(Hypertension. 1997;30:1078-1084.)
© 1997 American Heart Association, Inc.
Articles |
Norepinephrine Release in the Human Forearm
Effects of Epinephrine
From the Division of Clinical Pharmacology, Vanderbilt University School of Medicine, Nashville, Tenn.
Correspondence to Alastair J.J. Wood, MD, Division of Clinical Pharmacology, Vanderbilt University School of Medicine, Medical Research Bldg, Room 546, Nashville, TN 37232-6602.
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Abstract |
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Abstract It has been postulated that delayed facilitation of norepinephrine release by epinephrine is causally related to the development of hypertension. It has been proposed that a brief increase in epinephrine concentrations results in the uptake of epinephrine into the sympathetic nerve terminal. Subsequent rerelease of epinephrine stimulates presynaptic ß-adrenergic receptors, resulting in a prolonged increase in plasma norepinephrine (NE) concentrations, with amplified sympathetic responses and vasoconstriction. To determine whether such epinephrine-induced, delayed facilitation of NE release occurs in a vascular bed draining resistance vessels and, if it occurs, whether that facilitation differs in hypertension, we used a radioisotope dilution method to measure unstimulated and isoproterenol-stimulated forearm NE spillover before, during, and after a 50 ng/min infusion of epinephrine for 30 minutes directly into the brachial artery. No delayed facilitatory effects of epinephrine on forearm NE spillover were observed in either 6 normotensive (NT) or 8 borderline hypertensive (BHT) subjects (NT unstimulated forearm NE spillover preepinephrine 1.79±0.41 ng/min versus postepinephrine 2.36±0.65 ng/min, P=.38; BHT preepinephrine 2.24±0.70 ng/min versus postepinephrine 1.93±0.46 ng/min, P=.51; NT isoproterenol-stimulated forearm NE spillover preepinephrine 4.61±1.01 ng/min versus postepinephrine 4.4±0.98 ng/min, P=.9; BHT preepinephrine 4.04±1.36 ng/min versus postepinephrine 4.69±1.49 ng/min P=.5). We conclude that the short-term local infusion of epinephrine does not have a delayed facilitatory effect on forearm NE spillover in NT or BHT subjects. Therefore, the prolonged increase in NE concentrations after epinephrine infusion previously shown systemically, and not seen locally in the forearm, suggests that the delayed facilitatory response to epinephrine may occur in other organs.
Key Words: epinephrine • sympathetic nervous system • norepinephrine • receptors, adrenergic
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Introduction |
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Increased sympathetic activity and enhanced reactivity to stress have been reported in patients with both borderline and established hypertension, and it has been suggested that they play a role in the pathogenesis of hypertension.1 2 3 4 5 6 7 The mechanisms for these enhanced responses in hypertensive subjects are unknown, but it has been suggested that epinephrine, released from the adrenal medulla during physiological stress, is taken up into the sympathetic nerve terminal and later rereleased as a cotransmitter with NE. The epinephrine that has been rereleased stimulates further NE release through its action on presynaptic ß-adrenergic receptors, which are thought to be primarily ß2-adrenoceptors,8 and in this way the epinephrine may amplify and prolong sympathetic responses at a time when circulating epinephrine concentrations are no longer elevated.1 4 6 7
In vitro studies have confirmed that epinephrine is taken up into the sympathetic nerve terminal and subsequently rereleased.9 Considerable in vitro experimental evidence supports the existence of functional presynaptic ß-adrenergic receptors.10 11 12 13 More recently, we have shown,14 15 and others have confirmed,16 that prejunctional ß-adrenergic receptors facilitate the release of NE in vivo. Support for the functional importance of these mechanisms, and therefore their potential importance in the pathogenesis of human hypertension, comes from studies that have demonstrated that short-term, systemic infusion of epinephrine results in prolonged hemodynamic alterations,17 18 19 20 thought to be mediated through the delayed facilitatory effect of epinephrine on presynaptic ß-adrenergic receptor–mediated NE release.
However, because of the confounding reflex responses resulting from the systemic infusion of epinephrine, it has not been possible to examine adequately the hypothesis that epinephrine, through its presynaptic actions, has a delayed facilitatory effect on NE release in humans. Ideally, to limit the confounding effects of reflex sympathetic activation and alterations in parasympathetic activity that occur after systemic administration of epinephrine, the response to epinephrine should be examined after administration of the drug directly into the vascular bed of interest, in doses that have negligible systemic effects. In such a study, local infusion of epinephrine was found to facilitate neurogenic vasoconstriction, and it was inferred that the mechanism was delayed facilitation of NE release after infusion of epinephrine.21 We have examined directly the hypothesis that the short-term elevation of epinephrine concentrations locally in the forearm results in a prolonged facilitation of forearm NE spillover. In addition, we examined the responses in both healthy normotensive subjects and subjects with borderline hypertension because if delayed epinephrine-induced facilitation of forearm NE spillover occurred, we would expect it to be enhanced in BHT.
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Methods |
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Subjects
All subjects were white, nonsmoking males. A group of 6 healthy, NT volunteers, aged 33.8±2.3 years, and a group of 8 subjects with BHT, aged 33.4±1.9 years, were studied. Five subjects in the NT group and 5 subjects in the BHT were either employees or students of the university or had performed previous research studies. The results of the initial isoproterenol dose-response in these NT and BHT subjects, and differences in baseline systemic NE spillover, have been reported previously.3 Those initial studies were designed to demonstrate the phenomenon of presynaptic ß-adrenoceptor–mediated NE release and included another 7 NT subjects who did not participate in the present study. All subjects provided written informed consent, and the study protocol was approved by the Vanderbilt Committee for the Protection of Human Subjects. BHT was defined as a diastolic blood pressure intermittently above 90 mm Hg while the subject was receiving no antihypertensive therapy. Antihypertensive medications were discontinued 4 weeks before the study in the 4 borderline hypertensive subjects who were receiving antihypertensive treatment. Blood pressure was monitored once or twice weekly in these subjects, and if the diastolic blood pressure rose to 110 mm Hg or greater, subjects were to be excluded from the study. In no case did this occur. Apart from elevated blood pressure, no subject had clinically significant abnormalities on history, physical examination, or routine laboratory tests including complete blood count, prothrombin and partial thromboplastin times, renal and liver function tests, and electrocardiogram. Subjects did not take any medications for at least 4 weeks before the study and were maintained on a diet (provided by the metabolic kitchen of the Vanderbilt Clinical Research Center) that was free of caffeine and alcohol and provided 150 mmol Na+ and 70 mmol K+ per day for 5 days before the study.
Experimental Protocol
All experiments were performed in the morning with the subjects
resting supine in bed in the same temperature-controlled room. An
intravenous cannula was placed in an antecubital vein of
both arms. After we administered 1% lidocaine subdermally, we inserted
an 18-gauge polyurethane catheter (Cook Inc) into the brachial artery
of the nondominant arm for local infusions and blood sampling.
Arterial catheter patency was maintained with a 30-mL/h
saline infusion. By altering the concentration of drug, the total flow
rate through the cannula was maintained constant at 30 mL/h.
Arterial blood pressure was measured with a pressure
transducer (Hewlett Packard), and heart rate was recorded from a
continuous electrocardiographic monitor. After the arterial
line and intravenous catheters had been placed, subjects
rested quietly for 30 minutes. [3H]NE (56.9 Ci/mmol
norepinephrine L-[ring-2,5,6-3H],
DuPont NEN) was infused into the arm contralateral to the
arterial line. An initial loading dose of 25 µCi of
[3H]NE was administered over 2 minutes followed by a
constant infusion of 0.9 µCi/min. The [3H]NE was
prepared for human administration by the Vanderbilt Hospital
Radiopharmacy, and appropriate sterility and pyrogen testing was
performed. Immediately before use, [3H]NE was diluted to
a concentration of 2 µCi/mL in normal saline with 1 mg/mL
ascorbic acid added to the infusion solution. After baseline readings
obtained at 30 and 40 minutes, by which time [3H]NE
concentrations achieve steady state, forearm blood flow in response to
10 to 400 ng/min isoproterenol (Isuprel, Winthrop
Pharmaceuticals) was determined as described below. Isoproterenol was
infused intra-arterially in increasing doses using a
Harvard infusion pump (Harvard). Each dose of isoproterenol was infused
for 7 minutes with blood flow recordings performed during the
final 2 minutes.
After completion of the isoproterenol dose-response, a 40-minute washout period was allowed to elapse, during which responses returned to baseline and the present study was performed. Because facilitation of forearm NE spillover might be detected more easily under conditions of stimulation, we measured both unstimulated and isoproterenol-stimulated NE spillover before and after the epinephrine infusion. The dose of isoproterenol used to stimulate presynaptic ß-adrenergic receptor–mediated NE release was the dose of isoproterenol obtained from that individual's immediately preceding isoproterenol dose-response curve that increased forearm blood flow to approximately twice the baseline value. This dose of isoproterenol was used in the subsequent studies in that individual for stimulation of ß-adrenergic receptor–mediated NE spillover. The isoproterenol dose used to stimulate forearm ß-receptor–mediated responses varied between individuals (20, 40, or 60 ng/min), but the same dose was used for each individual subject. Preepinephrine forearm blood flow and NE spillover were determined before (unstimulated) and after a 7-minute infusion of isoproterenol (isoproterenol-stimulated) at the dose selected for each individual subject. Then 50 ng/min epinephrine was infused into the brachial artery for 30 minutes with determination of forearm blood flow and NE spillover during the final 2 minutes of the epinephrine infusion. The dose and duration of intra-arterial epinephrine were selected to reproduce the conditions of a previous study, which showed delayed enhancement of neurogenic vasoconstriction after the administration of intra-arterial epinephrine in a similar manner.21 After completion of the epinephrine infusion, saline was infused for the next 30 minutes. After this 30-minute washout, unstimulated and isoproterenol-stimulated responses were determined again (postepinephrine).
Forearm blood flow was measured was measured in the arm into which intra-arterial isoproterenol and epinephrine were infused using mercury-in-Silastic strain gauge plethysmography as described previously,14 and simultaneous arterial and venous blood samples were drawn for determination of endogenous and [3H]NE concentrations; this allowed the determination of NE kinetics (as described below) before the epinephrine infusion (preepinephrine unstimulated and preepinephrine isoproterenol-stimulated values), during the epinephrine infusion, and 30 minutes after the discontinuation of the epinephrine infusion (postepinephrine unstimulated and postepinephrine isoproterenol-stimulated values).
Catecholamine Measurements
[3H]NE was infused continuously throughout the
study as described above. Endogenous and
[3H]NE concentrations were measured to allow
determination of NE kinetics as we14 and
others22 have previously described. Samples were drawn
into cooled tubes with EGTA and reduced glutathione (Amersham Corp),
placed on ice, and centrifuged at 4°C. Samples of the
[3H]NE infusion solution were collected, stored, and
later assayed, as described below for the blood samples, to allow
determination of the actual rate of [3H]NE infusion.
NE and epinephrine concentrations were measured by HPLC using electrochemical detection with 3,4-dihydroxybenzylamine as the internal standard as we have described previously.23 The HPLC effluent coinciding with the NE peak was collected and counted by liquid scintillation. This allowed determination of plasma [3H]NE concentration without interference from tritiated metabolites. The intra- and inter-day coefficients of variation were 7.8 and 7.6%, respectively.
Determination of NE Kinetics
Calculations for the determination of NE kinetics using the
isotope dilution method22 24 were performed as follows:
fractional extraction (FE) of [3H]NE in the
forearm=(A*-V*)/A*, where A* and V* are the arterial and
venous concentrations of [3H]NE, respectively; forearm
spillover of NE=[(V-A)+(AxFE)]Q, where A and V are the
arterial and venous concentrations of
endogenous NE, respectively, and Q is the forearm plasma
flow derived from the hematocrit, the forearm blood flow, and the
forearm volume; forearm NE clearance=FExQ; NE plasma clearance from
the whole body (systemic clearance)=[3H]NE infusion
rate/A*; and the rate at which NE entered plasma for the whole body
(systemic spillover)=systemic clearancexA.
Data Analysis
Data are expressed as mean±SEM. Pre- and
postepinephrine unstimulated, epinephrine-stimulated,
and pre- and postepinephrine isoproterenol-stimulated values,
respectively, were compared by ANOVA and with post hoc
analysis, if indicated, performed using a two-tailed Student's
t test for paired data. The minimum level of statistical
significance was P<.05 (SPSS for Windows Release 6.0,
SPSS).
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Results |
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The characteristics of the NT and BHT hypertensive subjects studied are shown in Table 1
. Other than
blood pressure, the two groups did not differ with respect to age,
height, weight, forearm volume, forearm circumference, 24-hour sodium
excretion, baseline heart rate, or baseline forearm blood flow. Forearm
blood flow responses and NE kinetics for NT and BHT subjects are shown
in Tables 2 and 3,
respectively. The increased systemic NE spillover in BHT has been
reported previously3 ; in the present study, we
specifically set out to determine whether delayed facilitation of NE
release by epinephrine occurred and thus might explain the
increased sympathetic activity in BHT.
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Local infusion of epinephrine did not result in a delayed
facilitation of either unstimulated or isoproterenol-stimulated forearm
NE spillover. Unstimulated forearm NE spillover before and 30
minutes after epinephrine in the NT subjects did not differ
(preepinephrine 1.79±0.41 ng/min compared with
postepinephrine forearm NE spillover 2.36±0.65
ng/min, P=.38). In BHT, forearm NE spillover was
2.24±0.70 ng/min preepinephrine compared with
1.93±0.46 ng/min postepinephrine (P=.51).
The dose of isoproterenol selected had the desired stimulatory effect
on forearm NE spillover release, increasing it from
1.79±0.41 ng/min to 4.61±1.01 ng/min in NT and from
2.24±0.7 ng/min to 4.04±1.36 ng/min in BHT subjects.
However, isoproterenol-stimulated forearm NE spillover was not
increased 30 minutes after the epinephrine infusion
(postepinephrine: NT, 4.4±0.98 ng/min; BHT, 4.69±1.49
ng/min; preepinephrine: NT 4.61±1.01 ng/min
[P=.90] and BHT 4.04±1.36 ng/min
[P=.50]) (Fig 1, Tables 2
and 3).
Similarly, if the NT and BHT subjects were considered together (n=14),
both unstimulated forearm NE spillover before (2.05±0.43
ng/min) and after (2.12±0.37 ng/min) epinephrine
and isoproterenol-stimulated forearm NE spillover before
(4.29±0.87 ng/min) and after (4.56±0.92 ng/min)
epinephrine did not differ (P=.86 and
P=.75, respectively) (Fig 1). Thus, there was no evidence of
epinephrine-induced facilitation of forearm NE spillover in
either the basal or the stimulated state or in NT or BHT subjects; nor
was there evidence that such facilitation might occur selectively in
BHT subjects.
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In the present study, as previously reported,3 a blunted forearm blood flow response to ß-adrenergic stimulation was observed. The median dose of isoproterenol selected for stimulation was 20 ng/min in NT and 50 ng/min in BHT subjects (P=.14). Despite the higher dose of isoproterenol, the forearm blood flow response tended to be lower in the BHT group (5.7±0.8 mL/100 mL per minute) compared with the NT group (9.0±1.6 mL/100 mL per minute) (P=.07). Blunted vasodilation in response to a ß-adrenergic agonist was reflected further in the forearm blood flow response to epinephrine, which was blunted in BHT. The NT and BHT subjects received the same dose of intra-arterial epinephrine (50 ng/min), which resulted in a significantly greater forearm blood flow response in NT (3.4±0.44 to 12.1±1.8 mL/100 mL per minute) than in BHT subjects (2.9±0.36 to 5.3±1.0 mL/100 mL per minute) (P=.004). However, as we reported with the ß-adrenergic agonist isoproterenol3 and as Chang reported with epinephrine,25 forearm NE release during the intra-arterial infusion of epinephrine increased above unstimulated levels but did so to a similar degree in BHT and NT subjects (forearm NE spillover: NT 5.7±1.8 ng/min, BHT 4.1±1.4 ng/min; P=.49).
The doses of isoproterenol and epinephrine administered had no detectable systemic effects, producing alterations in neither heart rate nor blood pressure, confirming that the effects of the doses selected were limited primarily to the forearm. The venous concentration of epinephrine in the forearm into which epinephrine was infused was measured before, during, and 30 minutes after the epinephrine infusion. The lower limit of detection for epinephrine was 25 pg/mL. Subjects with epinephrine concentrations below the limit of detection were assigned that value. The epinephrine concentration increased significantly during the epinephrine infusion (350.9±70.3 pg/mL; P<.001), and epinephrine concentrations 30 minutes after discontinuing the epinephrine infusion (48.8±12.4 pg/mL) were similar to baseline concentrations (40.7±8.5 pg/mL; P=NS), indicating that the 30-minute infusion of epinephrine in the dose administered resulted in a moderate increase in local epinephrine concentrations that returned to preinfusion concentrations during the 30 minutes after discontinuation of the epinephrine infusion. Because resting epinephrine concentrations were low and below the threshold of detection in some BHT and NT individuals, we could not confidently compare the baseline epinephrine concentrations in the two groups.
Systemic NE spillover, reflecting overall sympathetic activity,
after epinephrine, in the absence of isoproterenol was higher
in both NT and BHT subjects, but this did not attain statistical
significance in either group (pre- and postepinephrine values,
respectively, in NT subjects: 269.1±18.2 and 347.3±38.4
ng/min, P=.08; BHT subjects: 485.4±83.2 and
553.5±101.0 ng/min, P=.09) (Tables 2 and 3).
However, in the combined groups, systemic NE spillover 30 minutes
after stopping the epinephrine infusion was greater
(465.1±64.7 ng/min) than before epinephrine
(392.7±55.4 ng/min) (P=.01) (Fig 2). In keeping with
the finding that systemic NE spillover was increased 30 minutes
after the epinephrine infusion was the observation that
postepinephrine baseline forearm blood flow (2.7±0.32 mL/100
mL per minute) was less than that preepinephrine (3.1±0.28
mL/100 mL per minute) (P=.04).
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Discussion |
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We examined the hypothesis that local administration of epinephrine in doses without systemic hemodynamic effects would have a delayed facilitatory effect on NE release in the forearm vasculature in NT subjects and in BHT subjects, a group in which this response, if indeed it occurred, would be expected to be enhanced. Our results indicate that (1) local infusion of epinephrine did not produce a sustained increase in NE spillover in the forearm and (2) there was no evidence of such delayed facilitation of NE spillover occurring selectively in BHT subjects.
Several previous studies have demonstrated that a short-term infusion of epinephrine was followed by sustained tachycardia and/or increase in blood pressure.17 18 19 These observations could not be accounted for by circulating levels of epinephrine because epinephrine has a half-life of less than a minute26 and because plasma concentrations of epinephrine return to baseline levels within minutes after the discontinuation of a systemic infusion.19 These studies have been interpreted as supporting the theory that epinephrine, through the process of local uptake, rerelease, and stimulation of presynaptic ß-adrenergic receptor–mediated NE release, had delayed facilitatory effects on NE release resulting in prolongation and amplification of sympathetic responses.
Not all studies support the suggestion that epinephrine has prolonged hemodynamic effects mediated through presynaptic ß-adrenergic receptors. Persson and colleagues27 found that systemic infusion of epinephrine had a delayed stimulatory effect on both muscle sympathetic nerve activity and NE spillover; they suggested that the hemodynamic changes and the changes in NE spillover occurring after the epinephrine infusion were due to reflex responses secondary to the decrease in central venous pressure noted when the epinephrine infusion was discontinued, and not due to direct stimulatory effects on presynaptic ß-adrenergic receptors. The systemic infusion of epinephrine also results in other secondary responses such as activation of the renin-angiotensin system,28 release of other stress hormones,29 and alterations in metabolic activity20 that may confound further the interpretation of the effects of systemic epinephrine administration on sympathetic activity.
Floras and colleagues,21 to circumvent the problems associated with the systemic infusion of epinephrine, studied forearm vascular responses, but not forearm NE kinetics, before and 30 minutes after the administration of 50 ng/min epinephrine directly into brachial artery for 40 minutes. They found that forearm vasoconstrictor responses to LBNP were enhanced 30 minutes after the intra-arterial infusion of epinephrine but not 30 minutes after the intra-arterial infusion of isoproterenol, a ß-adrenergic agonist that is not taken up and rereleased from the nerve terminal. In that study, vasoconstrictor responses after direct intra-arterial infusion of NE were not altered after epinephrine, implying that the enhanced vascular responses to LBNP observed after epinephrine were not due to increased vascular sensitivity to NE but rather to increased epinephrine-mediated release of NE.21 The methodology used in our study allowed the determination of local NE release and thus specific examination of that hypothesis.
We did not observe a delayed effect of epinephrine on forearm NE spillover after the intra-arterial administration of epinephrine. Several possible reasons for this, including limitations imposed by the study design, were considered. The physiological significance of minor changes in forearm spillover are uncertain and, considering the marked delayed hemodynamic effects noted after the systemic administration of epinephrine,17 18 19 20 it is likely that the effects of epinephrine on NE release would be substantial if indeed this was the mechanism that explained these physiological observations. Our data are derived from the study of 6 NT and 8 BHT subjects. The study had approximately 70%, 80%, and 98% power to detect a doubling of postepinephrine forearm spillover in the NT, BHT, and combined groups, respectively, and thus we cannot exclude the possibility that enhanced forearm NE spillover after epinephrine might occur in the NT group. However, the strategy of combining the BHT and NT groups (n=14) to determine whether there is evidence for a delayed facilitation of NE spillover after epinephrine is reasonable because facilitation of systemic responses following systemic infusion of epinephrine18 and enhancement of the vasoconstrictor effects of LBNP postepinephrine21 30 have been shown in both NT and BHT subjects. It has been postulated that epinephrine-facilitated delayed NE responses are important in the pathogenesis of hypertension.4 6 7 If so, one would expect such responses to be enhanced early in the disease process. Several studies have indicated that sympathetic activity, particularly early in the disease process, is enhanced in hypertensive subjects.2 3 Thus, although it might be hypothesized that the magnitude of the response differs, if a delayed facilitation of NE spillover occurred after epinephrine, it would do so in both groups. Combining the two groups in this manner fails to reveal evidence for enhancement of forearm NE spillover postepinephrine. The aim of the present study was first to document the presence or absence of a response, namely, delayed increase in NE spillover postepinephrine. Second, if this response occurred, we wished to then compare the magnitude of the increase in BHT and NT individuals. Thus, increasing the numbers of NT and or BHT subjects to allow comparison of the magnitude of the response between the two groups would be appropriate, if in fact a significant response had occurred. There was no facilitation of forearm NE spillover postepinephrine in NT, BHT, or combined groups, but it is of interest that in the combined groups we did detect a small but significant increase in systemic postepinephrine NE spillover, a finding that is compatible with previous data showing a delayed increase in systemic NE spillover after the systemic infusion of epinephrine.27 Our methodology was sensitive to these small changes in systemic NE spillover despite the low dose of epinephrine, which had no detectable systemic effects, suggesting that the methodology was sensitive enough to detect postepinephrine facilitation of forearm NE spillover if it occurred.
It is unlikely that an inadequate dose or duration of intra-arterial epinephrine infusion explains the lack of a delayed facilitatory effect of epinephrine on local sympathetic activity. The dose of epinephrine selected for intra-arterial infusion was chosen to achieve local concentrations of epinephrine in the forearm that were similar to circulating systemic concentrations of epinephrine during times of stress21 and, in other studies, this dose, infused into the brachial artery for 40 minutes, facilitated the vasoconstrictor responses to lower body negative pressure.21 We have shown that epinephrine concentrations 30 minutes after discontinuing the epinephrine infusion (48.8±12.4 pg/mL) were similar to baseline concentrations (40.7±8.5 pg/mL) (P=NS). Our interest was in showing that the epinephrine we had infused resulted in a temporary increase in local epinephrine concentrations that was no longer present when studies looking for delayed facilitation of responses were performed. We did not design the present study to specifically examine local rerelease of epinephrine from the nerve terminal. If we had shown a facilitation of forearm NE spillover, a double-isotope technique, with radioactive epinephrine looking for the very small increases in local epinephrine that would result from release of epinephrine theoretically sequestered in the nerve terminal, would be of interest.
In the present study, the infusion of a low dose of isoproterenol, rather than the application of LBNP, was used to stimulate presynaptic ß-adrenergic receptor–mediated release of NE. The strength of the isolated forearm methodology is that it excludes the contribution of systemic reflex responses. The stimulation of efferent nerve traffic, for example by LBNP, requires the activation of such systemic reflex responses—something we deliberately set out to avoid and which is a particular strength of the study. We have shown previously that the local infusion of isoproterenol increased forearm NE spillover in a dose-related manner14 without requiring the application of more generalized stimuli such as LBNP. However, we cannot exclude the possibility that under conditions of increased nerve traffic, in the forearm or another vascular bed, our findings may have been different.
It is unlikely that the preceding isoproterenol dose-response that was performed to determine the stimulating dose of isoproterenol for the subsequent studies altered our ability to detect a facilitation of forearm NE spillover postepinephrine. Isoproterenol, in contrast to epinephrine, is not taken up into the nerve terminal through uptake31 and, as such, has been used as a control for epinephrine by Floras and others who showed that the delayed, enhanced hemodynamic responses observed after epinephrine were not observed after isoproterenol.21 Data generated from our previous studies32 33 suggests that a temporal effect occurring over 1 hour (comparing pre- and postepinephrine) is not present and that baseline measurements are stable over 3 to 4 hours. Thus, it is unlikely that temporal changes occurring during the study affected the results. Ideally, one would like to have randomized the order in which placebo/epinephrine observations were obtained. However, this was not incorporated into the design, specifically because the duration of any carryover facilitatory effects of epinephrine (which we did not find) was unknown. In previous studies, hemodynamic changes up to 18 hours after a 6-hour infusion of epinephrine have been found,19 suggesting that a prolonged carryover effect was likely and, thus, the saline infusion always preceded the epinephrine infusion.
A potential disadvantage of using isoproterenol as the stimulus is that
it could compete with the epinephrine, hypothetically
rereleased from the nerve terminal, for presynaptic ß-adrenergic
receptor sites and therefore could obscure the effects of
epinephrine. This possibility is difficult to exclude; however,
the dose of isoproterenol selected as a stimulus was deliberately
chosen to be well down on the dose-response curve, thus allowing for
further response to additional ß-adrenergic receptor stimulation. It
is also difficult to exclude the possibility that in the presence of
the ß-adrenergic agonist isoproterenol, the 
-adrenergic receptor
effects of epinephrine (which would decrease presynaptic NE
release) predominated and obscured its stimulatory effects. Another
potential factor that could influence the isoproterenol-induced
increase in forearm NE spillover is shunting of blood flow away
from resistance vessels by isoproterenol. This appears to be unlikely
considering the profound effects of isoproterenol on forearm vascular
resistance.3
Prolonged exposure of ß-adrenoceptors to agonist has been shown both in vitro and in vivo34 35 to lead to a decrease in ß-adrenoceptor–mediated responses, a process known as desensitization. We have previously infused isoproterenol for 4 hours without producing decreases in ß-adrenergic receptor–mediated responses in the forearm,36 and it is therefore very unlikely that downregulation of ß-adrenergic receptors by the infusion of epinephrine resulted in desensitization to the stimulating dose of isoproterenol and consequently obscured enhanced responses after epinephrine.
The measurement of sympathetic activity in vivo is difficult, and the radioisotope dilution technique used in the present study, although not providing an exact measure of neuronal NE release, provides a quantifiable estimate of this release that is sensitive to pharmacological and physiological changes and that has been used extensively by numerous investigators.2 3 14 15 16 22 24 27 31 This technique thus provides an alternative to the use of a functional measure such as forearm blood flow to an indirect stimulus, such as LBNP, to measure changes in sympathetic response.
The effects of the epinephrine infused intra-arterially were restricted primarily to the forearm with no detectable systemic hemodynamic effects. However, systemic NE spillover was higher 30 minutes after epinephrine in both NT and BHT subjects, but this was significant only when all subjects were analyzed together. These delayed effects on systemic NE spillover are small, and we cannot exclude a time-related effect; nor has statistical correction for multiple testing been applied. However, these changes in systemic NE spillover could reflect the delayed facilitation of NE release by the small amount of epinephrine reaching the systemic circulation in organs other than the forearm, such as the heart and kidneys, and would be in keeping with previous studies that have noted a delayed increase in systemic NE spillover following systemic infusion of epinephrine.27 Several lines of evidence suggest that alterations in sympathetic sensitivity are not global but rather may be organ specific. For example, we have shown that salt depletion results in a significant increase in systemic NE spillover without alterations in forearm NE spillover,33 and Esler and colleagues,2 measuring organ-specific NE spillover, found that the increase in systemic NE spillover observed in hypertensive subjects was accounted for primarily by enhanced renal and cardiac NE spillover. Thus, the suggestion that epinephrine could result in a delayed facilitation of systemic, but not forearm, NE spillover would be in keeping with these observations. If epinephrine-induced facilitation of NE spillover occurs only in specific organs, then identification of the specific organs involved would be of considerable importance in elucidating the role of such a mechanism in the pathogenesis of hypertension.
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Acknowledgments |
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This work was supported in part by grants from the American Heart Association, Tennessee Affiliate, and a National Grant-in-Aid, and US Public Health Service grants HL-56251 and GM-5M01-RR00095. Dr Stein was supported in part by a Merck Sharp & Dohme International Fellowship in Clinical Pharmacology and is the recipient of a Pharmaceutical Manufacturers of America Faculty Development Award in Clinical Pharmacology.
Received August 9, 1996; first decision September 13, 1996; accepted April 18, 1997.
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