Localization of alpha 1-adrenoceptors in rat and human hearts by immunocytochemistry

Adrenergic Signaling in Circadian Control of Immunity

Circadian rhythms govern a multitude of physiologic processes, both on a cell-intrinsic level and systemically, through the coordinated function of multi-organ biosystems. One such system-the adrenergic system-relies on the catecholamine neurotransmitters, adrenaline and noradrenaline, to carry out a range of biological functions. Production of these catecholamines is under dual regulation by both neural components of the sympathetic nervous system and hormonal mechanisms involving the hypothalamus-pituitary-adrenal axis. Importantly, both neural and hormonal arms receive input from the body's central clock, giving rise to the observed rhythmic variations in catecholamine levels in blood and peripheral tissues. Oscillations in catecholamine signals have the potential to influence various cellular targets expressing adrenergic receptors, including cells of the immune system. This review will focus on ways in which the body's central master clock regulates the adrenergic system to generate circadian rhythms in adrenaline and noradrenaline, and will summarize the existing literature linking circadian control of the adrenergic system to immunologic outcomes. A better understanding of the complex, multi-system pathways involved in the control of adrenergic signals may provide immunologists with new insight into mechanisms of immune regulation and precipitate the discovery of new therapeutics.

α 1-Adrenergic Receptor Blockade by Prazosin Synergistically Stabilizes Rat Peritoneal Mast Cells

BACKGROUND

Adrenaline quickly inhibits the release of histamine from mast cells. Besides β 2-adrenergic receptors, several in vitro studies also indicate the involvement of α-adrenergic receptors in the process of exocytosis. Since exocytosis in mast cells can be detected electrophysiologically by the changes in the membrane capacitance (Cm), its continuous monitoring in the presence of drugs would determine their mast cell-stabilizing properties.

METHODS

Employing the whole-cell patch-clamp technique in rat peritoneal mast cells, we examined the effects of adrenaline on the degranulation of mast cells and the increase in the Cm during exocytosis. We also examined the degranulation of mast cells in the presence or absence of α-adrenergic receptor agonists or antagonists.

RESULTS

Adrenaline dose-dependently suppressed the GTP-γ-S-induced increase in the Cm and inhibited the degranulation from mast cells, which was almost completely erased in the presence of butoxamine, a β 2-adrenergic receptor antagonist. Among α-adrenergic receptor agonists or antagonists, high-dose prazosin, a selective α 1-adrenergic receptor antagonist, significantly reduced the ratio of degranulating mast cells and suppressed the increase in the Cm. Additionally, prazosin augmented the inhibitory effects of adrenaline on the degranulation of mast cells.

CONCLUSIONS

This study provided electrophysiological evidence for the first time that adrenaline dose-dependently inhibited the process of exocytosis, confirming its usefulness as a potent mast cell stabilizer. The pharmacological blockade of α 1-adrenergic receptor by prazosin synergistically potentiated such mast cell-stabilizing property of adrenaline, which is primarily mediated by β 2-adrenergic receptors.

Adrenergic regulation of innate immunity: a review

The sympathetic nervous system has a major role in the brain-immune cross-talk, but few information exist on the sympathoadrenergic regulation of innate immune system. The aim of this review is to summarize available knowledge regarding the sympathetic modulation of the innate immune response, providing a rational background for the possible repurposing of adrenergic drugs as immunomodulating agents. The cells of immune system express adrenoceptors (AR), which represent the target for noradrenaline and adrenaline. In human neutrophils, adrenaline and noradrenaline inhibit migration, CD11b/CD18 expression, and oxidative metabolism, possibly through β-AR, although the role of α₁- and α₂-AR requires further investigation. Natural Killer express β-AR, which are usually inhibitory. Monocytes express β-AR and their activation is usually antiinflammatory. On murine Dentritic cells (DC), β-AR mediate sympathetic influence on DC-T cells interactions. In human DC β₂-AR may affect Th1/2 differentiation of CD4+ T cells. In microglia and in astrocytes, β₂-AR dysregulation may contribute to neuroinflammation in autoimmune and neurodegenerative disease. In conclusion, extensive evidence supports a critical role for adrenergic mechanisms in the regulation of innate immunity, in peripheral tissues as well as in the CNS. Sympathoadrenergic pathways in the innate immune system may represent novel antiinflammatory and immunomodulating targets with significant therapeutic potential.

Potential targets for intervention in radiation-induced heart disease

Radiotherapy of thoracic and chest wall tumors, if all or part of the heart was included in the radiation field, can lead to radiation-induced heart disease (RIHD), a late and potentially severe side effect. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. The pathogenesis of RIHD is largely unknown, and a treatment is not available. Hence, ongoing pre-clinical studies aim to elucidate molecular and cellular mechanisms of RIHD. Here, an overview of recent pre-clinical studies is given, and based on the results of these studies, potential targets for intervention in RIHD are discussed.

Modulation of immune cell function by α(1)-adrenergic receptor activation

The sympathetic nervous system regulates human immune system functions through epinephrine (Epi) and norepinephrine (NE) activation of adrenergic receptors (AR) expressed on immunocompetent cell populations. The anti-inflammatory effects that are most often attributed to increased sympathetic activity have been shown to occur through β₂- and α₂-AR stimulation. However, dichotomous AR effects on immune system function are becoming increasingly apparent. Reports of α₁-AR expression on immune cell populations have been conflicting due to a lack of specific antibodies or subtype-selective receptor ligands. This has made α₁-AR identification difficult and further characterization of α₁-AR subtype expression limited. Nevertheless, there is some evidence suggesting an induction of α₁-AR expression on immunocompetent cells under certain physiological conditions and disease states. Also, the function of α₁-AR activation to modulate immune responses is just beginning to emerge in the literature. Changes in the secretion of inflammatory mediators as well as increased cell migration and differentiation have been described following α₁-AR stimulation on immunocompetent cells. These observations demonstrate the significance of α₁-AR activity in immune cell biology and emphasize the importance for understanding α₁-AR effects on the immune system.

Preclinical research into basic mechanisms of radiation-induced heart disease

Radiation-induced heart disease (RIHD) is a potentially severe side effect of radiotherapy of thoracic and chest wall tumors if all or part of the heart was included in the radiation field. RIHD presents clinically several years after irradiation and manifestations include accelerated atherosclerosis, pericardial and myocardial fibrosis, conduction abnormalities, and injury to cardiac valves. There is no method to prevent or reverse these injuries when the heart is exposed to ionizing radiation. This paper presents an overview of recent studies that address the role of microvascular injury, endothelial dysfunction, mast cells, and the renin angiotensin system in animal models of cardiac radiation injury. These insights into the basic mechanisms of RIHD may lead to the identification of targets for intervention in this late radiotherapy side effect.

Mast cell stabilization improves cardiac contractile function following hemorrhagic shock and resuscitation

Hemorrhagic shock (HS) is associated with cardiac contractile dysfunction. Mast cell (MC) degranulation is hypothesized to mediate the cardiodepressant effect. Cardiac function was assessed after HS and resuscitation (HS/R) with the administration of the MC stabilizers to prevent MC degranulation. Anesthetized male Sprague-Dawley rats were randomized to sham-operated control or HS/R groups and underwent 60 min of HS followed by 2 h of resuscitated reperfusion. Animals in the HS/R groups were randomized to receive cromolyn (5 mg/kg), ketotifen (1 mg/kg), or saline 15 min before shock. Hearts were excised following HS or 2 h of reperfusion, and function was assessed on a Langendorff apparatus. A second group of randomized animals had serial blood samples taken to assess MC degranulation by quantifying levels of serum beta-hexosaminidase. Hearts were excised at 0 min (before HS) and following 60 min of HS (before resuscitation) for a histological evaluation of MC density and degranulation. In vivo MC stabilization using ketotifen and cromolyn improved cardiac peak systolic pressure (P < 0.05), contractility (P < 0.05), and relaxation (P < 0.05) compared with that of HS controls. Serum beta-hexosaminidase increased during HS/R and was inhibited by MC stabilization (P < 0.05). Degranulation was inhibited when assessed by histochemistry and immune fluorescence. The inhibition of MC degranulation can significantly improve cardiac function following HS/R.

Resident cardiac mast cells and ischemia-reperfusion injury

BACKGROUND

The intense inflammatory reaction following reperfusion of ischemic myocardium has been implicated as a factor in the extension of myocardial injury. One of the therapeutic goals of modern cardiology is to design strategies to limit the infarct size following myocardial infarction. A sound understanding of the inflammatory cascade that involves the release of various proinflammatory mediators from cardiac cells is necessary before a specific intervention is pursued.

OBSERVATION

Summarized is the role of resident cardiac mast cells, which are noted to release inflammatory mediators, in ischemia-reperfusion-induced myocardial injury. Various pharmacologic interventions, such as disodium cromoglycate and ketotifen, that stabilize cardiac mast cells, or agents such as chlorpheniramine and cetirizine that prevent their degranulation during ischemia and reperfusion, may prove to be potential therapeutic agents to limit or salvage ischemia-reperfusion-induced injury.

CONCLUSION

On the basis of the effects of histamine H1 antagonists, adrenoceptor blockers, cellular calcium and nitric oxide modulators, as well as inhibitors of phosphodiesterase and mitogen-activated protein kinase on mast cells, cardiac resident mast cells may represent a novel target for the development of cardioprotective agents.

Modulation of intracellular Ca2+ via L-type calcium channels in heart cells by the autoantibody directed against the second extracellular loop of the alpha1-adrenoceptors

The effects of methoxamine, a selective alpha1-adrenergic receptor agonist, and the autoantibody directed against the second extracellular loop of alpha1-adrenoceptors were studied on intracellular free Ca2+ levels using confocal microscopy and ionic currents using the whole-cell patch clamp technique in single cells of 10-day-old embryonic chick and 20-week-old fetal human hearts. We observed that like methoxamine, the autoantibody directed against the second extracellular loop of alpha1-adrenoreceptors significantly increased the L-type calcium current (I(Ca(L))) but had no effect on the T-type calcium current (I(Ca(T))), the delayed outward potassium current, or the fast sodium current. This effect of the autoantibody was prevented by a prestimulation of the receptors with methoxamine and vice versa. Moreover, treating the cells with prazosin, a selective alpha1-adrenergic receptor antagonist blocked the methoxamine and the autoantibody-induced increase in I(Ca(L)), respectively. In absence of prazosin, both methoxamine and the autoantibody showed a substantial enhancement in the frequency of cell contraction and that of the concomitant cytosolic and nuclear free Ca2+ variations. The subsequent addition of nifedipine, a specific L-type Ca2+ channel blocker, reversed not only the methoxamine or the autoantibody-induced effect but also completely abolished cell contraction. These results demonstrated that functional alpha1-adrenoceptors exist in both 10-day-old embryonic chick and 20-week-old human fetal hearts and that the autoantibody directed against the second extracellular loop of this type of receptors plays an important role in stimulating their activity via activation of L-type calcium channels. This loop seems to have a functional significance by being the target of alpha1-receptor agonists like methoxamine.

Expression of alpha1-adrenergic receptor subtypes in heart cell culture

Three alpha1-AR subtypes have been cloned so far and are designated as alpha1a, alpha1b,, and alpha1d. Organ-specific distribution pattern and subtype-specific effects are known but not fully understood. To address a cell-type specific expression pattern in the heart we investigated expression pattern of alpha1-AR subtypes on RNA- and protein-level in heart tissue, cultured cardiomyocytes and non-myocytes of the rat. Each alpha1AR-subtype mRNA was present in neonatal and adult rat heart culture but the relative distribution pattern was significantly different. While the alpha1a-AR subtype is preferentially expressed in adult cardiomyocytes, the alpha1b-AR subtype was preferentially expressed in the non-myocyte cell fraction. The RT-PCR results were confirmed by Western-blotting (alpha1b) and immunocytochemical studies. Incubation with an alpha1-agonist (phenylephrine) for 72 h led to a significant reduction of the alpha1b-AR in neonatal heart cell culture on both mRNA and protein level. In contrast, incubation with an alpha1-antagonist (prazosin) induced a 1.6 fold upregulation of the alpha1a-AR mRNA without significant effects on radioligand binding and functional assay. The results indicate a distribution pattern of the alpha1-AR subtype which is specific for cell type and ontogeny of the rat heart and may be regulated by adrenergic agents.

Possible role of adrenergic component and cardiac mast cell degranulation in preconditioning-induced cardioprotection

The present study was designed to investigate the role of adrenergic component and cardiac mast cell degranulation in the cardioprotective effect of ischaemic preconditioning. Isolated rat hearts were subjected to 30 min of global ischaemia followed by 30 min of reperfusion. Ischaemic/norepinephrine (100 microm) preconditioning markedly reduced ischaemia-reperfusion-induced release of lactate dehydrogenase (LDH) and creatine kinase (CK) in coronary effluent and the incidence of ventricular premature beats (VPBs) and ventricular tachycardia/fibrillation (VT/VF) during the reperfusion phase. Moreover, ischaemic/norepinephrine preconditioning significantly reduced ischaemia-reperfusion-induced release of mast cell peroxidase (MPO), a marker of mast cell degranulation. Prazosin (0.1 microm), a alpha(1)adrenoceptor blocker, administered during ischaemic/norepinephrine preconditioning attenuated the cardioprotective and antiarrhythmic effect of ischaemic/norepinephrine preconditioning. MPO release increased immediately after ischaemic/norepinephrine preconditioning and the release was found to be inhibited in hearts subjected to ischaemic/norepinephrine preconditioning in the presence of prazosin. However, prazosin (0.1 microm) treatment per se produced cardioprotective and antiarrhythmic effects and reduced ischaemia-reperfusion-induced MPO release. These findings tentatively suggest that ischaemic preconditioning produced cardioprotective and antiarrhythmic effect by activating alpha(1)adrenoceptors and consequent degranulation of cardiac mast cells. Prazosin administered during ischaemic preconditioning abolished its ameliorative effect.