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Original Studies |
Laboratories of Metabolism (N.S., T.A., H.T., K.T.) and Applied Physiology (T.M., H.U.), Kyoto University Graduate School of Human and Environmental Studies; the Department of Metabolism and Clinical Nutrition, Kyoto University Faculty of Medicine (N.S., K.Y., T.A., H.T., Y.S.); and the Laboratory of Metabolism, Kyoto University Faculty of Integrated Human Studies (K.Y., K.T.), Kyoto 606-8501, Japan
Address all correspondence and requests for reprints to: Koichiro Yasuda, M.D., Ph.D., Laboratory of Metabolism, Kyoto University Faculty of Integrated Human Studies, Sakyo-ku, Kyoto 606-8501, Japan. E-mail: yasuda{at}metab.kuhp.kyoto-u.ac.jp
| Abstract |
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| Introduction |
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Recently, power spectral analysis of heart rate variability has attracted much interest as a noninvasive and very sensitive method to evaluate ANS activity (6, 7, 8, 9). It generates two major frequency components of heart rate variability, i.e. the high respiration-linked component (HI) and the low frequency component (LO), which can provide quantitative markers of parasympathetic nervous system (PNS) and SNS activities, respectively, and of their dynamic balance (6, 7, 8). For example, HI could be abolished upon vagal blockade by pharmacological drugs (e.g. glycopyrrolate or atropine), whereas the amplitude of LO was reduced (6, 7, 8). On the other hand, under conditions of ß-sympathetic blockade alone, the LO was reduced to some extent, but could not be totally abolished (6, 7, 8, 9). Combined vagal blockade and ß-sympathetic blockade abolished all heart rate fluctuations, leading to a metronome-like heartbeat (6). Akselrod et al. (7) also provided some evidence that the low frequency component of the R-R frequency spectrum was jointly mediated by PNS and SNS and appeared to compensate for blood pressure fluctuations at this frequency. These studies together with the earlier work thus suggest the possibility that the low and high frequency components of the R-R spectrum provide markers of the PNS-SNS balance that modulates heart rate variability. This measurement has been widely applied in various pathological conditions as well as physiological states; in diabetes, for example, it can be used for the early diagnosis of autonomic neuropathy (10).
The ß3-adrenergic receptor (ß3AR) plays a significant role in the control of lipolysis and thermogenesis in brown adipose tissue through ANS activity (11, 12, 13). A gene coding for human ß3AR was characterized in 1989 (14). A missense variant that replaced tryptophan with arginine at position 64 (Trp64Arg) was reported in 1995 from the study in Pima Indians (15). Since then, many studies in several populations suggest that the polymorphism was not uncommon.
It was reported that among Pima Indians, homozygotes for Trp64Arg had an earlier onset of noninsulin-dependent diabetes mellitus (NIDDM) and tended to have lower metabolic rates (15). In Finns, the variant was associated with abdominal obesity and resistance to insulin (16). Another study of Finns, however, reported Trp64Arg not to be associated with NIDDM or features of the insulin resistance syndrome (17). Similarly, in Japanese, both positive and negative results on the influence of the polymorphism have been reported (18, 19, 20, 21, 22, 23, 24, 25), but the reports that the variant had an influence in Japanese were obtained mainly from homozygotes, and there has been little recognition of the influence of heterozygotes, so the effects of Trp64Arg are not yet well defined.
As it is possible that the ß3AR polymorphism affects the entire ANS activity in which ß3AR functions, in this study we determined the prevalence of the polymorphism in a large number of subjects and then investigated the association of the ß3AR polymorphism with ANS activity by electrocardiogram (ECG) R-R interval power spectral analysis.
| Subjects and Methods |
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Two hundred and four male volunteers were studied after giving informed consent. All subjects were Japanese and were determined by interview not to be taking any medications. The mean age of the subjects was 20.1 ± 1.2 (±SD) yr. Blood sampling was carried out between 09001000 h after an overnight fast. At the same time, height and weight were measured, and family history was investigated by interview, including whether a subject had any relatives within the third degree who were or had been diagnosed as diabetic or were significantly obese [body mass index (BMI), >30].
Measurements of plasma glucose and insulin
Plasma obtained by centrifugation was used for measurements of glucose and immunoreactive insulin. Plasma glucose was measured by the glucose oxidase method (26). Immunoreactive insulin was determined by RIA using the polyethylene glycol method (27).
Determination of Trp64Arg polymorphism in the ß3AR gene
Genomic DNA was extracted from peripheral blood cells using a DNA Extractor WB Kit (Wako, Japan). PCR was carried out using 50 ng genomic DNA with the primers (sense, 5'-CGCCCAATACCGCCAACAC-3'; antisense, 5'-CCACCAGG AGTCCCATCACC-3') under conditions described previously (22).
The PCR products (210 bp) were digested with 60 U MvaI in a 200-µL volume and separated on a 3% agarose gel. Digestion of the normal sequence yields fragments of 99, 62, 30, 12, and 7 bp in length, whereas the Trp64Arg variant eliminates one of the MvaI sites, yielding a novel 161-bp product.
R-R interval power spectral analysis
Twenty-one subjects screened for the Trp64Arg polymorphism [10 normal homozygous (Trp/Trp), 10 heterozygous (Trp/Arg), and 1 variant homozygous (Arg/Arg)] were investigated for ANS activity by R-R interval power spectral analysis. Trp/Trp and Trp/Arg subjects were randomly chosen from the subjects without family history in each group. Only 1 Arg/Arg subject was found in this study; no statistical analysis could be made, and his ANS activity data are used only as a case report.
We investigated ANS activity during supine rest and postural change to standing in the morning (09001100 h) after an overnight fast. Subjects were at supine rest for 5 min and then stood up by the bedside for standing rest for another 5 min. The respiratory rate was controlled at 0.25 Hz by an electric metronome to avoid the parasympathetic component interfering with the low frequency component.
ECG R-R interval data obtained from the CM5 lead were digitized at 1000
Hz, and the derived R-R interval time series was then aligned in 2-Hz
sequence for power spectral analysis. The DC component and linear trend
were completely eliminated by digital filtering for bandpass between
0.030.5 Hz. After passing through the Hamming-type data window, power
spectral analysis by means of a fast Fourier transform was performed on
consecutive 240-s time series of R-R interval data obtained during the
tests. We analyzed low frequency (0.030.15 Hz; LO), high frequency
(0.150.4 Hz; HI), and total power (0.030.4 Hz; TOTAL) by
integrating the spectrum for the respective band width. Figure 1
shows examples of raw R-R interval data
(top) and the corresponding spectra (bottom). In
this spectrum, the black area shows the LO, and the
white area shows the HI. TOTAL reflects overall autonomic
nervous system activity. LO arises from combined SNS and PNS functions;
HI is from PNS activity only. The ratio of LO/HI (SNS index) reflects
SNS activity, and the ratio of HI/TOTAL (PNS index) reflects PNS
activity (28, 29).
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Significant differences were evaluated by Students t test.
| Results |
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One (0.5%) of the subjects was homozygous for the variant (Arg/Arg), and 49 (24.0%) were heterozygous (Trp/Arg) and 154 (75.5%) were homozygous for the normal (Trp/Trp) allele. The frequency of the Trp64Arg allele was 0.13.
Clinical characteristics
Table 1
shows clinical
characteristics of the subjects according to genotype. Subjects with
Trp/Arg did not differ from subjects without the variant
(P > 0.05) in any investigated characteristic: BMI
(22.2 ± 3.4 vs. 21.7 ± 2.7
kg/m2; Trp/Arg vs. Trp/Trp; mean ±
SD), plasma glucose (5.3 ± 0.6
vs. 5.1 ± 0.4 mmol/L), insulin (49.2 ± 9.6
vs. 47.4 ± 6.0 pmol/L), or family history of diabetes
or obesity (26.5 vs. 24.0%). In the subjects of ECG R-R
interval power spectral analysis, no significant difference in any of
the clinical characteristics was observed between the Trp/Trp and
Trp/Arg groups (see Table 1
).
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Figure 2
represents a typical set of
raw R-R intervals and the corresponding power spectral data obtained
from subjects with Trp/Trp (Fig. 2A
), Trp/Arg (Fig. 2B
), and Arg/Arg
(Fig. 2C
), respectively, during supine rest. Note that the mean
heart rate was subtracted from the original R-R interval data; thus,
only the R-R variability could be directly compared in this figure. It
can readily be seen that R-R variability in subjects with both Trp/Arg
and Arg/Arg was markedly reduced compared with that in the individual
with Trp/Trp. The corresponding R-R interval power spectra also
demonstrate vast differences in both LO and HI among the subjects.
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| Discussion |
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The oscillation of R-R interval reflects the ANS activity, the coefficient of variation score of which has been used for evaluation of ANS activity. TOTAL can also be an indicator of overall ANS activity. Diabetic neuropathy patients have significantly lower TOTAL (31). Our results indicate that the ANS activity of the Trp/Arg group was relatively low at rest as a whole, keeping the PNS-SNS balance. However, with standing, no such difference was found. In diabetic neuropathy patients, TOTAL was always lower than normal for any given physiological perturbation, and the responsiveness of both PNS and SNS activities thus remained nearly constant at a very low level (31). On the other hand, the carrier of Trp/Arg had a higher response in PNS and SNS activities to postural perturbation. This suggests that the low resting ANS activity was caused not by an organic defect of the nervous system, but by the imbalance of whole body ANS activity that was induced by the variant of ß3AR.
Of the biochemical effects of the Trp64Arg variant on ß3AR, the maximal adenylyl cyclase activity of the cell lines, hamster CHO-K1 and human HEK293, expressing variant ß3AR is lower than normal for various agonists (32). It is suggested that the variant ß3AR may be coupled less efficiently to the Gs protein than the normal ß3AR. In addition to lipolysis and thermogenesis in adipocytes, ß3AR modulates neural bronchomotor control, inducing relaxation of airway smooth muscle (33) and producing sustained peripheral vasodilation that is predominant in skin and fat (34, 35). Decreased effects of SNS activity caused by the variant ß3AR may be balanced not with up-regulation of SNS activity but by down-regulation of PNS activity, resulting in decreased TOTAL of Trp/Arg. On the other hand, the higher response of the PNS and SNS with standing may be the result of a compensatory action for the low basal activity.
In this study there were no significant differences in the clinical features in any of the groups we tested. Therefore, Trp64Arg alone does not seem to be a determining factor in obesity or NIDDM. The subjects of the previous studies showing adverse effects of Trp64Arg variant were mostly diabetic patients or obese individuals, and all were of middle age. However, the subjects of this study were young and in good health at the time of experiments, and their BMIs were almost ideal. There is a report that TOTAL declines with age (36). When the influence of aging is added to the effect of the variant of ß3AR, the defect in ANS activity may only then become obvious. The Trp64Arg variant has also been shown to be related to a low basal metabolic rate (BMR) in both Caucasian and Japanese subjects (26, 37) and possibly also in Pima Indians (15). The dysfunction of ANS may affect BMR, and low BMR is one of the risk factors for weight gain (1). The subjects of these studies were also in middle age and obese, so the complex risk factors, including the variant of ß3AR, may have become apparent.
In conclusion, the Trp64Arg variant, even heterozygous, decreased resting ANS activity, whereas the clinical characteristics did not differ between the groups with and without the variant. However, it is likely that the certain effects of the low ANS activity caused by the variant will become more obvious with aging or alteration of living environmental factors. We intend to continue a follow-up study to clarify the influence of Trp64Arg with aging. As R-R interval power spectral analysis is a very sensitive method for measuring ANS activity, as demonstrated by previous studies as well as the present experiment, it may be a useful tool to detect early neuronally mediated abnormal metabolic states.
| Acknowledgments |
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| Footnotes |
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Received November 4, 1998.
Revised February 3, 1999.
Accepted February 11, 1999.
| References |
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Arg mutation on human ß3-adrenoceptor activity. Eur J
Biochem. 247:11741179.[Medline]
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