Effect of vitamin E on carotid artery elasticity and baroreflex gain in young, healthy adults

Autonomic Neuroscience: Basic and Clinical 113 (2004) 63 – 70 www.elsevier.com/locate/autneu

Effect of vitamin E on carotid artery elasticity and baroreflex gain in young, healthy adults Pe´ter Studinger a, Beatrix Mersich a, Zsuzsanna Le´na´rd a, Aniko´ Somogyi b, Mark Kollai a,* a

Institute of Human Physiology and Clinical Experimental Research, Semmelweis University, Faculty of Medicine, H-1446 Budapest, P.O. Box 448, Hungary b 2nd Department of Internal Medicine, Semmelweis University, Budapest, Hungary Received 4 March 2004; received in revised form 22 April 2004; accepted 12 May 2004

Abstract In this study we tested the hypothesis that dietary vitamin E supplementation can improve carotid artery elasticity and cardio-vagal baroreflex gain in young, healthy individuals. A total of 20 subjects were studied in a double-blind, placebo-controlled, randomized study. Subjects in the active treatment group received 700 IU/day vitamin E for 1 month. Each subject was studied three times: before, during and 1 month after treatment. Plasma vitamin E levels were determined using high-performance liquid chromatography. Carotid artery diameter was measured by ultrasound and radial artery pressure by tonometry. Baroreflex function was assessed by time and frequency domain spontaneous indices. Plasma vitamin E levels increased by 123%, which was associated with a 20% increase in carotid artery compliance and a 30 – 60% increase in baroreflex indices. All these changes regressed 1 month after cessation of vitamin E supplementation. Significant correlations were observed across conditions (control, treatment and recovery), among plasma vitamin E concentrations, carotid artery compliance and distensibility values and two of the baroreflex gain indices in the treatment group. Our results demonstrate that vitamin E supplementation can increase carotid artery compliance and baroreflex gain in young, apparently healthy adults. D 2004 Elsevier B.V. All rights reserved. Keywords: Carotid artery; Baroreflex; Ultrasound; Vitamin E

1. Introduction Arterial baroreflex function is the most important shortterm regulator of arterial blood pressure, and the sensitivity (gain) of the reflex is relevant physiologically as a marker of autonomic control, and also clinically as a predictor of cardiovascular mortality (Mortara et al., 1997; LaRovere et al., 1998). Baroreceptors are strain-sensitive receptors embedded in the wall of the aorta and the carotid sinus; therefore, the baroreceptor stimulus is influenced by the elastic properties of the arterial wall (Andresen et al., 1978). In young, healthy subjects 60% of the variability in baroreflex gain was explained by changes in carotid artery compliance (Bonyhay et al., 1996). * Corresponding author. Tel.: +36-1-210-0306; fax: +36-1-334-3162. E-mail address: [email protected] (M. Kollai). 1566-0702/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.autneu.2004.05.003

The large elastic arteries are known to stiffen with age even in apparently healthy, normal subjects. This process starts early in life, and the carotid artery was shown to lose about 30% of its distensibility from the age of 6 to the twenties (Kawasaki et al., 1987; Van Merode et al., 1989). The mechanism of vascular stiffening in the young, healthy organism is not known, but oxidative stress is thought to be a contributive factor. In young, healthy men, large artery elastic parameters were found to be related to serum levels of oxidized low-density lipoprotein (LDL) independent of other serum lipid fractions (Toikka et al., 1999). Dietary antioxidants, such as vitamin E, can prevent the oxidative modification of LDL, and dietary vitamin E supplementation was shown to improve endothelial vasodilator function (Skyrme-Jones et al., 2000). However, the effect of vitamin E on carotid artery compliance and also on baroreflex function has not been studied.

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The aim of the present study was therefore to test the hypothesis that vitamin E supplementation can improve carotid artery elasticity in young, apparently healthy individuals, and vitamin E-induced improvement in carotid compliance is associated with increased cardio-vagal baroreflex gain. We also wanted to investigate if changes are reversible after cessation of vitamin E treatment.

2. Materials and methods 2.1. Subjects Twenty young, healthy volunteers (11 men, 9 women, age: 22.9 F 0.45 years) participated in the study. All were nonsmokers, free of overt autonomic or cardiovascular disease, had a body mass index < 25 kg/m2, resting brachial blood pressures < 135/85 mm Hg and were not taking regular medication. The subjects were asked not to take any other vitamin supplements and to maintain their usual diet and physical activity for the duration of the study. All subjects gave written informed consent, and the study was approved by the Ethical Committee of Semmelweis University. 2.2. Experimental design The subjects were randomized in a double-blind manner to receive 1 month of treatment with 700 IU/day of oral DLa-tocopherol acetate (Bioextra, Hungary) or placebo. The 10 –10 subjects were randomized into the active treatment and the placebo groups. Randomization was carried out with random numbers, and no further attempt was made to match the groups. Each subject was studied three times: before treatment, after 1 month of treatment and after another month of recovery period. 2.3. General procedure The subjects were studied in the early afternoon under standardized conditions, in a quiet room at a comfortable temperature. All fasted at least 2 h before testing and were asked to refrain from strenuous exercise or drinking alcohol or caffeine containing beverages for 24 h before the study. Upon arrival at the investigation unit, the subjects were equipped with measurement devices and then rested supine for about 15 – 20 min until the absence of evident heart rate and mean blood pressure trends demonstrated that satisfactory baseline conditions had been achieved. Heart rate, arterial blood pressure, respiration, carotid artery diameter and carotid distension were measured during the last 5 min of the resting period. To determine baroreflex gain, subjects were then asked to synchronize their respiratory rate with a metronome beating at 0.1 Hz for 10 min. Blood samples for fasting plasma lipid and vitamin E level measurements were taken on the same day in the morning hours.

2.4. Biochemical measurements Fasting venous samples were collected into vacuum tubes containing disodium salt of ethylenediamine tetraacetic acid (EDTA) and were put immediately on ice. Chilled blood was centrifuged with constant cooling, and the supernatant plasma was used for further analysis. Vitamin E content of plasma and its lipid fractions was determined by high-performance liquid chromatrography, using the method of Somogyi et al. (2000). Fasting plasma cholesterol, triglyceride, high-density lipoprotein- (HDL-) and LDL-cholesterol concentrations were measured by routine laboratory tests. 2.5. Blood pressure measurements Blood pressure was measured on the radial artery using applanation tonometry by means of a noninvasive, continuous blood pressure monitor (Colin CBM-7000, ADinstruments). Measurements made by an automatic microphonic sphygmomanometer were used to calibrate the radial pressure curve every 5 min. During data collection the servoreset mechanism of the Colin apparatus was turned off to permit continuous data acquisition. Central blood pressure was determined by the pulse wave analysis method using the SphygmoCor software package (SCOR, PWV Medical, Sydney, Australia). This method has been described in detail before (O’Rourke and Gallagher, 1996). In brief, the pressure waveforms obtained at the radial artery site were subject to a generalized transfer function to derive the corresponding central arterial waveforms. Central arterial pressure was computed from the latter. This method has been validated by direct catheter measurement of aortic pressure (Chen et al., 1997). 2.6. Carotid ultrasonography Common carotid artery wall thickness (IMT), carotid artery diameter and its change with the arterial pressure pulse were measured 1.5 cm proximal to the bifurcation by means of ultrasonography. The ultrasound device consisted of a vessel wall-tracking system (Wall Track System, Pie Medical, Maastricht, the Netherlands) combined with a conventional ultrasound scanner (7.5 MHz linear array, Scanner 200 Pie Medical) and has been described in detail before (Hoeks et al., 1990; Hoeks et al., 1997). Carotid artery diameter was recorded in five epochs, each containing four to eight distension pulses. 2.7. Other measurements RR-intervals (RRI) were measured from R-wave threshold crossings on continuously recorded ECGs, and respiration was recorded with an inductive system (Respitrace Ambulatory Monitoring).

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2.8. Data analysis 2.8.1. Carotid artery elasticity Baseline carotid artery compliance and distensibility coefficient (DC) were determined from the simultaneously recorded carotid distension waveforms and the derived central arterial pressure according to the following formulas: carotid compliance = DD/DP, and carotid DC = 2(DD/Dd)/ DP. DD, Dd and DP represent distension, diastolic diameter and pulse pressure, respectively. Carotid artery lumen crosssectional area (LCSA) and intima-media cross-sectional area (IMCSA) were calculated as LCSA = C(D d ) 2 /4, and IMCSA = C(Dd/2 + IMT)2 C(Dd/2)2. Incremental elastic modulus was determined as Einc=[3(1 + LCSA/IMCSA)]/ DC (Blacher et al., 1998). Baseline carotid compliance and distensibility coefficient yield information about the elastic behavior of carotid artery as a hollow organ, whereas Einc describes the properties of the wall structure, independent of the geometry. 2.8.2. Baroreflex gain We determined cardio-vagal baroreflex sensitivity by time-domain and frequency-domain analyses. We have calculated all spontaneous baroreflex indices from data recorded during controlled 0.1 Hz breathing. The recording periods were 10 min in duration. All analog signals were digitized and analyzed with the WinCPRS program (Absolute Aliens Oy, Finland) using a sampling rate of 500 Hz and stored in a personal computer for subsequent off-line analysis. The software detected the ECG R-wave and computed RRI and radial artery systolic blood pressure (SBP) time series and identified spontaneously occurring sequences in which SBP and RRI concurrently increased or decreased over three or more consecutive beats. Minimal accepted change was 1 mm Hg for SBP and 5 ms for RRI. Baroreflex sensitivity was calculated from up –up (sBRS+) and down – down (sBRS ) sequences as the slope of the

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regression line between SBP and RRI. Only sequences with a correlation coefficient >0.85 were considered. The number of all (up and down) blood pressure ramps and baroreflex sequences was also determined. Baroreflex effectiveness index that gives the fraction of all pressure ramps that were associated with corresponding lengthening or shortening of RR-interval was also calculated. When using frequency domain analysis, the signals were interpolated, resampled at the mean heart rate and their power spectra were determined using FFT-based methods. We determined low-frequency (LF) transfer function gain which expresses RRinterval and systolic pressure cross-spectral magnitude in the frequency range of 0.05 – 0.15 Hz frequency band, where coherence is greater than 0.5 (Parati et al., 2000). 2.8.3. Statistical analysis Data are expressed as means F 1 S.E.M. The Student’s unpaired t-test was used to compare baseline values between the placebo and the active treatment groups. Two-way repeated-measures ANOVA was used to compare the effects of treatment, with the Duncan’s all pairwise multiple comparison procedures as post hoc test. P values for treatment by period interaction effect, for treatment effect and for period effect were calculated. Relationships between variables were determined by linear regression analysis. Significance was accepted at p < 0.05. Statistical analyses were performed by the SigmaStat for Windows Version 2.03 (SPSS) program package.

3. Results 3.1. Biochemical characteristics Biochemical characteristics of subjects randomized to placebo or active vitamin E treatment are shown in Table 1 and in Fig. 1. There were no significant differences in any

Table 1 Hemodynamic, carotid artery and metabolic characteristics of subjects randomized into placebo and vitamin E treatment groups Placebo group

Heart rate (beats/min) Central systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Pulse pressure (mm Hg) Mean carotid diameter (mm) Carotid distension (Am) IMT (Am) Total cholesterol (mmol/l) LDL-cholesterol (mmol/l) HDL-cholesterol (mmol/l) Triglyceride (mmol/l) Plasma vitamin E (Amol/l) LDL-vitamin E (Amol/l)

Vitamin E treatment group

Control

Treatment

Recovery

Control

Treatment

Recovery

70 F 2.8 104 F 2.5 66 F 2.1 38 F 1.3 6.6 F 0.2 761 F 46 480 F 13 4.2 F 0.1 2.4 F 0.1 1.5 F 0.1 0.7 F 0.1 23.6 F 1.8 15.6 F 1.8

67 F 4.4 108 F 2.2 67 F 2.1 39 F 3.2 6.6 F 0.2 827 F 71 477 F 13 4.2 F 0.1 2.3 F 0.1 1.5 F 0.1 0.7 F 0.1 23.2 F 1.5 15.2 F 1.4

70 F 4.9 106 F 3.0 69 F 2.6 39 F 1.7 6.6 F 0.2 789 F 66 508 F 8.5 4.3 F 0.1 2.5 F 0.2 1.6 F 0.1 0.7 F 0.1 23.5 F 1.5 15.0 F 1.8

74 F 3.7 103 F 4.1 64 F 3.6 39 F 2.4 6.7 F 0.2 777 F 52 509 F 25 4.3 F 0.1 2.3 F 0.1 1.7 F 0.1 0.8 F 0.1 22.0 F 1.2 13.9 F 0.9

73 F 3.0 100 F 2.6 68 F 2.7 32 F 1.6a 6.6 F 0.2 764 F 51 503 F 22 4.5 F 0.2 2.5 F 0.2 1.6 F 0.1 0.8 F 0.1 49.1 F 2.9a 26.5 F 1.7a

70 F 3.4 98 F 2.1 63 F 1.4 35 F 1.5 6.5 F 0.2 740 F 42 527 F 22 4.4 F 0.1 2.3 F 0.1 1.6 F 0.1 0.8 F 0.1 22.0 F 0.9 13.7 F 0.8

IMT, carotid wall intima-media thickness; LDL, low-density lipoprotein; HDL, high-density lipoprotein. Results are expressed as mean F S.E.M. a Differences significantly different from control, at p < 0.05.

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placebo- and vitamin E-treated groups, and the characteristics of subjects in the placebo group did not change significantly throughout the investigation period. In response to vitamin E treatment, systolic blood pressure showed a tendency to decrease and diastolic blood pressure to increase, which changes resulted in significant reduction in pulse pressure ( p = 0.035 for treatment by period interaction effect). Heart rate, carotid diameter, carotid distension and IMT did not change. Carotid artery compliance and DC increased in 7 of the 10 subjects, with the group mean values increasing by about 20% ( p = 0.041 for treatment by period interaction effect for carotid artery compliance and p = 0.05 for treatment by period interaction effect for DC). Einc was not altered during treatment. Values of altered variables returned to control level during recovery. Changes in the vitamin E-treated group were not gender related. 3.3. Baroreflex gain Indices for baroreflex gain were determined by two different noninvasive techniques. Their mean values at baseline were not different in subjects randomized to placebo or treatment groups. Vitamin E supplementation induced substantial increases in all baroreflex gain indices ( p = 0.041 for period effect for BRS+, p = 0.004 for period effect for BRS and p = 0.042 for period effect for LF transfer function gain). During recovery baroreflex gain, values returned to control (Table 2 and Fig. 1). Gender did not have any effect on the observed changes. There was no time-dependent change in any of the baroreflex indices in the placebo group. Neither time nor treatment had an effect on the number of blood pressure ramps and baroreflex sequences. No changes in the baroreflex effectiveness index were observed either. Fig. 1. Changes in plasma vitamin E levels, carotid compliance and lowfrequency (LF) transfer function gain for individual subjects (left panels) and for group means (right panels) in the vitamin E treatment group. C, control values; E, values after 1 month of vitamin E treatment; R, values after 1 month of recovery. Error bars indicate S.E.M. *Differences significantly different from control at p < 0.05.

of the variables at baseline between the placebo- and vitamin E-treated groups. Furthermore, the characteristics of subjects in the placebo group did not change significantly after 4 and 8 weeks. In the treatment group, plasma vitamin E increased by 123% at the end of the supplementation period, then returned to control level at the end of the recovery period. This was accompanied by 91% increase in the plasma LDL vitamin E level, which also returned to control level during recovery. 3.2. Arterial elasticity Cardiovascular characteristics of subjects are shown in Tables 1 and 2 and in Fig. 1. There were no significant differences in any of the variables at baseline between the

3.4. Correlations between variables At baseline neither arterial elastic parameters nor baroreflex gain indices were related to plasma vitamin E level. Across conditions (control, treatment and recovery), however, significant correlations were observed among plasma vitamin E concentrations, carotid artery compliance and distensibility values and two of the baroreflex gain indices (Table 3). Carotid artery compliance was also related to baroreflex gain indices (Fig. 2) (r = 0.40, p < 0.05 for BRS+, r = 0.48, p < 0.05 for BRS , r = 0.54, p < 0.01 for LF transfer function gain).

4. Discussion This is the first study investigating the effects of dietary vitamin E supplementation on carotid artery compliance and cardio-vagal baroreflex gain in young, healthy individuals. Prior studies, when the cardiovascular effects of vitamin E have been investigated in clinical conditions,

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Table 2 Carotid artery elastic variables and baroreflex sensitivity indices in the placebo and treatment groups Placebo group

Compliance (Am/mm Hg) DC (10 3/mm Hg) Einc (103 mm Hg) N of R (ramps/10 min) N of S (sequences/10 min) BEI (%) BRS+ (ms/mm Hg) BRS (ms/mm Hg) LF transfer function gain (ms/mm Hg)

Vitamin E treatment group

Control

Treatment

Recovery

Control

Treatment

Recovery

20.3 F 0.9 6.2 F 0.3 1.8 F 0.1 114.5 F 5.1 90.4 F 6.0 79.2 F 4.0 17.1 F 2.0 15.6 F 2.1 12.1 F 2.0

21.5 F 1.7 6.9 F 0.6 1.6 F 0.2 117.9 F 4.7 94.2 F 7.9 80.1 F 4.2 15.5 F 2.0 15.2 F 2.0 13.5 F 2.3

20.3 F 1.1 6.5 F 0.3 1.6 F 0.1 117.1 F 4.8 89.5 F 6.8 76.4 F 4.5 16.5 F 2.3 16.0 F 1.8 13.3 F 3.6

20.6 F 1.3 6.6 F 0.4 1.7 F 0.1 117.0 F 5.6 92.1 F 4.3 78.6 F 3.8 16.0 F 3.6 12.5 F 1.9 11.4 F 2.7

24.6 F 1.7a 7.9 F 0.5a 1.5 F 0.1 107.5 F 5.3 85.8 F 9.3 79.8 F 5.2 21.3 F 4.6a 17.8 F 2.5a 18.5 F 3.4a

21.0 F 1.0 6.8 F 0.4 1.4 F 0.2 118.8 F 4.5 94.5 F 7.9 79.6 F 5.0 16.8 F 2.6 15.4 F 1.9 12.8 F 1.9

DC, distensibility coefficient; Einc, incremental elastic modulus; N of R, number of all (up and down) blood pressure ramps per 10-min recording; N of S, number of all (up – up and down – down) baroreflex sequences per 10-min recording, BEI, baroreflex effectiveness index; BRS+, baroreflex sensitivity for up – up sequences; BRS , baroreflex sensitivity for down – down sequences. Results are expressed as mean F S.E.M. a Differences significantly different from control, at p < 0.05.

produced controversial results. Observational studies indicated beneficial effect on cardiovascular morbidity and mortality (Gey et al., 1991; Riemersma et al., 1991; Stampfer et al., 1993; Rimm et al., 1993), which was, however, not supported by some of the controlled intervention trials (Vivekananthan et al., 2003). A recent review concluded that age and disease stage are likely to be important confounding factors (Ricciarelli et al., 2001). In the observational studies, vitamin E intake starts at an early age with normal vascular structure and function and the beneficial effect may result from a decades-long dietary pattern. In the intervention trials, the subjects of relatively old age or with previous cardiovascular disease may have already developed irreversible arterial damage, when vitamin E was supplemented over a relatively short period. It may be assumed, therefore, that the protective effects of vitamin E supplementation are more important earlier in life, at a time when pre-atherosclerotic vascular changes first become established and are likely to be more susceptible to modification.

coronary atherosclerosis (Crouse et al., 2002); and it importantly determines baroreflex function (Bonyhay et al., 1996). This study is the first to investigate changes in carotid artery elastic behavior during vitamin E treatment. We have demonstrated significant improvement in carotid artery elastic parameters in young, healthy subjects, resulting in about 20% increase in the group mean values. At baseline, no across-subject relationship was found between vitamin E levels and carotid elastic parameters, which extends the observation (Leeson et al., 2002) that neither plasma vitamin E nor total antioxidant status was significantly related to the distensibility of the brachial artery in young healthy subjects. This lack of relationship is likely to be explained by the small scatter of vitamin E plasma concentration data in subjects without vitamin E supplementation. On the other hand, we demonstrated a significant relation between plasma vitamin E level and carotid compliance, when baseline and treatment data were grouped together.

4.1. Carotid artery elasticity The significance of measuring carotid artery distensibility is manifold: it represents large artery elastic function (O’Rourke and Mancia, 1999); it provides a window on

Table 3 Correlation coefficient (r) values in the treatment group between plasma vitamin E concentrations, on the one hand, and carotid artery elastic parameters and baroreflex gain indices on the other, across control, treatment and recovery conditions

Vitamin E LDL-vitamin E

Compliance

DC

BRS +

BRS

LF transfer function gain

0.52** 0.51**

0.54** 0.43*

0.29 0.27

0.43* 0.42*

0.46* 0.39*

DC, distensibility coefficient; BRS+, baroreflex sensitivity for up – up sequences; BRS , baroreflex sensitivity for down – down sequences. Levels of significance: *p < 0.05, **p < 0.01.

Fig. 2. Correlation between carotid artery compliance and LF transfer function gain across conditions in the vitamin E-treated group (r = 0.54, p < 0.01). Full circles, control period; empty circles, active treatment period; full triangles, recovery period.

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Statistical analysis in the vitamin E-treated group revealed no difference in any of the investigated parameters between men and women. An earlier study reported differentiated effect of vitamin E treatment on carotid atherosclerosis in elderly men and women (Gale et al., 2001.), but in our study performed on young, healthy individuals, no such differences in carotid artery compliance were observed. We assume that the number of subjects in our study was too small to see minor gender-related differences, if there existed any. This issue requires further investigation. The mechanism of vitamin E-induced improvement in arterial elasticity is not known, but the antioxidant function of vitamin E may play a role. Oxidative stress is present in the healthy organism, and it may even participate in physiological control mechanisms (Droge, 2002). Oxidative stress contributes to the aging process of the arterial wall, and signs of oxidative damage can be identified already at young ages, with the difference between young and old specimens being only quantitative (Garcia Soriano et al., 2001; Szabo et al., 2002). Oxidative stress is usually associated with impaired endothelial function; therefore, we suggest that vitamin E may act by preserving endothelial function (Ricciarelli et al., 2001), protecting nitric oxide (NO) availability with the consequent relaxation of smooth muscle tone (Carr and Frei, 2000). This assumption is supported by the observation that vitamin E supplementation improved endothelial function in young diabetic patients (Skyrme-Jones et al., 2000). Structural changes in the arterial wall during vitamin E treatment seem unlikely, regarding the rapidity of increase in arterial compliance and that of decrease during recovery. Also, Einc, an index of wall material stiffness, did not change. With respect to another antioxidant, vitamin C, effects on central arterial stiffness in young healthy people are controversial. Eskurza et al. (2004) found no changes in carotid augmentation index calculated from carotid tonometric recordings after acute intravenous vitamin C administration, whereas Wilkinson et al. (1999) observed an increase in central arterial augmentation index derived from the radial arterial pressure by SphygmoCor after oral vitamin C supplementation. 4.2. Baroreflex function This study is also the first to investigate changes in baroreflex function during vitamin E supplementation. We found that 4 weeks of vitamin E supplementation produced significant improvement in baroreflex gain in healthy subjects in their early twenties. Antioxidant therapy may influence baroreflex function at several sites in the baroreflex arc, as it may increase barosensory arterial wall distensibility (Mullan et al., 2002), prevent the inhibition of baroreceptor firing caused by free radicals (Li et al., 1996) and improve the effectiveness of cardiac response (Chowdhary and Townend, 1999) by increasing the bioavailability of NO in the sinus node. In our study, the large increase in

baroreflex gain (30 –60%) as compared to that in carotid compliance (20%) raises the question whether changes in neural components of baroreflex function may contribute to the improvement in baroreflex gain. In order to answer this question, neural baroreflex gain needs to be determined directly. In studies with vitamin C, no changes were observed in baroreflex sensitivity in young, healthy subjects after acute administration of the vitamin, measured either by spontaneous indices (Nightingale et al., 2003) or by bolus phenylephrine (Monahan et al., 2004). The discrepancy in the results might be explained by different antioxidant treatment (vitamin E vs. vitamin C), the way it was administered (chronic oral vs. acute intravenous), the dose of the substance (physiological vs. pharmacological) and the method how baroreflex gain was measured (spontaneous indices vs. Oxford technique). 4.3. Limitations In this study, we failed to measure biomarkers of oxidative stress. However, we are unaware of any antioxidant-independent effect of vitamin E that could provide an alternative explanation to our conclusions. To determine carotid artery compliance and distensibility coefficients pulse pressure has to be measured locally, considering that pulse pressure is amplified towards the periphery. In this study, we calculated central arterial pressure from radial tonometric measurements using the generalized transfer function (SphygmoCor). Although carotid artery pressure is different from central arterial pressure, the difference is negligibly small (Chen et al., 1996). The SphygmoCor method has been proven to reconstruct faithfully the central arterial pulse wave, which then can be calibrated by the sphygmomanometrically determined diastolic and mean brachial pressure values (Chen et al., 1997). In a recent study, central arterial pressure calculated with SphygmoCor was found to be different from directly measured aortic catheter values (Smulyan et al., 2003). The observed difference was likely related to the calibration procedure, due to potential error in sphygmomanometric blood pressure measurements. Determination of carotid pulse pressure, by local tonometry, instead of using the generalized transfer function may not have solved the problem, since the calibration procedure is identical for both carotid tonometry and the SphygmoCor method. On the other hand, the SphygmoCor method is the only technique that allows continuous measurement of central arterial pressure noninvasively, which can be used for BRS determinations. In many earlier studies, peripheral systolic pressure was used to determine various indices of BRS. The use of peripheral systolic pressure to calculate BRS indices, however, represents a limitation (Avolio and O’Rourke, 2002), because with higher heart rates, the peripheral pulse amplitude is larger for any given central (carotid or aortic) pulse pressure. Therefore, during BRS

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determination, when heart rate is changing, the concurrent changes in systolic pressure will be different in central and peripheral arteries. We determined baroreflex gain at 0.1 Hz paced breathing. With the 0.1-Hz paced breathing protocol large amplitude oscillations were generated in arterial pressure and RR-interval, and the traditional approach of relating systolic pressure to RR-interval was used to estimate baroreflex gain. The 0.1-Hz paced breathing protocol has been used before to determine integrated baroreflex sensitivity in the supine position with high reproducibility and low failure rate (Davies et al., 1999). It is likely that slow deep breathing would exaggerate increases in pulmonary vagal afferent activity during inspiration and cause a corresponding increase in the respiratory sinus arrhythmia. This non-baroreflex mechanism may contribute to the measured baroreflex indices. However, besides increasing RR-interval variability, the amplitude of blood pressure changes is also increased by slow deep breathing: the oscillation of systolic arterial pressure can be as large as 10 –15 mm Hg, which is comparable to blood pressure changes induced by bolus phenylephrine injection. Thus, not only the stimulus for pulmonary stretch receptors, but also the stimulus for arterial baroreceptors is increased during slow deep breathing, although their relative contribution to the measured spontaneous indices still remains unclear.

5. Conclusion The present study demonstrates that a 4-week-long vitamin E supplementation can significantly increase carotid compliance and cardio-vagal baroreflex gain in young, healthy individuals. The observed changes in cardiovascular parameters regressed after the cessation of vitamin E administration. Based on our results, the supplementation of diet with vitamin E may be beneficial early in life, before the age- and disease-related changes in cardiovascular and autonomic nervous function become irreversible. The present investigation was carried out on 20 subjects with 10 subjects in the treatment arm. This study, therefore, should be considered as a pilot study because of the relatively small number of subjects, but our results indicate the need of a further, larger investigation to recommend vitamin E supplementation for young healthy adults.

Acknowledgements This work was supported by Hungarian National Research Fund, grant OTKA-29470, and the Hungarian Ministry of Welfare, grant ETT-144/2000.The authors acknowledge the skilled technical assistance of M. Herold and Z. Veres.

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