(Hypertension. 1995;25:1266-1269.)
© 1995 American Heart Association, Inc.
Articles |
From the University Department of Medicine and Therapeutics (S.U., H.L.E.) and MRC Blood Pressure Unit (J.J.M., J.M.C.C.), Western Infirmary, Glasgow, Scotland.
| Abstract |
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Key Words: angiotensin-converting enzyme alleles angiotensin I angiotensin II
| Introduction |
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| Methods |
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Angiotensin Infusion Study
All subjects were instructed to maintain a normal sodium diet
(approximately 150 mmol/d) for 3 days before the study day. After an
overnight fast and avoidance of alcohol and caffeine-containing
beverages, subjects reported to the clinical investigation and research
unit at 8 AM. After insertion of an intravenous cannula
into each forearm and a period of not less than 30 minutes of recumbent
rest, each subject received stepwise incremental doses of Ang I
(CLINALFA AG) and Ang II (CIBA-Geigy) (each administered as 1, 2.5, 5,
10, and 20 ng/kg per minute, 8 minutes for each dose) by intravenous
infusion (Braun Infusion Syringe Driver). There was a 1-hour washout
period between infusions, and infusion order was randomized. Blood
pressure and heart rate were measured each minute by a semiautomated
sphygmomanometer (Critikon, Johnson & Johnson Professional Products
Ltd), and the increase in blood pressure (from baseline) was calculated
for each dose step as the mean of the final five readings. From the
dose-response curve of each subject, pressor responses were then
characterized by calculation of PD20, ie, the
angiotensin dose that increased mean blood pressure by 20 mm Hg. The
detailed methodology for the analysis of pressor response studies
has been described previously.12 Blood samples were
obtained from the opposite arm before and after each incremental
infusion for measurement of ACE activity, Ang II, and aldosterone.
PCR Method
Genomic DNA was extracted from peripheral blood leukocytes
by standard methods.13 14 The template DNA was amplified
by PCR with 10 pmol of each primer (sense,
5'-CAGGAGACCACTCCCATCCTTTCT, and anti-sense,
5'-GATGTGGCCATCACATTCGTCAGAT), 3 mmol/L
MgCl2, commercial PCR buffer without
MgCl2, 0.5 mmol/L of each dNTP, and 1 U
Taq polymerase (Ang II reagents from Promega) for 30 cycles
at 94°C for 1 minute, 58°C for 1 minute, and 72°C for 2 minutes.
The PCR product was visualized after electrophoresis on 1.5% agarose
gels with ethidium bromide staining. Reanalysis of genotype
with 5% dimethyl sulfoxide added to the reaction confirmed the
original assignment.
Analysis of Blood Samples
Plasma Ang II concentrations were measured by radioimmunoassay
after extraction with Sep-Pak C18 cartridges.15 Ang I was
also measured to check for cross-reaction in the Ang II assay, although
this was less than 0.6%. Ang I levels did not significantly interfere
in the Ang II assay, but we took account of any such small
interference. Plasma aldosterone concentration was measured by
radioimmunoassay.16 Plasma active renin concentration was
measured by radioimmunoassay with the use of an antibody trapping
method.17 Serum ACE activity was measured by
high-performance liquid chromatography with the use of an artificial
substrate (Hip-His-Leu).18 Coefficients of variation of
assays were all less than 10%.
Statistical Analysis
Data are shown as median and range unless otherwise indicated.
Comparison of the two groups was by Mann-Whitney U test with
95% confidence intervals (CI); comparison of the differences in the
Ang II and aldosterone levels during Ang I infusion was by
repeated-measures ANOVA with 95% CI and Bonferroni correction.
| Results |
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Activity of the Renin-Angiotensin System at Baseline
Table 2 shows baseline activities and
concentrations for the different components of the renin-angiotensin
system. ACE activity in the subjects with the DD genotype was
significantly higher than in those with the II genotype,
whereas baseline aldosterone and active renin concentrations did not
differ significantly. Although not attaining conventional statistical
significance, basal Ang II levels in the DD subjects tended to be
higher than those in the II subjects (with the lower limit of the 95%
CI at almost zero). Basal Ang II levels and active renin concentrations
were significantly correlated in each group
(r2=.589, P<.012 for DD
group; r2=.810, P=.000 for II
group), with a tendency for the linear regression line of the
relationship between renin and Ang II to be steeper in DD subjects than
in II subjects (y=0.490x-1.6
versus y=0.236x+0.54). There was no
significant correlation between basal ACE activity and Ang II
level.
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Pressor Responses to Ang I and Ang II
Fig 1 shows PD20 values for the pressor
responses to Ang I and Ang II and also the ratio for the
PD20 values for Ang I and Ang II. PD20 for Ang
I was significantly lower in DD subjects than in II subjects (8.8
versus 14.8 ng/kg per minute; P=.0091; 95% CI, 1.3-8.7),
whereas PD20 for Ang II did not differ significantly
between the two groups (15.3 versus 16.5 ng/kg per minute;
P=NS; 95% CI, -8.2-4.8). The ratio of Ang I and Ang II
PD20 (which represents the PD20 for Ang
I adjusted for the pressor response to Ang II) was also lower in the DD
subjects than in the II subjects (0.85 versus 0.96; P=.0452;
95% CI, 0.005-0.57).
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Plasma Ang II Concentrations During Ang I Infusion
Fig 2 shows plasma Ang II concentrations during the
incremental Ang I infusion. Ang I infusion significantly increased
plasma Ang II concentrations in both groups, but across the dose range,
the levels produced in the DD subjects were consistently significantly
higher than those in the II subjects (P<.01 by ANOVA). This
difference was most apparent at the highest dose of 20 ng/kg per minute
(95% CI, 58-118 mol/L).
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Relationship Between Changes in Plasma Ang II Levels and Mean Blood
Pressure
Fig 3 shows the relationship between the changes in
plasma Ang II levels and mean blood pressure. In both groups, the
change in Ang II levels and the change in blood pressure were
significantly correlated (r2=.790,
P=.000 for DD; r2=.613,
P=.000 for II), with closely similar linear regression
equations (y=0.101x+4.2 for DD,
y=0.098x+3.6 for II).
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Aldosterone, Active Renin, and ACE Activity
Increases in plasma aldosterone during the Ang I and Ang II
infusions did not differ, and serum ACE activity did not change
significantly. Plasma active renin concentrations in both groups were
significantly reduced after Ang I and Ang II infusions, but there were
no significant differences between the two groups.
| Discussion |
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Our method for the assessment of the systemic effects of Ang I and of in vivo ACE activity is analogous to that used for testing of the effectiveness and duration of action of an ACE inhibitor drug by its ability to attenuate the pressor response to Ang I in humans.19 Factors other than circulating ACE activity will obviously affect the pressor response to Ang I, but our study design allows us to exclude the potential influence of a differential tissue sensitivity to the generated Ang II; thus, the PD20 for Ang I was adjusted for the PD20 for Ang II to show that the differential response to Ang I was not an indirect reflection of a differential sensitivity to Ang II that might arise because of differences between the two groups in their basal Ang II levels or, for example, by other unknown genetic tissue factors. Additionally, the likelihood of any potential effect of sodium intake on the pressor and aldosterone responses was minimized by prior adherence to a normal sodium diet confirmed by 24-hour urinary Na+ excretion before the study. Furthermore, it seems unlikely that the basal activity of the renin-angiotensin system would affect the pressor response to Ang I because the achieved Ang II level during the Ang I infusion was 100 times higher than the basal level. The significantly higher circulating Ang II levels in the course of the Ang I infusion probably explain the enhanced pressor response to Ang I in DD subjects, particularly as the relationships in the two groups between the changes in mean blood pressure and the changes in plasma Ang II levels during Ang I infusion were similar. However, enhanced tissue production of Ang II in the DD subjects20 after local uptake of Ang I obviously could not be estimated in this study, and it is possible that increased local generation of Ang II may explain the increased pressor effect in this group. The increases in Ang II during Ang I infusion achieved a significant treatment-time difference across the entire dose range, but with only the highest dose level studied achieving independent statistical significance. It is possible, therefore, that differences attributable to in vivo ACE activity are most evident when the renin-angiotensin system is activated and that there are relatively high circulating or tissue levels of Ang I.
Although basal Ang II levels in the two groups did not differ significantly, they tended to be higher in the DD subjects. This trend has been previously reported,7 although not in all such studies,9 and it is consistent with the concept of a greater regulatory contribution of ACE activity to Ang II generation (relative to renin activity) when the renin-angiotensin system is stimulated.
Despite higher plasma levels of Ang II in DD subjects, aldosterone concentrations during Ang I infusion did not differ significantly between the two groups. However, we designed the study principally to assess pressor responses, and the duration of each dose of Ang I was probably insufficient to achieve a steady-state aldosterone response to the generated Ang II.
In conclusion, our study has shown increased pressor responsiveness to Ang I, probably as a consequence of the generation of increased levels of Ang II, in normotensive men with the DD type of the ACE gene. This result may be relevant to the reported adverse cardiovascular risk conferred by the D allele as it provides a mechanistic rationale for the association between the ACE gene polymorphism and cardiovascular disease. Further studies focusing on pharmacological and physiological consequences of the ACE gene polymorphism (including possible differences in kinin metabolism and the responses to ACE inhibitor drugs) will be required to establish whether prevention and treatment of ischemic heart disease can be targeted to those patients.
| Footnotes |
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Received November 15, 1994; first decision December 16, 1994; accepted February 16, 1995.
| References |
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