclase within vascular smooth muscle cells and thus mediates vasorelaxation. Basal NO release can be stimulated by several mediators, such as acetylcholine, bradykinin, and substance P, and by mechanical stimuli, such as shear stress (for review, see References 8 and 9). Thus, endothelial NO formation may participate in the regulation of arterial blood pressure.10 11
Endothelial function has been considered to be selectively impaired in patients with arterial hypertension on the basis of the observation of a blunted flow response to muscarinergic agonists compared with a preserved response to sodium nitroprusside.12 13 14 15 16 These findings, however, have been reassessed recently, with controversial results.17 18 Thus, the prevalence and the underlying mechanisms of an impaired acetylcholine-induced vasodilation in arterial hypertension are unclear thus far. In the present investigation, we therefore sought to give an experimental answer to the following as yet unresolved questions: Is a blunted endothelium-dependent vasodilation in arterial hypertension caused (1) by an alteration of the signal-transduction pathway responsible for stimulation of endothelial NO synthesis, resulting in an impaired capacity of NO formation; (2) by an accelerated degradation of NO within the vessel wall; or (3) by reduction of overall dilatory function of vascular smooth muscle most likely due to changes in the vessel architecture? Furthermore, we investigated whether differences in the impairment of NO-mediated flow responses exist along the vascular tree. To address these issues, we comparatively studied the flow response to sodium nitroprusside, acetylcholine, bradykinin, and ischemia in resistance arteries and cutaneous microvessels of the forearm vasculature in normotensive and hypertensive subjects.
The final study sample consisted of 17 patients with a well-documented history of essential hypertension according to previously published criteria19 and 12 age-matched control subjects with normal values for arterial blood pressure documented in three consecutive measurements on different days. Patients were hospitalized for 3 days before investigation and took no antihypertensive medication for at least 2 days before the study. Five patients were excluded from the study because withdrawal of antihypertensive medication led to profound increases in blood pressure. Patients were free of concomitant diseases predisposing to alterations of endothelium-dependent vasomotion, such as diabetes mellitus, hypercholesterolemia, elevated plasma levels of uric acid, infections, or immunological disorders (for review, see Reference 5). The proportion of patients with borderline values for serum cholesterol (6.2 to 7.0 mmol/L) was comparable in the hypertensive (18%) and normotensive (22%) groups.
The normal volunteers were screened by clinical history, physical examination, electrocardiography at rest and during exercise, and routine chemical analysis. None of these subjects revealed present or past evidence of cardiovascular diseases known to affect vasomotion, such as hypertension, hypercholesterolemia, chronic heart failure, and diabetes mellitus.5 The protocol of the study was approved by the ethics committee of the Heinrich-Heine-University, and all subjects gave written informed consent before participating in the study.
All investigations were performed in an air-conditioned room at a temperature of 23°C with the subjects in a supine position. Participants in the study were instructed to refrain from smoking cigarettes or drinking alcohol and beverages containing caffeine for at least 12 hours before the investigation. The brachial artery of the nondominant arm was cannulated with a 2F catheter (model 115.09, Braun-Melsungen) for intra-arterial blood pressure monitoring and for local infusions. Continuous recording of arterial pressure was performed with a pressure transducer (Sirecust, model 380, Siemens) connected to the inflow line of the cannula. FBF (in mL·min−1·100 mL−1 tissue) was measured simultaneously in both arms, which were slightly elevated above the level of the right atrium, with mercury-in-rubber strain-gauge plethysmography (Periquant 833, Gutmann). The strain gauge plethysmograph was placed at the largest circumference of the forearm (mean, 273±4 mm; n=29). The upper arm congesting cuffs were inflated to 40 mm Hg for 5 seconds in each 10-second cycle to occlude venous outflow from the arms.20 For each determination, five measurements of FBF were performed and the results were averaged. Measurements for baseline FBF were started 20 minutes after cannulation of the brachial artery. PVR in the forearm was calculated by the ratio of mean arterial blood pressure and FBF. PVR (determined in mm Hg·min−1·100 mL−1 tissue ·mL−1) is expressed in the following as resistance units (U) (see Table 1⇓). Changes in PVR were expressed as a percentage of the baseline value to account for differences between normotensive and hypertensive subjects in vascular resistance at control. The circumference of the forearm, and thus its volume, was comparable in both groups. Therefore, the doses of vasodilator applied were assumed to yield comparable intravasal concentrations in both groups.
After resting control values of FBF had been measured, the subjects received local intra-arterial infusions of the respective vasodilators in increasing doses. The order of drug infusion varied among subjects. Drugs and saline were infused at flow rates of 0.6 to 2.0 mL/min by means of a constant-rate infusion pump (Braun). In the concentrations indicated, none of the substances exerted systemic vasodilatory effects, as was assessed by the parallel measurement of FBF in the control arm. In a preliminary subset, tachyphylactic flow responses caused by repetitive infusion of maximally effective doses of vasodilators could be excluded (data not shown). Furthermore, the length of the time interval between different interventions, which varied in increments of 5 minutes from 5 to 30 minutes, did not affect the vasodilatory response in FBF. Therefore, the final protocol was performed as follows. For each substance, the respective dose-response curves were derived in a randomized order for each subject. After each infusion, FBF was allowed to return to the steady state of baseline measurements, and after an additional period of 5 minutes the next infusion was started. Using this final setup for measuring FBF, we determined intraindividual variability of FBF at rest and during reactive hyperemia with a coefficient of variation <5±1% (determined in six control subjects). Measurements of FBF and skin blood flow were always performed by the same investigator.
Endothelial synthesis of NO was stimulated by bradykinin at doses of 0.006, 0.021, 0.063, and 0.210 μg/min, corresponding to 6 to 200 pmol/min, and by acetylcholine at doses of 1.1, 3.6, and 10.9 μg/min, corresponding to 2 to 20 nmol/min. Sodium nitroprusside was used to evaluate the sensitivity of the vascular smooth muscle compartment to exogenous NO and was infused at doses of 0.3, 1, 3, and 10 μg/min, corresponding to 1, 3.7, 10.1, and 37.6 nmol/min. To determine the overall dilatory capacity of resistance arteries, changes in FBF and PVR were measured during reactive hyperemia in 10-second intervals after a 3-minute no-flow ischemia induced by inflation of the upper arm cuff to suprasystolic values. The area under the curve of the flow response during reactive hyperemia was quantified for each patient by a computerized program (MicroCal Origin, version 2.94, MicroCal Software Inc) and determined as the “reactive hyperemia repayment” in FBF in milliliters during the first minute after relief of occlusion.
Skin Blood Flow Measurements
Relative BFV in the cutaneous microvasculature was assessed with LDF (Periflux, Perimed). The thermostat probe (36°C) was placed at the distal and volar site of the infused forearm, yielding reliable results with respect to spatial variations.21 LDF measures skin perfusion in a small volume of tissue (diameter, 1 mm) beneath the probe head. It therefore reflects blood flow in cutaneous microvessels, which comprise nutritional and superficial capillaries, and shunt and subpapillary plexus vessels.22 23 However, to which proportion these different vessel types contribute to the overall flow signal cannot be discerned by LDF. A reference baseline for the LDF recordings corresponding to zero perfusion was obtained by inflating the upper arm cuff to suprasystolic values (200 mm Hg) and recording the Doppler signal during this time. Skin blood flow was then expressed in arbitrary units as a percentage of the preset maximal gain of the LDF. The analog output from the LDF was continuously monitored with a strip-chart recorder (Rikadenki). BFV was not quantified during reactive hyperemia because motion artifact after relief of the upper arm cuff hampered exact quantification of BFV.
Standards of bradykinin (Sigma) were dissolved in sterile NaCl solution (0.9%) and then filtered through 0.2-μm filters (Waters-Millipore) at a flow bench under sterile conditions according to the recommendations of the European Pharmaceutical Association. Standards of acetylcholine were prepared from a sterile preparation (Acetylcholinum Opthalmicum, Dispersa). Sodium nitroprusside (Schwarz Pharma AG) was dissolved in a solution of sodium citrate (9 mg/mL) and then diluted in a 5% glucose solution to yield sterile standards, which were kept in opaque syringes.
Because the study population revealed a parametric distribution concerning basal hemodynamic parameters and maximal changes in FBF in the Shapiro-Wilk’s test, all data were expressed as mean±SEM unless otherwise stated; two-sided values of P<.05 were considered to be significant. The Mann-Whitney U test was used to evaluate differences between both groups concerning the clinical and laboratory parameters at baseline (Table 2⇓). A two-way ANOVA for repeated measures with consecutive post-hoc tests was used to test for (1) differences from control within each group concerning the dose-dependent increase in FBF during infusion of the respective vasodilator, (2) differences between the normotensive and hypertensive groups, and (3) differences between the respective vasodilators concerning their vasoeffective potency. Univariate linear regression models (Spearman’s rank) were performed to evaluate the relation between hyperemic peak flow and repayment. Calculation of the slope of the dose-response curve (LOGb) for increases in FBF elicited by the respective vasodilator in each patient was performed after logarithmic conversion. The data were processed using the software package SAS-PC (Statistical Analysis System, release 6.08, SAS Inc).
Baseline characteristics except blood pressure were similar in both groups. Mean arterial blood pressure under resting conditions was significantly higher in the hypertensive compared with the normotensive control group (Table 2⇑). The mean time since diagnosis of arterial hypertension in the hypertensive patients was 14±2 years. Previous antihypertensive treatment (9±3 years) included calcium channel antagonists (77%), thiazide diuretics (47%), β-adrenoceptor antagonists (28%), and angiotensin-converting enzyme inhibitors (12%). The combination of substance classes of these antihypertensive drugs previously prescribed to the patients varied as follows: one class (6%), two classes (59%), three classes (18%), and four classes (12%) of drugs. One patient had received no previous antihypertensive medication. Basal FBF tended to be slightly but not significantly lower in the hypertensive group compared with the control subjects (3.1±0.2 and 3.4±0.2 mL·min−1·100 mL−1 tissue, respectively). According to the elevated blood pressure, the PVR was significantly elevated in the hypertensive compared with the normotensive group (40±3 and 32±2 U, respectively; P<.001). BFV at rest in the cutaneous microvasculature amounted to 9±1 and 10±2 U for the normotensive and hypertensive groups, respectively.
Flow Responses to Exogenous NO
The NO donor sodium nitroprusside dilated peripheral resistance arteries in both groups significantly at each dose level tested. At the highest dose, the maximum blood flow was significantly lower in the hypertensive group compared with the normotensive group (9.2±0.8 and 13.3±0.8 mL·min−1·100 mL−1 tissue, respectively; Fig 1A⇓). In parallel, the minimally achieved PVR was 88% higher in the hypertensive group (Table 1⇑). The percent changes in PVR from resting value were significantly reduced in the hypertensive group compared with the control group only at the highest doses of sodium nitroprusside (Fig 1B⇓) but not over the entire dose-response curve, as seen with bradykinin and acetylcholine. Relative skin blood flow increased to a comparable extent in both groups after application of sodium nitroprusside.
Flow Responses to Endogenous NO
Acetylcholine and bradykinin increased basal FBF significantly at each dose infused in both groups. The acetylcholine-induced increase in FBF was significantly reduced in hypertensive patients compared with control subjects (146% versus 300% at the highest dose; Fig 2A⇓). Comparison of the percentage decrease in PVR between both groups, which eliminates the differences in resting PVR, also revealed significantly reduced acetylcholine-induced vasodilation in hypertensive patients compared with control subjects over the entire range of the dose-response curve (Fig 2B⇓). At the maximally effective dose of acetylcholine, patients with arterial hypertension revealed a 113% higher PVR compared with normotensive control subjects (Table 1⇑).
At the highest dose of bradykinin, the increase in FBF was considerably lower in hypertensive patients compared with normotensive control subjects (97% versus 329%, respectively; Fig 3A⇓). This was paralleled by a significantly elevated minimum PVR, which was more than three times higher in the hypertensive patients (Table 1⇑). Furthermore, the percent changes in PVR from control were significantly reduced in patients with arterial hypertension over the entire dose-response curve (Fig 3B⇓). In a set of control experiments, the bradykinin-induced changes in FBF were similar before and 20 minutes after intravenous application of 500 mg ASA, which in the concentration used effectively inhibits prostacyclin synthase24 (in mL·min−1·100 mL−1 tissue [n=3]: 14.9±0.4 before and 16.7±0.6 after application of ASA).
Acetylcholine increased skin blood flow in both groups in a dose-dependent manner to a comparable extent (Table 3⇓). In contrast, bradykinin-induced changes in BFV amounted to a maximum of almost 300% in the control subjects but to only 48% in patients with arterial hypertension (P<.05; Table 3⇓).
Flow Responses to Ischemia
The peak flow during reactive hyperemia amounted to 19.7±1.5 in the control group and only 13.1±0.8 mL·min−1·100 mL−1 tissue in the hypertensive group (Fig 4⇓). Thus, the minimum PVR obtained during reactive hyperemia was significantly higher (by 83%) in the patients with arterial hypertension compared with that in the normotensive control group (5.0±0.3 versus 9.1±0.7 U). The differences in FBF between both groups remained significant over almost the entire period of reactive hyperemia (Fig 4⇓). One minute after relief of occlusion, FBF returned to baseline values within both groups. The reactive hyperemia repayment was significantly reduced in the hypertensive group compared with the normotensive control group (5.3±0.3 versus 8.5±0.7 mL, respectively). In both groups, peak flow and repayment during reactive hyperemia were directly correlated, with r=.68 and r=.59, r2=45% and r2=36%, and P=.02 and P=.03 for the normotensive and hypertensive subjects, respectively.
Comparison of Vasodilatory Stimuli
In the control group, the extent of pharmacologically induced vasodilation was comparable for all three substances (Figs 1 through 3⇑⇑⇑). Comparison of the maximum FBF achieved with each of the NO-dependent vasodilators revealed that in patients with arterial hypertension the NO-dependent dilation of resistance arteries is significantly but not uniformly impaired: maximum FBF (mL·min−1·100 mL−1 tissue) amounted to only 6.1±0.8 for bradykinin, 7.6±0.6 for acetylcholine, and 9.2±0.8 for sodium nitroprusside (P<.01 versus control subjects, respectively; Fig 5⇓). The flow response to bradykinin was significantly more reduced compared with that following application of sodium nitroprusside (P<.05).
Calculation of [LOGb] as an index of the slope of the respective dose-response curves revealed similar results. In the normotensive group, values were comparable (5.0±0.4 for bradykinin, 4.2±0.7 for acetylcholine, and 4.8±0.4 for sodium nitroprusside). In contrast, [LOGb] in the hypertensive group was significantly reduced for all vasodilators, ranked in the following order: bradykinin (1.3±0.2) <acetylcholine (2.1±0.3) <sodium nitroprusside (2.9±0.3).
Finally, the maximum increase in FBF obtained with each of the pharmacological agents was compared with the degree of vasodilation achieved during reactive hyperemia. In the control group, the maximum increase in FBF elicited by each of the three vasodilators amounted consistently to two thirds of the maximum peak flow during reactive hyperemia (19.7±1.5 mL·min−1·100 mL−1 tissue): 68±7% for sodium nitroprusside, 68±5% for acetylcholine, and 70±7% for bradykinin. However, heterogeneous changes in flow were observed in the hypertensive group: The flow response after application of sodium nitroprusside normalized to that during reactive hyperemia was comparable to that in the normotensive control group (65±6%), whereas it amounted to only 55±4% for acetylcholine (P=NS versus normotensive control) and 43±6% for bradykinin (P<.05 versus normotensive control).
In the present study, we have made the following new observations. First, the endothelium-dependent, NO-mediated dilation of resistance arteries and cutaneous microvessels in the forearm vasculature of hypertensive patients is impaired to a varying degree, depending on the stimulated signal-transduction pathway. Second, the impairment of flow responses to exogenous and endogenous NO is indicative of a multifactorial process involved in blunted endothelium-dependent dilation of resistance arteries, suggesting that at least three different mechanisms may contribute to this phenomenon: (1) a reduced endothelial synthesis of NO caused either by a disturbed signal-transduction pathway and/or a reduced activity of NO synthase, (2) an accelerated degradation of NO within the vessel wall, and (3) alterations in the vessel architecture resulting in an overall reduced dilatory capacity of resistance arteries.
In the present study, four different stimuli were chosen to clearly discriminate within each individual the mechanisms contributing to the impairment of NO-mediated vasodilation. Endothelial synthesis of NO was stimulated by acetylcholine and bradykinin. The cholinergic vasodilation in the human forearm circulation is predominantly mediated by the M3-receptor subtype25 and is independent of the synthesis of prostanoids and prejunctional inhibition of adrenergic neurotransmission.12 13 Bradykinin acts on endothelial receptors to stimulate the synthesis of prostanoids and NO.8 9 The bradykinin-induced increase in FBF is significantly blunted by NG-monomethyl-l-arginine, which indicates that the flow response is in part mediated by endothelium-derived NO.16 26 Furthermore, in our study bradykinin-induced vasodilation appeared not to be mediated exclusively or predominantly by prostacyclin, since the flow response was comparable in the presence and absence of ASA, although the number of subjects in whom these experiments were performed was limited (n=3). The NO donor sodium nitroprusside relaxes vascular smooth muscle directly.27 28 Maximal FBF after ischemia is considered a crude but reliable index for the overall dilatory capacity of a given vascular bed.29 30 31 32 The degree of peak flow during reactive hyperemia after a defined period of ischemia is critically affected by morphological alterations of the vascular wall,4 whereas endothelium-derived factors do not appear to substantially modulate hyperemic peak flow, at least in this vascular bed.33 Therefore, the maximal flow response achieved with each of the NO-dependent vasodilators was compared and normalized to that obtained during reactive hyperemia.
Under baseline conditions, vascular resistance of the forearm circulation was significantly higher in the hypertensive than in the normotensive group because of the elevated blood pressure, whereas resting flow was only slightly diminished. This concurs with previous reports.12 13 15 Thus, at rest hypertension-associated alterations of the vascular structure and function leading to increased vessel tone appear almost compensated, at least in terms of blood flow. Nevertheless, the dilatory capacity and thus the maximal convective supply of nutrients, which become more important with increasing demand of peripheral tissue, might be affected considerably despite normal resting flow.
Flow Responses to Exogenous NO
In contrast to previous studies,12 13 14 15 16 the response to sodium nitroprusside was significantly reduced in hypertensive patients in our study. This difference is most likely explained by the different dosage regimens. As can be seen from Fig 1⇑, differences between the normotensive and hypertensive groups became significant only at the higher doses of sodium nitroprusside (3 and 10 μg/min), when 70% of the maximal flow response to this agent had been reached. This concurs with a very early report by Robinson et al.34 However, in the control groups of most of the more recent studies,12 13 14 15 16 the maximal increase in flow observed with NO donors was generally less than that with acetylcholine. When the different units given for changes in flow in these studies are taken into account, the maximal response in the normotensive control subjects to sodium nitroprusside in proportion to that of acetylcholine can be estimated as follows: 65% (FBF12 ), 70% (PVR13 ), 40% (percent change in FBF15 ), and 50% (percent change in PVR14 ). In contrast, in our own study, maximum FBF elicited by sodium nitroprusside equals that of acetylcholine (Fig 6). Taken together, these findings illustrate the necessity of constructing dose-response curves yielding equally effective increases in FBF for either NO donor, since the impairment of NO-mediated dilation of the vascular smooth muscle may be unmasked at high doses of NO donors only. Only in a very recent study were sodium nitroprusside and acetylcholine infused in equally effective doses (as assessed in healthy control subjects) in previously treated hypertensive patients.18 Similar to our findings, the flow response to acetylcholine and sodium nitroprusside was reduced to a comparable extent in hypertensive patients, although the absolute extent of impairment was less pronounced than in our study. This difference might be explained by a different severity and time course of arterial hypertension.
The control of the localized actions of NO is dictated by the relative kinetic rates of diffusion, intracellular and extracellular scavenging, and its reactivity of NO in target cells.35 The dilatory action of NO can be blunted by a disturbed diffusion due to increased wall thickness,36 a selective resistance of vascular smooth muscle to the vasodilator action of NO,35 or an enhanced inactivation of NO due to superoxide or hydroxyl radicals despite a preserved capacity of NO formation.37 In experimental hypertension, a preserved capacity of coronary endothelium to produce NO has been demonstrated.38 Furthermore, recent experimental data suggest that an increased oxidative stress within the vascular wall leads to a rapid inactivation of NO and thus an increase in vascular resistance.37 39 Therefore, it has been postulated that arterial hypertension predisposes to and accelerates atherosclerosis at least in part because of increased oxidative stress of the arterial wall resembling the pathomechanisms also observed in hypercholesterolemia (for review, see Reference 7).
In addition to the aforementioned mechanisms, structural alterations of the vascular wall may contribute at least in part to the impairment of NO-mediated flow responses. In the time course of arterial hypertension, the dilatory capacity of resistance arteries may be reduced by an increased wall-to-lumen ratio either due to growth of the wall into the lumen or to vascular remodeling without further growth of the vascular wall (for review, see References 1, 3, 4, and 40). This in turn limits the dilatory responses irrespective of the stimulated signal-transduction pathway. In line with this interpretation is our finding that in the forearm vasculature of hypertensive individuals the peak flow response during reactive hyperemia is blunted. A reduced global dilatory capacity of resistance arteries in hypertensive compared with normotensive subjects has been described very early after varying degrees of ischemia.30 31 32 Interestingly, in the present study the flow response to exogenous NO normalized to hyperemic peak flow was comparable in both groups. This finding indicates that in arterial hypertension the reduction in sensitivity of vascular smooth muscle toward exogenous NO does not exceed the impairment of reduction in global dilatory capacity.
Impaired Flow Responses to Endogenous NO
In the present study, the extent of impairment of acetylcholine-induced vasodilation in hypertensive patients was comparable to that observed in previously published reports at similar doses.12 13 14 15 Assessment of endothelial function by sole infusion of acetylcholine does not necessarily reflect all hypertension-induced alterations of endothelium-dependent vasomotion. An important finding of the present study is that endothelium-dependent vasodilation does not appear to be uniformly impaired in patients with arterial hypertension but rather depends on the site and type of endothelial receptor stimulated. In comparing the maximum increase in FBF and the slope of the dose-response curve, it became apparent that the flow response to bradykinin tended to be more severely impaired than that to acetylcholine, although this difference reached no statistical level of significance. Furthermore, in the cutaneous microvasculature of hypertensive patients, the flow response to bradykinin amounted to only about 50% of that in normotensive controls, whereas flow responses to acetylcholine and sodium nitroprusside were almost comparable in both groups. The fact that the acetylcholine-induced increase in skin blood flow was preserved, whereas the dilation of resistance arteries was significantly reduced in the same patients, is indicative of a distinct regulation of endothelium-dependent flow responses at different sites of the vascular bed in the forearm. Furthermore, this finding strongly contradicts the view that the flow signal obtained with LDF merely reflects changes in tone of resistance arteries farther upstream.
When comparing the endothelium-dependent vasodilation with that in response to sodium nitroprusside (Fig 6), it became evident that in hypertensive individuals the flow response to endogenous NO is more disturbed than that to exogenous NO. Thus, in addition to an accelerated inactivation of NO within the vessel wall, other mechanisms within the vascular endothelium must be affected in arterial hypertension. The different impairment of acetylcholine- and bradykinin-induced vasodilation might be caused by a selective alteration at the level of the endothelial receptors, the consecutive signal-transduction pathway, and/or the constitutive NO synthase itself, each of which may be affected differently during the course of arterial hypertension.
Implications and Pathophysiological Significance of a Reduced NO-Dependent Vasodilation
Impairment of vasomotor function in hypertension may be caused by an imbalance of vasodilating and vasoconstricting factors.41 Apart from the l-arginine–NO pathway, the vascular tone is regulated by a variety of autocrine and paracrine systems localized in the endothelial and smooth muscle cells, such as the vascular renin-angiotensin system, kallikrein-kinin system, natriuretic peptide system, endothelin, mechanosensitive ion channels, prostanoids, catecholamines, and endothelium-derived hyperpolarizing factor. Aside from their short-term effects on vascular tone, most of these substances also exert long-term effects on vasculature structure.2 The extent to which these different systems contribute to the hypertension-associated reduction of dilatory capacity in relation to the l-arginine–NO pathway cannot be discerned by the present study. However, it is obvious from the present data that the impairment of NO-mediated flow responses in patients with arterial hypertension represents a complex and multifactorial process, which is not at all adequately described by a general endothelial dysfunction. In addition to a reduced synthesis of NO, an accelerated inactivation of NO within the vascular wall and structural changes in the vessel architecture are likely to contribute to the phenomenon of an impaired endothelium-dependent vasodilation in hypertensive individuals. Another level of complexity in this matter is added by our finding that the degree of impairment in endothelium-dependent flow response is dependent on the stimulated receptor type and its site along the vascular bed. Thus, the development of future diagnostic and therapeutic strategies aimed to restore or preserve NO-mediated regulation of vascular tone in arterial hypertension has to take into account at least three different approaches: (1) the modulation of endothelial NO synthesis via different signal-transduction pathways, (2) the reduction of inactivation of NO such as antioxidative therapy, and (3) the regression of structural changes of the vascular wall.
Selected Abbreviations and Acronyms
|BFV||=||blood flow velocity|
|FBF||=||forearm blood flow|
|PVR||=||peripheral vascular resistance|
This work was supported by a grant from the Deutsche Forschungsgemeinschaft, Gerhard-Hess-Award Program, Ke 405/3.
Reprint requests to Dr Malte Kelm, Heinrich-Heine-Universität Düsseldorf, Medizinische Klinik und Poliklinik B, Abtl. f. Kardiologie, Pneumologie, Angiologie, Moorenstraβe 5, D-40225 Düsseldorf, Germany.
- Received August 9, 1995.
- Revision received September 8, 1995.
- Accepted November 21, 1995.
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