The alpha2 -adrenergic receptors in hypertension and heart failure: experimental and clinical studies

Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.

Synthesis and Structure−Activity Studies on N-[5-(1H-Imidazol-4-yl)-5,6,7,8-tetrahydro-1-naphthalenyl]methanesulfonamide, an Imidazole-Containing α1A-Adrenoceptor Agonist

Structure-activity studies were performed on the alpha(1A)-adrenoceptor (AR) selective agonist N-[5-(1H-imidazol-4-yl)-5,6,7,8-tetrahydro-1-naphthalenyl]methanesulfonamide (4). Compounds were evaluated for binding activity at the alpha(1A), alpha(1b), alpha(1d), alpha(2a), and alpha(2B) subtypes. Functional activity in tissues containing the alpha(1A) (rabbit urethra), alpha(1B) (rat spleen), alpha(1D) (rat aorta), and alpha(2A) (rat prostatic vas deferens) was also evaluated. A dog in vivo model simultaneously measuring intraurethral pressure (IUP) and mean arterial pressure (MAP) was used to assess the uroselectivity of the compounds. Many of the compounds that were highly selective in vitro for the alpha(1A)-AR subtype were also more uroselective in vivo for increasing IUP over MAP than the nonselective alpha(1)-agonists phenylpropanolamine (PPA) (1) and ST-1059 (2, the active metabolite of midodrine), supporting the hypothesis that greater alpha(1A) selectivity would reduce cardiovascular side effects. However, the data also support a prominent role of the alpha(1A)-AR subtype in the control of MAP.

Ghrelin Acts at the Nucleus of the Solitary Tract to Decrease Arterial Pressure in Rats

Ghrelin is an orexigenic peptide originally isolated from the stomach. Intracerebroventricular administration of ghrelin has been shown to elicit decreases in arterial pressure and renal sympathetic nerve activity in conscious rabbits. The aim of the present study was to determine the role of ghrelin in the brain stem in cardiovascular responses in rats. Unilateral microinjection of ghrelin into the nucleus of the solitary tract significantly decreased the mean arterial pressure and heart rate (-17.3+/-0.8 mm Hg and -13.6+/-3.5 bpm by 20 pmol). The microinjection of ghrelin into the nucleus of the solitary tract also suppressed the renal sympathetic nerve activity (-29.5+/-3.4%; P<0.0001). Pretreatment with intravenous injection of pentolinium (5 mg/kg), a ganglion-blocking agent, eliminated these cardiovascular responses induced by the microinjection of ghrelin (20 pmol) into the nucleus of the solitary tract; however, pretreatment with intravenous injection of atropine sulfate (0.1 mg/kg), an antagonist of muscarinic acetylcholine receptors, failed to prevent them. In contrast, unilateral microinjection of ghrelin into the area postrema, rostral, and caudal ventrolateral medulla caused no significant changes in the mean arterial pressure and heart rate. On the other hand, immunohistochemical study revealed that the receptor for ghrelin, the growth hormone secretagogue receptor, was expressed in the neuronal cells of the nucleus of the solitary tract and the dorsal motor nucleus of the vagus, but not in the cells of the area postrema. These results suggest that ghrelin acts at the nucleus of the solitary tract to suppress sympathetic activity and to decrease arterial pressure in rats.

α-2 Adrenergic Transmission and Human Baroreflex Regulation

We observed earlier that central α-2 adrenoceptor stimulation in mice greatly augments parasympathetic tone. To test the effects in humans, we assessed autonomic vasomotor tone and baroreflex regulation in 9 normal young adults on 2 occasions, once with and once without clonidine. We determined heart rate (HR), beat-by-beat blood pressure (BP), and muscle sympathetic nerve activity. HR variability was analyzed in the time and frequency domain. Pharmacological baroreflex slopes were determined using incremental phenylephrine and nitroprusside infusions. Clonidine lowered resting BP (122±4/73±3 versus 100±7/55±3 mm Hg, P <0.01), muscle sympathetic nerve activity (18±3 versus 4±2 bursts/min, P <0.01), and HR (62±3 versus 56±3 bpm, P <0.05). The baroreflex heart rate curve was reset to much lower HR values and showed no saturation at low HR. HR variability profoundly increased during clonidine plus phenylephrine (total power: 3224±843 versus 8943±2329 ms 2 , P <0.05). High-frequency power was 1451±520 at baseline and 6720±2475 ms 2 during baroreceptor loading ( P <0.05). The low-frequency/high-frequency ratio decreased (1.94±0.41 versus 0.69±0.10, P <0.05). In contrast, clonidine reduced resting sympathetic vasomotor tone and shifted the operating point of the sympathetic baroreflex to a flat part of the sympathetic baroreflex curve. The shift decreased the ability of the baroreflex to withdraw sympathetic vasomotor tone during baroreflex loading. These baroreflex changes were associated with a moderate increase in phenylephrine responsiveness. We conclude that α-2 adrenoceptor stimulation has a differential effect on baroreflex HR and vasomotor regulation. α-2 Adrenoceptor stimulation greatly augments baroreflex-mediated bradycardia, most likely by parasympathetic activation.

Functional Variant in the α 2B Adrenoceptor Gene, a Positional Candidate on Chromosome 2, Associates With Hypertension

In a genome-wide scan in Scandinavians, we found suggestive linkage between early-onset primary hypertension and a region on chromosome 2. The alpha(2B)-adrenoceptor gene, a candidate gene within this region, harbors a functional insertion/deletion (I/D) polymorphism of three glutamate residues. The aim of this study was to investigate if the DD genotype is associated with hypertension in Swedes. We performed an association study between the I/D polymorphism of the alpha(2B)-adrenoceptor and hypertension in the Skaraborg population. The material consists of all known patients with primary hypertension in Skara (n=772 nondiabetic subjects; n=171 normoalbuminuric type 2 diabetic subjects) and 817 population control subjects. We first compared genotype frequencies between patients with early-onset hypertension (aged 50 years or younger at onset) and subjects with normotension (blood pressure <120/80 mm Hg). Thereafter, the polymorphism was tested for association with hypertension at the population level. When comparing patients with early-onset hypertension and normotensive subjects, the DD versus II genotype was associated with early-onset hypertension when diabetic subjects were excluded from the analysis (OR=2.0; 95% CI=1.2 to 3.5) or when they were not excluded (OR=1.8; 95% CI=1.0 to 3.1). At the population level, the DD versus II genotype was weakly associated with nondiabetic hypertension (OR=1.4; 95% CI=1.0 to 1.8). Our data suggest that carriers of the DD versus II genotype of the alpha(2B)-adrenoceptor are at increased risk for hypertension. The genotypic effect is most evident when comparing groups corresponding to the upper and lower tails of the blood pressure distribution in the population; however, in nondiabetic hypertensive subjects it is weakly detectable even at the population level.

Pre-weaning carvedilol treatment in spontaneously hypertensive rats

Hypertension in spontaneously hypertensive rats (SHR) has been permanently abolished by aggressive treatment regimens targeted against the sympathetic nervous system and adrenal medulla, initiated during the pre-weaning period (guanethidine and nerve growth factor antiserum combined with either adrenal demedullation or prazosin treatment). To investigate the components of the sympatho-adrenal system involved, we treated pre-weaning SHR with the combined alpha(1)- and beta-adrenoceptor antagonist carvedilol (60 mg/kg/day s.c.; postnatal days 1-21). Carvedilol treatment significantly blocked adrenoceptors during the treatment period, delayed development (eye opening), reduced growth, and reduced arterial pressure and heart rate. However, there was only modest attenuation of the subsequent development of hypertension at 10 weeks of age (mean arterial pressure 129.5+/-1.8 versus 136.1+/-1.6 mm Hg in vehicle-treated littermates; P<0.05). Thus pre-weaning carvedilol treatment slightly but significantly attenuated the development of SHR hypertension at 10 weeks, suggesting that the profound antihypertensive effects of pre-weaning sympatho-adrenal ablation are attributable to factors other than alpha(1)- and beta-adrenoceptor-mediated effects of catecholamines during this period.

Insuffisances cardiaques d’origine médicamenteuse (en dehors des anthracyclines)

The principal drugs implicated in or disclosing cardiac insufficiency are drawn from a review of the literature and observations by the French national pharmacovigilance database, from 1984 to April 2003. Several pharmacological classes are identified: in addition to antimitotic drugs, such as anthracyclines, many drugs are implicated in cardiac insufficiency, e.g. immunomodulators, anti-inflammatory drugs (including coxibs), antiarrhythmic drugs, anaesthetic drugs, antipsychotic drugs, and antidiabetic drugs (including glitazones). It is usual to classify these drugs according to three categories: (i) drugs likely to cause cardiac insufficiency de novo (such as cyclophosphamide, paclitaxel, mitoxantrone, interferons, interleukin-2 etc.); (ii) drugs likely to worsen preexisting cardiac insufficiency (such as antiarrhythmics, beta-blockers, calcium antagonists, nonsteroidal and steroidal anti-inflammatory drugs, sympathomimetic drugs etc.); and (iii) drugs only occasionally causing cardiac insufficiency. This review shows that this classification is, in fact, artificial. If cardiac toxicity is a constant concern when using antimitotic drugs or some immunomodulator drugs, it is advisable to exercise caution in the use of many other drugs when treating patients with cardiac insufficiency, even if the clinical situation is well controlled. In particular, drug-drug interactions and patient medical history must be taken into account.

Effects of preweaning doxazosin treatment on adult pressure in the spontaneously hypertensive rat

1. The neonatal/preweaning period appears to represent a critical period of involvement of the sympathetic nervous system in the development of hypertension in spontaneously hypertensive rats (SHR). 2. We tested whether alpha1-adrenoceptor-mediated effects during the preweaning period are involved in the development of hypertension in the adult SHR. 3. Male SHR were treated with the alpha1-adrenoceptor antagonist doxazosin (10 mg/kg per day, s.c.) from postnatal day 1 to 21 inclusive. Direct conscious blood pressure and heart rate were measured via the caudal artery at 12 weeks of age. 4. Preweaning treatment with doxazosin had no significant effect on mean arterial blood pressure or heart rate in male SHR at 12 weeks of age. 5. These findings do not support the involvement of alpha1-adrenoceptor-mediated effects during the preweaning period in the development of hypertension in adult SHR.

Increased cardiac norepinephrine release in spontaneously hypertensive rats: role of presynaptic alpha-2A adrenoceptors

OBJECTIVES AND DESIGN

An increased sympathoadrenergic activation is thought to contribute to the maintenance of elevated blood pressure levels in hypertension. Therefore, the regulation of cardiac presynaptic sympathetic neurotransmission was investigated in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY).

METHODS AND RESULTS

Electrical field stimulation (1 min, 4 Hz) evoked a higher norepinephrine (NE) overflow from isolated perfused SHR than from WKY hearts (171 +/- 78 versus 111 +/- 27 nmol/g; means +/- SD, n = 7, P < 0.05). The difference in stimulation-evoked NE overflow was neither due to increased NE stores nor to a higher density of sympathetic nerve endings in SHR hearts. Furthermore, impairment of cardiac NE re-uptake was ruled out, as pharmacological inhibition of NE re-uptake by desipramine (300 nmol/l) similarly increased NE overflow from SHR (+ 54 +/- 17%) and WKY hearts (+ 59 +/- 18%). However, inhibition of presynaptic alpha-2 adrenoceptors (alpha-2R) with yohimbine (1 micromol/l) resulted in a significantly larger increase in NE overflow from WKY (+ 244 +/- 42%) than from SHR hearts (+ 162 +/- 47%, P < 0.05 versus WKY), indicating impairment of presynaptic inhibitory effect of alpha-2R in SHR. Supporting this notion, mRNA concentrations of alpha-2(A), the predominant presynaptic alpha-2R subtype, were reduced in SHR compared with WKY (738 +/- 251 versus 1468 +/- 518 mRNA molecules/10 ng, n = 7, P < 0.01), as quantified by competitive reverse transcription-polymerase chain reaction derived from left stellate ganglia.

CONCLUSIONS

The impairment of the alpha-2R mediated presynaptic negative feedback mechanism by a reduced expression of the alpha-2R subtype A may increase cardiac net secretion of NE in SHR and could therefore contribute to their hypertensive phenotype.

Drug Treatment of Congestive Heart Failure

Congestive heart failure (CHF), a syndrome with several pathologic backgrounds, is associated with a series of neuroendocrine (neurohumoral) processes that are triggered by the underlying problem of cardiac pump failure. Several important pathophysiologic systems are stimulated, including the reninangiotensin-aldosterone system (RAAS), the sympathetic nervous system (SNS), the release of vasopressin from the hypothalamus/pituitary axis, and the release of atrial-brain natriuretic peptides from the heart. These neuroendocrine mechanisms are not only highly relevant from diagnostic and pathophysiologic viewpoints, but are also realistic targets for drug treatment. Over the years, it has become clear that drug treatment of CHF with inotropic agents, aiming at the improvement of pump failure, offers but modest benefit. It appears to be preferable to use drugs to target the neuroendocrine processes, and this approach will be the goal of this review. Accordingly, the following therapeutic principles may be applied: 1. Suppression of the hyperactivity of the RAAS, by means of angiotensin-converting enzyme (ACE) inhibitors or angiotensin type 1-receptor antagonists (AT,-blockers); 2. Antagonism of the effects of aldosterone, by means of spironolactone and (possibly) other aldosterone antagonists; 3. Suppression of the hyperactivity of the SNS by means of f-blockers, and (possibly) centrally acting agents such as clonidine, which cause peripheral sympatho-inhibition; and 4. Vasopressin antagonists. The validity of these various concepts will be the subject of the present review.

Physiological significance of alpha(2)-adrenergic receptor subtype diversity: one receptor is not enough

Alpha(2)-adrenergic receptors mediate part of the diverse biological effects of the endogenous catecholamines epinephrine and norepinephrine. Three distinct subtypes of alpha(2)-adrenergic receptors, alpha(2A), alpha(2B), alpha(2C), have been identified from multiple species. Because of the lack of sufficiently subtype-selective ligands, the specific biological functions of these receptor subtypes were largely unknown until recently. Gene-targeted mice carrying deletions in the genes encoding for individual alpha(2)-receptor subtypes have added important new insight into the physiological significance of adrenergic receptor diversity. Two different strategies have emerged to regulate adrenergic signal transduction. Some biological functions are controlled by two counteracting alpha(2)-receptor subtypes, e.g., alpha(2A)-receptors decrease sympathetic outflow and blood pressure, whereas the alpha(2B)-subtype increases blood pressure. Other biological functions are regulated by synergistic alpha(2)-receptor subtypes. The inhibitory presynaptic feedback loop that tightly regulates neurotransmitter release from adrenergic nerves also requires two receptor subtypes, alpha(2A) and alpha(2C). Similarly, nociception is controlled at several levels by one of the three alpha(2)-receptor subtypes. Further investigation of the specific function of alpha(2)-subtypes will greatly enhance our understanding of the relevance of closely related receptor proteins and point out novel therapeutic strategies for subtype-selective drug development.

Role of the alpha1D-adrenergic receptor in the development of salt-induced hypertension

In an attempt to elucidate whether there is a specific alpha1-adrenergic receptor (alpha1-AR) subtype involved in the genesis or maintenance of hypertension, the alpha1D-AR subtype was evaluated in a model of salt-induced hypertension. The alpha1D-AR-deficient (alpha1D-/-) and control (alpha1D+/+) mice (n=8 to 14 in each group) were submitted to subtotal nephrectomy and given 1% saline as drinking water for 35 days. Blood pressure (BP) was monitored by tail-cuff readings and confirmed at the end point by direct intraarterial BP recording. The alpha1D-/- mice had a significantly (P=0.0004) attenuated increase in BP response in this protocol (baseline 94.6+/-2.8 versus end point 107.4+/-4.5 mm Hg) compared with that of their wild-type counterparts (alpha1D+/+), from a baseline 97.4+/-2.9 to an end point 139.4+/-4.5 mm Hg. Seven of 15 alpha1D+/+ mice died with edema, probably owing to renal failure, whereas 14 of 15 alpha1D-/- mice were maintained for 35 days. Body weight, renal remnant weight, and residual renal function were similar in the 2 groups, whereas the values of plasma catecholamines (epinephrine, norepinephrine, and dopamine) were higher in alpha1D+/+ than in the alpha1D-/- mice. These data suggest that alpha1D-AR plays an important role in developing a high BP in response to dietary salt-loading, and that agents having selective alpha1D-AR antagonism could have significant therapeutic potential in the treatment of hypertension.