Adrenergic receptors. Models for regulation of signal transduction processes

Role of adrenergic receptors in shock

Shock is a severe, life-threatening medical condition with a high mortality rate worldwide. All four major categories of shock (along with their various subtypes)-hypovolemic, distributive, cardiogenic, and obstructive, involve a dramatic mismatch between oxygen supply and demand, and share standard features of decreased cardiac output, reduced blood pressure, and overall hypoperfusion. Immediate and appropriate intervention is required regardless of shock type, as a delay can result in cellular dysfunction, irreversible multiple organ failure, and death. Studies have shown that dysfunction and downregulation of adrenergic receptors (ARs) are often implicated in these shock conditions; for example, their density is shown to be decreased in hypovolemic and cardiogenic shock, while their reduced signaling in the brain and vasculature decrease blood perfusion and oxygen supply. There are two main categories of ARs, α, and β, each with its subtypes and distributions. Our group has demonstrated that a dose of .02 mg/kg body wt of centhaquine (CQ) specifically activates α2B ARs on venous circulation along with the central α2A ARs after hypovolemic/hemorrhagic shock. Activating these receptors by CQ increases cardiac output (CO) and reduces systemic vascular resistance (SVR), with a net increase in blood pressure and tissue perfusion. The clinical trials of CQ conducted by Pharmazz Inc. in India have demonstrated significantly improved survival in shock patients. CQ improved blood pressure and shock index, indicating better blood circulation, and reduced lactate levels in the blood compared to in-use standard resuscitative agents. After successful clinical trials, CQ is being marketed as a drug (Lyfaquin^®) for hypovolemic/hemorrhagic shock in India, and United States FDA has approved the phase III IND application. It is anticipated that the phase III trial in the United States will begin in 2023. Thus, we have demonstrated that α2 ARs could be suitable targets for treating or managing hypovolemic/hemorrhagic shock. Further understanding of ARs in shock would help find new potential pharmacological targets.

Antagonism of α1-adrenoceptors by β3-adrenergic agonists: Structure-function relations of different agonists in prostate smooth muscle contraction

Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are poorly characterized, although mirabegron is used for treatment of lower urinary tract symptoms. Off-target effects of several β₃-adrenergic agonists include antagonism of α₁-adrenoceptors. Proposed, but unconfirmed explanations include phenylethanolamine backbones, found in some β₃-adrenergic agonists and imparting interaction with catecholamine binding pockets of adrenoceptors. Here, we examined effects of β₃-adrenergic agonists on contractions of human prostate tissues, including ZD7114 (without phenylethanolamine moiety), ZD2079 (phenylethanolamine backbone), BRL37344 and CL316243 (chloride-substituted phenylethanolamine deriatives). Prostate tissues were obtained from radical prostatectomy. Contractions by α₁-adrenergic agonists and electric field stimulation (EFS) were studied in an organ bath. ZD7114 (10 µM) right-shifted concentration responses curves for α₁-adrenergic agonists, resulting in increased EC₅₀ values for phenylephrine, methoxamine and noradrenaline up to one magnitude, without affecting E_max values. ZD7114 (10 µM) inhibited EFS-induced contractions, resulting in reduced E_max values. All effects of ZD7114 were resistant to the β₃-adrenergic antagonist L-748337, including increases in EC₅₀ values for α₁-adrenergic agonists, up to more than two magnitudes. Using 10 µM, neither ZD2079, BRL37344 or CL316243 affected α₁-adrenergic or EFS-induced contractions. At escalated concentrations, BRL37344 (200 µM) right-shifted concentration response curves for phenylephrine, increased EC₅₀ values for phenylephrine, and inhibited EFS-induced contractions, while CL316243 (300 µM) did not affect phenylephrine- or EFS-induced contractions. In conclusion, phenylethanolamine backbones are not decisive to impart α₁-adrenoceptor antagonism to β₃-agonists. Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are limited to off-target effects, including α₁-adrenoceptor antagonism by ZD7114 and BRL37344.

Differential α2A- and α2C-adrenoceptor protein expression in presynaptic and postsynaptic density fractions of postmortem human prefrontal cortex

BACKGROUND

Three different α₂-adrenoceptor (α₂-AR) subtypes have been described. The α_2A-AR and α_2C-AR subtypes are highly expressed in the human prefrontal cortex, where they modulate neurotransmission. However, due to the lack of subtype-selective ligands, the physiological relevance of both subtypes has not been fully resolved.

AIMS

In this context, the aim of the present study was to characterize the protein expression of both α₂-AR subtypes, in different synaptic fractions of postmortem human prefrontal cortex.

METHODS

A subcellular fractionation of the samples was performed and the protein expression of α_2A- and α_2C-ARs was measured in presynaptic membranes and postsynaptic density fractions by Western blot.

RESULTS

The results revealed that the α_2A-AR subtype is mainly located postsynaptically (95±3%) whereas the remaining 5±3% is in the presynapse. Conversely, the α_2C-AR subtype showed a similar distribution between pre- and postsynaptic membranes, with a slightly higher percentage present in the presynapse (60±2% vs. 40±2%).

CONCLUSIONS

These findings could explain some contradictory effects reported for α₂-AR agonists and antagonists in the human prefrontal cortex. Furthermore, the present data could contribute to elucidating the therapeutic potential of selectively targeting α_2A- or α_2C-AR subtypes.

THE ROLE OF β-ADRENERGIC RECEPTORS IN HEART FAILURE: DIFFERENTIAL REGULATION OF CARDIOTOXICITY AND CARDIOPROTECTION

β-adrenergic receptor blockers have demonstrated significant survival benefit and have become standard therapy for adults with dilated cardiomyopathy, although their efficacy in pediatric patients is still unproven. Recent data suggests that the two major cardiac β-adrenergic receptor subtypes (β1 and β2) couple differentially to intracellular signaling pathways regulating contractility and remodeling. This has led some to suggest that the β1 receptor is the "cardiotoxic subtype" whereas the β2 receptor is "cardioprotective." Given this paradigm, there could be situations where subtype selective β-blockade or even subtype selective β-stimulation might be beneficial. However, since most of these studies have been performed in isolated cardiomyocytes, their application to clinical practice is unclear. To better understand the roles of β1- vs. β2-receptors in the pathogenesis of clinical cardiomyopathy, we and others have taken advantage of several well-characterized murine models of cardiovascular disease. These studies demonstrate that β-receptor regulation of the balance between cardioprotection and cardiotoxicity is even more complex than previously appreciated: the role of each β-receptor subtype may vary depending on the specific cardiac stressor involved (e.g. ischemia, pressure overload, genetic mutation, cardiotoxin). Furthermore, the remodeling effects of β-receptor signaling have a temporal component, depending on whether a cardiac stress is acute vs. chronic.

Exercício físico, receptores β-adrenérgicos e resposta vascular

O exercício aeróbio promove efeitos benéficos na prevenção e tratamento de doenças como hipertensão arterial, aterosclerose, insuficiência venosa e doença arterial periférica. Os receptores β-adrenérgicos estão presentes em várias células. No sistema cardiovascular, promovem inotropismo e cronotropismo positivo cardíaco e relaxamento vascular. Embora os efeitos do exercício tenham sido investigados em receptores cardíacos, estudos focados nos vasos são escassos e controversos. Esta revisão abordará os efeitos do exercício físico sobre os receptores β-adrenérgicos vasculares em modelos animais e humanos e os mecanismos celulares envolvidos na resposta relaxante. Em geral, os estudos mostram resultantes conflitantes, onde observam diminuição, aumento ou nenhum efeito do exercício físico sobre a resposta relaxante. Assim, os efeitos do exercício na sensibilidade β-adrenérgica vascular merecem maior atenção, e os resultados mostram que a área de fisiopatologia vascular é um campo aberto para a descoberta de novos compostos e avanços na prática clínica.

The cAMP assay: a functional in vitro alternative to the in vivo Histamine Sensitization test

Safety requirements stipulate the performance of the in vivo Histamine Sensitization (HS) test for quality control of acellular pertussis (aP) vaccines. For reasons of reproducibility and animal welfare concern, an in vitro assay was developed. The assay reflects the mechanism of histamine sensitization and is based on cAMP production in A10 cells to residual pertussis toxin (PT). We showed that PT induces cAMP levels in a dose-dependent manner while the sensitivity of the assay equals the sensitivity of the HS test. Neither the individual components nor the combination vaccine DTaP-IP did affect the assay. The cAMP assay meets the criteria for specificity and sensitivity and therefore might be a promising candidate to replace the HS test.

Cellular and subcellular localization of alpha-1 adrenoceptors in the rat visual cortex

Noradrenaline is thought to play modulatory roles in a number of physiological, behavioral, and cellular processes. Although many of these modulatory effects are mediated through alpha-1 adrenoceptors, basic knowledge of the cellular and subcellular distributions of these receptors is limited. We investigated the laminar distribution pattern of alpha-1 adrenoceptors in rat visual cortex, using immunohistochemistry at both light and electron microscopic levels. Affinity-purified anti-alpha-1 antibody was confirmed to react only with a single band of about 70-80 kDa in total proteins prepared from rat visual cortex. Alpha-1 adrenoceptors were widely distributed though all cortical layers, but relatively high in density in layers I, II/III, and V. Immunoreactivity was observed in both neuronal perikarya and processes including apical dendrites. In double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-glial fibrillary acidic protein, anti-glutamic acid decarboxylase 65/67, anti-2-3-cyclic nucleotide 3-phosphodiesterase, and anti-tyrosine hydroxylase antibodies, alpha-1 adrenoceptors were found mainly in dendrites and somata of microtubule-associated protein 2-immunopositive neurons. About 20% of alpha-1 adrenoceptors were in GABAergic neurons. A small number of alpha-1 adrenoceptors were also distributed in axons of excitatory neurons, astrocytes, oligodendrocytes and noradrenergic fibers. Using an immunoelectron microscopic technique, numerous regions of alpha-1 adrenoceptor immunoreactivity were found in cell somata, on membranes of dendrites, and in postsynaptic regions. Moreover, a small number of immunoreaction products were also detected in axons and presynaptic sites. These findings provide the first quantitative evidence regarding the cellular and subcellular localization of alpha-1 adrenoceptor immunoreactivity in visual cortex. Moreover, the ultrastructural distribution of alpha-1 adrenoceptor immunoreactivity suggests that alpha-1 adrenoceptors are transported mainly into dendrites and that they exert effects at postsynaptic sites of neurons.

Comparative anatomy of α2 and β adrenoceptors in the adult and developing brain of the marine teleost the red porgy (Pagrus pagrus, Sparidae): [3H]clonidine and [3H]dihydroalprenolol quantitative autoradiography and receptor subtypes immunohistochemistry

The present study aimed to determine the anatomic distribution and developmental profile of alpha(2) and beta adrenoceptors (AR) in marine teleost brain. Alpha 2 and beta adrenoceptors were studied at different developmental stages by using [(3)H]clonidine and [(3)H]dihydroalprenolol, respectively, by means of in vitro quantitative autoradiography. Furthermore, immunohistochemical localization of the receptor subtypes was performed to determine their cellular distribution. Saturation studies determined a high-affinity component of [(3)H]clonidine and [(3)H]dihydroalprenolol binding sites. High levels of both receptors were found in preglomerular complex, ventral hypothalamus, and lateral torus. Dorsal hypothalamus and isthmus included high levels of alpha(2) AR, whereas pretectum and molecular and proliferative zone of cerebellum were specifically characterized by high densities of beta AR. From the first year of life, adult levels of both AR were found in most medial telencephalic, hypothalamic, and posterior tegmental areas. Decreases in both receptors densities with age were prominent in ventral and posterior telencephalic, pretectal, ventral thalamic, hypothalamic, and tegmental brain regions. Immunohistochemical data were well correlated with autoradiography and demonstrated the presence of alpha(2A), alpha(2C), beta(1), and beta(2) AR subtype-like immunoreactivity. Both the neuronal (perikaryal or dendritic) and the glial localization of receptors was revealed. The localization and age-dependent alterations in alpha(2) and beta AR were parallel to plasticity mechanisms, such as cell proliferation in periventricular thalamus, hypothalamus, and cerebellum. In addition, the biochemical characteristics, distribution pattern, and neuronal or glial specificity of the receptors in teleost brain support a similar profile of noradrenergic transmission in vertebrate brain evolution.

Central alpha2B-adrenergic receptor antisense in plasmid vector prolongs reversal of salt-dependent hypertension

OBJECTIVE

Previous studies have shown that a fully functional alpha(2B)-adrenergic receptor (AR) is necessary for the development of salt-induced hypertension. The current studies were designed to explore the effect of prolonged inhibition of central alpha(2B)-AR gene expression by antisense (AS) DNA on this hypertension.

METHODS

We developed a plasmid vector driven by a cytomegalovirus promoter, containing a green fluorescent protein reporter gene and AS for rat alpha(2B)-AR protein. Subtotally nephrectomized, salt-loaded hypertensive rats received intracerebroventricular injection of 500 microg of either the AS plasmid (n = 9) or sense plasmid (containing cDNA for alpha(2B)-AR), as control (n = 7).

RESULTS

The AS injection produced a fall in SBP from 201 +/- 4 to 171 +/- 5 mmHg within 12 h. The level of BP in the 3 days post-injection was 174 +/- 6, 181 +/- 4 and 184 +/- 6 mmHg on day 1, day 2 and day 3, respectively (P < 0.05), and returned gradually towards baseline in subsequent days, although it remained significantly lower for the 8 days of observation. The control sense plasmid injections produced no significant changes in blood pressure (BP). Neither group had histological evidence of neural tissue disruption.

CONCLUSIONS

These results indicate that protracted translational inhibition of the alpha(2B)-AR gene in the central nervous system can be obtained by AS DNA delivered via plasmid vector and lead to decreased generation of alpha(2B)-AR protein, which can partly reverse salt-induced hypertension for several days.

Cloning, pharmacological characterization, and polymorphism screening of the guinea pig beta(2)-adrenoceptor

In asthma, beta(2)-adrenoceptor agonist responsiveness has been associated with Arg16Gly and Gln27Glu polymorphisms of the beta(2)-adrenoceptor. Since the guinea pig is extensively used as an animal model for asthma, we investigated the occurrence of possible polymorphism of the guinea pig beta(2)-adrenoceptor. The guinea pig beta(2)-adrenoceptor coding region was amplified by sequence homology-based cloning. Homology of the translated protein with the human beta(2)-adrenoceptor was 88% with Ala at position 16 and Glu at position 27. Radioligand binding and cAMP- accumulation experiments of Chinese hamster ovary (CHO) cells transfected with the guinea pig beta(2)-adrenoceptor revealed a homogeneous population of functional receptors. Five degenerate single nucleotide polymorphisms were found within the beta(2)-adrenoceptor coding region of outbred Dunkin Hartley guinea pigs, at residues 354, 453, 483, 534 and 642. In conclusion, we have cloned the guinea pig beta(2)-adrenoceptor, which shows to be functional upon expression in a recombinant system and contains five single nucleotide polymorphisms dissimilar to human polymorphisms.

Effect of mild hypothermia on inodilator-induced vasodilation of pial arterioles in cats

BACKGROUND

Mild hypothermia has been proposed as a means of providing cerebral protection after traumatic brain injury. However, hypothermia has been shown to alter not only physiologic but also pharmacologic responses. The purpose of this study was to investigate whether mild hypothermia (3-4 degrees C temperature reduction) could alter cerebral vasodilation induced by inodilators, which are characterized by having an inotropic effect in addition to a vasodilatory effect. Isoproterenol (a beta-adrenergic receptor agonist), colforsin dapropate (an adenylate cyclase stimulant), and amrinone (a phosphodiesterase inhibitor) were chosen as inodilators.

METHODS

The cranial window technique, combined with microscopic video recording, was used. Forty-eight cats were randomly assigned to either a normothermic or a hypothermic group (33 degrees C). Isoproterenol, colforsin dapropate, or amrinone was topically applied in the cranial window and the diameter of pial arterioles was measured.

RESULTS

Topical administration of isoproterenol, colforsin dapropate, and amrinone produced a significant dilation in a dose-dependent manner during normothermia. The vasodilation induced by these inodilators was not affected by mild hypothermia.

CONCLUSION

The vasodilation induced by topical administration of isoproterenol, colforsin dapropate, and amrinone was not affected by mild hypothermia.

Epinephrine inhibits invasion of oral squamous carcinoma cells by modulating intracellular cAMP

In oral and maxillofacial surgery, epinephrine is routinely used for cancer resection and it is important to clarify the effects of this agent on cancer. We found here that the clinically relevant concentrations of epinephrine (10, 50 and 100 microg/ml) decreased the invasion ability of oral squamous carcinoma (Sa3) cells. In the Sa3 cells treated with epinephrine (10, 50 and 100 microg/ml), migration, morphological changes and formation of actin stress fibers were inhibited and intracellular cyclic adenosine monophosphate (cAMP) increased significantly. These findings suggest that epinephrine inhibits the invasion of cancer cells by modulating intracellular cAMP and that clinicians could use epinephrine effectively for the surgical resection of the cancer.

Do most antihypertensive agents have a sympatholytic action?

The sympathetic nervous system is a major modulator of cardiovascular functions. Over the past three decades, numerous studies, using various methodologies, have reported the existence of a variety of pre- and postsynaptic sympathetic dysfunctions in essential hypertension. Most of those abnormalities facilitate sympathetic neurotransmission, resulting in a chronic increase in the sympathetic tone and reactivity in an important proportion of hypertensive patients. The chronic sympathetic activation is also associated with major alterations in the balance between postsynaptic adrenergic receptors in cardiovascular tissues. Indeed, an attenuation of beta-adrenergic functions and a potentiation of alpha1-adrenergic functions have been demonstrated in human cardiovascular tissues, suggesting the development of a sympathetic postsynaptic alpha1 dominance during the development and evolution of hypertension. The chronic activation of the sympathetic system is deleterious and could contribute to the development of most cardiovascular complications associated with hypertension. One of the major aims of antihypertensive therapy should be to attenuate pre- or postsynaptic sympathetic tone. Most antihypertensive drugs have been found to improve either pre- or postsynaptic sympathetic functions in hypertensive patients. At the presynaptic level, the effects of antihypertensive drugs have been found to be more variable. At the postsynaptic level, all currently used antihypertensive drugs have been found to attenuate alpha1-adrenergic functions either by interfering directly with intracellular mechanisms underlying alpha1-adrenergic functions or indirectly by decreasing the release of norepinephrine from peripheral sympathetic nerves.

Chronic hypoxia stimulates periarterial sympathetic nerve development in chicken embryo

BACKGROUND

Epidemiological findings suggest an association between low-for-age birth weight and the risk to develop coronary heart diseases in adulthood. During pregnancy, an imbalance between fetal demands and supply may result in permanent alterations of neuroendocrine development in the fetus. We evaluated whether chronic prenatal hypoxia increases arterial sympathetic innervation.

METHODS AND RESULTS

Chicken embryos were maintained from 0.3 to 0.9 of the 21-day incubation period under normoxic (21% O(2)) or hypoxic conditions (15% O(2)). At 0.9 incubation, the degree of sympathetic innervation of the embryonic femoral artery was determined by biochemical, histological, and functional (in vitro contractile reactivity) techniques. Chronic hypoxia increased embryonic mortality (32% versus 13%), reduced body weight (21.9+/-0.4 versus 25.4+/-0.6 g), increased femoral artery norepinephrine (NE) content (78.4+/-9.4 versus 57.5+/-5.0 pg/mm vessel length), and increased the density of periarterial sympathetic nerve fibers (14.4+/-0.7 versus 12.5+/-0.6 counts/10(4) microm(2)). Arteries from hypoxic embryos were less sensitive to NE (pD(2), 5.99+/-0.04 versus 6. 21+/-0.10). In the presence of cocaine, however, differences in sensitivity were no longer present. In the embryonic heart, NE content (156.9+/-11.0 versus 108.1+/-14.7 pg/mg wet wt) was also increased after chronic hypoxia.

CONCLUSIONS

In the chicken embryo, chronic moderate hypoxia leads to sympathetic hyperinnervation of the arterial system. In humans, an analogous mechanism may increase the risk for cardiovascular disease in adult life.

Modification of beta-adrenoceptor signal transduction pathway by genetic manipulation and heart failure

The beta-adrenoceptor (beta-AR) mediated signal transduction pathway in cardiomyocytes is known to involve beta1- and beta2-ARs, stimulatory (Gs) and inhibitory (Gi) guanine nucleotide binding proteins, adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). The activation of beta1- and beta2-ARs has been shown to increase heart function by increasing Ca2+ -movements across the sarcolemmal membrane and sarcoplasmic reticulum through the stimulation of Gs-proteins, activation of AC and PKA enzymes and phosphorylation of the target sites. The activation of PKA has also been reported to increase phosphorylation of some myofibrillar proteins (for promoting cardiac relaxation) and nuclear proteins (for cardiac hypertrophy). The activation of beta2-AR has also been shown to affect Gi-proteins, stimulate mitogen activated protein kinase and increase protein synthesis by enhancing gene expression. Beta1- and beta2-ARs as well as AC are considered to be regulated by PKA- and protein kinase C (PKC)-mediated phosphorylations directly; both PKA and PKC also regulate beta-AR indirectly through the involvement of beta-AR kinase (betaARK), beta-arrestins and Gbeta gamma-protein subunits. Genetic manipulation of different components and regulators of beta-AR signal transduction pathway by employing transgenic and knockout mouse models has provided insight into their functional and regulatory characteristics in cardiomyocytes. The genetic studies have also helped in understanding the pathophysiological role of PARK in heart dysfunction and therapeutic role of betaARK inhibitors in the treatment of heart failure. Varying degrees of defects in the beta-AR signal transduction system have been identified in different types of heart failure to explain the attenuated response of the failing heart to sympathetic stimulation or catecholamine infusion. A decrease in beta1-AR density, an increase in the level of G1-proteins and overexpression of betaARK are usually associated with heart failure; however, these attenuations have been shown to be dependent upon the type and stage of heart failure as well as region of the heart. Both local and circulating renin-angiotensin systems, sympathetic nervous system and endothelial cell function appears to regulate the status of beta-AR signal transduction pathway in the failing heart. Thus different components and regulators of the beta-AR signal transduction pathway appears to represent important targets for the development of therapeutic interventions for the treatment of heart failure.

Noradrenaline-induced pigment aggregating response of melanophores in normal, denervated and reinnervated cichlid skin

The present study examines noradrenaline (NA) effects on melanophore pigment aggregation in normal, denervated and reinnervated teleost skin in vitro. Many axons were present in the melanophore-containing layer of normal skin. One week after a nerve crush lesion the skin was devoid of axons. By 1 month the skin was partly reinnervated. One day after nerve crush NA-sensitivity was markedly increased compared to controls. Sensitivity then approached normality but it remained elevated for at least one month. We conclude that melanophore supersensitivity develops very rapidly upon denervation and then gradually fades away during reinnervation.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Structure-function studies of the eighth hydrophobic domain of a serotonin receptor

The most prominent structural feature of the G protein-coupled receptor superfamily is their seven hydrophobic domains, which are postulated to form membrane-spanning alpha helices. Some members of the G protein-coupled receptor family, specifically several serotonin (5-HT) receptors, possess eight hydrophobic domains. The importance of this extra hydrophobic domain, located at the N terminus of the receptor, is unknown. This question was addressed by deleting the extra hydrophobic region from the 5-HT2C receptor and comparing its function and topology with those of the wild-type receptor. Immunofluorescence microscopy was used to determine the location of the N terminus of the epitope-tagged wild-type and mutant receptors. The N terminus of both receptors was extracellular, suggesting that the extra hydrophobic domain does not change the topology of this receptor and is unlikely to be a membrane-spanning alpha helix. Radioligand-binding studies in transfected cells and expression studies in Xenopus oocytes demonstrated that seven hydrophobic domains were sufficient for normal function in these assays. Interestingly, the mutant receptor, now containing seven hydrophobic domains, is expressed at higher levels in transfected cells than the wild-type receptor containing eight hydrophobic domains, suggesting that the extra hydrophobic domain does impact the activity of this receptor by regulating its expression.

Identification of alpha1B adrenergic receptor protein in gonadotropin releasing hormone neurones of the female rat

Noradrenaline is an important neurotransmitter which regulates GnRH release from the median eminence in the female rat during both basal GnRH secretion and the preovulatory or steroid hormone-induced GnRH-mediated LH surge. However, it is not clear at which sites in the brain this predominantly stimulatory influence is exerted nor is it known which adrenergic receptor subtype(s) mediate(s) the effects of noradrenaline. In order to determine if the GnRH neurones in the septum-diagonal band-preoptic area and/or their axon terminals in the median eminence are direct targets for noradrenaline, immunohistochemical triple-labelling studies were conducted to localize simultaneously GnRH peptide, dopamine-beta-hydroxylase and alpha1B adrenergic receptor protein. The results show that about 80% of all GnRH neurones examined contained patches of immunoreactive alpha1B adrenergic receptor protein at or near the plasma membrane and that some of these alpha1B adrenergic receptors were adjacent to dopamine-beta-hydroxylase containing axons. The GnRH neurones which did not contain alpha1B adrenergic receptors were preferentially located in the rostral portion of the septum and diagonal band while all GnRH neurones in the caudal septum, diagonal band and in the preoptic area expressed alpha1B adrenergic receptors. In the median eminence, a few alpha1B adrenergic receptor patches were seen in the external layer and these receptors were only rarely observed to be associated with GnRH containing axon terminals. The results suggest that the effects of noradrenaline on GnRH release are, at least in part, mediated by the activation of alpha1B adrenergic receptors which are located on most GnRH perikarya while the median eminence is not a likely site at which GnRH release is regulated by alpha1B adrenergic receptors.

PCR-based methods for identifying genetic variations in human alpha1B- and beta2-adrenergic receptors

alpha- and beta-adrenergic receptors belong to the superfamily of G-protein-coupled, seven transmembrane domain receptors and regulate a variety of cellular processes. Previous studies have demonstrated that changes in the amino acid sequence can result in substantial changes in the function of the receptors and it has been suggested that there may be an association between different disease states and the altered structure of alpha- and beta-adrenergic receptors. Accordingly, we have developed a simple PCR method for the identification of polymorphisms in the coding sequences of the human beta2-adrenergic receptor and the alpha1B-adrenergic receptor. This method may be useful for screening individual patients or at-risk populations for endocrine-metabolic disorders, as well as for asthma, cardiovascular disorders, and neuropsychiatric diseases.

Neuropeptide Y shifts equilibrium between alpha- and beta-adrenergic tonus in proximal tubule cells

Renal sympathetic nerves play a central role in the regulation of tubular Na+ reabsorption. Norepinephrine (NE) and neuropeptide Y (NPY) are colocalized in renal sympathetic nerve endings. The purpose of this study is to examine the integrated effects of these neurotransmitters on the regulation of Na+-K+-ATPase, the enzyme responsible for active Na+ reabsorption in renal tubular cells. Studies were performed on proximal tubular segments, which express adrenergic alpha- and beta-receptors, as well as NPY-Y2 receptors. It was found that alpha- and beta-adrenergic agonists had opposing effects on Na+-K+-ATPase activity. beta-Adrenergic agonists induced a dose-dependent inhibition of the Na+-K+-ATPase activity, whereas alpha-adrenergic agonists stimulated the enzyme. NPY abolished beta-agonist-induced deactivation of Na+-K+-ATPase and enhanced alpha-agonist-induced activation of Na+-K+-ATPase. The beta-adrenergic agonist appeared to inhibit Na+-K+-ATPase activity via a cAMP pathway. NPY antagonized beta-agonist-induced accumulation of cAMP. In our preparation, NE alone had no net effect but stimulated the Na+-K+-ATPase activity in the presence of beta-adrenergic antagonists, as well as in the presence of NPY. The results indicate that, in renal tissue, NPY determines the net effect of its colocalized transmitter, NE, by its ability to attenuate the beta- and enhance the alpha-adrenergic effect.

Alpha 2-adrenergic receptor activation inhibits calcitonin gene-related peptide expression in cultured dorsal root ganglia neurons

Calcitonin gene-related peptide (CGRP), a potent vasodilator, is produced in dorsal root ganglia (DRG) neurons which extend nerves peripherally to blood vessels and centrally to the spinal cord. We previously reported that neuronal CGRP expression is significantly reduced in the spontaneously hypertensive rat (SHR) which could contribute to the elevated BP. Other studies suggest that the enhanced activity of the sympathetic nervous system in the SHR may mediate, at least in part, this reduction in neuronal CGRP expression via activation of alpha 2-adrenoreceptors (alpha 2-AR) on DRG neurons. To test this hypothesis in vitro we employed primary cultures of adult rat DRG neurons. Neuronal cultures were initially exposed (24 h) to either the alpha 2-AR agonist UK 14,304 (10(-6) M) or vehicle; however, no changes in CGRP mRNA content or immunoreactive CGRP (iCGRP) release were observed. Using the rationale that in vivo DRG neurons receive a continuous supply of target tissue derived nerve growth factor (NGF), which stimulates CGRP synthesis, the cultured neurons were treated (24 h) with either vehicle, NGF (25 ng/ml) alone, or NGF plus UK. NGF treatment increased CGRP mRNA accumulation 5.5 +/- 0.9-fold (p < 0.001) and iCGRP release 2.9 +/- 0.4-fold (p < 0.001) over control levels. The stimulatory effects of NGF were markedly attenuated, but not abolished, by UK (NGF + UK vs. control, CGRP mRNA, 2.9 +/- 0.4-fold, p < 0.05; iCGRP, 1.7 +/- 0.2-fold, p < 0.05). These values were also significant (p < 0.05) when compared to NGF treatment alone. Experiments performed using the alpha 2-antagonist yohimbine confirmed that the effects of UK were mediated by the alpha 2-AR. These results, therefore, demonstrate that alpha 2-AR activation attenuates the stimulatory effects of NGF on CGRP expression in DRG neurons.

A beta-arrestin/green fluorescent protein biosensor for detecting G protein-coupled receptor activation

G protein-coupled receptors (GPCR) represent the single most important drug targets for medical therapy, and information from genome sequencing and genomic data bases has substantially accelerated their discovery. The lack of a systematic approach either to identify the function of a new GPCR or to associate it with a cognate ligand has added to the growing number of orphan receptors. In this work we provide a novel approach to this problem using a beta-arrestin2/green fluorescent protein conjugate (betaarr2-GFP). It provides a real-time and single cell based assay to monitor GPCR activation and GPCR-G protein-coupled receptor kinase or GPCR-arrestin interactions. Confocal microscopy demonstrates the translocation of betaarr2-GFP to more than 15 different ligand-activated GPCRs. These data clearly support the common hypothesis that the beta-arrestin binding of an activated receptor is a convergent step of GPCR signaling, increase by 5-fold the number of GPCRs known to interact with beta-arrestins, demonstrate that the cytosol is the predominant reservoir of biologically active beta-arrestins, and provide the first direct demonstration of the critical importance of G protein-coupled receptor kinase phosphorylation to the biological regulation of beta-arrestin activity and GPCR signal transduction in living cells. The use of betaarr2-GFP as a biosensor to recognize the activation of pharmacologically distinct GPCRs should accelerate the identification of orphan receptors and permit the optical study of their signal transduction biology intractable to ordinary biochemical methods.

Decreased transcript expression coincident with impaired glycosylation in the beta2-adrenergic receptor gene does not result from differences in the primary sequence

Variants of the S49 mouse lymphoma cell line exhibit multiple lesions along the pathway of cyclic AMP generation in response to beta2-adrenergic stimulation. Two such variants, beta(p) and beta(d), are characterized by decreased receptor binding and mRNA expression, 50% and 25% of wild-type receptor expression, respectively. The rate of beta2-adrenergic receptor synthesis was measured and found to be decreased in the beta d cells vis-a-vis the rate in wild type cells. The molecular mass of the beta2-adrenergic receptor in the S49 wild-type, beta(p) and beta(d) variant cells was estimated by labeling the receptor with the photoaffinity probe [125I]iodocyanopindololdiazirine. Receptor size was found to be 67,000 and 47,000 Da in the wild-type and 60,000 and 42,000 in the two variant cells. This 6 kDa discrepancy in mass was abolished upon treatment of labeled cell extracts with N-glycosidase F, suggesting the possibility of either N-terminal truncation or altered glycosylation of the receptor in the variant cells. To distinguish between these possibilities, we sequenced the beta2-adrenergic receptor gene and two kilobases of the 5'-non-coding region. No differences were found in the coding region of the gene from wild-type, beta(p) and beta(d) S49 cells suggesting that both the diminished expression and the decreased size of beta2-adrenergic receptor in the beta(p) and beta(d) S49 variants are related to impaired glycosylation of the receptor. This hypothesis was substantiated by the reduced retention of the variant cells' beta2-adrenergic receptor on immobilized WGA. Furthermore, growth of the S49 cells in the presence of the alpha-mannosidase II inhibitor, swainsonine, preferentially impaired the ability of the receptors derived from the variant cells to bind to WGA. These results imply that altered expression and glycosylation of G-protein-linked receptors occur as a consequence of one or more mutations outside the receptor's open reading frame.

Inhibition of microglial superoxide anion production by isoproterenol and dexamethasone

Microglia, like other tissue macrophages, are a component of the hypothalamic-pituitary endocrine-immune axis and, as such, are responsive to both neural and endocrine factors. Using cultured neonatal hamster microglia, we have examined the effect of isoproterenol, a beta-adrenergic agonist, and dexamethasone, a synthetic glucocorticoid, on superoxide anion production. For these experiments, microglia were pretreated with isoproterenol or dexamethasone and then induced to produce superoxide anion by exposure of the cells to phorbol myristate acetate (PMA). Our study demonstrates that the PMA-stimulated production of superoxide anion was decreased by acute (30 min) and chronic (24 h) pretreatment of the microglia with isoproterenol and was blocked by the beta-adrenergic receptor antagonist, propranolol. Since a rise in intracellular cAMP may be a prime factor in the inhibition of superoxide anion production in isoproterenol-treated cells, we used forskolin, a known activator of the adenylate cyclase in place of isoproterenol and re-investigate superoxide anion production. Short term exposures to forskolin produced a lower amount of superoxide anion than PMA-stimulated alone and, thus, mimicked the effect of isoproterenol. However, treatment with the same concentration of forskolin for 24 h prior to the induction of the NADPH oxidase did not significantly change PMA-stimulated superoxide anion production from untreated values. Thus, chronic exposure to forskolin produced a different effect than chronic exposure to isoproterenol. Isoproterenol and forskolin both increased immunoreactivity for the protein products of the early response genes, c-fos and c-jun. Pretreatment with dexamethasone for 24 h also inhibited superoxide anion production and was blocked by the protein synthesis inhibitor, cycloheximide. The simultaneous addition of varying concentrations of dexamethasone and 5 microM isoproterenol did not produce a greater inhibition in superoxide anion production than either agent alone. The down-regulation of microglial function by adrenergic agonists and by glucocorticoids provides a way in which the cytotoxicity of these immune cells can be reduced and may be a factor in the paracrine regulation of microglia.

Myocardial adenylyl cyclase type V and VI mRNA: differential regulation with age

Myocardial beta-adrenergic signal transduction diminishes with age. This decrease is not due to a decrease in the number of beta-adrenoceptors, but may be a result of an impaired capacity to activate adenylyl cyclase (AC). Forskolin-stimulated AC activity is diminished, and the number of forskolin binding sites is decreased, suggesting that the decrease in signal transduction with age is a result of fewer AC catalytic units. To investigate whether the decrease in AC with age is associated with diminished AC mRNA, we assessed AC type V and type VI mRNA in ventricles from 6-, 11-, and 24-month-old F-344 rats. The predominant mRNA species, type V, increased by 45% between 6 and 11 months of age but decreased to just below the 6-month level by age 24 months. In contrast, type VI mRNA decreased by 44% between 6 and 11 months of age and then increased to the 6-month level at age 24 months. The changes in type V and type VI mRNA did not parallel the decreases in the AC activity or forskolin binding sites with senescence. These data indicate that the steady-state levels of type V and VI AC mRNA are not reliable predictors of the efficacy of forskolin stimulation of AC activity at different ages.

Functional domains of the C-terminus of the rat angiotensin AT1A receptor

Previous work has shown that truncating the carboxyl terminus (C-terminus) of the rat angiotensin AT1A receptor to 309 amino acids abolished G-protein coupling and receptor internalization. This suggests that domains responsible for these functions lie beyond amino acid 309 of the C-terminus. The objective of this study was to determine the effect on angiotensin AT1A receptor function and regulation of deleting 41 amino acids from the C-terminus, which include the putative protein kinase C phosphorylation sites. Using site directed mutagenesis, the codon for Tyr319 was converted to a stop codon and the resulting truncated receptor permanently expressed in cultured human kidney cells. The properties of the truncated receptor were compared to those of the full length receptor. Expression of the truncated receptor was confirmed by sodium dodecyl sulphate polyacrylamide gel electrophoresis analysis of photolabelled membrane preparations. Angiotensin II activation of both full length and truncated receptors resulted in mobilization of inositol phosphates. However, whereas this was associated with rapid internalization of the full length receptor, the truncated receptor failed to internalize. Furthermore, pretreatment of cells with phorbol 12-myristate 13-acetate, a direct activator of protein kinase C, markedly attenuated the full length, but no the truncated receptor's ability to mobilise inositol phosphates. Thus, we conclude that the domain between amino acids 309 & 318 is important for G-protein coupling; that amino acids beyond 318 regulate internalization and one or more of the putative protein kinase C phosphorylation sites, present in the C-terminus of the angiotensin At1A receptor, actively regulate the receptor.

Characterization of 5' end of human thromboxane receptor gene. Organizational analysis and mapping of protein kinase C--responsive elements regulating expression in platelets

Platelet thromboxane receptors are acutely and reversibly upregulated after acute myocardial infarction. To determine if platelet thromboxane receptors are under transcriptional control, we isolated and characterized human genomic DNA clones containing the 5' flanking region of the thromboxane receptor gene. The exon-intron structure of the 5' portion of the thromboxane receptor gene was determined initially by comparing the nucleotide sequence of the 5' flanking genomic clone with that of a novel human uterine thromboxane receptor cDNA that extended the mRNA 141 bp further upstream than the previously identified human placental cDNA. A major transcription initiation site was located in three human tissues approximately 560 bp upstream from the translation initiation codon and 380 bp upstream from any previously identified transcription initiation site. The thromboxane receptor gene has neither a TATA nor a CAAT consensus site. Promoter function of the 5' flanking region of the thromboxane receptor gene was evaluated by transfection of thromboxane receptor gene promoter/chloramphenicol acetyltransferase (CAT) chimera plasmids into platelet-like K562 cells. Thromboxane receptor promoter activity, as assessed by CAT expression, was relatively weak but was significantly enhanced by phorbol ester treatment. Functional analysis of 5' deletion constructs in transfected K562 cells and gel mobility shift localized the major phorbol ester-responsive motifs in the thromboxane receptor gene promoter to a cluster of activator protein-2 (AP-2) binding consensus sites located approximately 1.8 kb 5' from the transcription initiation site. These studies are the first to determine the structure and organization of the 5' end of the thromboxane receptor gene and demonstrate that thromboxane receptor gene expression can be regulated by activation of protein kinase C via induction of an AP-2-like nuclear factor binding to upstream promoter elements. These findings strongly suggest that the mechanism for previously described upregulation of platelet thromboxane receptors after acute myocardial infarction is increased thromboxane receptor gene transcription in platelet-progenitor cells.

alpha2-adrenergic agonist-induced inhibition of cyclic AMP formation in transfected cell lines using a microtiter-based scintillation proximity assay

Human embryonic kidney cells (HEK-7) were transfected with the gene for the human alpha2C receptor (alpha2C4). Cells were grown in 96-well microtiter plates and cyclic AMP levels were measured by scintillation proximity assay, a modified radioimmunoassay technique. Radioactivity was quantified using a TopCount Scintillation detector. Cyclic AMP was increased in a dose-dependent manner by the addition of exogenous forskolin. The forskolin-induced enhancement of cyclic AMP was inhibited dose-dependently by the addition of alpha2-adrenergic agonists, and this inhibition was blocked by the addition of adrenergic antagonists. The extent of the inhibitory response caused by alpha2-adrenoceptor agonists was related to the receptor density in clonal cell lines derived from the transfected parental HEK-7 cells. By using cells grown in microtiter format, and employing the technological advantages of scintillation proximity assay and TopCount detection, it was possible to simultaneously evaluate the effects of multiple experimental permutations on cellular production of cyclic AMP with minimal disturbance of the cells and minimal and/or automated manipulation of the cyclic AMP formed. This combination of techniques should allow rapid testing of the actions of adrenergic agonists and antagonists on cells transfected with receptors linked to cyclic AMP formation.

Influence of alpha 1-adrenoceptor antagonism of ketanserin on the nature of its 5-HT2 receptor antagonism

1. In order to investigate alpha 1-adrenoceptor interactions with 5-HT2 receptors, the effect of ketanserin (which antagonizes both types of the above receptors) on responses to 5-hydroxytryptamine (5-HT) and phenylephrine (PE) was measured in isolated perfused rat tail arteries. 2. Ketanserin was found to be a competitive antagonist at 5-HT2 receptors (pA2 = 9.19) and alpha 1-adrenoceptors (pA2 = 7.52). 3. Ritanserin, an analogue of ketanserin with very weak alpha 1-adrenoceptor antagonist activity, was found to be a potent non-competitive 5-HT2 receptor antagonist. 4. In the presence of the potent alpha 1-adrenoceptor antagonist prazosin, ketanserin was a non-competitive 5-HT2 receptor antagonist. 5. Thus, for ketanserin to exhibit competitive 5-HT2 receptor antagonism, its coexisting alpha 1-adrenoceptor antagonist property appears mandatory.

Interactions between inhibitory and excitatory modulatory signals in single submucosal neurons

Intracellular recordings were made in submucosal neurons from the guinea pig ileum to study the actions of norepinephrine and somatostatin on slow depolarizations induced by 2-chloroadenosine (CADO) and substance P. Local application (by pressure) of CADO and substance P induced a slow depolarization that occurred concomitantly with an increase in input membrane resistance. Norepinephrine, UK-14304 (alpha 2-adrenoceptor agonist), and somatostatin blocked the excitatory responses induced by CADO in a concentration-dependent manner. The alpha 2-adrenoceptor antagonists idazoxan and yohimbine antagonized these inhibitory effects of UK-14304 and norepinephrine. UK-14304 also decreased depolarizations induced by forskolin, but not those induced by the adenosine 3',5'-cyclic monophosphate analogue 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Slow depolarizations induced by substance P were blocked neither by UK-14304 nor by somatostatin. It was previously shown that staurosporine (an inhibitor of various protein kinases) and KT-5720 (an inhibitor of protein kinase A) inhibited slow depolarizations induced by CADO. Here, substance P depolarizations were inhibited by staurosporine and calphostin C (a blocker of protein kinase C) but not by KT-5720. In conclusion, activation of alpha 2-adrenoceptors and somatostatin receptors selectively blocks excitatory responses induced by CADO, most likely by inhibition of adenylyl cyclase and via pertussis toxin-sensitive G proteins. Slow depolarizations induced by substance P are independent of adenylyl cyclase activation and involve activation of protein kinase C.

Antibodies to beta-adrenergic receptors disclosing agonist-like properties in idiopathic dilated cardiomyopathy and Chagas' heart disease

Recent studies confirm the existence of antibodies (Abs) to beta-adrenoceptors in patients with idiopathic dilated cardiomyopathy and Chagas' heart disease. These Abs can be shown to exert both stimulatory and inhibitory effects, which may play a role in the development of the cardiac abnormalities known to occur in these diseases, including advanced heart failure. The hypothesis is advanced that Chagas' heart disease and some forms of idiopathic dilated cardiomyopathy may represent, at least partially, a form of "adrenergic cardiomyopathy."

Enhancement of acute myocardial lesions by asthma drugs in rats

Asthma morbidity and mortality have risen significantly in the last 10 years. The reasons for the increase are multifactorial. One proposed explanation is possible myocardial toxicity arising from the use of beta-agonists alone or in combination with methylxanthines. Previous studies have shown that beta-agonists given alone and beta-agonist/methylxanthine combinations given at higher than recommended clinical doses induced dose-related cardiotoxicity and sudden death in rats. The objective of the present study was to determine whether or not beta-agonists given alone and in combination with methylxanthines at recommended clinical doses also induce cardiotoxicity and sudden death in rats. The beta-agonists, isoproterenol hydrochloride (15 micrograms/kg), fenoterol hydrobromide (40 micrograms/kg), and terbutaline hemisulfate (0.4 mg/kg) were given in single sc doses separately and concurrently with the methylxanthines aminophylline hydrate (20 mg/kg) and caffeine (40 mg/kg), which were given up to a susceptible animal model, the heavy Sprague-Dawley rat. beta-agonist-induced myocardial toxicity (necrosis) was observed. The toxicity was enhanced by aminophylline resulting in the sudden death (most likely due to ventricular fibrillation) of some animals. A decrease in serum iron levels was observed in rats of all beta-agonist and/or methylxanthine-treated groups.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Postnatal development of central nervous alpha 2-adrenergic binding sites: an in vitro autoradiography study in the tree shrew

The postnatal development of the alpha 2-adrenoceptor pattern was investigated by in vitro receptor autoradiography with the antagonist [3H]rauwolscine in the brains of tree shrews (Tupaia belangeri). At birth, high numbers of [3H]rauwolscine binding sites are diffusely distributed in the whole brain with exception of the neocortex which is very weakly labeled at this time. While the number of [3H]rauwolscine binding sites in the cerebellum decreases to low levels during the first three postnatal weeks, several brain regions show a significant increase in binding sites which are progressively concentrated in distinct nuclei. In the medulla oblongata, the diffuse labeling pattern changes so that binding sites become centralized in the dorsomedial nuclei. In the pons, similar changes can be observed with a moderate labeling of the locus coeruleus on postnatal day 10 and a strong labeling in the adult. In the thalamus, a transient appearance of high numbers of [3H]rauwolscine binding sites can be observed during the second and third postnatal week in specific nuclei. In the preoptic area and hypothalamus, there are only minor postnatal changes but the numbers of [3H]rauwolscine binding sites decrease between postnatal day 5 and adulthood. The high number of binding sites in the limbic system does not significantly change after birth. In the neocortex and the superior colliculus, the [3H]rauwolscine labeling pattern which is characteristic for the adult is achieved not before the third postnatal week. Competition experiments demonstrate that [3H]RAUW binds with high affinity to alpha 2-adrenoceptors in the postnatal as well as in the adult brain. Therefore, this study demonstrates region specific developmental profiles of the pattern of alpha 2-adrenoceptors in the postnatal tree shrew brain.

Synthesis and binding characteristics of two sulfhydryl-reactive probes for vasopressin receptors

The present study describes the synthesis and receptor binding affinities of the sulfhydryl-reactive vasopressin analogs deamino[Dab(N delta-N-maleoyl-beta-alanin e)4]AVP (1a) and deamino[Lys(N epsilon-N-maleoyl-beta-alanine)8VP (2a). The analogs were obtained by introducing the sulfhydryl-reactive maleoyl-beta-analyl group at the delta-amino group of Dab4 in deamino[Dab4]AVP (1) and at the epsilon-amino group of Lys8 in deamino[Lys8]VP (2), which were synthesized by the solid-phase method. Furthermore, the analog modified at Dab4 was prepared as tritium labeled compound (1b) after catalytic iodine tritium exchange at Tyr2 in deamino[Dab4]AVP. The sulfhydryl-reactive vasopressin analogs retained high binding affinity for the V2 vasopressin receptor in membranes derived from bovine kidney inner medulla. Apparent dissociation constants Kd of 45 nM (compound 1a) and 15 nM (compound 2a) were determined. Incubation of the ligand receptor complexes at pH 5.5 resulted in dissociation of the sulfhydryl-reactive vasopressin analogs from the V2 receptor. No indications of a covalent reaction between analogs 1a, 2a and 1b and sulfhydryl groups in or close to the hormone binding site of the V2 receptor were found.

Influence of aging on fluidity and coupling between beta-receptors and G-proteins in rat lung membranes

We tested the hypothesis that in rat lung membranes, the age-related decline in the percentage of beta-receptors coupled with high affinity to G-proteins, is due to limitation of the diffusion caused by a decrease in membrane fluidity. We measured both parameters simultaneously in a crude membrane preparation from lungs of rat of different age. In contrast to what is found in crude membrane preparations from rat liver and brain, in rat lung fluidity was increased upon aging. We conclude that the age-related alteration in coupling between receptor and G-protein is difficult to explain by alterations of membrane fluidity.