Catecholamine Biology Research Laboratory, Institut National de la Santé et de la Recherche Médicale, Broussais Faculty of Medicine, 75 270 Paris, France
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ABSTRACT |
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Norepinephrine
(NE) kinetics were investigated in freely moving (FM) and minimally
stressed (MS) rats with the isotope dilution technique. 1)
The mean NE spillover rate (NE-SOR) was 79 ± 6 ng · kg1 · min
1, and the
mean NE metabolic clearance rate (NE-MCR) 179 ± 9 ml · kg
1 · min
1
(n = 31). Thus the NE kinetics in FM and MS rats are
much faster than in human beings, probably related to a higher
sympathetic drive. 2) Whether the magnitude of NE-MCR is
related to the level of plasma NE concentration was investigated. No
significant correlation was calculated between plasma NE concentration
and NE-MCR in 31 control rats. When plasma NE concentration was varied
during either acute or chronic infusion of exogenous NE, NE-MCR
remained unchanged as long as animal hemodynamics were not altered.
When plasma NE concentration was high enough to increase mean arterial
pressure (MAP), NE-MCR was decreased. However, when MAP was increased
within comparable magnitude, NE-MCR was decreased during NE and
increased during epinephrine (Epi) infusion. Thus the existence of an
-/
-adrenergic mechanism involved in the regulation of NE-MCR
independent of known hemodynamic mechanisms is suggested. 3)
The "epinephrine hypothesis" was revisited in FM and MS rats. At
variance with humans, very high plasma Epi concentrations have to be
induced to increase NE-SOR in resting rats. Furthermore, NE-MCR was
also increased, accounting for the nonsignificant increase of plasma NE
concentration. Within the range of Epi concentrations with no effect on
NE-SOR, an increase of NE release was revealed when the presynaptic
2-adrenoreceptors were partially inhibited by yohimbine.
This suggests the existence of a second epinephrine hypothesis.
norepinephrine spillover rate; norepinephrine metabolic clearance rate; epinephrine; dopamine; epinephrine hypothesis; yohimbine
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INTRODUCTION |
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RECENTLY, Grassi and Esler (18) stated that assessment of the adrenergic neuronal function remains one of the major fields in cardiovascular research. Reviewing available techniques, they observed that radiotracer methods, allowing assessment of norepinephrine (NE) kinetics, and microneurography, allowing direct recording of sympathetic nerve traffic, have supplanted others. The radiotracer technique provides the investigator with an estimate of what comes in plasma, NE spillover rate (NE-SOR), and what comes out, metabolic clearance rate (NE-MCR), information of a broader physiological meaning than plasma NE concentration alone. Also, it allows a more refined understanding of sympathetic activity through estimates of regional contributions, at least in humans. Thus the radioisotope dilution technique has strongly improved our understanding of sympathetic activity in humans.
The aim of the present study was to investigate the relationship
between plasma catecholamine and the kinetics of NE in normotensive, freely moving rats. A three-step study was designed. 1) The
physiology of NE kinetics in the rat remains poorly known; previous
studies have dealt with NE kinetics in controls or in response to
antihypertensive drugs in spontaneously hypertensive rats (10,
21, 22). Our working hypothesis was to consider that NE kinetics
should be in agreement with a high sympathetic drive in the rodent
(28, 34, 36). 2) Whether the capacity of the
rat organism to clear NE, as judged with the NE-MCR, adapts itself to
plasma NE concentration was investigated in rats given various infusion
rates of exogenous NE during short (90 min) or long-lasting (7 days)
periods of time. Special attention was paid to investigating the NE
kinetics in the range of plasma catecholamine concentrations apparently
devoid of hemodynamic effect. 3) Epinephrine (Epi) was
proposed ("epinephrine hypothesis") to participate in the
presynaptic regulation of the release of NE, with conflicting
conclusions (1, 3, 16, 23, 24, 26, 29, 30, 33). It was
revisited in freely moving rats kept in apparently quiet conditions and
given increasing doses of exogenous Epi. The effect on NE-SOR appears
to be dose dependent; furthermore, on the basis of previous reports
(30, 31, 33, 35), we hypothesized a presynaptic regulation
of NE release under the control of a delicate
2-inhibition/
2-activation balance. The
Epi-induced
2-activation could become more efficient when the
2-inhibition was partially canceled by
yohimbine. Thus the rat appears to be a mammal with a high sympathetic
drive and with a tight control of presynaptic regulation of NE release.
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MATERIALS AND METHODS |
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Wistar rats (n = 154; Elevage Janvier, Le Genest Saint Isles, France) were housed in individual cages and fed ad libitum 24 h/day; lights went off at 1800 and on at 0600. Previous work has shown that anesthesia can create misleading information on sympathetic activity, making inappropriate any investigation carried out on anesthetized animals (25). All were studied in freely moving and apparently undisturbed conditions, equipped according to the following procedure.
Implantation of Catheters
All animals were anesthetized with pentobarbital sodium (60 mg/kg ip) and surgically implanted with femoral arterial and venous catheters for measurement of mean arterial pressure (MAP) and heart rate (HR) and for administration of tritiated NE, as well as other physiological or pharmacological compounds according to further-described experimental protocols. These catheters were routed subcutaneously to the back of the neck, exteriorized, and flushed regularly with sterile saline. A full week was allowed for recovery from anesthesia. At the end of the experiment, the animal was killed with an extraphysiological dose of pentobarbital sodium.Blood Pressure and HR Recordings
The femoral artery catheter was connected to a Gould pressure processor via a pressure transducer (Gould P231 D). MAP and HR were electrically integrated on the signal of instantaneous pressure by means of a Gould DC amplifier and a Gould Biotach, respectively. Between 20 and 30 min were allowed for equilibration of the preparation.Measurements of NE Kinetics
The isotope dilution technique described by Esler et al. (13) was used. Briefly, tritium-labeled norepinephrine ([3H]NE; [levo-ring-2,5,6-3H]NE, 40-60 Ci/mmol, 1 mCi/ml, New England Nuclear, Boston, MA) was diluted with 500 µl of 0.2 M acetic acid plus 50 µl of sodium sulfite (100 mg/ml), 350 µl of reduced glutathione (6 mg/ml) qsp 10 ml with 0.9% sodium chloride. The [3H]NE solution was infused at 27 µl/min intravenously i.e., 0.06 µCi · kgWhether alumina-extractable tritium measured after 90 min of [3H]NE infusion is appropriate to evaluate the kinetics of NE deserves comment. Although infused at tracer doses, [3H]NE is actively removed from the circulating volume by neuronal uptake, is translocated into storage vesicles, and enters the metabolic pathways of NE with production of 3H-labeled metabolites (9, 10, 19). Thus alumina-extractable 3H represents a mixture of various 3H-labeled compounds, in which [3H]NE accounts for a fraction only, a fraction that varies according to the time of [3H]NE infusion and the sites of blood sampling. Therefore, it appeared important to separate [3H]NE from other 3H-labeled metabolites, i.e., [3H]dihydroxyphenyl glycol ([3H]DHPG) and [3H]dihydroxymandelic acid ([3H]DOMA), and to evaluate the respective contribution of each fraction within the limits of our experimental protocols. Because the level of [3H]DOMA in plasma was shown to be very low, and to some extent negligible (10), special attention was paid to the plasma concentration of [3H]DHPG, having in mind that the [3H]DHPG/[3H]NE ratio increases (10) when plasma NE concentration is experimentally increased during infusion of exogenous NE.
HPLC procedure is the appropriate method (9) to
estimate plasma concentrations of both [3H]NE and
[3H]DHPG during infusion of [3H]NE. Thus a
pilot study was designed to validate our data. Three groups of
conscious rats were given either sodium chloride (control) or exogenous
NE infusion (500 or 3,000 ng · kg1 · min
1). The
arterial plasma sample obtained at the end of each experiment was
divided into two parts: one fraction was run according to procedures
routinely used in our laboratory (Paris), whereas the other was sent to
the Clinical Neuroscience Branch, National Institutes of Health (NIH),
Bethesda, MD (11, 27). One of us (E. Maignan) was kindly
welcomed to run the series of samples with the HPLC procedure at NIH.
The mobile phase [50 µM Na2PO4 · H2O, 130 µM EDTA, 3.6% acetonitrile, and octanesulfonic acid (48 mg/ml), adjusted to pH 3.1 with phosphoric acid] was filtered (0.22-µm membrane, Millipore, Bedford, MA), degassed, and pumped through the system at 1 ml/min. It was changed daily because of the presence of tritium.
Between 0.3 and 0.8 ml of plasma samples and 20 µl of infusate were mixed for 30 min with 5 mg of alumina (acid washed and stored at 100°C before use), DHBA as internal standard (2 ng), and 500 µl of Tris (pH 8.6). After brief centrifugation, the supernatant was discarded, and alumina was washed twice with milli-Q water. One hundred microliters of an acid solution [0.2 M phosphoric acid-0.2 M acetic acid (5-10%)] were added to the alumina, and the mixture was vortexed for 5 min to elute the catecholamines. A second elution was performed with 50 µl of the same acid solution to increase the recovery. After centrifugation, the supernatant (145 µl) was transferred into microsample vials, ready for injection by the autosampler.
The collected tritiated fraction was mixed with scintillation cocktail (Ecoscint A, National Diagnostic, Atlanta, GA), and tritium content was determined using a liquid scintillation analyzer (LS3801, Beckman). A part of the infusate was counted without extraction.
The mean (±SE) recovery of the DHBA internal standard was 77.7 ± 4.4%. Plasma concentrations of labeled and endogenous catecholamines were corrected according to the recovery of DHBA. Sensitivity of the assay was within the range of 1 and 4 pg of catecholamines measurable per milliliter of plasma. Interassay coefficients of variation, obtained using a quality control, were 4.3% for DHPG, 4.7% for NE, and 16.4% for epinephrine.
Endogenous catecholamines were measured according to a radioenzymatic assay routinely used in our laboratory (4). NE-SOR and NE-MCR were calculated according to Esler's procedure (12).
Experimental Protocols
Intravenous infusion of tritiated NE. Data obtained in freely moving rats given sodium chloride with or without tritiated NE are shown Table 2. No statistically significant difference was observed. In plasma, NE was slightly raised (+15%) but not significantly higher in rats given tritiated NE.
Study of NE clearance: infusions of exogenous NE.
In given experimental conditions, one may wonder whether the capacity
of the rat's organism to clear NE is dependent on the level of NE
released and circulating in plasma. Previous reports came to differing
conclusions. In humans, plasma NE concentration and NE clearance were
significantly negatively correlated for Gordon et al.
(17), whereas infusion of pressor doses of NE failed to
alter NE clearance for Ziegler et al. (37); in the rat, an
apparently nonsignificant 22% increase was reported in rats given
pressor doses of exogenous NE (10). Our working hypothesis was to consider that the capacity of rat's organism to clear NE could
increase as long as the hemodynamic state remained unaltered. Beyond a
still undefined range of plasma NE concentrations, when MAP increases,
the NE-MCR may be expected to decrease through various mechanisms where
cardiac output and blood flows are likely to have a major role
(12), whereas other mechanisms should not be excluded
(6). Intravenous infusion rates of exogenous NE were
empirically chosen to mimic various rates of NE delivered in plasma.
The first infusion rate (50 ng · kg1 · min
1) was chosen
to increase plasma NE concentration with no measurable effect on rat
hemodynamics (MAP and HR). A second group of animals was given NE at
500 ng · kg
1 · min
1. MAP
was not changed, and HR was slower. In a third group of rats, exogenous
NE was infused at 3,000 ng · kg
1 · min
1 to induce a
moderate hypertensive effect. Rats entering that series of experiments
were randomly assigned to four groups and given either sodium chloride
or exogenous NE infusion together with tritiated NE for a period of 90 min.
Effects of Exogenous Epi Infusion on NE Kinetics
Several years ago, it was hypothesized that Epi incorporated into nerve terminal stores could act to facilitate sympathetic neurotransmission via activation of presynapticEffect of 2-Antagonist Yohimbine on
2-Facilitation of NE Release Induced by Exogenous Epi
Infusion
Statistical Analysis
Data were expressed as means ± SE. NE-SOR and NE-MCR were normalized according to body weight. Differences were tested by either paired t-test or nonparametric Mann-Whitney test. P < 0.05 was accepted as the minimal level of significance. ![]() |
RESULTS |
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Methodology: Pilot Study
Because alumina-extractable tritium represents a mixture of several radioactive compounds, a mixture that makes NE kinetics data unclear, a pilot study was carried out to compare data obtained according to the procedure used in "Paris" (alumina-extractable tritium and radioenzymatic method when needed) to that used in the Clinical Neuroscience Branch (NIH), which combines an HPLC analysis of the radioactivity eluted from alumina ([3H]NE and [3H]DHPG) with HPLC measurement of catecholamines (later on defined as "NIH"). Data obtained are shown in Table 1.
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The second column of Table 1 reports data obtained when the radioactivity, expressed in disintegrations per minute, of given [3H]NE infusates was measured by direct scintillation counting in Paris and NIH. The two series should be identical; however, tritium infusion rates turned out to be 25% higher in Paris than in NIH. Whether Paris countings were contaminated by a degradation product was ruled out by data obtained in NIH: there was no difference in the radioactivity of infusates after alumina extraction compared with direct counting. Thus we have to consider that the difference was related to a difference in quench correction by the two scintillation counters. Scintillation counting carried out in Paris was used, when needed, to obtain reported data.
A statistically significant positive correlation was computed between tritium infusion rate and tritium concentration in plasma: y = 516 + 0.005x, P < 0.001, in 31 control rats. Thus variations in tritium infusion rates account for ~29.5% in variation of tritium concentration in plasma. As expected, tritium concentration measured in Paris (679 ± 77 dpm/ml) was significantly higher than [3H]NE measured in NIH (337 ± 39 dmp/ml). Three factors accounted for that difference: 1) higher infusion rate measured in Paris, likely to account for ~29.5%; 2) [3H]DHPG measured in NIH (20 ± 5 dpm/ml), which accounts for 5.6% of [3H]NE + [3H]DHPG in control animals; 3) an unknown fraction that accounted for ~18% of the radioactivity eluted in Paris.
NE kinetics were substantially different according to the methodology
used. NE-SOR was 136 ± 18 ng · kg1 · min
1 and NE-MCR
298 ± 32 ml · kg
1 · min
1 in NIH,
values significantly higher (P < 0.01) than the values measured in Paris: 82 ± 14 and 201 ± 28 ml · kg
1 · min
1,
respectively. Thus NE kinetics, according to the procedure in Paris,
were underestimated by 39.7% for NE-SOR and 32.5% for NE-MCR.
Because the ratio of DHPG to NE was shown to increase when exogenous NE
was infused (10), the present pilot study was also run in
rats given either 500 or 3,000 ng · kg1 · min
1 of
exogenous NE. Indeed, the percentage of radioactivity associated with
[3H]DHPG increased up to 10.1% during 500 and 18.9%
during 3,000 ng · kg
1 · min
1 infusion
rate of exogenous NE. Thus the radioactivity associated with
[3H]DHPG that could be considered as small in control
rats (5.6%) became substantial in rats given exogenous NE.
NE Kinetics in Resting Rats
Data obtained in a group of 31 FM and MS animals are shown in Table 2. The mean NE-SOR was 79 ± 6 ng · kg
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Effects of Exogenous NE Infusions
In rats given infusion of exogenous NE, NE-SOR are not reported, because their physiological meaning is difficult to assess in such experimental conditions.In rats given acute infusion of exogenous NE (Table
3), the NE-MCR remained unchanged within
a range of NE concentration from control to about a sixfold increase.
The upper limit of that stability appears to be indicated by the
slowing of HR. Beyond about six times the control level of NE
concentration, an increased MAP was observed, associated with a
significant decrease of NE-MCR. The statistically significant increase
of plasma dopamine concentration during intravenous infusion of NE at
50 ng · kg1 · min
1 was
unexpected; its physiological meaning remains unclear.
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In rats given chronic infusion (7 days) of exogenous NE, an apparent
increase of the animal's general metabolism is suggested by the
statistically significant decrease in body weight of those that were
given either 50 or 500 ng · kg1 · min
1 of
exogenous NE. NE-MCR remained unchanged (Table
4).
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Effects of Exogenous Epi Infusions
Data obtained are shown in Table 5. When plasma Epi concentration was increased up to ~3,200 pg/ml, plasma catecholamine concentrations as well as NE kinetics were not changed. When plasma Epi concentration was in the neighborhood of 20,000 pg/ml, MAP was significantly increased as expected, whereas HR was not. Both NE-SOR and NE-MCR were significantly increased, accounting for the nonsignificant increase of plasma NE concentration. Plasma dopamine concentration was significantly increased for a still-unknown reason.
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Effects of Exogenous Epi Infusion in Association With Yohimbine
Results are shown in Table 6. Data obtained during yohimbine (5,000 ng · kg
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In rats given combined infusions of Epi with yohimbine, plasma Epi levels were not different from Epi levels measured in rats given Epi infusion alone. However, their plasma NE concentration and NE-SOR were significantly increased when compared with animals receiving either Epi or yohimbine infusion alone. NE-SOR was about three times higher in rats given Epi + yohimbine compared with those given Epi alone. Changes in the NE-SOR accounted for changes in plasma NE concentration, as NE-MCR was not changed.
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DISCUSSION |
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The present study was carried out to investigate the relationship between the level of circulating catecholamines and the kinetics of NE in normotensive FM and MS rats. The isotope dilution technique was adapted to small-sized animals, and alumina-extractable tritium was used.
Methodologies and NE Kinetics in Control Rats
Measurement of catecholamines and their metabolites remains a challenge for the investigator. Three points deserve comment. 1) Although the radioenzymatic procedure runs the sample of plasma in its original physiological equilibrium (there is no need for protein precipitation with strong acid), the HPLC procedure was developed up to the point where it has become a standard procedure. Paired comparisons of the two procedures have reported conflicting conclusions. Although plasma NE concentration was reported to be 40% higher (P < 0.001) with radioenzymatic assay (8), it is shown to be 15% lower (P < 0.05) in the present study when compared with HPLC (Table 1). That contradiction remains unexplained at the moment. 2) A second difference could have a potential role in discussing data obtained in the present study. The radioenzymatic procedure reports plasma concentration of DOMA in the range of 4,000 pg/ml (7); although DOMA is clearly identified by thin-layer chromatography, a nonspecific contamination cannot be ruled out. On the other hand, HPLC indicates a plasma DOMA concentration below 10 pg/ml (10); in such a low range of plasma concentration, it is acceptable to consider that plasma DOMA is negligible from a physiological point of view. 3) Adsorption on alumina is a well-known procedure to extract catecholamines and their metabolites (2); it is commonly accepted that only diphenolic structures are adsorbed on alumina. Elution plus HPLC provides the investigator with two well-identified compounds ([3H]NE and [3H]DHPG); unfortunately, the whole quantity of radioactivity eluted from alumina was not available. Thus, after elution of alumina-extractable tritium, one has to deal with a mixture of [3H]NE + [3H]DHPG + a still-unknown fraction; that unknown fraction was tentatively evaluated as representing 18% of total radioactivity. Whether that unknown fraction represents [3H]DOMA cannot be established at the moment. Because alumina-extractable tritium is made of [3H]NE + [3H]DHPG (which contributes minimally in control animals) + an unknown fraction, it is understood that NE kinetics data obtained in Paris are 39.7 and 32.5% lower for NE-SOR and NE-MCR, respectively, than data obtained in NIH. Thus data on NE kinetics reported in the present study will be discussed in light of these methodological limitations.As shown in Table 2, intravenous infusion of radiolabeled NE had no
significant effect on either circulating catecholamines or systemic
hemodynamics. These data obtained in FM and MS rats (n = 31) indicate that their mean NE-SOR was 79 ± 6 ng · kg1 · min
1 and their
mean NE-MCR was 179 ± 9 ml · kg
1 · min
1. These
data, confirmed with those obtained in NIH, demonstrate a high NE
turnover in the rat, with high NE-SOR and high NE-MCR, whereas plasma
NE concentration is not very much different in the rat compared with
humans. In healthy humans, Esler et al. (14) have reviewed
several studies and concluded that venous NE-SOR ranges between 0.6 and
1.6 µg/min (i.e., between 8 and 23 ng · kg
1 · min
1 for 70 kg
body wt) and the venous NE-MCR between 2.3 and 4.8 l/min (i.e., between
32 and 68 ml · kg
1 · min
1
for 70 kg body wt). Our data on NE kinetics obtained in FM and MS rats
suggest a high sympathetic drive in the rodent, as already suggested
(28, 34, 36).
Clearance of NE
The concept of clearance was developed by renal physiologists in the early 1940s to define the volume of plasma cleared from a given compound by a unit of time. It was extended by endocrinologists to cover the whole series of mechanisms that participate in the clearing process of a given hormonal agent. The concept of metabolic clearance has been applied to circulating NE (10, 12, 14). Besides neuronal reuptake, known to have a predominant role in NE catabolism, several other mechanisms (as distinct from urinary excretion, protein binding, platelet conjugation, etc.) are assumed to have a role in the MCR of NE. Whether that metabolic process can be activated by an increased level of plasma NE is not clear. In humans, plasma NE concentration and MCR were reported to be negatively correlated (17), although this was not confirmed (37), whereas in the rat, NE clearance tended to increase during infusion of a pressor dose of exogenous NE (10). Our working hypothesis was to consider that the capacity of the rat organism to clear NE could increase as long as the hemodynamic state remained unaltered. Three series of data are reported. Figure 1 shows a lack of significant correlation between plasma NE and NE-MCR in 31 FM and MS rats; it suggests that the whole capacity of Wistar rats to clear NE is independent of plasma NE level, at least in resting conditions. A second series of experiments was carried out in rats given exogenous NE infusion to set up plasma NE concentrations at various levels. In acute conditions (Table 3), the NE-MCR remained unchanged when plasma NE was increased up to ~2,500 pg/ml, concentrations measured in the rat during swimming exercise (31). This is in agreement with previously reported work. Careful examination of the data of Eisenhofer et al. (10) reveals no apparent change in either DHPG or MHPG concentrations in rats given exogenous NE infusion when plasma NE remained <2,000-3,000 pg/ml. A step further, we suspected that activation of the NE-MCR could be time dependent, and exogenous NE was chronically infused for 7 days, with no effect on NE-MCR (Table 4). Thus the whole capacity of the rat organism to clear NE is not increased when plasma NE is experimentally increased as long as the hemodynamic state is not altered. Nonetheless, we previously reported a statistically significant correlation between arterial concentration and urinary excretion of NE in the dog (5). Whether the isotope dilution technique is not sensitive enough to identify small variations of NE-MCR should be considered. In the third series of experiments, plasma NE was increased in such a way that MAP was increased (Table 3), and NE-MCR was shown to be significantly reduced. A first interpretation is to consider that hemodynamic changes account for the decreased NE-MCR: either a decrease of cardiac output (12), a reduction of peripheral blood flow (14), or both. However, it deserves further comment. When plasma NE is experimentally increased, especially when it counterbalances the NE synaptic-plasma concentration gradient, NE stimulates both postsynaptic mechanisms, leading to an increase of MAP and presynaptic neuronal uptake, leading to a greater activation of the deamination pathway to form DHPG. Table 1 shows a significant increase of DHPG concentration in rats given exogenous NE at 3,000 ng · kgEpinephrine Hypothesis Revisited
Several years ago, Majewski (24) hypothesized that Epi incorporated into sympathetic neuronal stores could act to facilitate sympathetic neurotransmission via activation of presynapticThe epinephrine hypothesis was further investigated in the lower range
of plasma Epi concentrations not having, by themselves, an effect on
NE-SOR (Table 5). The presynaptic membrane presents, apparently,
structural features sensitive to 2-adrenergic stimuli, leading to a facilitation of the release of NE and
2-adrenoceptors that have a counterregulatory role, a
counterregulation that appears to be predominant in the rat (29,
30, 32, 34). We have speculated on the existence of a
delicate presynaptic regulation of the release of NE on the basis of an
-/
-adrenergic equilibrium. Such an equilibrium could be species
dependent; the
-component could be much more predominant in the rat,
accounting for a much higher Epi concentration (Table 5) than in humans
(22), to trigger an increase of NE release. If so, it
could be swung toward a predominant
-component by architectural
reordering of the presynaptic membrane with an
2-adrenoreceptor inhibitor, yohimbine, for example. Our
results support such a working hypothesis. A 500 ng · kg
1 · min
1 infusion
rate of Epi, which had no effect by itself on the NE-SOR (Table 6),
increased it when infused in association with yohimbine (Fig.
2). Thus, besides a faster NE kinetics,
the rat presents a quantitatively different regulation of the
presynaptic component of the NE release.
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In summary, the present work was carried out to study the relationship
between plasma catecholamine and the NE kinetics in the rat.
1) In freely moving rats, likely to be at rest, mean NE-SOR
and NE-MCR were both much higher values than were previously reported
in humans, in agreement with a strong sympathetic activity in the
rodent. 2) Several lines of results tend to demonstrate that
the NE-MCR is independent of plasma NE concentration as long as the
hemodynamic state is not altered. When MAP is increased during infusion
of exogenous NE, NE-MCR was decreased, suggesting a predominant role
for hemodynamic factors in the control of NE-MCR. 3)
Nonetheless, besides the hemodynamic, another mechanism should be
considered. The NE-MCR appears also under the control of an -/
-adrenergic equilibrium, because for a comparable hypertensive effect, NE-MCR was decreased during NE and increased during Epi infusion. 4) The epinephrine hypothesis was revisited and
was shown to be concentration dependent, revealed only at very high plasma Epi concentration. 5) A second epinephrine hypothesis
is proposed: Epi can reveal a stimulating effect on NE-SOR, even at low
plasma concentration, when presynaptic
2-adrenoceptors are partially inhibited by yohimbine.
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ACKNOWLEDGEMENTS |
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We thank Dr. Graeme Eisenhofer (Clinical Neurochemistry Laboratory, National Institutes of Health) for professional support and hospitality. In addition, we would like to thank all the staff of the Clinical Neurochemistry Laboratory for kind and efficient support.
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FOOTNOTES |
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This work was supported by a grant-in-aid from the Centre National d'Etudes Spatiales.
Address for reprint requests and other correspondence: J.-L. Cuche, 104 rue de la République, 50 600 Saint Hilaire du Harcouët, France (E-mail: Jean-Louis.Cuche{at}wanadoo.fr).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 13 March 2001; accepted in final form 29 May 2001.
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