Direct insertion and fluorescence studies of rhodamine-labeled β-adrenergic receptors in cell membranes

Summary Previous studies utilizing the fluorescence of propanolol as a probe for the beta-adrenergic receptor showed that this receptor is motionally constrained. To further study the betaadrenergic receptorin situ we have reinserted rhodamine-labeled beta-receptors into cell membranes. This report presents documentation of their insertion and physiologic viability. Beta-receptors were purified by affinity chromatography (10,000-fold), then fluorescently labeled with tetramethyl rhodamine isothiocyanate, repurified (55,000-fold) and incubated with rat pancreatic islet cells. The binding of³H-dihydroalprenolol by these cells was increased from aB _max of 168±2 to 309±20 fmol/mg protein with no change inK _d . Various treatments which remove peripheral membrane proteins, e.g. NaOH, lithium diiodosalicylate, and trypsinization, did not alter binding by the cells with inserted receptors indicating that the receptors inserted into cell membranes. In islet cells treated with Koshland's reagent I, beta-adrenergic binding was completely abolished, but following incubation with isolated beta-receptors, the cells bound beta-adrenergic radioligand with aB _max of 100 fmol/mg protein, indicating functionality on the part of the inserted receptors. Furthermore, insertion of isolated receptors into frog erythrocytes resulted in increased production of cAMP in response to added isoproterenol. In pancreatic islet cells, incubation with labeled receptors caused the fluorescence to shift in wavelength with increased intensity indicating a shift from an aqueous to a lipid environment, probably into the membrane. Using fluorescence (P), it was found that the inserted receptors became motionally constrained to aP of 0.38 (limitingP _o=0.42) during the first 15 min, remaining so for at least 2 hr. Colchicine (5 g/ml) caused a decrease inP to 0.18 indicating release of constraint. Isoproterenol (10^–5M) caused a rapid decrease toP=0.15. This effect was blocked by propranolol. Propranolol itself (10^–5M) had no effect. These results indicate that the labeled receptors rapidly insert into cell membranes and also support the view that agonist activation of the receptor causes an increase in receptor mobility, presumably due to release of constraint from cytoskeleton elements.     

Characterization of postsynaptic β-adrenergic receptors and presynaptic muscarinic cholinergic receptors in the rat spleen

The β-adrenergic and muscarinic cholinergic receptors in the splenic homogenates of control and 6-hydroxydopamine (6-OHDA) treated rats were characterized. The specific binding of [³H]dihydroalprenolol (DHA) and [³H]quinuclidinyl benzilate (QNB) in the rat spleen were saturable and of high affinity and showed pharmacological specificity of splenic β-adrenergic and muscarinic cholinergic receptors. Following 6-OHDA treatment, the Bmax value for specific [³H](-)DHA binding to the rat spleen was significantly increased by 26 percent and 22 percent compared to control at 2 and 3 weeks without a change in the Kd. In contrast, there was a 38 percent decrease in the Bmax for [³H](-)QNB in the 6-OHDA treated rat spleen at 2 and 3 weeks respectively without a change in the Kd. The Bmax value at 5 weeks was significantly greater than that at 2 or 3 weeks. The splenic norepinephrine (NE) concentration was markedly reduced by the 6-OHDA treatment at 1 to 3 weeks, while there was a significant recovery in the splenic NE concentration at 5 weeks. Thus, our results strongly suggest that we are biochemically localizing muscarinic cholinergic receptors on the sympathetic nerves of the rat spleen and that the β-adrenergic receptors of the spleen are localized postsynaptically.     

Subcellular distribution of α1-adrenergic receptors in rat liver

The distribution of α₁-adrenergic receptors in rat liver subcellular fractions was studied using the α₁-adrenergic receptor ligand [³H]prazosin. The highest number of [³H]prazosin binding sites was found in a plasma membrane fraction followed by 2 Golgi and a residual microsomal fraction, the numbers of binding sites were 1145, 845, 629 and 223 fmol/mg protein, respectively. When the binding in these fractions was compared with the activity of plasma membrane ‘marker’ enzymes in the same fractions a relative enrichment of [³H]prazosin binding sites was found in the residual microsomes and one of the Golgi fractions. Photoaffinity labelling with ¹²⁵I-arylazidoprazosin in combination with SDS-polyacrylamide gel electrophoresis revealed the specific binding to 40 and 23 kDa entities in a Golgi fraction, while in plasma membranes the binders had an apparent molecular mass of 36 and 23 kDa. When [³H]prazosin was injected in vivo into rat portal blood followed by subcellular fractionation of liver, a pattern of an initial rapid decline and thereafter a slow decline of radioactivity was noted in all fractions. Additionally, in the two Golgi fractions a transient accumulation of radioactivity occurred between 5 and 10 min after the injection. The ED₅₀ values for displacement of [³H]prazosin with adrenaline was lowest in the plasma membrane fraction, followed by the residual microsomes and Golgi fractions, the values were 10⁻⁶, 10⁻⁵ and 10⁻⁴ mol/l, respectively. On the basis of lack of correlation between distribution of α₁-adrenergic antagonist binding and adenylate cyclase activity, differences in the molecular mass of α₁-adrenergic antagonist binders, differences in the kinetics of in vivo binding and accumulation of [³H]prazosin and also differences in agonist affinity between plasma membrane and Golgi fractions, it is concluded that α₁-adrenergic receptors are localized to low-density intracellular membranes involved in receptor biosynthesis and endocytosis.     

Thyroid dependent maturation of β-adrenergic receptors in the rat lung

β-Adrenergic receptors were identified in membranes of fetal and postnatal rat lung with (?)-[³H]dihydroalprenolol, [³H]DHA. β-Receptor number (Bmax) increased 11-fold from day 18 of gestation to adult levels by day 28 of postnatal life. The increase of β-adrenergic receptors occurring between postnatal days 15 and 28 was dependent on thyroxine (T4) in propylthiouracil treated pups. β-Adrenergic receptors on day 28 were identical in euthyroid (PTU + T4) as compared to normal control pups (489±31 and 491±30 femtomoles·mg^?1) however receptors were markedly reduced in 28 day hypothyroid pups (PTU alone), Bmax = 294±21.5, m±S.E. p<0.01. Treatment of the hypothyroid pups with T4 for three days on postnatal day 25 increased β-adrenergic receptors approximately two-fold by day 28. This thyroid hormone dependent increase in lung β-adrenergic receptors occurs between postnatal days 15 and 28 coincident with the known increase in thyroid gland activity in the rat pup.     

Development of covalent antagonists for β1- and β2-adrenergic receptors

The selective covalent tethering of ligands to a specific GPCR binding site has attracted considerable interest in structural biology, molecular pharmacology and drug design. We recently reported on a covalently binding noradrenaline analog (FAUC37) facilitating crystallization of the β₂-adrenergic receptor (β₂AR^H2.64C) in an active state. We herein present the stereospecific synthesis of covalently binding disulfide ligands based on the pharmacophores of adrenergic β₁- and β₂ receptor antagonists. Radioligand depletion experiments revealed that the disulfide-functionalized ligands were able to rapidly form a covalent bond with a specific cysteine residue of the receptor mutants β₁AR^I2.64C and β₂AR^H2.64C. The propranolol derivative (S)-1a induced nearly complete irreversible blockage of the β₂AR^H2.64C within 30 min incubation. The CGP20712A-based ligand (S)-4 showed efficient covalent blocking of the β₂AR^H2.64C at very low concentrations. The analog (S)-5a revealed extraordinary covalent cross-linking at the β₁AR^I2.64C and β₂AR^H2.64C mutant while retaining a 41-fold selectivity for the β₁AR wild type over β₂AR. These compounds may serve as valuable molecular tools for studying β₁/β₂ subtype selectivity or investigations on GPCR trafficking and dimerization.     

Biological maturation and β-adrenergic effectors: development of β-adrenergic receptors in rabbit heart

Summary The -adrenergic receptor, transduction processes and catalytic activity of the adenylate cyclase enzyme complex have been investigated in rabbit heart at different stages of biological maturation. The binding of [³H]-dihydroalprenolol to a washed membrane preparation isolated from rabbit ventricular muscle was used to characterize -adrenergic receptors. Significant age-related differences were noted in -receptor affinity (K_d) and density (RD) of neonatal and adult animals; the adult K_d was 3.7-fold greater and the RD 2-fold higher than the neonates. No significant differences in these parameters were detected among the 27-day old fetus and the 1- and 7-day old neonates. Age-dependent differences in agonist isoproterenol affinity for the receptor were not observed in contrast to the significant changes in antagonist (DHA) affinity.Age-related changes in receptor affinity were also quantitated by determining the inhibitory potency of alprenolol on isoproterenol stimulated adenylate cyclase enzyme activity. A decreased affinity of the -adrenergic receptor for alprenolol in the adult heart was indicated by a 3.7-fold greater K_i for the adult than the 1-day old neonate. Ontogenic variations in the coupling efficiency between the receptor and catalytic components of the adenylate cyclase complex were also evaluated. The K_d of the -adrenergic receptor for isoproterenol and the EC50 for adenylate cyclase stimulation were determined under similar conditions. The corresponding coupling index (K_d/EC50) was found to be 2.4-fold greater in the 1-day old neonate than adult, suggesting that for a given percentage increase in adenylate cyclase activity, a lower percentage of -adrenergic receptor sites need be occupied in the neonate. These data extend previous studies (29) and indicate all components of the rabbit heart adenylate cyclase enzyme complex (i.e., the -adrenergic receptor, the GTP-dependent transduction event, and the catalytic subunit) exhibit significant developmental changes.     

Immunohistochemical localization of β1-adrenergic receptors in the liver of male and female F344 rat

The distribution of beta(1)-adrenergic receptors in the liver of Fischer 344 (F344) rat has been examined by an immunohistochemical method. The study was carried out on formalin-fixed and paraffin-embedded livers from young adult, middle-aged, and old female and male F344 rats. An antibody specific for the beta(1)-adrenoreceptor subtype was used. A positive reaction was found in the liver parenchyma of female and male rats from all age groups. Within the liver lobule, a clear zonation is observed, with the beta(1)-adrenoreceptor positivity most evident in pericentral zone hepatocytes and a gradual fading of the immunostaining from pericentral to periportal zone hepatocytes, which may be completely negative. Immunoreactivity is localized on the cell membrane and on the membrane of peripheral cytoplasmic vesicles, and is mostly confined to the cell side facing vascular space. The intensity of immunostaining seems to be slightly higher in the 6- and 10-month-old female rats as compared to the matched male rats and to the senescent female rats. No age-related changes in the intensity of immunostaining are appreciable in male rats. However, no definite conclusion could be drawn about the existence of gender-related differences or age-related changes in the density of beta(1)-adrenoreceptors. A low density of beta1-adrenoreceptor was observed in the spontaneous preneoplastic lesions of the livers from senescent rats.     

Serotoninergic agonist and antagonist are modulators of the α2-adrenoceptor activity in rat brain cortex membranes

The influence of activation and inhibition of serotonin receptors by serotonin (5HT) and miancerin on binding of specific nonselective α₂-antagonist [³H]RX821002 in rat cerebral cortex membranes was studied. It was shown that the ligand-receptor interaction for α₂-adrenoceptors corresponded to the model suggesting the presence of one pool of receptors and binding of two ligand molecules to the receptor. The parameters of the [³H]RX821002 binding to α₂-adrenoceptors were as follows: K _d = 1.57 ± 0.276 nM, B _max = 7.24 ± 1.63 fmol/mg protein, n = 2. In the case of activation of 5HT-receptors by serotonin, the character of ligand binding was different: two pools of receptors were detected with the parameters K _d1 = 0.82 ± 0.06; K _d2 = 2.65 ± 0.22 nM; B _m1 = 1.65 ± 0.23; B _m2 = 4.20 ± 0.11 fmol/mg protein; n = 2. The affinity of high-affinity receptors increased twofold and the affininty of low-affinity receptors decreased by 69% as compared to control values. The concentration of high-affinity receptors decreased 4.4-fold, and of low-affinity, 1.7-fold. The value of maximal reaction (B _max) decreased by 20%. In the case of miancerin-induced inhibition of 5HT-receptors the character of ligand binding also changed; two pools of receptors were detected with the following parameters: K _d1 = 0.48 ± 0.09; K _d2 = 3.79 ± 0.71 nM; B ₁ = 0.63 ± 0.17; B ₂ = 4.75 ± 0.21 fmol/mg protein; n = 2. The affinity of high-affinity receptors pool increased by 70% and that of low-affinity receptors decreased by 76% as compared to control values. The concentration of active high-affinity and low-affinity α₂-adrenoceptors decreased by 70% and 141%, respectively. The total amount of the receptors (B _max) decreased by 26%. The data suggest that α₂-adrenoceptors in rat cerebral cortex exist as dimers. Modulatory effects of serotonin and miancerin on specific antagonist binding to α₂-adrenoceptors may be accomplished by altering the character and binding parameters of the nonselective α₂-antagonist [³H]RX821002.     

Serotoninergic agonists and antagonists are the modulators of α1-Adrenoceptor activity in rat brain cortical membranes

The effects of activation and inhibition of serotonin receptors by serotonin (5-HT) and mianserin on the specific nonselective α₁-antagonist [³H]prazosine binding in rat cerebral cortex membranes was studied. It was shown that the ligand-receptor interaction of α₁-adrenoceptors corresponded to the model suggesting the presence of one pool of receptors and the binding of two ligand molecules to the receptor. The parameters of [³H]prazosine binding to α₁-adrenoceptors were as follows: K _d =1.85 ± 0.16 nM, B _max = 31.1 ± 0.3 fmol/mg protein, n = 2. In case of activation of 5HT-receptors by serotonin, the character of ligand binding was different: two pools of receptors were detected with the parameters K _d1 = 0.61 ± 0.04, K _d2 = 3.82 ± 0.15 nM, B _m1 = 6.6 ± 0.7, B _m2 = 25.6 ± 0.4 fmol/mg protein, n = 2. The sensitivity of the high-affinity pool increased threefold and the sensitivity of the low-affinity pool decreased twofold as compared to the control. The value of maximal reaction (B _max) did not change. In the case of inhibition of 5HT-receptors by mianserin, radioactive ligand is bound to α₁-adrenoceptors according to the same model as in the control conditions. The affinity of α₁-adrenoceptors to [³H]prazosine decreases twofold and the concentration increases (K _d = 3.97 ± 0.12 nM, B _max = 40.0 ± 0.5 fmol/mg protein). The data suggest that α₁-adrenoceptors in rat cerebral cortex exist as a dimer. The modulatory effects of serotonin and mianserin on the specific binding of [³H]prazosine to α₁-adrenoceptors was detected, manifesting itself as changes in the binding parameters and in the general character of ligand-receptor interactions.     

Hypokalemia modulatesα- andβ-adrenoceptor bindings in rat skeletal muscle

Changes in the population of adrenergic alpha- and beta-receptors were examined in rat soleus muscles during hypokalemia by their direct determination using radiolabeled ligands. Only beta-adrenoceptors were detected in the normal rat muscles. Hypokalemia led to a pronounced decrease in beta-adrenoceptors, the number of [3H]DHA binding sites, by 50%, as compared with that in the normal rats. There was a genesis of alpha 1-adrenoceptors in hypokalemic rat muscles, since the competitive potency of adrenergic drugs against [3H]prazosin binding was in the order prazosin much greater than phentolamine greater than (+/-)-noradrenaline greater than yohimbine much greater than (+/-)-isoproterenol. The reduction of [3H]DHA binding sites was accompanied by an increase of an approximately equal amount in high-affinity [3H]prazosin binding sites. The Kd determined by kinetic analysis of [3H]prazosin binding was calculated from the ratio K-1/K1 that gave a value of 3.05 nM, which generally agreed with the 1.83 nM determined by saturation experiments (Scatchard plot). This phenomenon of a reduction in the beta-adrenoceptors and the occurrence of alpha 1-adrenoceptors in muscles during hypokalemia is discussed. alpha- and beta-adrenoceptors on soleus muscle membrane may play important but opposite roles in modulating potassium release from the muscle cells.     

Affinity of different α1-agonists and antagonists for the α1-adrenoceptors of rabbit and rat liver membranes

In membranes prepared from rabbit liver, competition with [³H] prazosin by different α₁-agonists and antagonists revealed different affinities in comparison to the results obtained on rat liver membranes, and showed a good correlation with the affinity of the same compounds for the cloned α_1c-adrenoceptor subtype. The potencies observed on rat liver membranes were well correlated with the affinity observed for the cloned α_1b-adrenoceptors. These results confirm that rabbit and rat liver membranes preparations can be utilized to evaluate the affinity of compounds for these α₁-adrenergic subtypes.     

Similar atypical β-adrenergic receptors mediate in vitro rat adipocyte lipolysis and colonic motility inhibition

We investigated the putative common nature of the rat atypical β-adrenoceptors mediating white adipocyte lipolysis and proximal colon motility inhibition, using the non-selective antagonist alphenolol and agonist isoprenaline and the selective agonists SR 58611A and BRL 37344. Results in either isolated intestinal and fat tissues were consistent with: isoprenaline acting through both typical (β₁, β₂) and atypical β-adrenoceptors; SR 58611A and BRL 37344 acting solely through the latter. The identical pA₂ values obtained with alprenolol, irrespective of the tissue and the selective agonist (SR 58611A or BRL 37344) used, support the high functional homology of the atypical β-adrenoceptors in rat colon and adipocytes.     

Molecular comparison of α2-adrenergic receptors from rat adrenocortical carcinoma and human blood platelet

Summary We have previously described a simple two-step purification technique to isolate ₂-adrenergic receptors from the rat adrenocortical carcinoma (Jaiswal, R. K. and Sharma, R. K. (1985) Biochem. Biophys. Res. Commun. 130, 58–64). Utilizing this technique we have now achieved 77 000-fold purification to apparent homogeneity of ₂-adrenergic receptors from human platelets. We have compared the biochemical characteristics of these receptors with those from the rat, which were purified 40000-fold to homogeneity.The [¹²⁵I] receptor proteins from two sources showed: (a) a single radioactive band with a Mr of 64000 as evidenced by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDSPAGE); and (b) a single symmetrical peak with a pl of 4.2 by isoelectric focusing polyacrylamide gel electrophoresis. Both proteins showed typical ₂-adrenergic binding characteristics with specific binding activities of 13.85 nmol/mg and 14.17 nmol/mg protein. These values are close to the theoretical binding activity of 15.6 nmol/mg protein for 1 mol of the ligand binding 1 mol of the receptor protein. These results attest to the purity of the receptors, to its Mr of 64000, and to its acidic nature. However, the peptide maps of the radioiodinated ₂-adrenergic receptors from rat adrenocortical carcinoma and human blood platelets reveal some distinct differences which may relate to the differences in the pharmacological specificities between rodent and nonrodent ₂-adrenergic receptors.Abbreviations PAC p-aminoclonidine - PMSF Phenylmethyl-sulfonylfluoride - DTT Dithiothreitol - HPLC High Performance Liquid Chromatography     

Inhibition of associative long-term depression by activation of β-adrenergic receptors in rat hippocampal CA1 synapses

The aim of this study was to investigate the role of β-adrenergic receptors in modulating associative long-term depression (LTD) at CA1 synapses in rat hippocampal slices. Standard extracellular electrophysiological techniques were employed to record field excitatory post-synaptic potential (fEPSP) activity and to induce associative LTD. Two independent Schaffer collateral pathways were elicited in hippocampal CA1 areas. In one (weak) pathway, the stimulating intensity was adjusted to elicit small fEPSP activity (20–30% of the maximum response). In contrast, 80–90% of the maximum response was evoked in the other (strong) pathway. Associative LTD of weak pathway could be induced by paired stimulation of weak and the strong pathways, repeated 100 times at 0.167 Hz. The associative LTD of weak pathway was NMDA receptor- and phophatase 2B dependent, because bath application of 50 μM D, L-AP5 or 10 μM cypermethrin blocked its induction. Bath application of 1 μM isoproterenol inhibited associative LTD, and this effect was blocked by timolol, suggesting the involvement of β-adrenergic receptors. The inhibitory effect of β-adrenergic receptors on LTD induction was blocked in slices pretreated with inhibitors of protein kinase A and mitogen-activated protein kinase, suggesting that these signal cascades are downstream effectors following activation of β-adrenergic receptors. Nevertheless, bath application of timolol or cypermethrin alone did not have significant effect on associative LTD induction, suggesting neither endogenous function of β-adrenergic receptor nor endogenous PKA activity does have a role in associative LTD induction.     

Crosstalk between adenosine A1 and β1-adrenergic receptors regulates translocation of PKCε in isolated rat cardiomyocytes

Adenosine A(1) receptor (A(1)R)-induced translocation of PKCε to transverse (t) tubular membranes in isolated rat cardiomyocytes is associated with a reduction in β(1)-adrenergic-stimulated contractile function. The PKCε-mediated activation of protein kinase D (PKD) by endothelin-1 is inhibited by β(1)-adrenergic stimulated protein kinase A (PKA) suggesting a similar mechanism of A(1)R signal transduction modulation by adrenergic agonists may exist in the heart. We have investigated the influence of β(1)-adrenergic stimulation on PKCε translocation elicited by A(1)R. Immunofluorescence imaging and Western blotting with PKCε and β-COP antibodies were used to quantify the co-localization of PKCε and t-tubular structures in isolated rat cardiomyocytes. The A(1)R agonist CCPA increased the co-localization of PKCε and t-tubules as detected by imaging. The β(1)-adrenergic receptor agonist isoproterenol (ISO) inhibited this effect of CCPA. Forskolin, a potent activator of PKA, mimicked, and H89, a pharmacological PKA inhibitor, and PKI, a membrane-permeable PKA peptide PKA inhibitor, attenuated the negative effect of ISO on the A(1)R-mediated PKCε translocation. Western blotting with isolated intact hearts revealed an increase in PKCε/β-COP co-localization induced by A(1)R. This increase was attenuated by the A(1)R antagonist DPCPX and ISO. The ISO-induced attenuation was reversed by H89. It is concluded that adrenergic stimulation inhibits A(1)R-induced PKCε translocation to the PKCε anchor site RACK2 constituent of a coatomer containing β-COP and associated with the t-tubular structures of the heart. In that this translocation has been previously associated with the antiadrenergic property of A(1)R, it is apparent that the interactive effects of adenosine and β(1)-adrenergic agonists on function are complex in the heart.     

Molecular basis of ligand dissociation in β-adrenergic receptors

The important and diverse biological functions of β-adrenergic receptors (βARs) have promoted the search for compounds to stimulate or inhibit their activity. In this regard, unraveling the molecular basis of ligand binding/unbinding events is essential to understand the pharmacological properties of these G protein-coupled receptors. In this study, we use the steered molecular dynamics simulation method to describe, in atomic detail, the unbinding process of two inverse agonists, which have been recently co-crystallized with β(1) and β(2)ARs subtypes, along four different channels. Our results indicate that this type of compounds likely accesses the orthosteric binding site of βARs from the extracellular water environment. Importantly, reconstruction of forces and energies from the simulations of the dissociation process suggests, for the first time, the presence of secondary binding sites located in the extracellular loops 2 and 3 and transmembrane helix 7, where ligands are transiently retained by electrostatic and Van der Waals interactions. Comparison of the residues that form these new transient allosteric binding sites in both βARs subtypes reveals the importance of non-conserved electrostatic interactions as well as conserved aromatic contacts in the early steps of the binding process.     

Characteristics of β-adrenergic receptors in bovine corneal epithelium: Comparison of fresh tissue and cultured cells

The characteristics of the β-adrenergic receptors in homogenates of fresh tissue and cultured bovine corneal epithelium were compared using [³H]dihydroalprenolol. High affinity, specific binding sites were observed in both preparations. Fresh tissue exhibited a higher binding site density (165 fmol/mg protein) than did cells in culture (57 fmol/mg protein). Studies with various β-adrenergic agonists and antagonists indicated that binding characteristics were typical of β-adrenergic receptors, predominantly of the β₂ subtype. These results demonstrate that β-adrenergic receptors exist in both fresh and cultured bovine corneal epithelium and that these receptors are qualitatively and quantitatively similar.     

The α1-adrenergic receptors: diversity of signaling networks and regulation

The α(1)-adrenergic receptor (AR) subtypes (α(1a), α(1b), and α(1d)) mediate several physiological effects of epinephrine and norepinephrine. Despite several studies in recombinant systems and insight from genetically modified mice, our understanding of the physiological relevance and specificity of the α(1)-AR subtypes is still limited. Constitutive activity and receptor oligomerization have emerged as potential features regulating receptor function. Another recent paradigm is that β arrestins and G protein-coupled receptors themselves can act as scaffolds binding a variety of proteins and this can result in growing complexity of the receptor-mediated cellular effects. The aim of this review is to summarize our current knowledge on some recently identified functional paradigms and signaling networks that might help to elucidate the functional diversity of the α(1)-AR subtypes in various organs.     

The α1-adrenergic receptors: diversity of signaling networks and regulation

The α₁-adrenergic receptor (AR) subtypes (α_1a, α_1b, and α_1d) mediate several physiological effects of epinephrine and norepinephrine. Despite several studies in recombinant systems and insight from genetically modified mice, our understanding of the physiological relevance and specificity of the α₁-AR subtypes is still limited. Constitutive activity and receptor oligomerization have emerged as potential features regulating receptor function. Another recent paradigm is that βarrestins and G protein-coupled receptors themselves can act as scaffolds binding a variety of proteins and this can result in growing complexity of the receptor-mediated cellular effects. The aim of this review is to summarize our current knowledge on some recently identified functional paradigms and signaling networks that might help to elucidate the functional diversity of the α₁-AR subtypes in various organs.