(Hypertension. 1995;26:70-77.)
© 1995 American Heart Association, Inc.
Articles |
In Vivo Characterization of Muscarinic Receptor Subtypes That Mediate Vasodilatation in Patients With Essential Hypertension
From the Department of Nephrology, University Hospital Leiden (T.A.B., P.C.C.); Department of Pharmacotherapy, Academic Medical Centre, Amsterdam (T.A.B., M.G.C.H., M.P., P.A. van Z.); and Department of Clinical Pharmacy, University Hospital Leiden (P.V.) (the Netherlands).
Correspondence to Tobias A. Bruning, Department of Nephrology, Building 1, C3-P, Room 27, University Hospital Leiden, PO Box 9600, 2300 RC Leiden, Netherlands. E-mail bruning@ RullF2.LeidenUniv.NL.
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Abstract |
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Abstract Attenuated cholinergic vasodilatation has been suggested as an endothelium-related mechanism involved in essential hypertension. We investigated the role of muscarinic (M) receptor subtypes in the forearm resistance vasculature. In eight white men with essential hypertension and eight matched normotensive control subjects (age of both groups, 47±4 years; mean±SEM), we infused the nonselective agonist methacholine in the presence of saline and the antagonists atropine (nonselective), pirenzepine (M1-selective), and AF-DX 116 (M2-selective) into the brachial artery and measured forearm blood flow and forearm vascular resistance using venous occlusion plethysmography. Affinity constants (pKb values) were determined from calculated plasma concentrations of the infused compounds and EC50 values. Sodium nitroprusside was given as an endothelium-independent control, and minimal forearm vascular resistance after 10 minutes of ischemia was used as a marker of structural vascular changes. Hypertensive patients showed higher minimal forearm vascular resistance, indicating structural vascular changes. However, sodium nitroprusside– and methacholine-induced vasodilatation was similar in both groups, with apparent EC50 values (log moles per liter; mean±SEM) of -7.32±0.13 and -7.51±0.21 in hypertensive patients and -7.37±0.13 and -7.45±0.02 in control subjects, respectively. Atropine, pirenzepine, and AF-DX 116 caused a shift to the right of the concentration-response curve of methacholine, with apparent pKb values of 8.63±0.08, 6.81±0.13, and 5.51±0.29 in hypertensive individuals and 8.62±0.10, 6.98±0.08, and 5.49±0.09 in control subjects, respectively. Again, there were no statistically significant differences in these pharmacological parameters between hypertensive patients and normotensive subjects. The affinity constants and rank order for potency of the muscarinic antagonists, atropine>pirenzepine>AF-DX 116, indicate that cholinergic vasodilatation in the forearm vascular bed is predominantly mediated by the M3 receptor subtype. Despite indications for structural vascular changes in hypertensive subjects, the vasodilator responses to both sodium nitroprusside and methacholine were unchanged in these patients. Essential hypertension is not associated with changes in the pharmacological characteristics of muscarinic receptors in forearm resistance vasculature.
Key Words: receptors, muscarinic • hypertension, essential • forearm • plethysmography • endothelium-derived relaxing factor
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Introduction |
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The observation by Furchgott and Zawadzki1 that acetylcholine-induced vascular relaxation requires an intact endothelium has caused a renewed interest in the role of acetylcholine and the muscarinic receptors in this process. The vasodilator response to acetylcholine is assumed to be mediated at least in part by muscarinic receptor–induced release of endothelium-derived relaxing factor, now known to be nitric oxide.2 In the human forearm, a well-established in vivo model for peripheral resistance vasculature, acetylcholine-induced vasodilatation has been shown also to depend on nitric oxide release.3 4 In various conditions, such as primary and secondary hypertension,5 6 7 heart failure,8 9 hypercholesterolemia,10 11 12 atherosclerosis,13 and insulin-dependent diabetes,14 the vascular responses to cholinergic agonists in this particular vascular bed have been shown to be impaired. For inducement of endothelium-dependent vasodilatation, acetylcholine has been used in most studies,5 6 7 9 11 12 14 whereas methacholine has been used less frequently.8 10 12 13 Numerous observations in various vascular preparations in both animals and humans have led to the widely accepted assumption that hypertension is associated with and possibly caused by endothelial dysfunction (for reviews, see References 15 through 1815 16 17 18 ). However, this view has recently been challenged by experiments in both hypertensive rats19 and patients with essential hypertension.20 So far the type of muscarinic receptors involved in cholinergic vasodilatation in resistance vessels has received little attention. Furthermore, the important question of whether the identity of these muscarinic receptors is affected by hypertension has not been addressed. At present, molecular cloning studies have recognized five different muscarinic receptor genes, classified as m1 through m5.21 The discovery of selective antagonists has enabled the pharmacological identification of four functional muscarinic receptor subtypes, M1 through M4.22 23 24 25 Pharmacological studies with selective antagonists in isolated conduit arteries of various species suggest that the M3 receptor predominantly mediates the endothelium-dependent vasodilator response to nonselective cholinergic agonists, such as acetylcholine and the more stable compounds methacholine and carbachol.26 Resistance vessels are of particular importance for the regulation of blood pressure (BP) and as a target for antihypertensive drugs. We have recently demonstrated that the M3 receptor predominates in the cholinergic vasodilator response in perfused rat mesenteric resistance vessels27 and in the forearm resistance vasculature of healthy human volunteers.28
We here report that there is no difference between hypertensive patients and normotensive control subjects in cholinergic vasodilatation or in the pharmacological characteristics of the muscarinic receptor subtype involved.
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Methods |
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Subjects
Eleven white male patients with essential hypertension (
145/95
mm Hg, Korotkoff phase V) were selected. Each patient had been treated
for at least 5 years with one or more antihypertensive agents. Their
medical history, physical examination, and routine laboratory tests did
not show any other abnormalities. The patients were asked to
discontinue all medication 4 to 6 weeks before the day of the study.
Patients in whom the withdrawal of antihypertensive treatment was
considered hazardous (mostly because of persisting elevated BP despite
medication) were excluded from the study. After therapy was
discontinued, 3 of the 11 patients were excluded because they did not
show a rise in BP to hypertensive values. Eight healthy white males
matched for age, height, and body weight were selected as control
subjects (Table 1). Control subjects were normotensive
(
140/85 mm Hg, Korotkoff phase V); had no family history of
hypertension or any evidence of present or past hypertension,
cardiovascular, endocrine, or neurological disorders;
and were taking no medication. BP was measured in the physician's
office with standard techniques after the subject had been seated for
at least 5 minutes.29 Each BP value was the mean of three
bilateral readings. Twelve hours before the experimental procedures the
subjects refrained from smoking, alcohol, and caffeine-containing
beverages. The protocol was approved by the Medical Ethics Committee of
the Leiden University Hospital, and informed consent was obtained from
all subjects.
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Procedures
All experiments were performed in a quiet room kept at 22° to
24°C. During the experiments the subjects were in the supine position
with both forearms stabilized slightly above the level of the heart.
After local anesthesia of the skin (1% lignocaine) the
brachial artery of the nondominant arm was cannulated in the cubital
fossa. The cannula (Autocath 1453.13, Plastimed) was used for drug
infusion with a constant-rate infusion pump (Harvard 22, Harvard
Apparatus, Ltd) and for intra-arterial
recording of BP with a Statham P23Id pressure transducer (Gould
Inc). Heart rate (HR) was derived from a continuously recorded
one-lead electrocardiogram. The forearm blood flow
(FBF) in both arms was measured at 15-second intervals by R
wave–triggered venous occlusion plethysmography (EC-2 plethysmograph,
Hokanson Inc) with the use of mercury-in-Silastic strain gauges and a
rapid cuff inflator (Hokanson E-10). Tracings of the
electrocardiograph, BP, and FBF were directly recorded on a
polygraph (Mingograph 803, Siemens-Elema). A personal computer (model
AT3, IBM) extended by an analog-digital convertor (model DT 2801, Data
Translation Inc) was used for R wave–triggered control of the rapid
cuff inflator and for on-line analysis of FBF,
intra-arterial BP, and HR.30 31 Forearm
vascular resistance (FVR) was calculated from each separate FBF
measurement and the mean values of the concomitantly recorded
intra-arterial BP, respectively. During all experiments
both hands were excluded from the circulation with the use of small
wrist cuffs inflated to 40 mm Hg above systolic BP. Baseline
recordings were started 1 minute after inflation of these cuffs.
Forearm and hand volumes were measured by water displacement. Venous
blood was taken for the determination of serum cholesterol
and glucose. Serum cholinesterase activity was measured by the method
of Rappaport et al32 (420-CC kit, Sigma Chemical Co). The
experiments started at least 45 minutes after cannulation of the
brachial artery. The total duration of the study was between 5 and 7
hours for each subject. Between the various infusions, the wrist cuffs
were deflated, and sufficient time (40 to 60 minutes) was allowed for
FBF to return to baseline levels.
Drugs and Solutions
The following compounds were infused into the brachial artery:
methacholine HBr (Brunschwig), atropine sulfate (Bufa), pirenzepine
2-hydrochloride, and AF-DX 116
(11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydroxy-6H-pyrido[2,3-b][1,4]benzodiazepin-6-on;
both generous gifts from Dr Karl Thomae, Biberach a/d Riss, FRG). All
drugs were dissolved in 0.9% saline. All commercially obtained
compounds were analyzed before use. All solutions were prepared
aseptically from sterile stock solutions and ampoules on the day of the
study and stored at 4°C until used.
Experimental Protocols
Sixteen subjects divided into two groups (hypertensive patients,
n=8; normotensive control subjects, n=8) participated in the current
investigations. Fig 1 summarizes the general design of
the experimental protocols. Control infusions of the agonist
methacholine were always performed together with a continuous infusion
of vehicle (0.9% saline at 0.4 mL/min). All cumulative-dose infusions
lasted 16 minutes and consisted of four dose steps of 4 minutes each.
This 4-minute interval proved sufficient to allow FBF to reach a
steady-state effect. The concomitant continuous infusions of either
vehicle or antagonist lasted 21 minutes and started 5
minutes before the cumulative-dose infusions. Infusions of sodium
nitroprusside (SNP) and methacholine in the presence of vehicle always
preceded the experiments with the antagonists. The
experiments were performed in a fixed order, as shown in Fig 1.
Baseline values were recorded during 3 minutes before each
experiment. The average values of FBF, FVR, HR, and
intra-arterial BP obtained from six consecutive
recordings during the last 1.5 minutes of each infusion step were
used for analysis. The muscarinic receptor involved in the
vasodilator response to the nonselective agonist methacholine was
characterized with the use of the competitive antagonists
pirenzepine (M1-selective; ie,
M1>M3>M2) and AF-DX 116
(M2-selective; ie,
M2>M1>M3) in essential
hypertensive patients and normotensive control subjects. Atropine, a
nonselective muscarinic antagonist, was used for
comparison. SNP was given as an endothelium-independent
vasodilator control agent that does not directly interfere with the
parasympathetic nervous system or with muscarinic receptors. The
minimal vascular resistance after 10 minutes of forearm
ischemia33 was determined as a measure of
structural vascular changes before any of the infusion experiments were
performed.
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Infusion Experiments
We constructed dose-response curves for the vasodilator effects
of SNP (0.1 to 100 ng/kg per minute IA) and methacholine (0.1 to 100
ng/kg per minute IA). Three experiments with methacholine were
performed in combination with saline serving as control before each
successive experiment with methacholine and antagonist. By
this procedure complete washout of antagonist was
controlled (Fig 1). The first control infusion experiment with
methacholine was performed before any antagonist was
administered. We repeated the methacholine dose-response curve (dose
range, 0.3 to 300 ng/kg per minute IA) in the presence of a
continuous-dose infusion of the M2-selective
antagonist AF-DX 116 (4000 ng/kg per minute IA). The second
infusion experiment with methacholine in the presence of saline served
as control for the dose-response curve with methacholine (dose range,
0.3 to 300 ng/kg per minute IA) in the presence of a continuous-dose
infusion of the M1-selective antagonist
pirenzepine (500 ng/kg per minute IA). The third control dose-response
curve with methacholine preceded the infusion of methacholine in the
presence of the nonselective antagonist atropine (50 ng/kg
per minute IA).
Minimal FVR
Before the infusion experiments the minimal FVR after 10 minutes
of forearm ischemia33 was determined by the
simultaneous measurement of peak FBF and
intra-arterial BP. FBF was measured in the noninfused
control arm to avoid the influence of the intra-arterial
cannula in the infused arm.34 Ischemia was induced
by inflating the pressure cuffs placed around the upper arms to 40
mm Hg above systolic BP for 10 minutes. Immediately after release of
the cuffs, FBF and intra-arterial BP were measured during
the next 5 minutes.
Calculations
The average of six consecutive FBF measurements performed during
the last 1.5 minutes of each dose step was used for analysis.
Drug plasma concentrations (Cplasma, micromoles per
liter) were calculated from the infusion rate (IR, nanograms per
kilogram per minute), body weight (W, kilograms), hematocrit (Ht),
forearm volume (V, milliliters), FBF (milliliters per 100 mL per
minute), and molecular weight of the drug (MW, daltons):
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The average calculated Cplasma and corresponding average relative FVR values were used to construct concentration-response curves for each individual subject by means of a curve-fitting computer program (GraphPad Software) based on the relationship:
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where E is the effect (percent change in FVR) observed with a calculated agonist of concentration A (moles per liter); Emax (percent change in FVR) is the maximally attainable effect; EC50 (moles per liter) is the apparent concentration at which a half-maximal effect is seen; and the exponent P describes the slope of the relationship (Hill coefficient). The pKb affinity constants were calculated for each individual subject using the EC50 values and calculated plasma concentrations of the antagonists estimated at the EC50 of the concentration-response curve (see Reference 2828 ). The pKb values were derived from the equation log Kb=log[B]-log(CR-1), where [B] is the concentration of antagonist35 and CR is the concentration ratio that represents the EC50 value of the agonist concentration-response curve in the presence of the antagonist divided by the EC50 value of the agonist concentration-response curve in the presence of vehicle.35
Statistical Analysis
Results are given as mean±SEM unless indicated otherwise. FVR
values are expressed as percentage change from baseline (see Fig 1).
The error in the calculated concentrations in the
concentration-response curves was determined by averaging all
individually calculated plasma concentrations, resulting in mean
concentrations with abscissa-oriented SEM limits, which are given
horizontally in the figures. To compare the relative potency of the
endothelium-dependent vasodilator methacholine with the
endothelium-independent vasodilator SNP on a molar
basis, we calculated the ratios of the apparent EC50 values
and expressed these as percentages with 95% confidence
intervals.36 Wilcoxon's signed rank test for matched
pairs and ANOVA were used to evaluate the statistical significance of
the data. Two-sided probability values less than .05 were regarded as
significant.
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Results |
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Table 1 gives clinical and hemodynamic baseline characteristics of the subjects studied. Baseline values of FBF and FVR established in the experiments with the antagonists and the corresponding control experiments were all in the same range. During administration of AF-DX 116 into the brachial artery, a significant increase in HR was observed in both groups (17±2% for hypertensive patients, P<.01; 20±5% for normotensive control subjects, P<.05), reflecting a systemic parasympathicolytic effect. In all other experiments, inconsistent changes in HR, FBF, and FVR in the control arm were observed. Mean arterial pressure tended to decrease slightly (
6%) during successive infusion
steps (Table 2) but was not significantly different
between the various experiments.
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Minimal FVR After 10 Minutes of Ischemia
Maximal FBF after 10 minutes of forearm ischemia was not
different between the two groups. Minimal FVR, however, was higher in
hypertensive patients (P<.05, Table 1), suggesting
structural vascular changes. Both FBF and FVR returned to baseline
values within 5 minutes after forearm occlusion was discontinued.
Cumulative-Dose Infusions of SNP and Methacholine
Both the endothelium-independent vasodilator SNP
(dose range, 0.1 to 100 ng/kg per minute IA) and the
endothelium-dependent muscarinic receptor agonist
methacholine (dose range, 0.1 to 100 ng/kg per minute IA)
dose-dependently decreased FVR. Both compounds proved equipotent at a
molar basis, as demonstrated in Fig 2 and Table 3. The vasodilator responses to SNP and methacholine
were the same in hypertensive patients and normotensive control
subjects (Fig 2 and Table 3). In both groups subsequent control
infusions of the agonist methacholine caused similar vasodilator
responses, with the same EC50 values and maximal effects
(Fig 3 and Table 3). The mean ratio of the response to
methacholine to the response to SNP (EC50 of
methacholine/EC50 of SNPx100%) was 104% in hypertensive
subjects and 102% in normotensive control subjects. The 95%
confidence interval for the difference in this ratio between
hypertensive patients and normotensive control subjects was -8% to
+12%.
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Cumulative-Dose Infusions of Methacholine in the Presence of the
Muscarinic Antagonists Atropine, Pirenzepine, and AF-DX
116
The muscarinic receptor antagonists atropine
(nonselective), pirenzepine (M1-selective), and AF-DX 116
(M2-selective) all caused parallel rightward shifts of the
concentration-response curves of methacholine (Fig 3). None of the
three antagonists alone induced any changes in FBF or FVR.
Fig 3 also shows that atropine (50 ng/kg per minute IA) caused a
greater shift than pirenzepine, whereas pirenzepine (500 ng/kg per
minute IA) was more potent than AF-DX 116 (4000 ng/kg per minute IA).
Thus, the rank order for potency of the three muscarinic receptor
antagonists used was atropine>pirenzepine>AF-DX 116. The
differences between the three muscarinic receptor
antagonists, as visualized by the shifts shown in Fig 3,
were also reflected by the calculated apparent pKb
values in both hypertensive patients and normotensive control subjects
(Table 4). The pKb values were
calculated after adjustment for the increase in FBF caused by
methacholine.28 There were no significant differences
between the values obtained for atropine, pirenzepine, and AF-DX 116
observed in the different groups of subjects studied (hypertensive
versus normotensive).
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Discussion |
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In this study we investigated the functional responses to the endothelium-dependent vasodilator methacholine and the endothelium-independent vasodilator SNP in forearm resistance vessels of patients with essential hypertension and matched normotensive control subjects. In addition, we characterized the muscarinic receptor subtype involved, using a classic pharmacological approach that we recently introduced for this experimental model in healthy volunteers.28
Preserved Vasodilatation to Methacholine in Patients With Essential
Hypertension
The present data do not support the hypothesis that
endothelium-dependent vasodilatation is altered in
essential hypertension. This contrasts with reports showing impaired
vasodilator responses to cholinergic agonists in the resistance
vasculature of the forearm of hypertensive
humans.5 6 7 37 38 The clinical characteristics of our study
populations were similar to those reported in other
studies,6 7 38 hypertension being well established and in
the same range. The use of SNP and methacholine in each participant,
together with the calculation of plasma concentrations, permits
comparisons of the potency of these compounds on a molar basis within
individual subjects.28 39 To detect possible blunting of
the response to methacholine relative to the response to SNP, we
calculated the difference between the ratio of the EC50 of
methacholine to the EC50 of SNP in hypertensive individuals
and normotensive control subjects.36 The resulting 95%
confidence interval of this difference, -8% to +12%, indicates that
missing a true reduction of more than 8% in the sensitivity to
methacholine relative to that of SNP in our experiments is unlikely,
despite the relatively small size of our study population. The widely
held assumption that the release of endothelium-derived
nitric oxide is impaired in hypertension mainly results from numerous
animal experiments.40 41 42 43 44 Recently, however, Angus
and Lew45 reviewed these data and challenged the concept
of a generalized involvement of endothelial dysfunction
in essential hypertension. Acetylcholine has been predominantly used to
demonstrate endothelial dysfunction of isolated conduit
arteries in various animal models of experimental
hypertension.40 41 42 46 47 48 An attenuated response to
acetylcholine in resistance vessels of animals with hypertension is
less conclusive.19 49 50 51 52 The human forearm vascular bed is
widely recognized as a suitable model for the functional and
pathophysiological analysis of resistance
vessels in vivo.3 4 5 6 7 8 9 10 11 12 13 14 30 53 Although the basal release of
nitric oxide can be blocked effectively with the use of
L-arginine analogues,3 4 it should be
emphasized that in the forearm model the precise role of the
endothelium is difficult to assess because the latter
cannot be removed. Therefore, any conclusion with respect to the
endothelial function in this vascular bed requires
extrapolation from in vitro experiments. The use of an unstable agonist
such as acetylcholine in an in vivo model such as the human forearm
further complicates the interpretation of responses
obtained.45 54 55 Differences found by other researchers
using acetylcholine in the human forearm model may be explained by
differences in cholinesterase activity. However, in our experiments we
could not detect differences in plasma cholinesterase activity of
previously treated patients with essential hypertension compared with
normotensive control subjects. We have preferred to use the
nonselective muscarinic receptor agonist methacholine rather than
acetylcholine, because methacholine is known to be a more stable
compound in plasma, being barely susceptible to degradation by
esterases.56 57
Pharmacological Characterization of the Muscarinic Receptor
Subtype Mediating Vasodilatation
At present there is substantial evidence for the existence of
at least three functionally different receptor subtypes, that is,
M1, M2, and
M3.22 23 24 25 For the pharmacological
identification of muscarinic receptors, antagonists are
more useful than muscarinic receptor agonists because the agonists
available are rather unselective.26 Three
antagonists with limited selectivity are currently
available for use in humans. Atropine, a nonselective muscarinic
receptor antagonist and the most potent
antagonist in animal studies and in vitro experiments, has
an affinity constant (pA2) of 8.90 in isolated rat thoracic
aorta58 and of 9.86 in rat mesenteric resistance
vessels.27 The apparent pKb values
obtained in the present study are somewhat lower than the values
found in vitro but are not different from those found in the forearm
resistance vessels of healthy volunteers (apparent
pKb of 8.4728 ). Pirenzepine, a
muscarinic receptor antagonist with reasonably high
affinity for M1 receptors, displayed only a moderate
affinity in the forearm of healthy humans (apparent
pKb of 7.0428 ). This value is similar
to the values found in hypertensive patients and matched normotensive
control subjects and is in agreement with affinities found for the
M3 receptor subtype in isolated rat thoracic aorta
(pA2 of 6.7558 ) and pulmonary artery
(pA2 of 6.9659 ). Affinities of pirenzepine for
the M3 receptor in rat submandibular gland23
and for the endothelial M3 receptor in cat
middle cerebral artery (pA2 of 7.5260 ) and
rabbit thoracic aorta (pA2 of 7.5461 ) are
somewhat higher than those established in the human forearm. AF-DX 116
displays a relatively high affinity for the cardiac M2
receptor (pA2 of 7.0523 ). In isolated rat
mesenteric resistance vessels it exhibits low affinity for the receptor
mediating the cholinergic vasodilator response (pKb
value of 6.1927 ). Similarly, the pKb
values found in the present investigations are relatively low and
comparable to those reported earlier for healthy subjects
(5.6728 ), indicating that M2 receptors do not
play an important role in cholinergic vasodilatation in this vascular
bed. The apparent pKb affinity values found in the
present study were not different between the groups and were
similar to what we observed previously in healthy
volunteers.28 This is also in line with data of Hendriks
et al,19 who did not detect any differences between
spontaneously hypertensive rats and Wistar-Kyoto rats in the potencies
of various selective muscarinic receptor antagonists in the
perfused rat mesenteric bed. This also illustrates the accuracy of the
methods used in these experiments, which approaches that of experiments
with isolated blood vessels. The apparent pKb values
observed in the present study should not be interpreted as an
absolute measure for receptor affinity and can only be considered as an
estimate of the relative potencies of the antagonists used.
The order of the affinity values, atropine>pirenzepine>AF-DX 116, and
the low affinities of pirenzepine and AF-DX 116 compared with atropine
indicate a primary role for the M3 receptor in cholinergic
vasodilatation in the resistance vessels of the human forearm, which is
not altered in essential hypertension. However, since a functional role
for the m4 and m5 receptor subtypes has not yet
been established, the involvement of the M3 receptor can be
postulated only by inference. The fact that none of the
antagonists caused any changes in FBF or FVR suggests that
at rest basal cholinergic tone is not present in the vascular bed
of the forearm. One might assume that if vasodilatation mediated by
muscarinic receptors or the endothelium is altered in
hypertension, the best place to uncover such an alteration would be in
a vascular bed exhibiting higher resistance in hypertensive as opposed
to normotensive subjects. However, in our investigations resting FVR
was not significantly different between the two groups. On the other
hand, minimal FVR was significantly higher in the hypertensive group,
suggesting the presence of structural vascular changes in hypertensive
patients.33 34 However, the vasodilator responses to
methacholine or SNP were not different between hypertensive and
normotensive individuals. This contrasts with several other forearm
studies in which acetylcholine was used.5 6 7 Our
observations are supported by a recent report by Cockcroft and
coworkers,20 who have convincingly shown that in
previously treated and untreated patients with essential hypertension,
the vasodilator responses to the cholinergic agonists carbachol and
acetylcholine compared with the responses to SNP did not differ from
those in normotensive control subjects.
Our findings do not argue against observations of an attenuated release of basal endothelium-derived nitric oxide in patients with hypertension.53 However, because we found no attenuated response to methacholine in hypertensive patients, other evidence for an attenuated response to cholinergic agonists in hypertension remains at best controversial. In addition, our present data in hypertensive patients and normotensive control subjects are consistent with our previous finding that the M3 receptor subtype predominates in cholinergic vasodilatation in the vascular bed of the human forearm but show that its pharmacological characteristics are apparently not altered in essential hypertension.
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Acknowledgments |
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The authors gratefully acknowledge Dr A.F. Cohen, Centre for Human Drug Research, Leiden, Netherlands, for kindly providing hypertensive subjects; and Eugenie A.P. Kuypers, Irene M. Teepe-Twiss, Henri C.R. Brandenburg, Gerard Jan Blauw, Harry D. Batink, and Michiel J.B. Kemme for technical assistance.
Received January 16, 1995; first decision February 16, 1995; accepted March 24, 1995.
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