Modulation of peripheral sympathetic nerve transmission

The past 15 years have been witness to a remarkable growth in knowledge regarding the modulation of "sympathetic traffic" to neuroeffector organs, including vascular tissue. The release of norepinephrine from peripheral sympathetic neurons is now known to be under both negative and positive feedback control. Norepinephrine, when released from peripheral neurons, acts on presynaptic alpha 2-receptors to inhibit further neurotransmission. Vascular postsynaptic alpha 2-receptors, sensitive to circulating catecholamines, subserve vasoconstriction. The antihypertensive agents clonidine, guanabenz and guanfacin likely reduce blood pressure by acting centrally on alpha 2 postsynaptic neurons to limit sympathetic transmission to blood vessels. Clonidine can produce venoconstriction and thereby improve orthostatic hypotension by activating venous alpha 2-receptors. Additional presynaptic dopaminergic receptors (DA2), muscarinic receptors (acetylcholine), opioid receptors, prostaglandin receptors, adenosine receptors (A1) and histamine (H2) receptors are present on sympathetic nerve membranes and, when engaged with the appropriate ligand, can limit the exocytotic process. Gamma-aminobutyric acid and serotonin demonstrate similar roles in reducing sympathetic nerve activity. In contrast to these inhibitory presynaptic mechanisms, facilitation of norepinephrine release appears to occur by way of neuronal angiotensin II receptor activation and perhaps through stimulation of sympathetic nerve membrane beta 2-receptors. An appreciation of these inhibitory and facilitator mechanisms is useful in the treatment of a variety of clinical conditions, including hypertension, heart failure, orthostatic hypotension, septic shock and a number of common withdrawal syndromes.     

Is there any desensitization of presynaptic alpha 2-adrenergic receptors in hypertension? Experimental and clinical studies]

Several authors have discussed an alteration of adrenergic receptivity in arterial hypertension. De Champlain (Hypertension 1990; 8: S77-S85) suggested that postsynaptic alpha 1-adrenergic functions became dominant while beta-adrenergic functions are attenuated in arterial hypertension. However, the status of presynaptic alpha 2-adrenoceptors remains unknown. The present study investigates presynaptic alpha 2-adrenoceptors in hypertension through the measurement of plasma levels of noradrenaline after administration of yohimbine, an alpha 2-adrenoceptor antagonist, in essential hypertension. Yohimbine (0.2 mg/kg per os) induced a 73% increase of plasma levels of noradrenaline in hypertensive patients (n = 12) and a 178% one in normotensive subjects (n = 6, p < 0.05). A similar significant difference was found in experimental neurogenic hypertension observed in awake dogs 3 weeks after sinoaortic denervation: the increase in plasma concentrations of noradrenaline after yohimbine (0.5 mg/kg i.v.) was +279% in hypertensive versus +642% in normotensive dogs (p < 0.05). The results show that the magnitude of the yohimbine-induced sympathetic activation is lower in hypertensives than in normotensives. They suggest the existence of a presynaptic alpha 2-adrenoceptor desensitization in arterial hypertension. The abnormality of this presynaptic inhibitory mechanism can increase the sympathetic tone and help to develop and maintain arterial hypertension.     

Influence of calcium-entry blockade on vasoconstrictor responses in feline mesenteric vascular bed

The subtypes of postjunctional alpha-adrenoceptors activated by neuronally released and exogenous norepinephrine and the source of calcium used for vasoconstrictor responses were investigated in the feline mesenteric vascular bed. Under constant flow conditions, intra-arterial injections of phenylephrine and UK14304, alpha 1- and alpha 2-adrenoceptor agonists, increased mesenteric arterial perfusion pressure in a dose-related manner. Prazosin, an alpha 1-antagonist, reduced vasoconstrictor responses to phenylephrine without altering responses to UK14304. Yohimbine, an alpha 2-antagonist, reduced responses to UK14304 without altering responses to phenylephrine. The same pattern of blockade was observed in animals pretreated with 6-hydroxydopamine to destroy the integrity of adrenergic terminals. Responses to phenylephrine and UK14304 were reduced by nitrendipine, a calcium-entry blocking agent, and this agent decreased vasoconstrictor responses to sympathetic nerve stimulation, tyramine, and norepinephrine. Responses to sympathetic nerve stimulation were selectively blocked by prazosin, but responses to norepinephrine were selectively blocked by yohimbine. Vasoconstrictor responses to tyramine were reduced by both prazosin and yohimbine. Nitrendipine also reduced responses to angiotensin II, U46619, a prostaglandin endoperoxide analogue, Bay K 8644, and potassium chloride. These data suggest the presence of alpha 1- and postjunctional alpha 2-adrenoceptors and support the hypothesis that norepinephrine released by nerve excitation acts mainly on alpha 1-receptors but that exogenous norepinephrine acts primarily on alpha 2-receptors. However, norepinephrine released by tyramine acts on both receptor subtypes. Nitrendipine inhibited responses to the alpha 1- and alpha 2-adrenoceptor agonists as well as those to nerve released and exogenous norepinephrine, the calcium agonist, Bay K 8644, and to other vasoconstrictor agents. These data suggest that in the feline mesenteric vascular bed, an extracellular source of calcium ions is required for vasoconstriction induced by a variety of mechanisms including activation of alpha 1- and postjunctional alpha 2-adrenoceptors.     

Hepatic alpha 1-adrenergic receptor alteration in a rat model of chronic sepsis

Catecholamine therapy is often ineffective in reversing the peripheral vasodilatation and hypotension of septic shock. This suggests that catecholamines might not be able to activate alpha 1-adrenergic receptors to cause vasoconstriction. Despite elevations in endogenous catecholamines, hypoglycemia is also a complication of human sepsis, suggesting that among many other causes, hepatic alpha 1-receptors might be altered. To better understand the pathophysiologic basis for this pharmacologic dilemma, we studied the effect of experimental sepsis on alpha 1-adrenergic receptors in hepatic tissue, a rich source of alpha 1-receptors, from septic and control Sprague-Dawley rats. alpha 1-adrenergic receptors were measured with [3H]-prazosin and data analyzed by a computerized nonlinear least-square regression algorithm. Twenty-four hours following cecal ligation with puncture, a decreased number of alpha 1-adrenergic receptors was noted in crude and purified plasma membrane fractions (23 and 40% reductions respectively) from septic animals. No changes in either agonist or antagonist affinity for receptors from septic animals were noted. These data indicate that the catecholamine refractoriness seen in septic shock may be a result of alterations in alpha 1-adrenergic receptor number or receptor-effector coupling.     

The alpha 2-adrenergic receptors: molecular structure and in vivo function]

The adrenergic system plays an essential role in the regulation of cardiovascular homeostasis. The endogenous catecholamines adrenaline and noradrenaline mediate their biological actions via activation of nine different adrenergic receptor subtypes, three alpha 1-receptors (alpha 1A, alpha 1B, alpha 1D), three alpha 2-receptors (alpha 2A, alpha 2B, alpha 2C) and three beta-receptors (beta 1, beta 2, beta 3). Only a few subtype-selective ligands exist which can be used to determine the physiological and pathophysiological significance of these individual receptor subtypes. Recent progress in mouse molecular genetics has led to the generation of transgenic models carrying deletions in individual adrenergic receptor genes ("knockout mice"). These mouse models were used to determine the specific functions of the three alpha 2-receptor subtypes. alpha 2A-receptors mediate the central antihypertensive action of the alpha 2-agonists, clonidine and moxonidine. Stimulation of vascular alpha 2B-receptors causes a transient vasoconstriction. The release of noradrenaline from sympathetic nerves is controlled by presynaptic alpha 2A- and alpha 2C-receptors. Both presynaptic alpha 2-receptors are essential, as deletion of alpha 2A- and alpha 2C-receptors leads to cardiac hypertrophy and failure due to chronically enhanced catecholamine release. These studies demonstrate the power of mouse molecular genetics to determine the physiological significance of adrenergic receptor subtype diversity and point out novel strategies for subtype-selective drug development.     

Recent developments in noradrenergic neurotransmission and its relevance to the mechanism of action of certain antihypertensive agents

This report reviews a number of significant developments in the fields of noradrenergic transmission and adrenergic receptors which suggest that, in addition to the classical postsynaptic adrenoceptors, there are also presynaptic adrenoceptors that help modulate the release of norepinephrine (NE) from peripheral as well as central noradrenergic nerve endings during nerve stimulation. In particular, stimulation of presynaptic alpha-adrenoceptors reduces this release of transmitter and the reverse is observed after blockade of these receptors. Clearcut pharmacological differences exist between the postsynaptic alpha 1-adrenoceptors that mediate the responses of certain organs and the presynaptic alpha 2-adrenoceptors that modulate the NE release during nerve stimulation. Therefore, subclassification of alpha-adrenoceptors into alpha 1 and alpha 2 subtypes is warranted but must be considered to be independent of the anatomical location of these receptors. Some noradrenergic nerve endings have also been shown to possess beta-adrenergic receptors, the stimulation of which increases the quantity of transmitter released by nerve impulses. Physiologically, these receptors could be activated by circulating epinephrine (E) and be involved in essential hypertension. A third type of catecholamine receptor found at the noradrenergic nerve ending is the inhibitory dopamine (DA) receptor, which might be of significance in the development of new antihypertensive agents. Application of these new concepts of noradrenergic neurotransmission and the subclassification of alpha-adrenoceptors to the treatment of hypertension is presented. Clonidine, for example, appears to be a potent alpha 2-adrenoceptor agonist; the central receptor involved in its antihypertensive action is pharmacologically an alpha 2-type but located postsynaptically. Clonidine also induces activation of peripheral presynaptic alpha 2-adrenoceptors, which might contribute to its cardiovascular action. The antihypertensive effects of alpha-methyldopa are related to the formation of alpha-methylnorepinephrine, a preferential alpha 2-adrenoceptor agonist, which can stimulate peripheral presynaptic alpha 2-adrenoceptors leading to a decrease of NE release and a reduction in sympathetic tone. Prazosin is a new antihypertensive agent the mechanism of action of which involves a selective blockade of postsynaptic alpha 1-adrenoceptors. This drug does not antagonize several effects of clonidine that are mediated via alpha 2-adrenoceptors. The mechanisms presently considered to account for the antihypertensive activity of beta-adrenoceptor blocking agents are numerous. It is proposed that blockade of peripheral presynaptic facilitatory beta-adrenoceptors could be of significance in the antihypertensive action of these drugs.     

Norepinephrine-induced plasma dopamine decrease in man: pharmacological evidence of the involvement of alpha 2-adrenoceptors.

OBJECTIVE: The purposes of this study were: (1) to test whether intravenous infusion of norepinephrine can affect plasma dopamine levels; and (2) to explore to what extent dopamine-2 or alpha 2-receptors play a role in this response. DESIGN: Norepinephrine infusion in man was performed to test whether the increase in norepinephrine during sympathetic stimulation can affect dopamine release. Specific antagonists of presynaptic dopamine-2 and alpha 2-receptors were administered to test the receptor(s) involved in this possible regulatory phenomenon. METHODS: Plasma catecholamine levels were investigated in seven normal subjects before and after administration of domperidone (dopamine-2 antagonist), yohimbine (alpha 2-antagonist) and norepinephrine. RESULTS: Both oral domperidone and yohimbine induced a significant increase in both plasma norepinephrine and plasma dopamine. Norepinephrine infusion induced a significant decrease in plasma dopamine. Pretreatment with domperidone only partially counteracted this inhibitory effect of norepinephrine infusion, whereas yohimbine fully counteracted it. CONCLUSIONS: Our data show that norepinephrine may act as a hormone at plasma concentrations as low as 450 pg/ml. The norepinephrine-induced plasma dopamine decrease seems to be alpha 2-adrenoceptor-mediated. This norepinephrine effect may be involved in the physiologic decrease in plasma dopamine that we demonstrated in the upright position in normal subjects.     

The effect of warming on adrenergic neurotransmission in canine cutaneous vein

The effect of warming on adrenergic neurotransmission was examined in canine cutaneous veins. Isometric tension was recorded from rings of saphenous veins of the dog in organ chambers filled with physiological salt solution. During contractions caused by potassium or prostaglandin F2 alpha, warming from 37 to 41 degrees C caused an augmentation. During contractions caused by stimulation of the adrenergic nerves, and by exogenous norepinephrine, warming caused a relaxation. The relaxation with warming was not altered by the beta-adrenergic antagonist, propranolol, or by inhibitors of extraneuronal and neuronal uptake of norepinephrine. During contractions evoked by the alpha 2-adrenergic agonists, alpha-methyl norepinephrine and B-HT 920, warming caused a relaxation, whereas during contractions due to the alpha 1-adrenergic agonists, cirazoline , methoxamine, ST 587, and phenylephrine, it caused an augmentation. The relaxation caused by warming during norepinephrine-induced contractions was prevented by the preferential alpha 2-antagonists yohimbine and rauwolscine, but not by the preferential alpha 1-antagonist, prazosin. In strips of saphenous vein incubated with [3H] norepinephrine , warming did not affect the release of labeled transmitter evoked by nerve stimulation. These experiments indicate that warming directly enhances contractility of vascular smooth muscle, while depressing the responsiveness of cutaneous vessels to sympathetic nerve activation by a selective inhibitory effect on postjunctional alpha 2-adrenoceptors. Relaxation with warming is greater during nerve stimulation than during administration of exogenous norepinephrine, which may be due to a predominance of postjunctional alpha 2-adrenoceptors in the neuromuscular junction.     

Interaction between cardiac sympathetic drive and heart rate in heart failure: modulation by adrenergic receptor genotype

OBJECTIVES: In the present study, we aimed to evaluate the effect of adrenergic receptor polymorphisms on the response of myocardium to measured levels of cardiac adrenergic drive, and to evaluate whether polymorphisms of presynaptic adrenoceptors modified the rate of cardiac and systemic release of norepinephrine. BACKGROUND: Heightened sympathetic activity plays an important pathophysiologic role in congestive heart failure (CHF). Recently several functionally relevant polymorphisms of the alpha(2)-, beta(1)-, and beta(2)-adrenoceptors have been identified, and specific genotypes have been associated with the incidence or clinical severity of CHF. These adrenoceptors are known to be located both pre-synaptically (alpha(2) and beta(2)) and post-synaptically (beta(1) and beta(2)), raising the possibility that their association with clinical measures in CHF could be mediated either by modulation of the cardiac response to a given level of adrenergic drive or by altering norepinephrine release from sympathetic nerve terminals. METHODS: We determined the beta(1)-, beta(2)-, and alpha(2C)-adrenoceptor genotype in 60 patients with severe CHF in conjunction with measurement of cardiac and systemic sympathetic activity using the radiotracer norepinephrine spillover method. RESULTS: We showed a strong relationship (r = 0.67, p < 0.001) between heart rate and the level of cardiac adrenergic drive, and heart rate for a given level of cardiac adrenergic drive was substantially greater in patients with the Arg/Arg16 beta(2)-adrenoceptor polymorphism (p = 0.02), whereas no such relationship existed for polymorphisms of the beta(1)-adrenoceptor. The genotype of the alpha(2C)- and beta(2)-adrenoceptors showed no relationship to the rate of norepinephrine release from cardiac sympathetic nerves. CONCLUSIONS: For the first time, we show that beta(2)-adrenoceptor polymorphisms significantly influence the relationship between heart rate and cardiac adrenergic drive in CHF, but do not affect the rate of norepinephrine release from sympathetic nerve terminals.     

Alterations in rat aortic alpha 1-adrenoceptors and alpha 1-adrenergic stimulated phosphoinositide hydrolysis in intraperitoneal sepsis

We investigated the alterations of rat aortic alpha 1-adrenoceptors and alpha 1-adrenergic stimulated phosphoinositide (PI) metabolism in intraperitoneal sepsis. An analysis of [125I]-hydroxyethylaminotetralone (HEAT) binding to alpha 1-adrenoceptors on rat aortic membranes revealed decreased numbers of receptors without changes in affinity. The maximum number of binding sites decreased from 349 +/- 35 fmol/mg to 146 +/- 16 fmol/mg (P less than 0.05 vs. control). PI metabolism was similarly attenuated in aortae from septic rats. The norepinephrine-stimulated hydrolysis of [32P]-phosphatidylinositol-4,5-bisphosphate was significantly decreased in aortae from septic rats as was the alpha 1-adrenoceptor stimulated accumulation of [3H]-inositol monophosphate. Finally, the basal labeling of [32P]-phosphatidylinositol-4,5-bisphosphate but not of [32P]-phosphatidylinositol or [32P]-phosphatidic acid was significantly diminished. These results imply that signal transduction induced by alpha 1-adrenoceptor agonists in rat aorta is significantly altered in intraperitoneal sepsis. These findings may help define the mechanisms of depressed aortic contractility in models of sepsis and endotoxic shock.     

Opioid-noradrenergic interactions in the neurohypophysis. II. Does noradrenaline mediate the actions of endogenous opioids on oxytocin and vasopressin release?

The function of noradrenaline in the rat neurohypophysis was investigated by examining the effects of selective adrenergic receptor agents on electrically evoked release of oxytocin, arginine vasopressin, and noradrenaline using the [3H]-noradrenaline technique. Since endogenous opioids in the neurohypophysis suppress release of both neurohormones and of noradrenaline, we assessed the role of noradrenaline in mediating opioid actions on neurohormone secretion by examining modification of the action of the opioid antagonist naloxone by adrenergic receptor agents. The data suggest (1) that in addition to opioid receptors, 'presynaptic' alpha 2-receptors regulate release from neurohypophysial noradrenaline terminals; (2) noradrenaline released from neurohypophysial terminals acts on beta- and alpha 1-receptors to facilitate both oxytocin and arginine vasopressin release: this action only becoming evident at elevated levels of endogenous noradrenaline release attained following removal of presynaptic opioid or alpha 2-regulation, and (3) opioid peptides within the neurohypophysis act to inhibit oxytocin and, to a lesser extent, arginine vasopressin secretion, partly through inhibiting release of facilitatory noradrenaline. We propose a model in which opioids act in the neurohypophysis both independently of noradrenaline via kappa-receptors on neurosecretory terminals or pituicytes and also via interaction with the noradrenaline system.     

Mapping of adrenergic receptors in the trachea by autoradiography

We investigated the distribution of adrenergic receptors in ferret trachea using autoradiography. [3H]Dihydroalprenolol, used to identify beta-adrenoceptors, revealed a high density of specific binding sites over surface epithelium and submucosal glands, with less labelling of smooth muscle. [3H]prazosin labelling showed that alpha 1-receptors were numerous in glands and epithelium, but sparse in smooth muscle. Comparison of adrenergic receptor densities in tracheal sections from the same animals showed a rank order for submucosal glands of alpha 1 greater than beta, for epithelium beta greater than alpha 1 and for smooth muscle beta greater than alpha 1. Within the submucosal glands, alpha 1- and beta-adrenergic receptors were differentially distributed, with alpha 1-receptors being significantly more numerous over serous than mucous cells and beta receptors being significantly more numerous over mucous than serous cells. This technique provides insight into adrenergic regulation of airway function and should be useful in investigations of how relative receptor densities may be altered in disease.     

Down-regulation of vascular alpha1-adrenoceptors does not account for the loss of vascular responsiveness to catecholamines in experimental cholestasis.

AIMS/BACKGROUND: Vascular hyporesponsiveness to sympathomimetic stimulation occurs in jaundice. Recently, we reported that this vascular adrenergic hyporesponsiveness was associated with the loss of reactivity of vascular alpha1-adrenoceptors. This study examines the possibility that the vascular adrenergic hyporesponsiveness is due to down-regulation of vascular alpha1-adrenoceptors. METHODS: This question was addressed by measuring the changes in the plasma norepinephrine (NE) and epinephrine (E) concentrations, determined by high performance liquid chromatography, and the affinity and number of alpha1-adrenoceptors determined by a competitive antagonist radioligand binding assay in vascular smooth muscle membranes prepared from 3-day bile duct ligated (BDL) rats. The results were compared to data obtained from 3-day bile duct manipulated (sham-operated; SO) and control (C) rats. RESULTS: Compared to C and SO rats, the plasma concentrations of NE and E in the BDL rats were significantly elevated reflecting increased sympathetic nervous system activity. BDL did not change either the affinity or the number of vascular alpha1-adrenoceptors. CONCLUSIONS: Since the affinity and number of vascular alpha1-adrenoceptors were unchanged in the face of elevated plasma concentrations of catecholamines in the BDL rats, we have concluded that down-regulation of vascular alpha1-adrenoceptors does not account for the vascular adrenergic hyporesponsiveness in experimental cholestasis.     

Presynaptic alpha 2-adrenoceptor mediated regulation of norepinephrine release in perfused mesenteric vasculatures in young and adult spontaneously hypertensive rats

The present study was designed to evaluate the role of the presynaptic alpha 2-adrenoceptor in the pathogenesis of hypertension. Norepinephrine overflow during sympathetic nerve stimulation and its changes by presynaptic alpha 2-adrenoceptor inhibition were examined in the perfused mesenteric vasculatures of young and adult spontaneously hypertensive rats (SHR) compared with age-matched Wistar Kyoto rats (WKY). Electrical sympathetic nerve stimulation caused significantly greater overflow of endogenous norepinephrine from the adrenergic nerve terminals in young SHR than in age-matched WKY. Yohimbine, an alpha 2-adrenoceptor blocking agent, facilitated norepinephrine overflow from the adrenergic nerve terminals. The effects of yohimbine on norepinephrine overflow and pressor responses to electrical nerve stimulation were less in young SHR than in age-matched WKY. Norepinephrine overflow in adult SHR was similar to that in adult WKY, and differences in the effect of yohimbine on norepinephrine overflow between SHR and WKY were not marked at this chronic stage. These results suggest that enhanced norepinephrine overflow in the mesenteric vasculatures can be observed only in young SHR; this may be due in part to an impaired negative feed-back mechanism on the nerve terminals by presynaptic alpha 2-adrenoceptors.     

Role of endocytosis in the activation of the extracellular signal-regulated kinase cascade by sequestering and nonsequestering G protein-coupled receptors

Acting through a number of distinct pathways, many G protein-coupled receptors (GPCRs) activate the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) cascade. Recently, it has been shown that in some cases, clathrin-mediated endocytosis is required for GPCR activation of the ERK/MAPK cascade, whereas in others it is not. Accordingly, we compared ERK activation mediated by a GPCR that does not undergo agonist-stimulated endocytosis, the alpha(2A) adrenergic receptor (alpha(2A) AR), with ERK activation mediated by the beta(2) adrenergic receptor (beta(2) AR), which is endocytosed. Surprisingly, we found that in COS-7 cells, ERK activation by the alpha(2A) AR, like that mediated by both the beta(2) AR and the epidermal growth factor receptor (EGFR), is sensitive to mechanistically distinct inhibitors of clathrin-mediated endocytosis, including monodansylcadaverine, a mutant dynamin I, and a mutant beta-arrestin 1. Moreover, we determined that, as has been shown for many other GPCRs, both alpha(2A) and beta(2) AR-mediated ERK activation involves transactivation of the EGFR. Using confocal immunofluorescence microscopy, we found that stimulation of the beta(2) AR, the alpha(2A) AR, or the EGFR each results in internalization of a green fluorescent protein-tagged EGFR. Although beta(2) AR stimulation leads to redistribution of both the beta(2) AR and EGFR, activation of the alpha(2A) AR leads to redistribution of the EGFR but the alpha(2A) AR remains on the plasma membrane. These findings separate GPCR endocytosis from the requirement for clathrin-mediated endocytosis in EGFR transactivation-mediated ERK activation and suggest that it is the receptor tyrosine kinase or another downstream effector that must engage the endocytic machinery.     

Alpha- and beta-adrenergic receptor activity in circulating blood cells of patients with phaeochromocytoma: effects of adrenalectomy

In many tissues adrenergic responsiveness may be impaired by chronic exposure to adrenergic agonists. We examined the effects of high levels of circulating norepinephrine and epinephrine to alpha 2- and beta 2-adrenoceptors on platelets and mononuclear leucocytes (MNL) of patients with phaeochromocytoma. When compared with controls the density of beta 2-receptors prior to removal of the tumour was diminished by 67% (500 +/- 160 sites/cell, n = 7 vs 1500 +/- 350 sites/cell, n = 29; P less than 0.001). Binding affinities for [3H]dihydroalprenolol (apparent KD = 0.45 +/- 0.16 nmol/l in phaeochromocytoma, 0.52 +/- 0.19 nmol/l in controls) were not significantly different in MNL preparations from the two groups. MNL adenylate cyclase activities in response to 10 mumol/l (-) isoproterenol was lower in the patients (7.5 vs 22 pmol cAMP/10(6) MNL/10 min; P less than 0.001). Conversely, the number of alpha 2-adrenoceptors on platelets and the affinity of these receptors for [3H]yohimbine did not differ between patients and controls. Ensuing adrenalectomy the regeneration of the beta-adrenergic cAMP-system results from a two-phase process. A prompt but moderate restoration was followed by a gradual rise to the normal range within about 1 to 2 months indicating the dynamic character of the beta 2-adrenoceptor cAMP-system in MNL. We conclude that the downregulation observed in MNL, which appears not to be operative in human platelets, represents only one mechanism of feedback control in the sympatho-adrenal-system protecting tissues from prolonged stimulation by excessive catecholamines.     

Subtype-selective down-regulation of rat renal cortical alpha- and beta-adrenergic receptors by catecholamines

In the current studies, we have explored agonist-mediated down-regulation of adrenergic receptors in vivo. We infused catecholamines from sc implanted osmotic minipumps and examined the effects of the resultant increases in circulating levels of catecholamines on rat renal cortical alpha- and beta-adrenergic receptor subtypes, as assessed in radioligand binding studies. Infusion of epinephrine or norepinephrine (at 150 micrograms/kg X h) elevated plasma levels of each catecholamine 10- to 20-fold and decreased renal cortical alpha 1-receptor number about 50% without changing alpha 2-receptor number. Isoproterenol infusion (150 micrograms/kg X h) raised plasma levels of this catecholamine, but had no effect on the number of either alpha 1- or alpha 2-receptors. Renal cortical beta-adrenergic receptor number was decreased by infusion of all three catecholamines. However, the beta 1- and beta 2-adrenergic receptors were altered selectively by the different agonists. Infusion of norepinephrine decreased both beta 1- and beta 2-receptor number, but was more effective for the beta 1-receptors; this result was somewhat at variance with that we previously reported for rats bearing transplanted pheochromocytomas. The decrease in beta-receptor number due to epinephrine infusion was largely due to loss of the renal cortical beta 2-receptors. Infusion of isoproterenol decreased the number of both beta 1- and beta 2-receptors (69% and 75%, respectively). Infusion of norepinephrine maximally decreased the number of alpha 1-, beta 1-, and beta 2-receptors within 2 days, and the t 1/2 for receptor loss was about 12 h. beta-Receptors lost in response to isoproterenol infusion could not be recovered in a pellet prepared by high speed centrifugation of the supernatant derived from the preparation of renal cortical membranes. These results indicate that adrenergic receptor subtypes are differentially down-regulated by elevated levels of circulating catecholamines and that this differential loss of receptors depends on the nature of the receptor subtype, the agonist, and perhaps also whether catecholamines are infused rather than increased by pheochromocytoma.     

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

This is a brief overview of experimental and clinical studies exploring the hemodynamic functions of the alpha2A and alpha2B adrenergic receptor (AR) subtypes in animals submitted to genetic manipulations or gene treatment, as well as the clinical effects of central sympathetic suppression with the alpha2-AR agonist clonidine in patients with ischemic heart disease and/or heart failure. The animal experiments have led us to conclude that the sympathetic outflow is regulated by activation of the presynaptic alpha2A-AR subtype, which is the predominant alpha2-AR subtype in the central nervous system and exerts a sympathoinhibitory (hypotensive) action; on the contrary, activation of the central alpha2B-AR elicits a sympathoexcitatory response (such as seen in salt-induced hypertension, which requires functionally intact alpha2B-AR). Since there are no selective pharmacologic agents yet capable of discriminating among alpha2-AR subtypes, clinical studies utilize clonidine, the central sympathetic suppressant effect of which has been used for 35 years to treat hypertension. In small clinical trials, clonidine was used successfully for treatment of acute or chronic heart failure, acute myocardial infarct or hypertensive cardiomyopathy with subclinical diastolic dysfunction. We speculate that future development of agents capable of selectively activating the alpha2A-AR or blocking the alpha2B-AR may further improve our capability to treat hypertension, ischemic heart disease and heart failure.     

Effect of selective blockade of catecholaminergic alpha and beta receptors on histamine-induced release of corticotropin and prolactin.

We investigated the role of adrenergic receptors in histamine (HA)-induced release of corticotropin (ACTH) and prolactin (PRL) in conscious male rats. Specific alpha- or beta-receptor antagonists were administered intracerebroventricularly in doses of 1 mmol at time -20 min, and HA (270 nmol), the H1 receptor agonist 2-thiazolylethylamine (2-TEA; 2,180 nmol) or the H2 receptor agonist 4-methylHA (4-MeHA; 790 nmol) were administered intracerebroventricularly at -15 min. The animals were decapitated at 0 min, and plasma was analyzed for ACTH and PRL. Administration of HA and the histaminergic agonists stimulated ACTH secretion equally, while only HA and the H2 receptor agonist stimulated PRL secretion. Pretreatment with the adrenergic receptor antagonists had no effect on the ACTH response to the histaminergic compounds. In contrast, the PRL response to HA or 4-MeHA was inhibited or prevented by the alpha-receptor antagonists phenoxybenzamine and phentolamine, the alpha1-receptor antagonist prazocin, the beta-receptor antagonist propranolol and the beta1-receptor antagonist atenolol, whereas the alpha2-receptor antagonist yohimbine or the beta2-receptor antagonist ICI-118-551 had no effect. The study indicates that histaminergic neurons interact with the catecholaminergic neuronal system in regulation of PRL secretion, and that this interaction is dependent upon activation of alpha1- and beta1-receptors. In contrast, histaminergic neurons stimulate ACTH secretion independently of adrenergic receptor activation.     

Critical role for the alpha-1B adrenergic receptor at the sympathetic neuroeffector junction

The alpha-1 adrenergic receptors (alpha(1)ARs) are critical in sympathetically mediated vasoconstriction. The specific role of each alpha(1)AR subtype in regulating vasoconstriction remains highly controversial. Limited pharmacological studies suggest that differential alpha(1)AR responses may be the result of differential activation of junctional versus extrajunctional receptors. We tested the hypothesis that the alpha(1B)AR subtype is critical in mediating sympathetic junctional neurotransmission. We measured in vivo integrated cardiovascular responses to a hypotensive stimulus (induced via transient bilateral carotid occlusion [TBCO]) in alpha(1B)AR knockout (KO) mice and their wild-type (WT) littermates. In WT mice, after dissection of the carotid arteries and denervation of aortic baroreceptor buffering nerves, TBCO produced significant pressor and positive inotropic effects. Both responses were markedly attenuated in alpha(1B)AR KO mice (change systolic blood pressure 46+/-8 versus 11+/-2 mm Hg; percentage change in the end-systolic pressure-volume relationship [ESPVR] 36+/-7% versus 12+/-2%; WT versus KO; P<0.003). In vitro alpha(1)AR mesenteric microvascular contractile responses to endogenous norepinephrine (NE; elicited by electrical field stimulation 10 Hz) was markedly depressed in alpha(1B)AR KO mice compared with WT (12.4+/-1.7% versus 21.5+/-1.2%; P<0.001). In contrast, responses to exogenous NE were similar in alpha(1B)AR KO and WT mice (22.4+/-7.3% versus 33.4+/-4.3%; NS). Collectively, these results demonstrate a critical role for the alpha(1B)AR in baroreceptor-mediated adrenergic signaling at the vascular neuroeffector junction. Moreover, alpha(1B)ARs modulate inotropic responses to baroreceptor activation. The critical role for alpha(1B)AR in neuroeffector regulation of vascular tone and myocardial contractility has profound clinical implications for designing therapies for orthostatic intolerance.