Imidazoline receptors in the heart: characterization, distribution, and regulation

Dexmedetomidine Exerts a Negative Chronotropic Action on Sinoatrial Node Cells Through the Activation of Imidazoline Receptors

ABSTRACT

Dexmedetomidine (DEX), an α2-adrenoreceptor (α2-AR) and imidazoline receptor agonist, is most often used for the sedation of patients in the intensive care unit. Its administration is associated with an increased incidence of bradycardia; however, the precise mechanism of DEX-induced bradycardia has yet to be fully elucidated. This study was undertaken to examine whether DEX modifies pacemaker activity and the underlying ionic channel function through α2-AR and imidazoline receptors. The whole-cell patch-clamp techniques were used to record action potentials and related ionic currents of sinoatrial node cells in guinea pigs. DEX (≥10 nM) reduced sinoatrial node automaticity and the diastolic depolarization rate. DEX reduced the amplitude of hyperpolarization-activated cation current (If or Ih) the pacemaker current, even within the physiological pacemaker potential range. DEX slowed the If current activation kinetics and caused a significant shift in the voltage dependence of channel activation to negative potentials. In addition, efaroxan, an α2-AR and imidazoline I1 receptor antagonist, attenuated the inhibitory effects of DEX on sinoatrial node automaticity and If current activity, whereas yohimbine, an α2-AR-selective antagonist, did not. DEX did not affect the current activities of other channels, including rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs), L-type Ca2+ current (ICa,L), Na+/Ca2+ exchange current (INCX), and muscarinic K+ current (IK,ACh). Our results indicate that DEX, at clinically relevant concentrations, induced a negative chronotropic effect on the sinoatrial node function through the downregulation of If current through an imidazoline I1 receptor other than the α2-AR in the clinical setting.

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

Circulatory effect of TCS-80, a new imidazoline compound, in rats

BACKGROUND

Synthesis and hypotensive properties of centrally acting imidazoline agents: 1-[(imidazolidin-2-yl)imino]-1H-indazole (Marsanidine) and 7-chloro-1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indazole (TCS-80) were tested in rats. We have recently synthesized two novel Marsanidine analogues which decrease blood pressure and heart rate in rats: 1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indole (TCS-54), and 7-chloro-1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indole (TCS-213). Among all these analogues, compound TCS-80 exhibits the highest affinity to I1-imidazoline receptors and the lowest α2/I1 selectivity ratio. The observed cardiovascular effects of the compounds might be mediated through α2-adrenergic and I1-imidazoline receptors and subsequent decrease of the symphathetic nerve activity. The present studies were performed to determine whether α2-adrenergic and/or I1-imidazoline receptors are involved in the decrease of blood pressure and heart rate induced by Marsanidine, TCS-54, TCS-80, and TCS-213 in rats.

METHODS

Anesthetized rats were infused iv with the tested compounds and selective α2-adrenoceptor antagonist, RX821002, or nonselective α2-adrenergic/I1-imidazoline receptor antagonist, Efaroxan. The mean arterial blood pressure and heart rate were monitored directly and continuously throughout the experiment.

RESULTS

Efaroxan inhibited the hypotensive effect of TCS-80 stronger than RX821002. The degree of inhibition of the hypotensive effect of the remaining compounds was similar for both antagonists. The presence of Efaroxan and RX821002 diminished the heart rate decrease induced by all compounds administration, though the influence on the maximal chronotropic effect was attenuated significantly in the TCS-80 and TCS-213 treated animals only.

CONCLUSION

Our results indicate that hypotensive and negative chronotropic activities of all tested compounds are mediated by both the α2-adrenergic and I1-imidazoline receptors. Moreover, the circulatory effect of TCS-80 might be mediated to relatively higher degree by the I1-imidazoline receptors than by the α2-adrenergic ones.

Cystathionine-γ lyase-derived hydrogen sulfide mediates the cardiovascular protective effects of moxonidine in diabetic rats

Blunted cystathionine-γ lyase (CSE) activity (reduced endogenous H2S-level) is implicated in hypertension and myocardial dysfunction in diabetes. Here, we tested the hypothesis that CSE derived H2S mediates the cardiovascular protection conferred by the imidazoline I1 receptor agonist moxonidine in a diabetic rat model. We utilized streptozotocin (STZ; 55mg/kg i.p) to induce diabetes in male Wistar rats. Four weeks later, STZ-treated rats received vehicle, moxonidine (2 or 6mg/kg; gavage), CSE inhibitor DL-propargylglycine, (37.5mg/kg i.p) or DL-propargylglycine with moxonidine (6mg/kg) for 3 weeks. Moxonidine improved the glycemic state, and reversed myocardial hypertrophy, hypertension and baroreflex dysfunction in STZ-treated rats. Ex vivo studies revealed that STZ caused reductions in CSE expression/activity, H2S and nitric oxide (NO) levels and serum adiponectin and elevations in myocardial imidazoline I1 receptor expression, p38 and extracellular signal-regulated kinase, ERK1/2, phosphorylation and lipid peroxidation (expressed as malondialdehyde). Moxonidine reversed these biochemical responses, and suppressed the expression of death associated protein kinase-3. Finally, pharmacologic CSE inhibition (DL-propargylglycine) abrogated the favorable cardiovascular, glycemic and biochemical responses elicited by moxonidine. These findings present the first evidence for a mechanistic role for CSE derived H2S in the glycemic control and in the favorable cardiovascular effects conferred by imidazoline I1 receptor activation (moxonidine) in a diabetic rat model.

Immunodetection and subcellular distribution of imidazoline receptor proteins with three antibodies in mouse and human brains: Effects of treatments with I1- and I2-imidazoline drugs

Various imidazoline receptor (IR) proteins have been proposed to mediate the effects of selective I1- and I2-IR drugs. However, the association of these IR-binding proteins with classic I1- and I2-radioligand binding sites remains somewhat controversial. In this study, three IR antibodies (anti-NISCH and anti-nischarin for I1-IRs; and anti-IRBP for I1/I2-IRs) were used to immunodetect, characterize and compare IR protein patterns in brain (mouse and human; total homogenate, subcellular fractionation, grey and white matter) and some cell systems (neurones, astrocytes, human platelets). Various immunoreactive IRs (specific molecular weight bands coincidently detected with the different antibodies) were related to I1-IR (167 kDa, 105/115 kDa and 85 kDa proteins) or I2-IR (66 kDa, 45 kDa and 30 kDa proteins) types. The biochemical characterization of cortical 167 kDa protein, localized in the membrane/cytosol but not in the nucleus, indicated that this I1-IR also forms part of higher order nischarin-related complexes. The contents of I1-IR (167 kDa, 105/115 kDa, and 85 kDa) proteins in mouse brain cortex were upregulated by treatment with I1-drugs (moxonidine, efaroxan) but not with I2-drugs (BU-224, LSL 61122). Conversely, the contents of I2-IR (66 kDa, 45 kDa and 30 kDa) proteins in mouse brain cortex were modulated by treatment with I2-drugs (decreases after BU-224 and LSL 61122, and increases after idazoxan) but not with I1-drugs (with the exception of moxonidine). These findings further indicate that brain immunoreactive IR proteins exist in multiple forms that can be grouped in the already known I1- and I2-IR types, which are expressed both in neurones and astrocytes.

Moxonidine modulates cytokine signalling and effects on cardiac cell viability

Regression of left ventricular hypertrophy and improved cardiac function in SHR by the centrally acting imidazoline I1-receptor agonist, moxonidine, are associated with differential actions on circulating and cardiac cytokines. Herein, we investigated cell-type specific I1-receptor (also known as nischarin) signalling and the mechanisms through which moxonidine may interfere with cytokines to affect cardiac cell viability. Studies were performed on neonatal rat cardiomyocytes and fibroblasts incubated with interleukin (IL)-1β (5 ng/ml), tumor necrosis factor (TNF)-α (10 ng/ml), and moxonidine (10(-7) and 10(-5) M), separately and in combination, for 15 min, and 24 and 48 h for the measurement of MAPKs (ERK1/2, JNK, and p38) and Akt activation and inducible NOS (iNOS) expression, by Western blotting, and cardiac cell viability/proliferation and apoptosis by flow cytometry, MTT assay, and Live/Dead assay. Participation of imidazoline I1-receptors and the signalling proteins in the detected effects was identified using imidazoline I1-receptor antagonist and signalling protein inhibitors. The results show that IL-1β, and to a lower extent, TNF-α, causes cell death and that moxonidine protects against starvation- as well as IL-1β -induced mortality, mainly by maintaining membrane integrity, and in part, by improving mitochondrial activity. The protection involves activation of Akt, ERK1/2, p38, JNK, and iNOS. In contrast, moxonidine stimulates basal and IL-1β-induced fibroblast mortality by mechanisms that include inhibition of JNK and iNOS. Thus, apart from their actions on the central nervous system, imidazoline I1-receptors are directly involved in cardiac cell growth and death, and may play an important role in cardiovascular diseases associated with inflammation.

Moxonidine improves cardiac structure and performance in SHR through inhibition of cytokines, p38 MAPK and Akt

BACKGROUND AND PURPOSE

Regression of left ventricular hypertrophy by moxonidine, a centrally acting sympatholytic imidazoline compound, results from a sustained reduction of DNA synthesis and transient stimulation of DNA fragmentation. Because apoptosis of cardiomyocytes may lead to contractile dysfunction, we investigated in spontaneously hypertensive rats (SHR), time- and dose-dependent effects of in vivo moxonidine treatment on cardiac structure and function as well as on the inflammatory process and signalling proteins involved in cardiac cell survival/death.

EXPERIMENTAL APPROACH

12 week old SHR received moxonidine at 0, 100 and 400 µg·kg(-1)·h(-1) , s.c., for 1 and 4 weeks. Cardiac function was evaluated by echocardiography; plasma cytokines were measured by elisa and hearts were collected for histological assessment of fibrosis and measurement of cardiac proteins by Western blotting. Direct effects of moxonidine on cardiac cell death and underlying mechanisms were investigated in vitro by flow cytometry and Western blotting.

KEY RESULTS

After 4 weeks, the sub-hypotensive dose of moxonidine (100 µg) reduced heart rate and improved global cardiac performance, reduced collagen deposition, regressed left ventricular hypertrophy, inhibited Akt and p38 MAPK phosphorylation, and attenuated circulating and cardiac cytokines. The 400 µg dose resulted in similar effects but of a greater magnitude, associated with blood pressure reduction. In vitro, moxonidine inhibited norepinephrine-induced neonatal cardiomyocyte mortality but increased fibroblast mortality, through I(1)-receptor activation and differential effects on downstream Akt and p38 MAPK.

CONCLUSIONS AND IMPLICATIONS

While the antihypertensive action of centrally acting imidazoline compounds is appreciated, new cardiac-selective I(1)-receptor agonists may confer additional benefit.

Functional and molecular effects of imidazoline receptor activation in heart failure

AIMS

Heart failure is a progressive deterioration in heart function associated with overactivity of the sympathetic nervous system. The benefit of inhibition of sympathetic activity by moxonidine, a centrally acting imidazoline receptor agonist, was questioned based on the outcome of a failing clinical trial. The following studies measured cardiac structure and hemodynamics and mechanisms underlying moxonidine-induced changes, in cardiomyopathic hamsters, where the stage of the disease, dose, and compliance were controlled.

MAIN METHODS

Male BIO 14.6 hamsters (6 and 10 months old, with moderate and advanced heart failure, respectively) received moxonidine at 2 concentrations: low (2.4 mg/kg/day) and high (9.6 mg/kg/day), or vehicle, subcutaneously, for 1month. Cardiac function was measured by echocardiography, plasma and hearts were collected for histological determination of fibrosis and apoptosis, as well as for measurement cytokines by Elisa and cardiac proteins by Western blotting.

KEY FINDINGS

Compared to age-matched vehicle-treated BIO 14.6, moxonidine did not reduce blood pressure but significantly reduced heart rate and improved cardiac performance. Moxonidine exerted anti-apoptotic effect with differential inflammatory/anti-inflammatory responses that culminate in attenuated cardiac apoptosis and fibrosis and altered protein expression of collagen types. Some effects were observed regardless of treatment onset, although the changes were more significant in the younger group. Interestingly, moxonidine resulted in upregulation of cardiac imidazoline receptors.

SIGNIFICANCE

These studies imply that in addition to centrally mediated sympathetic inhibition, the effects of moxonidine may, at least in part, be mediated by direct actions on the heart. Further investigation of imidazolines/imidazoline receptors in cardiovascular diseases is warranted.

Acute hemodynamic changes after rapid intravenous bolus dosing of dexmedetomidine in pediatric heart transplant patients undergoing routine cardiac catheterization

BACKGROUND

Dexmedetomidine is a highly selective α(2)-adrenoceptor agonist with sedative, anxiolytic, and analgesic properties that has minimal effects on respiratory drive. Its sedative and hypotensive effects are mediated via central α(2A) and imidazoline type 1 receptors while activation of peripheral α(2B)-adrenoceptors result in an increase in arterial blood pressure and systemic vascular resistance. In this randomized, prospective, clinical study, we attempted to quantify the short-term hemodynamic effects resulting from a rapid i.v. bolus administration of dexmedetomidine in pediatric cardiac transplant patients.

METHODS

Twelve patients, aged 10 years or younger, weighing ≤40 kg, presenting for routine surveillance of right and left heart cardiac catheterization after cardiac transplantation were enrolled. After an inhaled or i.v. induction, the tracheas were intubated and anesthesia was maintained with 1 minimum alveolar concentration of isoflurane in room air, fentanyl (1 μg/kg), and rocuronium (1 mg/kg). At the completion of the planned cardiac catheterization, 100% oxygen was administered. After recording a set of baseline values that included heart rate (HR), systolic blood pressure, diastolic blood pressure, central venous pressure, systolic pulmonary artery pressure, diastolic pulmonary artery pressure, pulmonary artery wedge pressure, and thermodilution-based cardiac output, a rapid i.v. dexmedetomidine bolus of either 0.25 or 0.5 μg/kg was administered over 5 seconds. The hemodynamic measurements were repeated at 1 minute and 5 minutes.

RESULTS

There were 6 patients in each group. Investigation suggested that systolic blood pressure, diastolic blood pressure, systolic pulmonary artery pressure, diastolic pulmonary artery pressure, pulmonary artery wedge pressure, and systemic vascular resistance all increased at 1 minute after rapid i.v. bolus for both doses and decreased significantly to near baseline for both doses by 5 minutes. The transient increase in pressures was more pronounced in the systemic system than in the pulmonary system. In the systemic system, there was a larger percent increase in the diastolic pressures than the systolic pressures. Cardiac output, central venous pressure, and pulmonary vascular resistance did not change significantly. HR decreased at 1 minute for both doses and was, within the 0.5 μg/kg group, the only hemodynamic variable still changed from baseline at the 5-minute time point.

CONCLUSION

Rapid i.v. bolus administration of dexmedetomidine in this small sample of children having undergone heart transplants was clinically well tolerated, although it resulted in a transient but significant increase in systemic and pulmonary pressure and a decrease in HR. In the systemic system, there is a larger percent increase in the diastolic pressures than the systolic pressures and, furthermore, these transient increases in pressures were more pronounced in the systemic system than in the pulmonary system.

Hemodynamic and cardiac effects of chronic eprosartan and moxonidine therapy in stroke-prone spontaneously hypertensive rats

The renin-angiotensin and sympathetic nervous systems play critical interlinked roles in the development of left ventricular hypertrophy, fibrosis, and dysfunction. These studies investigated the hemodynamic and cardiac effects of monoblockade and coblockade of renin-angiotensin and sympathetic nervous systems. Stroke-prone spontaneously hypertensive rats (16 weeks old; male; n=12 per group) received the sympatholytic imidazoline compound, moxonidine (2.4 mg/kg per day); the angiotensin-receptor blocker eprosartan (30 mg/kg per day), separately or in combination; or saline vehicle for 8 weeks, SC, via osmotic minipumps. Blood pressure and heart rate were continuously measured by radiotelemetry. After 8 weeks, in vivo cardiac function and structure were measured by transthoracic echocardiography and a Millar conductance catheter, and the rats were then euthanized and blood and heart ventricles collected for various determinations. Compared with vehicle, the subhypotensive dose of moxonidine resulted in lower (P<0.01) heart rate, left ventricular hypertrophy, cardiomyocyte cross-sectional area, interleukin 1 beta, tumor necrosis factor-alpha, and mRNA for natriuretic peptides. Eprosartan reduced pressure (P<0.01), as well as extracellular signal-regulated kinase (ERK) 44 phosphorylation, Bax/Bcl-2, and collagen I/III, and improved left ventricular diastolic function (P<0.03). Combined treatment resulted in greater reductions in blood pressure, heart rate, left ventricular hypertrophy, collagen I/III, and inhibited inducible NO synthase and increased endothelial NO synthase phosphorylation, as well as reduced left ventricular anterior wall thickness, without altering the other parameters. Thus, in advanced hypertension complicated with cardiac fibrosis, sympathetic inhibition and angiotensin II blockade resulted in greater reduction in blood pressure and heart rate, inhibition of inflammation, and improved left ventricular pathology but did not add to the benefits of angiotensin II blockade on cardiac function.

Control of left ventricular mass by moxonidine involves reduced DNA synthesis and enhanced DNA fragmentation

BACKGROUND AND PURPOSE

Left ventricular hypertrophy (LVH) is a maladaptive process associated with increased cardiovascular risk. Regression of LVH is associated with reduced complications of hypertension. Moxonidine is an antihypertensive imidazoline compound that reduces blood pressure primarily by central inhibition of sympathetic outflow and by direct actions on the heart to release atrial natriuretic peptide, a vasodilator and an antihypertrophic cardiac hormone. This study investigated the effect of moxonidine on LVH and the mechanisms involved in this effect.

EXPERIMENTAL APPROACH

Spontaneously hypertensive rats were treated with several doses of moxonidine (s.c.) over 4 weeks. Blood pressure and heart rate were continuously monitored by telemetry. Body weight and water and food intake were measured weekly. Measurements also included left ventricular mass, DNA content, synthesis, fragmentation, and apoptotic/anti-apoptotic pathway proteins.

KEY RESULTS

The decrease in mean arterial pressure stabilized at approximately -10 mm Hg after 1 week of treatment and thereafter. Compared to vehicle-treated rats (100%), left ventricular mass was dose- and time-dependently reduced by treatment. This reduction remained significantly lower after normalizing to body weight. Moxonidine reduced left ventricular DNA content and inhibited DNA synthesis. DNA fragmentation transiently, but significantly increased at 1 week of moxonidine treatment and was paralleled by elevated active caspase-3 protein. The highest dose significantly decreased the apoptotic protein Bax and all doses stimulated anti-apoptotic Bcl-2 after 4 weeks of treatment.

CONCLUSIONS AND IMPLICATIONS

These studies implicate the modulation of cardiac DNA dynamics in the control of left ventricular mass by moxonidine in a rat model of hypertension.

Receptors involved in moxonidine-stimulated atrial natriuretic peptide release from isolated normotensive rat hearts

Imidazoline I1-receptors are present in the heart and may be involved in atrial natriuretic peptide (ANP) release. The following studies investigated whether moxonidine (an antihypertensive imidazoline I1-receptor and alpha2-adrenoceptor agonist) acts directly on the heart to stimulate ANP release, and to characterize the receptor type involved in this action. Perfusion of rat (200-225 g) isolated hearts with moxonidine (10(-6) and 10(-5) M), for 30 min, resulted in ANP release (83+/-29 and 277+/-70 ng/30 min, above basal, respectively), significantly (P<0.01) different from perfusion with buffer (-6+/-31 ng/30 min). ANP release stimulated by moxonidine (10(-6) M) was inhibited by co-perfusion with the antagonists, AGN192403 (imidazoline I1-receptor), phenoxybenzamine (alpha2>alpha1-adrenoceptors), and prazosin (alpha1>alpha2-adrenoceptors), but increased by rauwolscine (alpha2-adrenoceptors). Perfusion with 10(-5) M brimonidine (full alpha2-adrenoceptor agonist) inhibited moxonidine-stimulated ANP release. Similarly, moxonidine (10(-6) M) tended to reduce coronary flow, but significantly increased coronary flow in the presence of brimonidine, which was vasoconstrictive when perfused alone. Coronary flow was reduced by 10(-5) M each, brimonidine>clonidine>moxonidine; while similar bradycardia was observed with clonidine and moxonidine, but not with brimonidine. In conclusion, these results argue in favor of moxonidine acting primarily on imidazoline I1-receptors to release ANP, with both alpha2-adrenoceptor and imidazoline I1-receptors exerting inhibitory inter-relation. In contrast, the coronary vasodilatory effect of moxonidine requires full activation of alpha2-adrenoceptor. The sympatholytic and ANP-releasing effects of moxonidine appear to be mediated by cardiac imidazoline receptors that may be differentially localized. Most importantly, moxonidine can stimulate ANP release from the heart without contribution of the central nervous system.

The use of dexmedetomidine to facilitate acute discontinuation of opioids after cardiac transplantation in children

OBJECTIVE

To report the use of dexmedetomidine to facilitate rapid opioid and benzodiazepine withdrawal in children with transplanted hearts and to review the receptor physiology and pharmacodynamic impact of dexmedetomidine on the denervated heart.

DESIGN

Case series.

SETTING

Intensive care unit at a tertiary pediatric medical center.

PATIENTS

The series included a 6-month-old infant with pulmonary atresia who had a 3-month exposure to high-dose opioids and benzodiazepines and had undergone cardiac transplantation 4 wks before the use of dexmedetomidine and a 7-yr-old boy who had been sedated while undergoing extracorporeal membrane oxygenation for 3 wks before transplantation and started to receive dexmedetomidine 3 days after transplantation.

INTERVENTION

Administration of dexmedetomidine to facilitate the discontinuation of opioids and benzodiazepine.

MAIN RESULT

Successful rapid withdrawal from opioids and benzodiazepines while maintaining hemodynamic stability.

CONCLUSION

To our knowledge, this report describes the first use of dexmedetomidine to facilitate opioid withdrawal in children with a cardiac transplant. Dexmedetomidine allowed for the preservation of satisfactory hemodynamic parameters during acute withdrawal from opioids in children with denervated hearts.

Imidazoline receptors in the heart: a novel target and a novel mechanism of action that involves atrial natriuretic peptides

Chronic stimulation of sympathetic nervous activity contributes to the development and maintenance of hypertension, leading to left ventricular hypertrophy (LVH), arrhythmias and cardiac death. Moxonidine, an imidazoline antihypertensive compound that preferentially activates imidazoline receptors in brainstem rostroventrolateral medulla, suppresses sympathetic activation and reverses LVH. We have identified imidazoline receptors in the heart atria and ventricles, and shown that atrial I1-receptors are up-regulated in spontaneously hypertensive rats (SHR), and ventricular I1-receptors are up-regulated in hamster and human heart failure. Furthermore, cardiac I1-receptor binding decreased after chronic in vivo exposure to moxonidine. These studies implied that cardiac I1-receptors are involved in cardiovascular regulation. The presence of I1-receptors in the heart, the primary site of production of natriuretic peptides, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), cardiac hormones implicated in blood pressure control and cardioprotection, led us to propose that ANP may be involved in the actions of moxonidine. In fact, acute iv administration of moxonidine (50 to 150 microg/rat) dose-dependently decreased blood pressure, stimulated diuresis and natriuresis and increased plasma ANP and its second messenger, cGMP. Chronic SHR treatment with moxonidine (0, 60 and 120 microg kg(-1) h(-1), sc for 4 weeks) dose-dependently decreased blood pressure, resulted in reversal of LVH and decreased ventricular interleukin 1beta concentration after 4 weeks of treatment. These effects were associated with a further increase in already elevated ANP and BNP synthesis and release (after 1 week), and normalization by 4 weeks. In conclusion, cardiac imidazoline receptors and natriuretic peptides may be involved in the acute and chronic effects of moxonidine.

Augmentation of moxonidine-induced increase in ANP release by atrial hypertrophy

Imidazoline receptors are divided into I1 and I2 subtypes. I1-imidazoline receptors are distributed in the heart and are upregulated during hypertension or heart failure. The aim of this study was to define the possible role of I1-imidazoline receptors in the regulation of atrial natriuretic peptide (ANP) release in hypertrophied atria. Experiments were performed on isolated, perfused, hypertrophied atria from remnant-kidney hypertensive rats. The relatively selective I1-imidazoline receptor agonist moxonidine caused a decrease in pulse pressure. Moxonidine (3, 10, and 30 μmol/l) also caused dose-dependent increases in ANP secretion, but clonidine (an α2-adrenoceptor agonist) did not. Pretreatment with efaroxan (a selective I1-imidazoline receptor antagonist) or rauwolscine (a selective α2-adrenoceptor antagonist) inhibited the moxonidine-induced increases in ANP secretion and interstitial ANP concentration and decrease in pulse pressure. However, the antagonistic effect of efaroxan on moxonidine-induced ANP secretion was greater than that of rauwolscine. Neither efaroxan nor rauwolscine alone has any significant effects on ANP secretion and pulse pressure. In hypertrophied atria, the moxonidine-induced increase in ANP secretion and decrease in pulse pressure were markedly augmented compared with nonhypertrophied atria, and the relative change in ANP secretion by moxonidine was positively correlated to atrial hypertrophy. The accentuation by moxonidine of ANP secretion was attenuated by efaroxan but not by rauwolscine. These results show that moxonidine increases ANP release through (preferentially) the activation of atrial I1-imidazoline receptors and also via different mechanisms from clonidine, and this effect is augmented in hypertrophied atria. Therefore, we suggest that cardiac I1-imidazoline receptors play an important role in the regulation of blood pressure.

Chronic treatment with rilmenidine in spontaneously hypertensive rats: differences between two schedules of administration

Rilmenidine is one of the lead compounds of the second generation of centrally acting antihypertensive drugs. In the first part of this study, 2 routes of administration of chronic treatment (1 month) with rilmenidine were compared. In conscious and pentobarbital-anesthetized spontaneously hypertensive rats (SHR), rilmenidine was delivered intraperitoneally either 250 microg/kg b.i.d. or 500 microg/kg/d infusion by means of minipumps. The possibility of rilmenidine-induced desensitization of central (brain cortex) and/or peripheral (kidney) alpha2-adrenoreceptors was studied in saturation experiments with the classic alpha2-adrenergic antagonist [H]rauwolscine. In the second part of this study, the cardiovascular and cardiac antihypertrophic effects of the most efficient procedure were investigated. The discontinuous administration of the drug was more effective than infusion. In rats treated with rilmenidine b.i.d., mean blood pressure was reduced by nearly 15% when no reduction occurred in SHRs treated with minipumps. With the first schedule of administration, plasma concentration of the drug reached a maximum of approximately 30 ng/ml when it was only 12 ng/ml with the continuous infusion of the same dose. Anesthesia with pentobarbital potentiated the antihypertensive effect of rilmenidine in rats treated discontinuously and unmasked an antihypertensive action in rats receiving the drug with minipumps. In saturation binding experiments, no significant changes in adrenergic receptors were observed in kidney membrane preparations. In contrast, in brain cortical membranes a reduction by about 50% of the Bmax of [H]rauwolscine value was observed in rats treated discontinuously with rilmenidine. In contrast, a 400% increase of the Bmax was observed in the brain of rats treated with minipumps. Over the one-month period of the second study, the discontinuous treatment with the 500 microg/kg/d dose of rilmenidine was still able to reduce blood pressure, at least at the peak concentration time, but did not induce any significant reduction of the ventricular mass. In conclusion, rilmenidine has only weak antihypertensive effects in conscious SHRs, even at doses higher than those that are active in rabbits and humans. As a consequence, it lacks significant cardiac antihypertrophic effects in this species. Pharmacokinetic data show that the rapid plasma withdrawal of this drug may explain this particular feature in rats.

Imidazoline Receptors but Not α2-Adrenoceptors Are Regulated in Spontaneously Hypertensive Rat Heart by Chronic Moxonidine Treatment

We have recently identified imidazoline I(1)-receptors in the heart. In the present study, we tested regulation of cardiac I(1)-receptors versus alpha(2) -adrenoceptors in response to hypertension and to chronic exposure to agonist. Spontaneously hypertensive rats (SHR, 12-14 weeks old) received moxonidine (10, 60, and 120 microg/kg/h s.c.) for 1 and 4 weeks. Autoradiographic binding of (125)I-paraiodoclonidine (0.5 nM, 1 h, 22 degrees C) and inhibition of binding with epinephrine (10(-10)-10(-5) M) demonstrated the presence of alpha(2)-adrenoceptors in heart atria and ventricles. Immunoblotting and reverse transcription-polymerase chain reaction identified alpha(2A)-alpha(2B)-, and alpha(2C), and -adrenoceptor proteins and mRNA, respectively. However, compared with normotensive controls, cardiac alpha(2) -adrenoceptor kinetic parameters, receptor proteins, and mRNAs were not altered in SHR with or without moxonidine treatment. In contrast, autoradiography showed that up-regulated atrial I(1)-receptors in SHR are dose-dependently normalized by 1 week, with no additional effect after 4 weeks of treatment. Moxonidine (120 microg/kg/h) decreased B(max) in right (40.0 +/- 2.9-7.0 +/- 0.6 fmol/unit area; p < 0.01) and left (27.7 +/- 2.8-7.1 +/- 0.4 fmol/unit area; p < 0.01) atria, and decreased the 85- and 29-kDa imidazoline receptor protein bands, in right atria, to 51.8 +/- 3.0% (p < 0.01) and 82.7 +/- 5.2% (p < 0.03) of vehicle-treated SHR, respectively. Moxonidine-associated percentage of decrease in B(max) only correlated with the 85-kDa protein (R(2) = 0.57; p < 0.006), suggesting that this protein may represent I(2)-receptors. The weak but significant correlation between the two imidazoline receptor proteins (R(2) = 0.28; p < 0.03) implies that they arise from the same gene. In conclusion, the heart possesses I(1)-receptors and alpha(2)-adrenoceptors, but only I(1)-receptors are responsive to hypertension and to chronic in vivo treatment with a selective I(1)-receptor agonist.

Moxonidine, an Antihypertensive Agent, is Permissive to α1-Adrenergic Receptor Pathway in the Rat-Tail Artery

To investigate whether alpha1-adrenergic receptors were involved in the contractile response of tail arteries to moxonidine, isolated ring segments of tail arteries from male adult Sprague-Dawley rats were studied. Moxonidine (EC50 = 1.3 microM) and the alpha1-agonist phenylephrine (EC50 = 2.5 microM) increased tension development in the rat-tail artery similarly. The response to moxonidine (1 microM) could be blocked by both alpha1-adrenoceptor blockers prazosin (IC50 = 1 nM), and urapidil (IC50 = 14 nM), and also by alpha2-adrenoceptor blockers, yohimbine (IC50 = 49 nM) and efaroxan (IC50 = 49 nM). Combination drug treatment (urapidil and yohimbine, or yohimbine and prazosin) was more effective in blocking the contractile response to moxonidine, than treatment with prazosin or urapidil alone. Comparison of pA2 values for prazosin in the presence of moxonidine (9.35) or phenylephrine (10.2) confirm that alpha1-adrenergic receptors are involved in the contractile response of rat-tail artery to moxonidine.

Presynaptic effects of moxonidine in isolated buffer perfused rat hearts: role of imidazoline-1 receptors and alpha2-adrenoceptors

Numerous studies support the concept that centrally acting antihypertensive drugs, such as imidazolines, mediate sympathoinhibition not only via activation of central nervous alpha2-adrenoceptors (alpha2-AR) but also via imidazoline-1 receptors (I1-R). An additional presynaptic involvement in sympathetic neurotransmission of imidazolines, via I1-R independent of alpha2-AR, is still controversial and remains to be clarified in the heart. Concentration response curves on endogenous norepinephrine (NE) overflow evoked by stimulation of epicardial postganglionic sympathetic nerves in isolated buffer-perfused rat hearts were performed for brimonidine, moxonidine, rauwolscine, 2-endo-amino-3-exo-isopropylbicyclo[2.2.1]heptane (AGN192403), and efaroxan. To unmask an I1-R-mediated effect of moxonidine, hearts were pre-exposed in additional experiments with brimonidine or rauwolscine with or without AGN192403 or efaroxan, respectively. Moxonidine reduced stimulated NE overflow (log EC50: -6.15 +/- 0.14). AGN192403, a selective ligand at I1-R, had no influence on the dose-response curve of moxonidine (log EC50: -6.01 +/- 0.25). After pre-exposure to brimonidine [ stimulation 1 (S1) + stimulation 2 (S2); 10(-5) M], the inhibitory action of moxonidine was potentiated compared with control (32.0 +/- 2.8 versus 73.13 +/- 3.0%) and completely abolished with AGN192403 or efaroxan. This effect was also totally inhibited by pre-exposure with indomethacin (10(-7) M) and tricyclodecan-9-yl-xanthogenate (D-609), an inhibitor of phosphatidylcholine-selective phospholipase C (PC-PLC; 10(-7) M). Conversely, moxonidine was without modulating efficacy under alpha2-AR-blockade by rauwolscine. In summary, we demonstrate that moxonidine reduces NE release independently of I1-R, demonstrating the prominent effect of alpha2-AR in cardiac tissue under basal conditions. We also demonstrate that I1-R are involved in NE release under conditions of alpha2-AR-stimulation involving both a pathway of prostaglandins and PC-PLC.