(Hypertension. 1999;34:1066-1072.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
Impaired Atrial M2-Cholinoceptor Function in Obesity-Related Hypertension
From Laboratoire de Pharmacologie Médicale et Clinique, INSERM U317, Faculté de Médecine, 37 allées Jules Guesde 31073 Toulouse Cedex, France.
Correspondence to Michel Pelat, Laboratoire de Pharmacologie Médicale et Clinique, INSERM U317, Faculté de Médecine, 37 allées Jules Guesde 31073 Toulouse Cedex, France. E-mail pharmed{at}cict.fr
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Abstract |
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Abstract—The aim of this study was to investigate the activity of the parasympathetic limb of the baroreflex arch in a canine model of obesity-related hypertension. Twelve male beagle dogs were randomized into 2 groups. Six dogs were fed with normal canine food and 6 were submitted to a 10-week high-fat diet (HFD). We have evaluated the consequences of HFD on heart rate (HR) and blood pressure (BP) circadian cycles and methylscopolamine dose-response curves. Binding of [3H]-AF-DX 384 and adenylyl cyclase activity were investigated to determine the density and functionality of M2-cholinoceptors on right atrial membranes from control and HFD dogs. HFD induced a significant increase in body weight (15±1 vs 12±1 kg), systolic BP (161±5 vs 145±4 mm Hg), diastolic BP (92±3 vs 79±2 mm Hg), and HR (96±4 vs 81±3 bpm). Circadian rhythms of HR and BP observed in the baseline period were abolished after 9 weeks of HFD. After propranolol (1 mg/kg) pretreatment, the dose of methylscopolamine able to induce 50% maximum tachycardia was significantly increased after 9 weeks of HFD (7.4±0.3 vs 4.7±0.1 µg/kg). In the control group, the experimental period failed to modify these parameters. The numbers of M2-cholinoceptors measured in right atrial membranes were significantly lower in HFD than in control groups (54±6 vs 27±6 fmol/mg protein). The ability of carbachol to inhibit isoproterenol-stimulated adenylyl cyclase activity was significantly lower in HFD than in control groups (IC50=47±12 vs 6.4±1.4 µmol/L). However, the basal activity of adenylyl cyclase was unchanged by HFD. HFD decreases M2-cholinoceptor number and function in cardiomyocytes. This could explain the abolition of circadian rhythm of HR and the changes in chronotropic effect brought about by methylscopolamine.
Key Words: obesity • receptors, muscarinic • diet • adenylyl cyclase
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Introduction |
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Epidemiological, clinical, and experimental data have established a clear relation between obesity and arterial hypertension.1 2 3 4 However, because of the methodological limitations of human studies, the nature of the pathophysiological mechanisms of the blood pressure (BP) increment still has not been elucidated. Nevertheless, some studies suggest an involvement of insulin resistance as well as sympathetic nervous activity in obesity-related hypertension.4 5 In a model obtained with the use of a high-fat diet (HFD),6 various abnormalities have been reported that can explain the increase in BP, such as renal4 or autonomic nervous system dysfunctions.7 8 9 10 As observed in obese humans,11 12 13 some authors have reported an increase in sympathetic nervous activity during weight gain in dogs.8 10 14 Moreover, a decrease in the parasympathetic drive to the heart has been reported in obese humans15 16 and in obese dogs.7 8 10 A recent study from our group has investigated time-course changes of autonomic nervous activity in this model and showed that parasympathetic tone is reduced in the steady state of obesity-related arterial hypertension.10 Nevertheless, the nature and the consequences of cardiac muscarinic cholinoceptor changes in obesity-related hypertension remain unknown.
The aim of the present study was to investigate the activity of the parasympathetic limb of the baroreflex arch in an experimental model of dogs made obese and hypertensive by a long period of an ad libitum HFD. For this purpose, we have investigated in vivo the evolution of circadian rhythm of heart rate (HR) and the response to different doses of methylscopolamine (a peripheral muscarinic cholinoceptor antagonist). Concurrently, we have performed an in vitro approach to evaluate the density of atrial M2-cholinoceptors and their ability to modulate adenylyl cyclase activity.
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Methods |
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Animals and General Procedure
Twelve adult beagle dogs initially weighing 10 to 12 kg were included in this study. They were all submitted to a light-dark cycle of 12 hours. A totally implantable telemetry measurement system (Data Science International) that allows continuous and long-term recording of BP and HR was surgically installed under general anesthesia (1 mg/kg IM acepromazine plus 10 mg/kg IV tiletamine-zolazepam) before the beginning of the study. After a recovery period (3 weeks), the dogs were randomized into 2 different groups. The control group (n=6) was fed with 19.5±2.2 g/kg of normal canine diet (Royal Canin M25*) during the entire experimental period. After the baseline period (3 weeks), the second group (HFD group, n=6) was fed ad libitum with a HFD for 10 weeks. As previously described,17 the hypercaloric hyperlipidic diet (containing
42% of saturated fatty acids longer than
C16) was a normal canine diet (19.5±2.2 g/kg)
mixed with uncooked beef fat. Body weight was measured twice per week
during the entire study. All animal procedures were performed in
accordance with the official regulations of the French Ministry of
Agriculture.
BP and HR Measurements
In individual cages, a receiver unit (RL200, Data Science
International) was used to detect and amplify the signal from each
implantable transmitter and convert it into a series of digital pulses
to be decoded and evaluated by computer.18
Systolic and diastolic BPs and HR were obtained
from the femoral artery pressure waveform. BP and HR signals were
digitalized at 500 Hz. Systolic and diastolic BPs
and HR were computed for each cycle and extracted at 2 Hz, then stored
in a compatible IBM-PC for further analysis. BP and HR
measurements were performed twice per week, between 9 and 10
AM, on quiet, unrestrained animals in their cages.
Circadian Rhythm of BP and HR
In control and HFD groups, BP and HR signals were continuously
recorded for 24 hours at the end of the baseline period and after 9
weeks of the experimental period. To study the circadian cycle of BP
and HR, 2 periods were chosen (day from 6 AM to 7
PM and night from 11 PM to 6 AM).
Mean values of BP and HR were calculated for each 60-minute sequence of
recording. During the entire recording period, the
animals remained quiet and unrestrained in their cages.
Methylscopolamine Dose Response
To investigate the in vivo function of heart muscarinic
cholinoceptors, a dose-response experiment was performed with different
doses of methylscopolamine (a peripheral nonselective
muscarinic antagonist, ie, inducing an increase in HR).
This experiment was performed before and after 1, 3, 5, 7, and 9 weeks
of HFD and during the baseline period and after 9 weeks of HFD and
control groups, respectively.
Fifteen minutes before the first methylscopolamine injection, a blockade of sympathetic endogenous stimulation of the heart with propranolol (1 mg/kg IV) pretreatment was performed in all animals. After stabilization of the HR, 6 cumulatives doses (2.5, 5, 7.5, 10, 20, and 30 µg/kg IV) of methylscopolamine were administered, and HR was continuously recorded. Methylscopolamine dose-response curves were analyzed to determine maximal tachycardia (Emax) and ED50 (dose of methylscopolamine able to induce 50% of the maximal effect).
Binding Cardiomyocyte Studies
Preparation of Right Atrial Membranes
At the end of the experimental period (10 weeks after the
baseline period), right atrial tissue was obtained from both control
and HFD groups. Tissues were minced with scissors and
homogenized in 10 volumes of ice-cold buffer (Tris-HCl
5 mmol/L, EDTA 5 mmol/L, pH 7.4) with an Ultra Turrax for 10
seconds at 24 000 rpm and 20 seconds at 12 000 rpm at 1-minute
intervals in an ice-cold tube. The homogenate was diluted
in the same buffer and centrifuged at 100g for 15
minutes at 4°C. The supernatant was collected and centrifuged
at 20 000 rpm for 15 minutes. The final pellet was resuspended and
centrifuged at 20 000 rpm for 15 minutes in ice-cold
incubation buffer (Tris-HCl 50 mmol/L, MgCl2
0.5 mmol/L, pH 7.4). The resulting pellet was finally resuspended
in the required volume of incubation buffer and used immediately.
Binding studies were conducted in a final volume of 400 µL. Protein
content was determined by the Bradford method with bovine serum
albumin as standard.19
Saturation Experiment
As previously described by Caulfield and
Birdsall,20 determination of the number of
M2-cholinoceptors in atrial membranes was
performed with the use of 2,3-dipropylamino-3H
([3H]-AF-DX 384), a specific
antagonist of the M2-cholinoceptor
subtype. Membranes were incubated in the presence of 8 increasing
concentrations of [3H]-AF-DX 384 ranging from
0.125 to 6 nmol/L for 30 minutes. Nonspecific binding of
[3H]-AF-DX 384 was defined as the radioactivity
bound to membranes that was not displaced by a high concentration
(10 µmol/L) of atropine. At the end of incubation, the reaction
was stopped by the addition of 4 mL of ice-cold incubation buffer
followed by rapid filtration under reduced pressure through Whatmann
GF/C glass fiber filters placed on a Millipore manifold. The filters
were then washed twice with 10-mL portions of ice-cold incubation
buffer. The radioactivity retained on the filters was measured in the
presence of liquid scintillation medium (Emulsifier Safe, Packard) with
the use of a Packard beta-counter at an efficiency of 50%. Specific
binding was defined as total binding minus nonspecific binding.
Specific binding was directly proportional to the protein
concentration. Data were analyzed with a Scatchard plot
allowing Bmax and
Kd calculations.
Competition Experiments
Competition experiments were performed with
[3H]-AF-DX384 binding (5 nmol/L) in the
presence of increasing concentrations of carbachol, a nonselective
muscarinic agonist (ranging from 10-11 to
10-5 mol/L). Buffer, incubation procedures,
rapid filtration, and radioactivity retained on filters were realized
as described above. Curves were analyzed with a 2-site model
according to n Hill. Competition curves were analyzed to
calculate IC50 (eg, calculated concentration of
carbachol able to inhibit 50% of maximal specific binding) for both
high- and low-affinity agonist binding sites.
Evaluation of Adenylyl Cyclase Activity
As described by Salomon et al,21 adenylyl cyclase
activity was evaluated in right atrial membranes from both dog groups.
Briefly, 20 to 30 µg of membranes were incubated in a final volume of
100 µL incubation buffer (40 mmol/L Tris-HCl, 100 µmol/L
EGTA, 1.5 mmol/L MgCl2, pH 7.4) containing
0.1 µmol/L guanosine 5'-triphosphate, 1 mmol/L cyclic
adenosine 3', 5'-monophosphate (cAMP), 0.5 mmol/L
isobutylmethylxanthine (IBMX), 0.2 mmol/L
adenosine 5'-triphosphate (ATP), 5 mmol/L creatine
phosphate, 70 IU/mL creatine kinase, 0.2% bovine serum
albumin, and 0.5 to 1 µCi
[
32P]-ATP. Direct and indirect adenylyl
cyclase stimulation was achieved with forskolin, a direct adenylyl
cyclase agonist (10 µmol/L), or isoproterenol, a nonselective
ß-adrenoceptor agonist (10 µmol/L), respectively. For the
inhibition of adenylyl cyclase activity, membranes were incubated with
5 increasing concentrations of carbachol ranging from
10-8 to 10-5 mol/L in the
presence of forskolin (10 µmol/L) or isoproterenol (10
µmol/L).
Incubations were performed for 10 minutes at 30°C, and the reaction was stopped by adding 20 µL of 2.2N HCl and 10 nCi [3H]-cAMP. The incubates were poured off in Dowex columns and eluted with 100 mmol/L ammonium acetate buffer (pH 7.4). The radioactivity contained in the eluate was measured by liquid scintillation using a Packard beta-counter. All results are expressed as picomoles of cAMP formed per milligram of membrane proteins per minute.
Drugs
[3H]-AF-DX 384,
[(±)-5,11-dihydro-11-[(2 to 2-[dipropylamino)
methyl]-1-piperidinyl-ethyl) amino] carbonyl-6H-pyrido
(2,3-b)1,4-benzodiazepine-6-one] (specific radioactivity 100 to 160
Ci/mmol), [3H]-adenosine 3',5'-cyclic
monophosphate (specific radioactivity 25 to 40 Ci/mmol), and
[32P] adenosine 5'-triphosphate were
from New England Nuclear (Boston). Guanosine-5'-triphosphate,
adenosine 3',5'-cyclic monophosphate, creatine kinase, and
creatine phosphate were purchased from Boehringer-Mannheim.
Carbamylcholine chloride, (-)scopolamine methyl bromide,
(-)isoproterenol, forskolin, bovine serum albumin (fraction
V), and 3-isobutyl 1-methylxanthine (IBMX) were from Sigma Chemical Co.
(±)Propranolol was a generous gift from Zeneca
Pharma.
Statistical Analysis
Dose-response curves were fitted by a nonlinear regression to
sigmoidal curves to calculate Emax and
ED50 with the use of the program Prism (Graph Pad
Software). [3H]-AF-DX 384 binding or
competition curves were analyzed by iterative curve fitting
(Graph Pad Software).
All results are depicted as mean±SEM. All statistical comparisons were performed after examination of homoscedasticity. Multiple comparisons were realized with the use of ANOVA, followed when required by Dunnett’s post hoc test. Single comparisons between control and HFD groups (at the same experimental time) and between day and night values (in the same experimental group) were performed by unpaired and paired Student’s t tests, respectively. A value of P<0.05 was considered significant.
The linear regression between 2 parameters was also determined with 1-way ANOVA. The intraclass coefficient derived from the within-pair and between-pair mean square was computed. A value of P<0.05 was considered significant.
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Results |
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Body Weight, Systolic and Diastolic BPs, and HR
During the baseline period, mean values of HR (78±6 vs 81±3 bpm), systolic BP (143±4 vs 145±4 mm Hg), and diastolic BP (79±5 vs 79±2 mm Hg) were not significantly different in control and HFD groups, respectively. HFD induced a significant body weight increase in dogs (12.0±0.7 vs 14.9±0.6 kg, P<0.005). This gain was associated with a significant increase in systolic BP (161±5 mm Hg) and diastolic BP (92±3 mm Hg) and in HR (96±4 bpm) after 9 weeks of hypercaloric regimen. In the control group, body weight (11.5±0.5 vs 12.0±0.5 kg), systolic BP (148±5 mm Hg) and diastolic BP (82±3 mm Hg), and HR (82±4 bpm) remained unchanged during the entire experimental period.
Circadian Variability of BP and HR
During the baseline period, night values of HR and
systolic and diastolic BPs were significantly lower
than day values in both control and HFD groups (Figure 1 and
Table).
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) and HR
(•) over 24 hours obtained before (top) and after 9 weeks of HFD
(bottom) in HFD group (n=6). Values of systolic BP and HR were
calculated for each 60-minute sequence of recording. Values are
expressed as mean±SEM.
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In the control group, differences in HR and BP between day and night were still observed after 9 weeks of normal regimen. In the HFD group, circadian cycles of HR and BP were abolished after 9 weeks of hypercaloric hyperlipidic regimen. Moreover, after 9 weeks of HFD, night and day values of BP and HR were significantly higher than baseline values.
Methylscopolamine Dose-Response Experiments
In both groups, pretreatment with propranolol
induced a significant decrease in HR (Figure 2). In the control group, the change in
HR was unaffected during the entire experimental period (
HR=-23±2
and -25±4 bpm). In the HFD group, the propranolol-induced
decrease in HR was significantly increased starting from the third week
(HR=-21±2, -24±2, -30±1, -34±1, -35±1, and -39±2 bpm at
baseline period and after 1, 3, 5, 7, and 9 weeks of HFD,
respectively).
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).
Propranolol hydrochloride (1 mg/kg IV) pretreatment was
administered 15 minutes before methylscopolamine. After stabilization
of HR, 6 doses (2.5, 5, 7.5, 10, 20, and 30 µg/kg IV) of
methylscopolamine were administered. Effects of methylscopolamine are
expressed as mean of HR increase±SEM.In controls, methylscopolamine induced a dose-dependent increase in HR (Emax=113±12 bpm; ED50=5.0±0.1 µg/kg). These values remained unchanged after 9 weeks of normal canine diet (Emax=107±5 bpm; ED50=4.8±0.1 µg/kg). In the HFD group, the baseline methyl-scopolamine dose-response curve was not significantly different from that of control dogs (Emax=111±9 bpm; ED50=4.8±0.1 µg/kg). Methylscopolamine ED50 was similar at baseline until the third week of HFD (5.7±0.4, 5.4±0.2) and increased significantly (7.0±0.8, 6.7±0.3, and 7.4±0.4 µg/kg, P<0.05, respectively) from the fifth to the ninth weeks of HFD. During HFD, HR values (before propranolol pretreatment) were positively related to the logarithm of the methylscopolamine ED50 (r=0.915 P<10-4). HFD failed to induce significant changes in maximal induced tachycardia (Emax=102±14, 101±5, 94±7, 88±3, and 95±2 bpm after 1, 3, 5, 7, and 9 weeks, respectively).
[3H]-AF-DX384 Binding
In saturation experiments, M2-cholinoceptor
Bmax measured in the right atrium of dogs
submitted to a hyperlipidic hypercaloric diet was significantly lower
than in control dogs (27±6 vs 54±6 fmol/mg of membrane protein), with
no change in the values of Kd (1.0±0.2 vs
1.2±0.3 nmol/L). In atrium cardiomyocyte membranes from
control dogs, competition of [3H]-AF-DX384
binding by increasing concentrations of carbachol resulted in a
biphasic curve compatible with the presence of 2 sites with different
affinities for muscarinic agonist. High-affinity receptors
represented 29±2% (IC50=2.0±0.5
nmol/L) and low-affinity receptors 71±3%
(IC50=1.0±0.3 µmol/L) of the total atrial
population of M2-cholinoceptors.
In the HFD group, the percentage of high- and low-affinity receptors (33±2% and 67±3%, respectively) was not significantly different from control values. However, the IC50 for both high- and low-affinity agonist binding sites were significantly right-shifted in obese hypertensive dogs (274±40 nmol/L and 61±3.0 µmol/L, respectively; Figure 3). Moreover, M2-cholinoceptor Bmax was significantly correlated with both HR values and the logarithm of methylscopolamine ED50 (r=-0.761, P=0.0027, and r=0.680, P=0.013, respectively).
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Adenylyl Cyclase Activity
In control atrial membranes, compared with basal value (58±15
pmol cAMP/min/mg membrane protein), incubation with isoproterenol or
forskolin induced a significant increase in adenylyl cyclase activity
(126±17 and 148±17 pmol cAMP/min/mg membrane protein, respectively,
P<0.05). In atrial membranes from HFD dogs, basal adenylyl
cyclase activity as well as the stimulating effects of both
isoproterenol and forskolin were not significantly different from
control dogs (68±5, 104±5, and 148±11 pmol cAMP/min/mg membrane
protein, respectively).
The ability of carbachol to inhibit the activity of adenylyl cyclase stimulated by isoproterenol (Figure 4, top) or forskolin (Figure 4, bottom) was significantly lower in obese hypertensive dogs (IC50=47±12 and 172±49 µmol/L, respectively) than in controls (IC50=6.4±1.4 and 1±0.3 µmol/L, respectively). The logarithm of methylscopolamine ED50 was also correlated with the logarithm of the agonist IC50 evaluated after isoproterenol stimulation (r=0.835, P=0.0003).
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Discussion |
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Our results show that the decrease in the activity of the efferent parasympathetic limb of the baroreflex can be explained by an impairment in cardiac muscarinic cholinoceptor function in obesity-related hypertension. For the first time, a decrease in both number and agonist affinity of atrial M2-cholinoceptors without any functional change in adenylyl cyclase per se is described in obesity-related hypertension.
In this model of the obese hypertensive dog, which also presents sodium and fluid retention, an increase in free fatty acid plasma levels, and insulin resistance without hyperglycemia,6 10 14 the first result to be discussed is the abolition of the circadian cycle of HR and BP. This modification, probably because of a decreased parasympathetic activity, appears to be specifically related to the appearance of obesity-related hypertension. It was not observed in lean and normotensive dogs, in which, as previously reported,18 we found a 10% reduction in both HR and BP during the night period, which was stable over the experimental period. Circadian rhythm of BP and HR is mainly related to changes in autonomic nervous system activity during the night. In fact, treatment with atropine was shown to abolish the nocturnal decrease of HR in rats, thus indicating the physiological role of the parasympathetic nervous system in this periodic oscillation.22 In the same way, in patients with autonomic failure with parasympathetic dysfunction, abolition of the circadian rhythm of HR is an early event and is related to dysautonomia.23 Arterial hypertension during hyperlipidic hypercaloric diet is associated with both diurnal and nocturnal tachycardia caused by parasympathetic tone decrease. In the same experimental model, our group has previously reported10 a decrease in the energy of the high-frequency band of spectral variability of HR, which is considered to represent the parasympathetic drive to the heart.24
To further investigate the parasympathetic activity, we studied the cardiac responses to administration of a nonselective muscarinic antagonist, methylscopolamine. For this purpose, and to eliminate changes caused by modifications of the sympathetic tone elicited by high-fat diet, the animals were pretreated with propranolol. The results show that the methylscopolamine dose producing 50% of the maximal effect was significantly increased from the 5th week of the HFD. The changes in parasympatholytic response was previously investigated by Van Vliet et al7 and Truett et al.8 However, these authors only investigated the effects of a single dose of atropine. There is no current simple explanation for the increase in methylscopolamine ED50. An alteration of the central cholinergic receptors that participate in BP control does not hold because methylscopolamine does not cross the blood-brain barrier.25 It is not related to the excess-tachycardia phenomenon, which was present and similar in both groups at the end of the experimental period (data not shown). This apparent paradoxic response to muscarinic antagonist was also reported with atropine in humans with autonomic failure.26 Whatever the case, it can be considered as an index of a decreased parasympathetic nervous system activity. Finally, the most probable explanation for the reduced responses to methylscopolamine would be a change in peripheral muscarinic receptivity in the heart that we tried to investigate in the in vitro approaches of this study.
In vitro results show that the number of M2-cholinoceptors on atrial cardiomyocyte membranes was reduced in dogs submitted to the HFD. Similar findings were reported in dogs with heart failure27 or in atrial myocytes from failing human heart28 but were never described in obesity-related hypertension. This is consistent with the observed decrease in cardiac parasympathetic tone. Despite the lack of change in the Kd values of [3H]-AF-DX384, binding of the agonist appears to be profoundly modified in HFD dogs. In fact, in competition experiments of [3H]-AF-DX384 binding by carbachol, the percentage of high- and low-affinity M2-cholinoceptors was not different in lean normotensive and HFD dogs, which suggests that receptor coupling to Gi protein was unchanged. However, competition curves were dramatically shifted to the right in HFD dogs, and the IC50 values for both high- and low-affinity M2-cholinoceptors were increased. As for the methylscopolamine experiments, this finding suggests that obesity-related hypertension is associated with a decrease in number and affinity of peripheral M2-cholinoceptors.
The determination of adenylyl cyclase activity first gives further insight into the results of the binding studies. The ability of carbachol to inhibit the stimulated adenylyl cyclase activity is reduced in obese hypertensive dogs. The activity of adenylyl cyclase, in basal conditions as well as after isoproterenol or forskolin stimulation, remains similar in obese hypertensive and normotensive dogs; this functional alteration was not due to a change in adenylyl cyclase activity per se. As for in vivo and competition experiments, the observed changes in adenylyl cyclase activity could also be explained by a reduction of atrial M2-cholinoceptor number or by a loss in their affinity for agonists.
As in human right atrium,29 30 31 32 the
M2-cholinoceptor subtype predominates in canine
heart.33 34 Through inhibitory G
proteins,29 muscarinic receptors inhibit adenylyl cyclase
activity and decrease heart rate.30 32 Different
hypotheses could explain cardiac muscarinic receptor changes observed
in obese hypertensive dogs, such as an increase in cholinoceptor
catabolism or a decrease in the level of the gene
transcription.35 36 Cytokines (eg, tumor necrosis
factor-) as well as endogenous
catecholamines or insulin have been proposed to
downregulate M2-cholinoceptors through a decrease
in transcription level.36 37 These factors are known to be
overexpressed in obesity associated or not associated with
arterial hypertension6 10 38 and could be the
initial cause of the impaired atrial
M2-cholinoceptors inducing a decrease in vagal
tone in arterial hypertension induced by a HFD.
Although further studies could be performed to confirm these M2-cholinoceptor changes in obese hypertensive humans, our study supports that impaired M2-cholinoceptor function is associated with tachycardia and changes in short- and long-term (eg, circadian) BP and HR variabilities in obesity-related hypertension. These abnormalities are well known as risk factors for cardiovascular death related to human arterial hypertension.39 40 41 The pathophysiological relevance of the decrease in parasympathetic tone is unclear, even though an association between high HR and high arterial rigidity has been reported in hypertensive subjects.42 In a transversal study, Van Vliet et al7 report that the development of arterial hypertension by itself could contribute to the impairment of cardiac parasympathetic function.7 However, the early aspect (eg, before 5 weeks of HFD) of this decrease and its links with both food intake and the free fatty acid plasma levels suggest an important initial role of parasympathetic tone in the physiopathological mechanism of obesity-related hypertension.9 10 This decrease in parasympathetic tone could reduce the capacity of the autonomic nervous system to buffer BP changes. Endogenous catecholamines are able to downregulate M2-cholinoceptors.36 So, as suggested by propranolol blockade, early sympathetic overactivity rather than arterial hypertension could lead to changes in vagal tone in obesity-related hypertension.
In summary, we found that the decrease in parasympathetic tone induced by HFD in dogs is related to an impaired atrial M2-cholinoceptor without changes in adenylyl cyclase activity per se. The data suggest that atrial M2-cholinoceptor changes could explain the parasympathetic tone decrease and thus the abolition of circadian cycles of HR and BP observed in obese hypertensive dogs and play a major role in maintaining high values in arterial blood pressure as well as in cardiovascular death related to arterial hypertension.
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Acknowledgments |
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We would like to express special thanks to Dr Marie-Thérèse Canal, Nathalie Troislouche-Laplace, and Jean-Michel Duplantier for their excellent technical assistance.
Received February 26, 1999; first decision March 22, 1999; accepted June 28, 1999.
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