| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Cardiovascular Research Center, II University of Naples, 80131 Naples, Italy
Address all correspondence and requests for reprints to: Dr. Domenico Cozzolino, M.D., via Pansini, 5, 80131 Napoli, Italy. E-mail: cozzolino_domenico{at}libero.it.
 |
Abstract |
|---|
 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
Objective: The objective of the study was to investigate the effects of a high-dose infusion of ß-endorphin, an opioid peptide, on blood pressure and on the hormonal profile in healthy subjects and in hypertensive patients and the mediation played by opioid receptor agonism.
Design, Setting, and Participants: According to a randomized double-blind design, 11 healthy subjects (controls) and 12 hypertensive inpatients (mean age, 38.9 and 40.4 yr, respectively) received 1-h iv infusion of ß-endorphin (250 µg/h) and, on another occasion, the same infusion protocol preceded by the opioid antagonist naloxone (8 mg).
Main Outcome Measures: Hemodynamic and hormonal measurements were performed at established times during the infusion protocols.
Results: At baseline, circulating ß-endorphin, norepinephrine, and endothelin-1 in hypertensive patients were significantly (P < 0.05) higher than in controls. In controls, ß-endorphin reduced blood pressure (P < 0.01) and circulating norepinephrine (P < 0.02) and increased plasma atrial natriuretic factor (P < 0.003) and GH (P < 0.0001). In hypertensive patients, ß-endorphin decreased systemic vascular resistance (P < 0.0001), blood pressure (P < 0.0001), and plasma norepinephrine (P < 0.0001) and endothelin-1 (P < 0.0001) and raised circulating atrial natriuretic factor (P < 0.0001), GH (P < 0.0001), and IGF-I (P < 0.0001). These hemodynamic and hormonal responses to ß-endorphin in hypertensive patients were significantly (P < 0.0001) greater than in controls but were annulled in all individuals when naloxone preceded ß-endorphin infusion.
Conclusions: High doses of ß-endorphin induce hypotensive and beneficial hormonal effects in humans, which are enhanced in essential hypertension and are mediated by opioid receptors.
|
Introduction |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
There is evidence of an involvement of the endogenous opioid system in essential hypertension. Sarcolemma of cardiomyocytes from hypertensive rats exhibits a higher concentration of opioid receptors than from normotensive rats (8). Morphine has been shown to reduce blood pressure; the hypotensive response was slight in normotensive rats but enhanced in a hypertensive animal setting (9). Fontana et al. (10) reported a higher plasma concentration of some opioid peptides in hypertensive subjects and their normotensive relatives. Furthermore, opioid binding sites are up-regulated in the brain and spinal cord of spontaneously hypertensive rats (11).
To date, little evidence is available as to whether the increased plasma concentrations of endogenous opioid peptides in hypertensive patients simply represent a biological marker, or that these substances play a key role in regulating blood pressure; and opioid peptides have any effect on hormones with vasoactive properties in these patients. We recently demonstrated that pharmacological plasma levels of ß-endorphin, the major endogenous circulating opioid peptide, improved mechanical ventricular function, reduced systemic vascular resistance (SVR), and beneficially affected the neurohormonal pattern in patients with idiopathic dilated cardiomyopathy (12).
The aims of this study were to evaluate whether ß-endorphin infused at high doses was able to affect blood pressure homeostasis and the hormonal profile in healthy normotensive subjects and in patients with essential hypertension and whether these effects were mediated by opioid receptors by administering naloxone before ß-endorphin infusion.
|
Subjects and Methods |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
The study was approved by the Ethics Committee of II University of Naples (Italy). Twelve inpatients with established essential hypertension and 11 control subjects gave their written informed consent to participate in the study after a clear and detailed explanation of its experimental nature. The procedures followed in this study were in accordance with our institutional guidelines. The duration of essential hypertension (according to the JNC 7 Report, stages 1 and 2) in the patients investigated was at least 18 months, and the diagnosis was made after the exclusion of relevant causes (endocrine, vascular, or neurological) of secondary hypertension. None of the patients had previously been treated with ß-blockers nor had any of them been taking other antihypertensive drugs over the last 72 h before the study. Moreover, none of the patients had other major diseases. All patients were on an isocaloric and slightly hyposodic (sodium intake, 2.5–3 g/d) diet. The control group was composed of healthy normotensive subjects, with no family history of essential hypertension, and was recruited from hospital staff (eight nurses and three physicians). The subjects’ clinical characteristics are given in Table 1
.
|
All subjects familiarized themselves with the experimental procedure (infusion protocols, hemodynamic measurements, venous sampling, etc.) and were studied after an overnight fast between 0800 and 1200 h. All subjects were initially equilibrated for 45 min to a thermoneutral (26–27 C) room temperature while lying supine. During this time, an iv cannula was immediately introduced into one antecubital vein and kept patent by a slow infusion of 150 mmol/liter NaCl (saline) solution. Venous blood samples for laboratory analyses were obtained from this cannula. Two basal samples were obtained 60 min after this venous insertion, and the mean of the two served as baseline values for statistical comparisons. Then, another iv cannula was introduced into the other antecubital vein just before the start of infusions and served for drug administrations. On two separate occasions 1 wk apart, all subjects received in random order two infusion protocols.
ß-Endorphin infusion protocol. Human synthetic ß-endorphin was dissolved in saline solution containing 3 g/liter human albumin in the morning of the test day and administered at a constant rate of 250 µg/h for 60 min by means of a programable pump.
Naloxone plus ß-endorphin infusion protocol. An iv bolus (8 mg) of naloxone preceded by 5-min administration of ß-endorphin, which was prepared and infused at a constant rate of 250 µg/h for 60 min as above described.
The volume of liquids infused (1 ml/min) in each subject was similar during the two studies. Blood samples and hemodynamic recordings were taken every 15 min during infusions of ß-endorphin or naloxone plus ß-endorphin.
On another day, a control study with saline solution (placebo) was administered to seven hypertensive patients and to six controls.
Hemodynamic measurements
In each subject, heart rate (HR) and finger arterial pressure were noninvasively and continuously determined by a plethysmographic technique (Finapres; Ohmeda, Englewood, CA) that has been shown to be as accurate as intraarterial blood pressure measurements (13). Data were elaborated by software that allowed systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure, and HR to be expressed in graphs. In all subjects, SVR was determined. SVR was calculated as: (mPA – mPRA)/cardiac output x 80, where mPA is the mean aortic pressure derived by plethysmographic method as DBP + 1/3(SBP – DBP), and mPRA is the mean right atrial pressure (considered equal to zero mm Hg in each subject). Cardiac output was calculated by multiplying stroke volume by HR. Stroke volume was calculated by echocardiography equipped with 2.5–3.5-MHz transducer (Apogee CX; Interspec, Inc., Ambler, PA). Stroke volume was determined as: LVDv – LVSv, where LVDv and LVSv are end-diastolic and end-systolic volume of left ventricle, respectively. Left ventricular volumes were calculated by two-dimensional apical two- and four-chamber views using the area-length method (14). All measurements were calculated from an average of three consecutive cardiac cycles. In our laboratory, the intraobserver mean coefficient of variation of stroke volume, as assessed in seven healthy subjects by repeating the measurements 14 d apart, was 4.6 ± 4.6%.
Hormone assays
Samples for ß-endorphin, endothelin-1, ANF, GH, and IGF-I were collected in tubes containing a mixture (0.1 ml/ml blood) of EDTA-aprotinin solution (5000 U/ml, Trasylol; Bayer, Leverkusen, Germany; 1.2 g/liter disodium EDTA) and immediately centrifuged for 15 min at 4 C at 2000 x g. All plasma samples were stored at –20 C until assayed.
Plasma ß-endorphin was determined by RIA. The intra- and interassay variability and sensitivity were 4.5%, 6.8%, and 0.2 pmol/liter, respectively.
Plasma norepinephrine was partially purified by batch alumina extraction, separated using ion-pairing reverse-phase HPLC (µBondapak C18-column, Powerline 600A chromatography system, and WISP 700 as autoinjector; Waters, Milford, MA), and quantified by a current produced on exposure of the column effluent to oxidizing and then reducing potentials connected in series (Coulochem 5100A; ESA, Bedford, MA). Recovery through the alumina extraction step, calculated using dihydroxybenzylamine as an internal standard, ranged from 60–70%, and each sample was corrected for its recovery. The intra- and interassay variability and sensitivity were 4.1%, 9.8%, and 20 pmol/liter, respectively.
For endothelin-1 assay, the resulting plasma was separated on c-18 columns. The eluate was evaporated in a speed vacuum concentrator and stored at –20 C until assayed by RIA. The intra- and interassay variability and sensitivity were 4.5%, 6.8%, and 0.1 pmol/liter, respectively.
Plasma ANF was determined, after chromatographic preextraction, by RIA. The intra- and interassay variability and sensitivity were 5.1%, 11.4%, and 0.5 pmol/liter, respectively.
Plasma GH was determined by IRMA. The intra- and interassay variability and sensitivity were 3.2%, 3.7%, and 0.15 µg/liter, respectively.
Plasma IGF-I was determined by IRMA. The intra- and interassay variability and sensitivity were 2.5%, 5.9%, and 0.3 ng/ml, respectively.
Laboratory staff were blinded as to the infusion code.
Statistical analysis
Data were analyzed statistically using a software package (version 11.0, SPSS, Inc., Chicago, IL). Baseline was defined as the mean of two measurements taken at time zero. The change from baseline in each parameter was calculated as the measurement at peak minus baseline during infusions of ß-endorphin and naloxone plus ß-endorphin and was indicated as 
value. Within each group of subjects, two-way ANOVA for repeated measures was used to test for within-treatment changes (to compare baseline values with responses to ß-endorphin and naloxone plus ß-endorphin infusions at 15, 30, 45, and 60 min) and between-treatment differences (to compare values at corresponding time points). The integrated areas under the curves (UCAs) of each parameter above (or under) baseline were calculated (sum of time points 0–60 min) and analyzed by one-way ANOVA to compare the differences in responses to ß-endorphin between the two groups of subjects.
A value of P < 0.05 was considered statistically significant.
All data are presented as mean values ± SD.
|
Results |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
. At baseline, blood pressure values and plasma levels of ß-endorphin, norepinephrine, and endothelin-1 in hypertensive patients were significantly (P < 0.05) higher than in normotensive subjects (Table 1). ß-Endorphin infusion protocol
Hemodynamic measurements.
In normotensive subjects, ß-endorphin infusion induced a significant decrease in SBP ( value, –5.7 ± 6.6 mm Hg, P < 0.01 vs. baseline) and DBP ( value, –2.4 ± 6.2 mm Hg, P < 0.01 vs. baseline) with a nadir at 60 min and a not significant change in SVR [ value, –76 ± 115 dyn/sec·cm–5, P = not significant (NS) vs. baseline] (Fig. 1). On the contrary, not significant changes in stroke volume, HR, and cardiac output ( values, +2.5 ± 7.2 ml, –1.1 ± 6.6 beats/min, +117 ± 479 ml/min, respectively; P = NS vs. baseline) were registered throughout the period of ß-endorphin infusion.
|
value, –15.3 ± 8.4 mm Hg, P < 0.0001 vs. baseline) and DBP ( value, –8.6 ± 5.8 mm Hg, P < 0.0001 vs. baseline) with a nadir at 60 min (Fig. 1). A similar pattern of response to ß-endorphin infusion was found in SVR, which significantly decreased when compared with baseline ( value, –174 ± 198 dyn/sec·cm–5, P < 0.0001) (Fig. 1). Stroke volume significantly ( value, +2.2 ± 7.9 ml, P < 0.005 vs. baseline) rose after 60 min of ß-endorphin infusion. No significant changes in HR and cardiac output ( values, –1.1 ± 2.2 beats/min, +55 ± 618 ml/min, respectively; P = NS vs. baseline) were found throughout the period of ß-endorphin infusion.
When the total (sum of points from 0–60 min) hemodynamic responses to ß-endorphin infusion in the two groups of subjects were compared, it was found that the UCAs of SBP and DBP, SVR, stroke volume, and cardiac output in hypertensive patients were significantly (P < 0.0001) more pronounced than in normotensive subjects (Fig. 2).
|
). A significant reduction in plasma levels of norepinephrine ( value, –162 ± 111 pmol/liter, P < 0.02 vs. baseline) was found upon ß-endorphin infusion with a nadir at 60 min, whereas the change in plasma endothelin-1 was not significant ( value, –0.2 ± 0.7 pmol/liter, P = NS vs. baseline) (Fig. 3). ß-Endorphin infusion significantly increased the venous ANF concentration ( value, +1.7 ± 1.3 pmol/liter, P < 0.003 vs. baseline), with a peak at the end of ß-endorphin infusion (Fig. 4). During ß-endorphin infusion, a progressive rise in plasma levels of GH ( value, +1.4 ± 0.8 µg/liter, P < 0.0001 vs. baseline) was found, whereas IGF-I remained substantially unchanged ( value, +41 ± 66 ng/ml, P = NS vs. baseline) (Fig. 4).
|
|
). Significant decreases in plasma levels of norepinephrine ( value, –440 ± 260 pmol/liter, P < 0.0001 vs. baseline) and endothelin-1 ( value, –1.30 ± 0.73 pmol/liter, P < 0.0001 vs. baseline) were found upon ß-endorphin infusion with a nadir at 60 min (Fig. 3). ß-Endorphin infusion significantly raised the venous ANF concentration ( value, +5.5 ± 1.27 pmol/liter, P < 0.0001 vs. baseline), with a peak at the end of ß-endorphin administration (Fig. 4). ß-Endorphin infusion provoked progressive and marked increments in plasma levels of both GH and IGF-I with a peak at 60 min, which were statistically significant when compared with baseline ( values, +4.02 ± 0.41 µg/liter, P < 0.0001; +125 ± 51 ng/ml, P < 0.0001, respectively) (Fig. 4).
The UCAs of norepinephrine, endothelin-1, ANF, GH, and IGF-I in hypertensive patients were significantly (P < 0.0001) greater than in normotensive subjects when the sums (from 0–60 min) of the hormonal responses to ß-endorphin infusion in the two groups of subjects were compared (Fig. 2).
Naloxone plus ß-endorphin infusion protocol
Hemodynamic measurements.
Intravenous bolus of naloxone before ß-endorphin infusion did not change SBP and DBP, SVR, stroke volume, HR, and cardiac output in both normotensive subjects and hypertensive patients (Fig. 1).
Hormone assays.
In both normotensive subjects and hypertensive patients, naloxone plus ß-endorphin infusion caused a marked and sustained increase in plasma levels of ß-endorphin, which were virtually identical with those measured during infusion of ß-endorphin alone (Fig. 3).
Naloxone given before ß-endorphin infusion in both normotensive subjects and hypertensive patients had not significant effects on plasma levels of norepinephrine, endothelin-1, ANF, GH, and IGF-I (Figs. 3 and 4).
The administration of ß-endorphin, alone or after naloxone, was well tolerated by all subjects.
No significant changes in any parameter investigated were found during the placebo infusion protocol in six normotensive subjects and in seven hypertensive patients (data not shown).
|
Discussion |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
The findings of this study can be summarized as follows. A high-dose infusion of ß-endorphin induced a decrease in blood pressure, a decremental trend of plasma norepinephrine and endothelin-1, a plasma increase in ANF, and an activation of the GH/IGF-I axis in all individuals. These effects were enhanced in hypertensive patients. Naloxone given before ß-endorphin infusion reversed these effects in all subjects.
Because basal plasma levels of ß-endorphin, norepinephrine, and endothelin-1 in hypertensive patients were significantly higher than in normotensive subjects, it was hypothesized that such enhanced opioid and sympathetic drives represented an early feature of the natural history of essential hypertension in humans. This hypothesis is supported by a previous study in healthy offspring with hypertensive heredity who showed a slight increase in basal plasma levels of some opioid peptides at rest that exaggeratedly rose together with blood pressure and plasma norepinephrine during exercise tests (10). Data on basal plasma levels of endothelin in patients with hypertension are inconclusive. Nevertheless, there is evidence that endothelin is promptly activated under some stress conditions in subjects prone to hypertension (15).
In the present study, ß-endorphin infusion produced pharmacological concentrations (approximately 200 pmol/liter) of ß-endorphin in plasma, which approached those seen during strong stressful conditions. High doses of ß-endorphin infused for a short period caused a progressive reduction of blood pressure and SVR in all subjects investigated. Consistent with these results are the findings of other authors indicating the hypotensive effects of some opioid peptides in several animal species and humans, probably through central and peripheral mechanisms including inhibition of sympathetic outflow with subsequent vasodilation (16). Accordingly, a very recent study from our laboratory documented the ability of ß-endorphin to lower peripheral vascular resistance in patients with dilated cardiomyopathy (12). Moreover, there is evidence that the endogenous opioid system blunts pressor responses in patients with acute respiratory failure (17).
The mechanisms whereby ß-endorphin causes such hemodynamic effects in subjects investigated in this study are not completely defined. One can speculate that a contribution to the hypotensive response to ß-endorphin infusion could be given by circulating factors, including a number of hormones with vasoactive properties. Three considerations support this view: the pressor effects were paralleled by plasma hormonal changes during ß-endorphin infusion, a consistent ß-endorphin-mediated reduction in plasma levels of vasoconstrictive neurohormones norepinephrine (in all subjects) and endothelin-1 (in hypertensive patients), and a contemporary ß-endorphin-induced increase in plasma concentrations of vasorelaxing peptides ANF and GH in all subjects.
In the present study, ß-endorphin inhibited circulating catecholamines. Accordingly, there is evidence of an opioid peptide-mediated presynaptic inhibition of norepinephrine release from neuronal endings (18). Moreover, µ-opioid receptor stimulation decreases sympathetic nervous system activity in humans (4). Similarly, ß-endorphin infusion induced a total reduction in plasma endothelin-1 in our subjects. These data are in keeping with a previous work showing a negative opioid-mediated effect on endothelin release from porcine aortic endothelial cells (19). The ß-endorphin-induced positive response of ANF, with potent vasodilating and diuretic effects (20), found in our subjects is in line with the conclusions from other reports which demonstrated a stimulatory action of ß-endorphin on ANF production in both healthy subjects (21) and patients with dilated cardiomyopathy during mental stress (22). A similar positive plasma response of GH (and IGF-I in hypertensive patients) was observed during ß-endorphin infusion. This hormonal response might be secondary to a direct ß-endorphin-induced stimulation on GH production, in accordance with the findings from experimental animals when they received µ-receptor opiate agonist morphine (23). Interestingly, an acute activation of the GH/IGF-I axis has recently been shown to lower blood pressure and peripheral vascular resistance in humans by stimulating the endothelial nitric oxide pathway (24).
In summary, the current data led us to postulate that the lowering effect of ß-endorphin on blood pressure was, at least in part, the net result of a particular hormonal combination represented by a decrease in vasoconstrictive neurohormones and a contemporary increase in vasorelaxing hormones.
The exaggerated pressor and hormonal effects of ß-endorphin in hypertensive patients are a major finding of this study.
It is reasonable to hypothesize that this enhanced opioid-mediated inhibition of blood pressure in hypertensives could be a consequence of a hypersensitivity to opioid receptor agonists or an augmented expression of opioid receptors in central cardiovascular nuclei of this type of patient. This interpretation is congruent with a previous experimental study showing an overexpression of µ- and/or 
-opioid receptors in some brain areas, including hypothalamus, hippocampus, and midbrain of hypertensive rats (25).
Similarly, an increased sensitivity of some central and peripheral organs to ß-endorphin might account for the intriguing and exaggerated hormonal responses upon ß-endorphin infusion in hypertensive patients. Several considerations reinforce this hypothesis. Because patients with essential hypertension exhibit insulin resistance (26), and given that hypersensitivity of the endocrine pancreas to ß-endorphin has already been reported in other conditions of insulin resistance, i.e. type 2 diabetes mellitus (27) and obesity (28), it may be postulated that an enhanced sensitivity to opioids of a number of endocrine organs and/or neuronal systems is operative in hypertensive patients.
Here, the opioid antagonist naloxone with high affinity for µ-receptors, given iv soon before ß-endorphin infusion, failed to produce any hemodynamic and hormonal response, as already reported (5). In other words, naloxone abolished the hemodynamic and hormonal responses registered when ß-endorphin was administered alone. In keeping with our findings is the ability of naloxone to blunt the pressor and neurohormonal effects during acute stress in patients with dilated cardiomyopathy (22). The current data suggest the hypothesis that the effects of ß-endorphin are mediated by opioid receptor activation.
Although further investigations are needed to clarify the precise significance of our findings, we suggest that, to avoid an undesired drop of blood pressure, anesthesiologists should consider whether patients scheduled for surgical procedures or analgesic therapy are suffering from hypertension or not when opioid drugs are planned.
We conclude that ß-endorphin administered at pharmacological doses for a short period has hypotensive effects in humans. Such effects are enhanced in patients with essential hypertension, are, at least in part, sustained by inhibition of vasoconstrictive neurohormones norepinephrine and endothelin-1, as well as by a contemporary stimulation of vasorelaxing peptides ANF and GH/IGF-I, and are mediated by opioid receptor agonism.
|
Footnotes |
|---|
Abbreviations: ANF, Atrial natriuretic factor; DBP, diastolic blood pressure; HR, heart rate; NS, not significant; SBP, systolic blood pressure; SVR, systemic vascular resistance; UCA, area under the curve.
Received December 27, 2004.
Accepted June 2, 2005.
|
References |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
opioid peptide receptors in spontaneously hypertensive rats. Peptides 12:779–785[CrossRef][Medline]
opioid receptor and opioid receptor antagonist-insensitive mechanisms. J Pharmacol Exp Ther 272:1–7
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
, P < 0.05 vs. respective value after naloxone plus ß-endorphin infusion.
, P < 0.05 vs. normotensives.
