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Introduction |
2-Adrenoceptors
mediate many of the cardiovascular, metabolic, and endocrine functions
of the endogenous catecholamines noradrenaline and adrenaline (Ruffolo
et al., 1993
). The quantitative responsiveness to
2-adrenoceptor stimulation can vary
considerably between and within individuals. Such variation is partly
related to receptor regulation phenomena that have been demonstrated,
e.g., in essential hypertension (Insel, 1996) and end-stage renal
disease (Daul et al., 1987). Moreover, variability of
2-adrenoceptor responsiveness also may relate
to polymorphisms in the genes encoding these receptors (Freeman et al.,
1995; Svetkey et al., 1996).
Because many physiological parameters are under a mixed control by
multiple adrenoceptor types, in vivo studies on genetic or
disease-induced alterations of human
2-adrenoceptors require the administration of
selective exogenous agonists. In this regard, most previous studies
have relied on clonidine, azepexole (also known as B-HT 933), and
-methylnoradrenaline. The systemic use of clonidine is complicated
by several factors. First, clonidine has central effects, e.g., blood
pressure lowering, which may, at least partly, occur independent of
2-adrenoceptors (Ernsberger and Haxhiu, 1997).
Second, clonidine is only a partial agonist at
2-adrenoceptors and may, in some cases, even
act as an antagonist (Michel et al., 1989). Third, the selectivity of
clonidine for 2- over
1-adrenoceptors is poor (Blöchl-Daum et
al., 1991). Azepexole has been studied less intensively, but many of
the above problems may apply because it is structurally related to
clonidine (Ernsberger et al., 1987). In contrast,
-methylnoradrenaline is a catecholamine derivative; therefore, it is
unlikely to cross the blood-brain barrier, and central effects have not
been reported for this compound after systemic administration. In in
vitro studies -methylnoradrenaline consistently has been found to be
selective for 2- over
1-adrenoceptors with selectivity factors
ranging between 8- and 300-fold (Ruffolo et al., 1988). Therefore,
-methylnoradrenaline infusions have been used by several
investigators to study human 2-adrenoceptor
function in vivo (FitzGerald et al., 1981; Elliott and Reid, 1983;
Murphy et al., 1984; Schäfers et al., 1990; MacGilchrist et al.,
1991; Krum et al., 1992). On the other hand, -methylnoradrenaline has more pronounced effects on systolic than diastolic blood pressure (DBP) (Murphy et al., 1984; Schäfers et al., 1990), which is inconsistent with a claimed
2-adrenoceptor-mediated vasoconstriction and
points to a possible involvement of cardiac 
-adrenoceptors.
The present study was designed to validate the use of
-methylnoradrenaline as a tool to study
2-adrenoceptor responsiveness in human in
vivo. Therefore, we have identified the adrenoceptor type mediating
-methylnoradrenaline responses on cardiac and vascular function and
circulating concentrations of noradrenaline, free fatty acids, glucose,
insulin, gastrin, and growth hormone by using selective antagonists.
Additionally, we have determined the reproducibility of various
hemodynamic and metabolic parameters that are under adrenergic control.
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Materials and Methods |
Study Protocol.
Six young male volunteers (mean age,
25.6 ± 3.8 years; mean weight, 71.6 ± 4.7 kg) participated
in this placebo-controlled, randomized, single-blind study after having
given informed, written consent. All subjects were drug-free and were
judged to be healthy on the basis of medical history, physical
examination, electrocardiogram, and routine laboratory screening. The
study protocol had been approved by the Ethics Committee of the
University of Essen Medical School and was in accordance with the
principles laid down in the Declaration of Helsinki.
We examined the cardiovascular and metabolic effects of
-methylnoradrenaline administered by i.v. infusion in four
incremental dose steps of 0.1, 0.2, 0.4, and 0.8 µg × kg
1 × min1 for 10 min
at each dose level. For safety reasons, infusions of
-methylnoradrenaline were terminated if systolic blood pressure rose
by more than 50 mm Hg, DBP rose by more than 30 mm Hg, or heart rate
increased by more than 50 beats/min (bpm) or fell (with propranolol
pretreatment) below 40 bpm. During each study day i.v. infusions of
-methylnoradrenaline were performed after subjects had been given
the following pretreatments: 1) placebo, given as 0.9% NaCl, 10 ml of
loading dose, followed by a slow i.v. maintenance infusion, 2)
1-adrenoceptor blockade with 2 mg of doxazosin
p.o. (Cardular; Pfizer, Karlsruhe, Germany), 3)
2-adrenoceptor blockade with yohimbine, given
as a loading dose of 32 µg × kg1 by
slow i.v. injection over 10 min followed by an i.v. maintenance dose of
1 µg × kg1 × min1, and 4) -adrenoceptor blockade with
propranolol (Dociton; Zeneca GmbH, Plankstadt, Germany) given as an
i.v. loading dose of 62.5 µg × kg1 over
10 min followed by a maintenance infusion of 0.45 µg × kg1 × min1. For all
i.v. treatments the infusions were started immediately after the
administration of the loading dose and were maintained until the end of
the -methylnoradrenaline infusion. For all treatments the
maintenance infusion was infused at an identical infusion rate of 20 ml × h1. The chosen doses of propranolol
and doxazosin have been shown previously in young healthy volunteers to
suppress the effects of i.v. isoprenaline on blood pressure and heart
rate and to abolish the effect of i.v. noradrenaline on DBP,
respectively (Schäfers et al., 1997; Werner et al., 1997). The
chosen dose of yohimbine effectively blocks
2-adrenoceptors because it elevates resting blood pressure, enhances the blood pressure response to an isometric hand-grip test, and antagonizes the adrenaline-induced
2-adrenoceptor-mediated aggregation of human
platelets ex vivo (Goldberg et al., 1983); however, the exact degree of
2-adrenoceptor blockade by this yohimbine dose
is not known. The i.v. loading dose injections of placebo, yohimbine,
and propranolol were administered 45 min before the start of the
-methylnoradrenaline infusion. The doxazosin tablet was given 120 min before the commencement of i.v. -methylnoradrenaline so that the
agonist infusion was done during the time of maximal plasma levels of
doxazosin (Vincent et al., 1983).
During pretreatment with either placebo, doxazosin, or yohimbine, the
preset safety limit of an increase in systolic blood pressure of 50 mm
Hg was reached in most subjects during the highest dose of 0.8 µg × kg1 × min1 so that infusions of
-methylnoradrenaline were terminated. With propranolol pretreatment,
infusion of -methylnoradrenaline resulted in an increase in DBP (see
below) with a secondary reflex bradycardia. Therefore, with propranolol
only two subjects received the dose of 0.8 µg × kg1 × min1 and only
three subjects completed the dose of 0.4 µg × kg1 × min1. For these
reasons ANOVA for the comparison between placebo and doxazosin and
placebo and yohimbine, respectively, was restricted to the dose levels
of 0.1, 0.2, and 0.4 µg × kg1 × min1, and, for the comparison between placebo
and propranolol, ANOVA was restricted to the doses of 0.1 and 0.2 µg × kg1 × min1 only. Accordingly, all figures are
restricted to the first three dose levels from 0.1 to 0.4 µg × kg1 × min1.
To assess the between-day variability of the cardiovascular and
metabolic responses induced by an i.v. infusion of
-methylnoradrenaline, placebo was administered on two different
study days so that each subject participated in a total of 5 study days
that were at least 1 week apart. Treatments were allocated randomly
with the two placebo days always separated by 2 weeks.
Hemodynamic Measurements.
Hemodynamics were assessed
noninvasively with direct measurements of heart rate (HR), blood
pressure, systolic time intervals, transthoracic impedance, and pulse
transmission time. Stroke volume, cardiac output (CO), total peripheral
resistance (TPR), and pulse-wave velocity were calculated from the
directly measured parameters. For analysis purposes, TPR and DBP were
chosen as the primary parameters for vasoconstrictor tone and
pulse-wave velocity was chosen as the parameter for vascular
compliance, whereas CO, stroke volume, systolic time intervals, and HR
were used as the primary parameters of cardiac function, respectively.
Blood pressure (mm Hg) was measured with a standard mercury
sphygmomanometer, with the disappearance of Korotkow's sound defined
as DBP. Systolic time intervals were measured according to standard
techniques (Lewis et al., 1977; Li and Belz, 1993) from simultaneous
recordings of an electrocardiographic lead, a phonocardiogram, and a
carotid pulse tracing at high paper speed (100 mm × s1) using a Siemens-Cardirex multichannel ink
jet recorder (Siemens Medizintechnik, Erlangen, Germany) as described
previously (Schäfers et al., 1994, 1997). From these recordings
we determined the duration of the RR-interval, i.e. the duration
between two R-waves of the electrocardiogram, from which HR was
calculated, the duration of electromechanical systole, and the
duration of left ventricular ejection time. The duration of the
pre-ejection period was calculated by subtraction of left ventricular
ejection time from electromechanical systole. The duration of the
electromechanical systole was corrected for HR to yield
QS2c (Schäfers et al., 1994). Pulse
transmission time was determined noninvasively from pressure tracings
over the carotid and femoral artery as described previously
(Breithaupt-Grögler et al., 1997). Aortic pulse-wave velocity
then was calculated as the ratio between the distance traveled by the
pulse wave and pulse transmission time (Breithaupt-Grögler et
al., 1997). Stroke volume (ml) was measured by impedance cardiography,
applying the standard approach with circular tape electrodes and
graphical signal analysis according to Kubicek's equation (Kubicek et
al., 1966). A "Kardio-Dynagraph" was used to record changes in
transthoracic impedance (Heinz Diefenbach Elektromedizin, Frankfurt,
Germany). CO (l × min1) was calculated as
CO = HR × stroke volume/1000. TPR (dyne × s × cm5) was calculated as mean arterial pressure
divided by CO × 80, where mean arterial pressure was defined as
DBP plus one-third of the pulse pressure.
Baseline measurements of hemodynamic parameters were performed after 30 min of supine rest (baseline 1 recordings). Immediately before the
start of the -methylnoradrenaline infusion another measurement was
performed to determine the effects of the antagonist treatments; these
values also served as baseline for the subsequent infusion of
-methylnoradrenaline (baseline 2 recordings). Blood pressure was
measured five times each at baseline, immediately before
-methylnoradrenaline infusion, and during the last 5 min of each
dose step of the agonist infusion, i.e., throughout minute 5 to 10. Recordings of systolic time intervals, transthoracic impedance, and
pulse transmission time were always performed after the last blood
pressure measurement. At each time point five cardiac cycles were
analyzed, and the mean of these five cycles and of the five blood
pressure recordings was taken for the analysis of the dose-response curve.
Neurohumoral Measurements.
At baseline 1 and 2 and at the
end of the 0.1- and 0.4-µg × kg1 × min1 doses of the -methylnoradrenaline
infusion, blood was drawn from an antecubital vein for the measurement
of blood glucose, plasma concentrations of noradrenaline and
adrenaline, gastrin, free fatty acids, and serum concentrations of
insulin and growth hormone. Plasma catecholamines were analyzed
by HPLC with electrochemical detection; measurements of concentrations
of free fatty acids, insulin, gastrin, and growth hormone were
performed with commercially available kits obtained from Wako (Neuss,
Germany), Biochem Immunosystems (Freiburg, Germany), IBL (Hamburg,
Germany), and Nichols Institute (San Juan Capistrano, CA), respectively.
Chemicals.
-Methylnoradrenaline was obtained from
Research Biochemical International (Natick, MA) and prepared by our
hospital pharmacy as a sterile stock solution of 1 mg × ml1 in physiological saline using sodium
pyrophosphate as preservative. The -methylnoradrenaline stock
solution was stable for at least 2 weeks with recovery rates of 95 to
99%. The solutions for i.v. administration were freshly prepared on
each study day by dilution in 0.9% saline. Yohimbine was obtained from
Caelo (Hilden, Germany) and prepared as a stock solution of 2 mg × ml1 in 0.9% saline by our hospital
pharmacy. Solutions for i.v. administration were freshly prepared on
each study day.
Data Analysis.
The intraday variability of test parameters
under resting conditions was assessed by the coefficient of variation
of a total of six measurements obtained at regular intervals over a
period of 75 min during the first placebo day within each subject. The interday variability of resting parameters was assessed by calculation of the coefficient of variation of the baseline 1 measurements, i.e.,
the baseline after 30 min of complete rest immediately before administration of the antagonists, of the 5 study days.
Because there were only 2 placebo days, calculation of a coefficient of
variation was not meaningful for calculation of the interstudy-day
reproducibility of -methylnoradrenaline responses. Therefore, this
variability was assessed by calculating the mean difference between the
responses to 0.4 µg × kg1 × min1 -methylnoradrenaline during the two
placebo days ± the S.D. of this mean difference. Additionally,
the responses during the 2 placebo days were compared by paired
t test.
The antagonist-induced alterations of baseline parameters were compared
with the mean alterations of both placebo days by paired, two-tailed
t tests. Possible effects of antagonists on the
-methylnoradrenaline-induced changes were analyzed by two-way ANOVA
of the entire dose-response curve range from 0.1 to 0.4 µg × kg1 × min1, comparing
each antagonist treatment against placebo. Multiple comparison
corrections for the three different treatments (doxazosin, propranolol,
and yohimbine) were not performed; therefore, the resulting
P values are to be interpreted in a descriptive manner. P < .05 (two-tailed) was considered statistically
significant. All quoted P values for comparison by ANOVA
refer to main treatment effects. Serum insulin data during
-methylnoradrenaline were log-transformed before statistical
analysis to achieve homogeneity of variances. All values are shown as
mean ± S.E.M. if not stated otherwise.
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Results |
Data Reproducibility.
Under supine resting conditions, the
intraday coefficient of variation for cardiovascular parameters ranged
from 0.7% for QS2c to 6.6% for TPR (Table
1). Coefficients of variation for intraday variability were not calculated for the hormonal and metabolic
parameters because only two measurements were available for each study
day. The interday coefficient of variation for the baseline 1 measurements of the five study days ranged from 1.2% for
QS2c to 6.9% for cardiac output (Table 1). The
interday coefficient of variation for the metabolic and hormonal
parameters generally were larger, ranging from 11.8% for glucose to
41.5% for noradrenaline (Table 1).
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TABLE 1
Intra- and interday variability of cardiovascular, metabolic, and
hormonal parameters under resting conditions and during
-methylnoradrenaline infusion
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The variability of the agonist-induced changes in hemodynamic,
hormonal, and metabolic parameters was assessed by a comparison of the
data obtained with 0.4 µg × kg1 × min1 -methylnoradrenaline on the 2 placebo
days (Table 1). There was no significant difference between the 2 placebo days in the response to -methylnoradrenaline for any of the
parameters. For example, the mean differences of
QS2 and plasma noradrenaline were 
2.5 ± 8.5 ms and 10 ± 49 pg × ml1, respectively.
Antagonist Effects on Baseline Parameters.
Baseline 1 measurements, i.e., values after 30 min of supine rest immediately
before administration of the study treatments, are shown in Table
2. On the placebo days, none of the
parameters was altered significantly at baseline 2, i.e., relative to
baseline 1 (data not shown). The
1-adrenoceptor antagonist, doxazosin, did not
cause statistically significant changes in resting hemodynamics or
hormonal and metabolic parameters (Table 2). In contrast, the
2-adrenoceptor antagonist, yohimbine,
significantly increased systolic blood pressure (10.3 mm Hg),
pulse-wave velocity (0.71 m × s1), HR
(4.6 bpm), plasma noradrenaline (121 pg × ml1), and serum insulin (2.2 µU × ml1) and shortened QS2c
(8.1 ms). The -adrenoceptor antagonist, propranolol, significantly
lowered heart rate (4.7 bpm) and systolic blood pressure (1.5 mm Hg).
This was associated with a prolongation of pre-ejection period, but
this difference failed to reach statistical significance with the given
number of observations. None of the antagonists affected baseline
values for free fatty acids or gastrin (Table 2). For plasma adrenaline
and serum growth hormone 46 and 64% of all baseline measurements were
below the limits of detection of 10 pg × ml1 and 0.5 ng × ml1, respectively. Therefore, the influence of
adrenoceptor antagonists on these parameters was not analyzed.
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TABLE 2
Effects of placebo and adrenoceptor antagonists on cardiovascular,
metabolic, and hormonal parameters under resting conditions
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Antagonist Effects on -Methylnoradrenaline-Induced Changes.
Intravenous infusion of -methylnoradrenaline dose-dependently
increased HR (Fig. 1), systolic blood
pressure (Fig. 2), pulse pressure (data
not shown), pulse-wave velocity (data not shown), and CO (Fig.
3); shortened QS2c
(Fig. 4) and pre-ejection period (data
not shown); and reduced DBP (Fig. 5) and
TPR (Fig. 6). -Methylnoradrenaline also increased serum insulin (Fig. 7),
whole blood glucose (Fig. 8), serum free
fatty acids (Fig. 9), and serum gastrin
(Fig. 10) and decreased plasma
noradrenaline (Fig. 11). In the three
subjects, where serum growth hormone levels were above the limits of
detection (i.e., >0.5 ng × ml1) during
at least one of the two placebo days at baseline 2, -methylnoradrenaline reduced serum growth hormone (data not shown).
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Fig. 1.
Effects of i.v. infusion of -methylnoradrenaline
on heart rate after pretreatment with placebo (Pla), doxazosin (Dox),
yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 2.
Effects of i.v. infusion of -methylnoradrenaline
on systolic BP after pretreatment with placebo (Pla), doxazosin (Dox),
yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 3.
Effects of i.v. infusion of -methylnoradrenaline
on CO after pretreatment with placebo (Pla), doxazosin (Dox), yohimbine
(Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 4.
Effects of i.v. infusion of -methylnoradrenaline
on the duration of QS2c after pretreatment with placebo
(Pla), doxazosin (Dox), yohimbine (Yo), and propranolol (Pro).
Means ± S.E.M.; n = 6 for each treatment.
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Fig. 5.
Effects of i.v. infusion of -methylnoradrenaline
on diastolic BP after pretreatment with placebo (Pla), doxazosin (Dox),
yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 6.
Effects of i.v. infusion of -methylnoradrenaline
on TPR after pretreatment with placebo (Pla), doxazosin (Dox),
yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 7.
Effects of i.v. infusion of -methylnoradrenaline
on serum insulin after pretreatment with placebo (Pla), doxazosin
(Dox), yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 8.
Effects of i.v. infusion of -methylnoradrenaline
on blood glucose after pretreatment with placebo (Pla), doxazosin
(Dox), yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 9.
Effects of i.v. infusion of -methylnoradrenaline
on free fatty acids after pretreatment with placebo (Pla), doxazosin
(Dox), yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 10.
Effects of i.v. infusion of -methylnoradrenaline
on serum gastrin after pretreatment with placebo (Pla), doxazosin
(Dox), yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.;
n = 6 for each treatment.
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Fig. 11.
Effects of i.v. infusion of -methylnoradrenaline
on plasma noradrenaline after pretreatment with placebo (Pla),
doxazosin (Dox), yohimbine (Yo), and propranolol (Pro). Means ± S.E.M.; n = 6 for each treatment.
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The 1-adrenoceptor antagonist, doxazosin,
slightly but significantly blunted the increase in systolic blood
pressure (P < .05; Fig. 2). Doxazosin treatment did
not significantly affect the -methylnoradrenaline-induced changes of
HR (Fig. 1), CO (Fig. 3), QS2c (Fig. 4), DBP
(Fig. 5), pulse pressure (data not shown), pulse-wave velocity (data
not shown), TPR (Fig. 6), or any of the hormonal and metabolic
parameters (Figs. 7-11).
The 2-adrenoceptor antagonist, yohimbine,
significantly enhanced the -methylnoradrenaline-induced fall in DBP
(P < .05; Fig. 5) but did not significantly affect the
fall in TPR (Fig. 6) or the changes of other hemodynamic parameters
(Figs. 1-4). Although neither yohimbine nor doxazosin significantly
affected the -methylnoradrenaline-induced elevation of HR, this
increase was significantly greater during yohimbine than during
doxazosin treatment in a direct, pairwise comparison (P < .01; Fig. 1). Yohimbine significantly blunted the increase in
glucose (P < .01; Fig. 8), prevented the fall in
plasma noradrenaline (P < .05; Fig. 11), and enhanced
the rise in serum insulin (P < .05; Fig. 7). In a
direct comparison between the -adrenoceptor antagonists,
-methylnoradrenaline increased glucose significantly more during
doxazosin than during yohimbine treatment (P < .05;
Fig. 8). Moreover, -methylnoradrenaline decreased plasma
noradrenaline during doxazosin and increased it during yohimbine
treatment (P < .05 for direct comparison of doxazosin versus yohimbine; Fig. 11).
The -adrenoceptor antagonist, propranolol, completely prevented the
shortening of QS2c (P < .001;
Fig. 4) and pre-ejection period (P < .05; data not
shown) by -methylnoradrenaline and converted the fall in DBP
(P < .01; Fig. 5) and TPR (P < .05; Fig. 6) into increases. Similarly, the increases in HR
(P < .01; Fig. 1) and CO (P < .001;
Fig. 3) were reversed to decreases after -adrenoceptor blockade, and
the increase in pulse pressure was markedly reduced (P < .0001; data not shown). Propranolol treatment prevented the rise in
serum insulin (P < .01; Fig. 7) and blunted the
increase in free fatty acids (P < .01; Fig. 9) and
gastrin (P < .05; Fig. 10) but had no significant
effect on glucose (Fig. 8) and noradrenaline (Fig. 11) during
-methylnoradrenaline infusion.
After placebo and -adrenoceptor blockade by doxazosin and yohimbine,
-methylnoradrenaline did not increase plasma growth hormone levels
(most of which were below the limit of detection at baseline; see
above). After propranolol pretreatment, -methylnoradrenaline caused
minor increments of growth hormone in four subjects of 1.0, 10.3, 0.8, and 0.2 ng × ml1, respectively, at the
0.4-µg × kg1 × min1 dose level.
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Discussion |
Data Reproducibility.
Some cardiovascular parameters are
assessed directly (e.g., blood pressure) but represent complex
physiological events, whereas others (e.g., TPR) are obtained
indirectly, i.e., are derived mathematically from directly measured
parameters, but are presumed to represent primary physiological events.
Thus, TPR and QS2c are primary indicators of
vascular and cardiac function, respectively, whereas blood pressure
represents the integration of inotropic and chronotropic events,
vascular compliance, and vascular smooth muscle tone.
Under resting conditions the intra- and interday variability of the
cardiovascular parameters generally was smaller than that of the
hormonal and metabolic parameters, and, expectedly, the intraday
variability was consistently smaller than the interday variability.
Among the hemodynamic parameters the variability was smallest for
QS2c as an indicator of cardiac effects.
Accordingly, QS2c can detect changes of systolic
performance more sensitively than M-mode echocardiography, Doppler
echocardiography, or impedance cardiography (de Mey et al., 1992).
Under stimulated conditions, a quantitative assessment of data
variability was more difficult, but there was no systematic change in
the responses to 0.4 µg × kg1 × min1 -methylnoradrenaline between the two
placebo days, and the rank order of variability was similar to that
under resting conditions. Taken together these data can be used for
power calculations for future clinical pharmacological studies.
Adrenoceptor Antagonist Effects on Resting Parameters.
Cardiac
function predominantly is regulated via -adrenoceptors, whereas
vasomotor tone is enhanced via both 1- and
2-adrenoceptors and reduced via
-adrenoceptors. In contrast, neuronal noradrenaline release is
inhibited by 2-adrenoceptors and stimulated by
-adrenoceptors (Langer, 1977; Ruffolo et al., 1993). The present
effects of the 1-adrenoceptor antagonist,
doxazosin, the 2-adrenoceptor antagonist, yohimbine, and the -adrenoceptor antagonist, propranolol, are consistent with these known functions.
In the present and previous studies (Goldberg et al., 1983;
Schäfers et al., 1997) yohimbine predominantly increased systolic blood pressure with only little effect on DBP and TPR. This can be
explained by an increase in stroke volume, a reduction in vascular compliance, or both. Because yohimbine had no significant effect on
stroke volume or CO, a reduced vascular compliance appears to be more
likely. This hypothesis is supported by the significant acceleration of
pulse-wave velocity, suggesting a decrease in vascular compliance
(Belz, 1995). Whether yohimbine affects vascular compliance directly
or, e.g., via central mechanisms, remains to be determined.
Only little is known regarding the adrenoceptor types that control
glucose, insulin, free fatty acid, growth hormone, and gastrin levels
in humans in vivo. In mice and rats, pancreatic insulin release can be
inhibited by 2-adrenoceptor stimulation (Angel
et al., 1990; Niddam et al., 1990). Our yohimbine data indicate that
this mechanism is operative and tonically activated by endogenous
catecholamines in humans. On the other hand, our propranolol data
indicate that -adrenoceptors involved in the control of insulin
release are not tonically active in human. Whereas previous studies
have demonstrated adrenergic modulation of lipolysis (Burns et al.,
1981; Wright and Simpson, 1981; Tarkovacs et al., 1994), growth hormone
secretion (Brown et al., 1985, 1988), and gastrin release (Intorre et
al., 1994), our data indicate that none of the parameters is tonically
controlled by endogenous catecholamines under resting conditions. Thus,
multiple adrenoceptor types are involved in the control of
cardiovascular, metabolic, and hormonal function in humans in vivo,
some of which appear to be tonically active under resting conditions.
The relative contributions of adrenoceptor types differ considerably
between parameters.
-Methylnoradrenaline Effects.
-Methylnoradrenaline is a
standard tool to assess human peripheral
2-adrenoceptor function in humans in vivo (see
Introduction). In contrast to clonidine (Brown et al., 1988),
-methylnoradrenaline lacked consistent effects on plasma growth
hormone levels in the present study, confirming that
-methylnoradrenaline does not cross the blood-brain barrier.
Although -methylnoradrenaline is a potent and efficacious
2-adrenoceptor agonist in vitro, the
assumption that it is selective for
2-adrenoceptors in human in vivo has not been
validated to our knowledge. Our study confirms that
-methylnoradrenaline increases systolic blood pressure more than DBP
(Murphy et al., 1984; Schäfers et al., 1990), indicating a
cardiac rather than a vascular effect. The use of primary indicators of
cardiac and vascular function and of selective antagonists has allowed
us to investigate the underlying mechanisms in more detail.
-Methylnoradrenaline markedly shortened QS2c
and increased CO, indicating enhanced cardiac systolic performance
because of a positive inotropic effect (Lewis et al., 1977; Johnson et
al., 1981). Both changes were fully suppressed by the -adrenoceptor antagonist, propranolol, but were not affected by the -adrenoceptor antagonists. Thus, -methlynoradrenaline markedly stimulates cardiac -adrenoceptors. This stimulation is quantitatively similar to that
of the full -adrenoceptor agonist, isoprenaline, as determined under
similar conditions in our laboratory (Schäfers et al., 1994).
Although these data clearly demonstrate -adrenoceptor stimulation by
-methylnoradrenaline, they do not allow conclusions about
selectivity over 2-adrenoceptors because the
latter do not contribute to inotropy.
On the other hand, -methylnoradrenaline surprisingly decreased DBP
and TPR, indicating a vasodilating rather than the expected vasoconstricting action on vascular smooth muscle, and this was blocked
by propranolol. Thus, even in the vasculature, where - and
-adrenoceptors coexist, -adrenoceptor stimulation dominated the
overall functional response to -methylnoradrenaline. Indeed, -methylnoradrenaline-induced vasoconstriction was detected only in
the presence of propranolol, although vasoconstriction is the prototypical vascular response to stimulation of both
1- and 2-adrenoceptors (Jie et al., 1987b; Ruffolo et
al., 1993). Because, in contrast to yohimbine,
1-adrenoceptor blockade by doxazosin had
hardly any effect on the hemodynamic, metabolic, and hormonal responses
to -methylnoradrenaline, we suggest that the vasoconstriction observed in the presence of propranolol was mediated by stimulation of
vascular 2-adrenoceptors. The contribution of
2-adrenoceptors to the control of vascular
tone in humans in vivo clearly has been shown by previous studies (Jie
et al., 1987b). Although we are fully aware that our data do not
provide conclusive evidence for this suggestion, it is supported by the
following observations. First, the significant potentiation of the fall
in DBP in the presence of yohimbine is consistent with blockade of
vasoconstricting 2-adrenoceptors. Second,
prototypical 1-adrenoceptor agonists such as
noradrenaline and phenylephrine cause vasoconstriction even without
concomitant -adrenoceptor blockade upon systemic i.v. administration
(Schäfers et al., 1997, 1999).
A different situation appears to exist for the prejunctional regulation
of noradrenaline release, which can be inhibited by 2-adrenoceptors and stimulated by
-adrenoceptors (Langer, 1977; Brown et al., 1985; Jie et al., 1987a;
Ruffolo et al., 1993). Despite the dominance of -adrenoceptor
effects postjunctionally in the vasculature, -methylnoradrenaline
lowered plasma noradrenaline, and this was blocked by yohimbine but not
affected significantly by propranolol. Thus, -methylnoradrenaline
behaves as a predominant -adrenoceptor agonist at postjunctional
cardiac and vascular receptors but as a predominant
2-adrenoceptor agonist at prejunctional receptors. We speculate that vastly different receptor reserves for the
two pre- and postjunctional pathways may explain this differential
action of -methylnoradrenaline.
The metabolic and endocrine parameters also exhibited a complex
regulation pattern. Thus, -methylnoradrenaline-induced increases of
serum insulin were enhanced by yohimbine and fully suppressed by
propranolol, demonstrating a dual control by stimulatory - and
inhibitory 2-adrenoceptor pathways. On the
other hand, in the absence of adrenoceptor blockade,
-methylnoradrenaline elevated blood glucose despite increased
insulin concentrations. This indicates that additional mechanisms may
be involved in the elevation of blood glucose by
-methylnoradrenaline, e.g., stimulation of glucagon or activation of
glycogenolysis. Irrespective of the nature of the underlying
physiological mechanisms, this rise in blood glucose did not involve
1- or -adrenoceptors because blockade of
1- or -adrenoceptors had no effect. In
contrast, yohimbine antagonized the increase in blood glucose,
indicating involvement of 2-adrenoceptors possibly by their effect on insulin release, which was enhanced in the
presence of yohimbine.
-Methylnoradrenaline increased free fatty acids and gastrin. Both
effects were blunted by propranolol but not affected by doxazosin or
yohimbine, indicating mediation solely via -adrenoceptors. Taken
together, these data demonstrate that some metabolic and hormonal
responses to i.v. -methylnoradrenaline are controlled mostly, if not
exclusively, by 2-adrenoceptors, e.g., plasma noradrenaline and blood glucose; some are controlled by
-adrenoceptors, e.g., free fatty acids and gastrin; whereas others
depend on the balance between inhibitory 2-
and stimulatory -adrenoceptors, e.g., insulin. Interestingly, none
of the metabolic responses to -methylnoradrenaline in vivo appears
to involve 1-adrenoceptors, although this
adrenoceptor type is well expressed, e.g., in the human liver
(Garcia-Sainz et al., 1995).
Conclusions.
Taken together, our data indicate that
-methylnoradrenaline is selective for 2-
over 1-adrenoceptors, but its overall effects on hemodynamic and metabolic parametery, with the exception of plasma
noradrenaline and glucose levels, are mediated largely via
-adrenoceptors. For most parameters that are under dual
2- and -adrenoceptor control, e.g., DBP,
TPR, and insulin, -methylnoradrenaline behaves as a predominant
-adrenoceptor agonist. However, for the regulation of plasma
noradrenaline, which is also under dual 2- and
-adrenoceptor control, the 2-adrenoceptor
actions of -methylnoradrenaline dominate. This profile of
-methylnoradrenaline resembles that of the endogenous catecholamine,
adrenaline, which has predominant inotropic and vasodilator properties.
Because the DBP response to noradrenaline involves mainly
1-adrenoceptors (Schäfers et al., 1997),
we propose that -methylnoradrenaline may be selective for
2- relative to
1-adrenoceptors in humans in vivo. However,
its 2-adrenoceptor-stimulating effects on the cardiovascular system can be revealed only during concomitant -adrenoceptor blockade. With these precautions, it may be the agonist of choice for studies of peripheral
2-adrenoceptor function in humans in vivo.
We thank Mr. H. Strobel for preparing -methylnoradrenaline
solutions for infusions and M. B. Michel-Reher, H. Sporkmann, and
A. Versbach for their skillful technical assistance.
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-Methylnoradrenaline in Humans1
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Abstract
Introduction
Evolving concepts and clinical implications.
N Engl J Med
334:
580-585
