The beta 2-adrenergic receptors of human epidermoid carcinoma cells bear two different types of oligosaccharides which influence expression on the cell surface

Dependence of neurotrophic factor activation of Trk tyrosine kinase receptors on cellular sialidase

A direct link between receptor glycosylation and activation following natural ligand interaction has not been observed. Here, we discover a membrane sialidase-controlling mechanism that depends on ligand binding to its receptor to induce enzyme activity which targets and desialylates the receptor and, consequently, causes the induction of receptor dimerization and activation. We also identify a specific sialyl alpha-2,3-linked beta-galactosyl sugar residue of TrkA tyrosine kinase receptor, which is rapidly targeted and hydrolyzed by the sialidase. Trk-expressing cells and primary cortical neurons following stimulation with specific neurotrophic growth factors express a vigorous membrane sialidase activity. Neuraminidase inhibitors, Tamiflu, BCX1812, and BCX1827, block sialidase activity induced by nerve growth factor (NGF) in TrkA-PC12 cells and by brain-derived neurotrophic factor (BDNF) in primary cortical neurons. In contrast, the neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid, specific for plasma membrane ganglioside Neu3 and Neu2 sialidases has no inhibitory effect on NGF-induced pTrkA. The GM1 ganglioside specific cholera toxin subunit B applied to TrkA-PC12 cells has no inhibitory effect on NGF-induced sialidase activity. Neurite outgrowths induced by NGF-treated TrkA-PC12 and BDNF-treated PC12(nnr5) stably transfected with TrkB receptors (TrkB-nnr5) cells are significantly inhibited by Tamiflu. Our results establish a novel mode of regulation of receptor activation by its natural ligand and define a new function for cellular sialidases.

Prostaglandin E2 receptor EP3alpha subtype: the role of N-glycosylation in ligand binding as revealed by site-directed mutagenesis

Functional mouse prostaglandin E2 (PGE2) receptor EP3alpha subtype has been expressed in insect cells using a baculovirus system (Huang C. and Tai H.-H. Biochem J 1995; 307: 493-498). EP3alpha receptor has two potential sites (Asn-X-Ser/Thr), Asn 16 and Asn 193, for N-glycosylation. The role of glycosylation in ligand binding of the EP3alpha receptor was investigated by site-directed mutagenesis. Asn was mutated to Gln in each of the two potential glycosylation sites in the EP3alpha receptor. Recombinant wild-type and mutant EP3alpha receptors were expressed in insect cells using baculovirus. Ligand binding assay indicated that the affinity of PGE2 binding was reduced by 50% in the Gln 193 mutant EP3alpha receptor, while the specificity of ligand binding was unaltered. The affinity for PGE2 binding was not affected in the Gln 16 mutant EP3alpha receptor. However, its specificity was partially changed as the EP3-specific agonist became less effective in displacing the [3H]-PGE2 binding to the mutant receptor. These results indicated that N-glycosylation of the EP3alpha receptor could partially affect the affinity and specificity of the ligand binding.

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.

Structure and functional significance of the carbohydrates of the LH/CG receptor

The LH/CG appears to contain 1-4 bi- or multi-antennary complex-type N-linked oligosaccharide side chains, which appear to locate apart from the hormone-binding regions. The exact sites to which the N-linked chains are attached remain to be delineated. The carbohydrates of the mature membrane-inserted receptor do not contribute to either specific high-affinity ligand-binding or signal transduction of the receptor. Thus, the polypeptide core of the receptor is responsible for both high affinity binding and dictating the hormone specificity. Moreover, the deglycosylated receptor, once inserted to the plasma membrane in a functionally mature form, retains its functional conformation or permits the conformational change that is required for coupling of the receptor to effector enzymes. Addition of oligosaccharides to the nascent LH/CG receptor but not their subsequent conversion to complex-type ones appears to be required for acquiring the hormone-binding conformation. On the other hand, neither addition of oligosaccharides to the nascent receptor, nor their further maturation are needed for the transport of the receptor to the plasma membrane. Thus, one function of the N-linked oligosaccharides in the LH/CG receptor appears to be to direct the proper folding of the receptor.

Analysis of the role of N-glycosylation in cell-surface expression and binding properties of angiotensin II type-2 receptor of rat pheochromocytoma cells

We previously demonstrated that the AT2 receptor is a glycoprotein containing N-linked oligosaccharide side chains and that the marked disparity between the sizes of AT2 receptors from different tissues was related to different degrees of N-glycosylation. In the present study, we used an inhibitor of N-glycosylation, tunicamycin, as well as an endoglycosidase, glycopeptidase-F, to examine the contribution of carbohydrate moieties to the ligand-binding properties, cell-surface expression and apparent molecular mass of AT2 receptors of rat pheochromocytoma cells (PC-12 cells). Photoaffinity labelling of cell-surface AT2 receptors revealed that PC-12 cells grown in the presence of tunicamycin expressed, in addition to the previously described 140 kDa receptor, lower-molecular-mass receptors of 63 kDa, 47 kDa and 32 kDa. Lectin affinity chromatography revealed that the 63 kDa and the 47 kDa receptors are partially glycosylated and that the 32 kDa receptor is completely deglycosylated. Competitive binding experiments were carried out on tunicamycin-treated cells that expressed predominantly the 63 kDa or the 47 kDa receptors. Both receptor forms exhibited a high affinity for angiotensin II, although a slight decrease (of about 2-fold) was consistently observed on tunicamycin-treated cells as compared with control cells. Endoglycosidase digestion of AT2 receptors of PC-12 cells also yielded smaller receptor forms of 47 kDa and 32 kDa. Similarly, angiotensin II showed a high but slightly decreased binding affinity (of about 2-fold) for deglycosylated membranes as compared with control membranes. In conclusion, the stepwise action of tunicamycin suggests the presence of at least three N-linked oligosaccharide side chains on the AT2 receptor of PC-12 cells. These oligosaccharide side chains have a minor contribution to the affinity of the receptor. Interestingly, glycosylation is not an essential requirement for the expression of AT2 receptor at the surface of PC-12 cells.

Anomalous behavior of CGP 12177A on beta 1-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta 1- and beta 2-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta 3-adrenergic receptor. Using hamster cells stably expressing either the human beta 1-, human beta 2- or rat beta 1-adrenergic receptor, we found that CGP 12177A behaved as an agonist of beta 1-adrenergic receptors. Whereas at low concentrations, CGP 12177a behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta 1-adrenergic receptor expression. Thus, at low receptor of densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta 2-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta 1-adrenergic receptors indicated that approximately 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas approximately 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.

Switching from alpha 1- to beta-subtypes in adrenergic response during primary culture of adult-rat hepatocytes as affected by the cell-to-cell interaction through plasma membranes

The alpha 1-adrenergic response was predominant over the beta-adrenergic one in adult rat hepatocytes, when the responses were measured as the agonist-induced generations of Ins(1,4,5)P3 and cyclic AMP, respectively. During primary culture of the adult rat hepatocytes, the beta-adrenergic response developed rapidly, whereas the alpha 1-response decreased gradually. Such receptor-subtype switching did not occur unless the cells were cultured under conditions favourable for cell growth, i.e. at low cell density (10(4) cells/cm2). The switching was prevented progressively as the cell culture density was increased up to 20-fold or the low-density culture was achieved by addition of increasing amounts of liver plasma membranes. The gradual decrease in alpha 1-response was accounted for by a concurrent decrease in the receptor site density, whereas rapid development of the beta-response definitely preceded the increase in beta-ligand binding sites during the culture. This rapid development of the beta-response reflected enhanced coupling of the receptor to G-protein during the early stage of culture, as evidenced by the progressively developed ability of GTP to lower the affinity of beta-agonist binding to membranes prepared from these short-time-cultured hepatocytes.

Antibodies for the immunochemistry of the human beta 3-adrenergic receptor

Based on the amino acid sequence deduced from the recently cloned human beta 3-adrenergic receptor (hu beta 3AR) gene, polyclonal antibodies were prepared against synthetic peptides, corresponding to regions of hu beta 3AR presumed to be exposed at the outer or the inner side of the membrane on the basis of the putative three-dimensional structure of the previously characterized beta 1 and beta 2 adrenergic receptors. Affinity-purified antibodies directed against N-terminal, extracellular or intracellular loops and C-terminal peptides reacted specifically with the hu beta 3AR and not with either the human beta 1 or beta 2 adrenergic receptor. Using these antibodies, it was demonstrated that the receptor is present at the surface of Chinese Hamster Ovary (CHO) cells transfected with the hu beta 3AR gene; in addition, the presence of the receptor protein was established in a human tissue (gall bladder). Immuno-affinity chromatography of solubilized CHO hu beta 3AR-containing cell membranes allowed the isolation of hu beta 3AR protein with an overall yield of 30%. The degree of purity of the receptor was more than 80%, as assessed by N-terminal sequencing of the protein eluted from the column. Sequence analysis demonstrated the absence of a methionine residue at the N-terminal position, and suggested that the side chain of the asparagine residue at position 7 is glycosylated.

Structure, function, and regulation of adrenergic receptors

Adrenergic receptors for adrenaline and noradrenaline belong to the large multigenic family of receptors coupled to GTP-binding proteins. Three pharmacologic types have been identified: alpha 1-, alpha 2-, and beta-adrenergic receptors. Each of these has three subtypes, characterized by both structural and functional differences. The alpha 2 and beta receptors are coupled negatively and positively, respectively, to adenylyl cyclase via Gi or Gs regulatory proteins, and the alpha 1 receptors modulate phospholipase C via the Go protein. Subtype expression is regulated at the level of the gene, the mRNA, and the protein through various transcriptional and postsynthetic mechanisms. Adrenergic receptors constitute, after rhodopsin, one of the best studied models for the other receptors coupled to G proteins that are likely to display similar structural and functional properties.

Significance of the carbohydrate moiety of the rat ovarian luteinizing-hormone/chorionic-gonadotropin receptor for ligand-binding specificity and signal transduction

The contribution of the carbohydrate moiety of the rat ovarian luteinizing-hormone (LH)/chorionic-gonadotropin (CG) receptor to ligand-binding specificity and signal transduction was investigated by using glycosidases. Purified membranes from pseudo-pregnant rat ovaries were treated with neuraminidase or peptide N-glycosidase F, to remove terminal sialic acids and N-linked oligosaccharides of the receptor, respectively. Ligand blotting and densitometric scanning of the autoradiograms showed that 90-95% of the receptors were deglycosylated, and that desialylation was virtually complete. Neither the desialylated nor the deglycosylated receptors were able to bind human follicle-stimulating hormone or bovine thyroid-stimulating hormone, as revealed by competition binding experiments. The 50% effective dose of hCG for adenylate cyclase activation, as determined by measuring the formation of cyclic [32P]AMP from [alpha-32P]ATP for 15 min at 30 degrees C, was similar in the control and deglycosylated membranes: 10.2 +/- 3.3 nM and 12.2 +/- 3.8 nM respectively. The same was true for the time course of the basal, hCG- and forskolin-stimulated enzyme activity. In addition, removal of oligosaccharides from the receptor did not restore the ability of desialylated hCG, nor of the deglycosylated hormone, to stimulate adenylate cyclase. In conclusion, the carbohydrate moiety of the native membrane-inserted rat ovarian LH/CG receptor does not contribute to the ligand-binding specificity, and it is not required for the functional coupling of the occupied receptor and the adenylate cyclase system. These functions are associated with the polypeptide portion of the receptor.

High level functional expression of human beta 1-adrenergic receptor in baculovirus-infected cells screened by a rapid in situ procedure

A novel screening assay for the identification of baculovirus infected cells expressing membrane receptors was developed by using a replica transfer technique. Sf9 cells were cotransfected with wild type baculoviral DNA and the transfer vector pVL941-beta 1 containing the coding region of the human beta 1-adrenergic receptor gene. Infected cells embedded in agarose were incubated with [125I]-iodocyanopindolol and transferred onto filters that were subsequently autoradiographed. This procedure resulted in the isolation of recombinant baculoviruses that expressed beta 1-adrenergic receptors. Binding assays carried out with [125I]-ICYP indicated that more than 600,000 receptors were expressed per cell, the highest level noted so far for this receptor in genetically engineered cells. Sf9 cells expressing the beta 1-AR were analysed by ligand binding, competition experiments, adenylyl cyclase stimulation and photoaffinity labeling. These cells express a homogenous population of receptors and display the known pharmacological properties of beta 1-AR in human tissues.

The cannabinoid receptor: biochemical and cellular properties in neuroblastoma cells

The cannabinoid receptor that has been pharmacologically characterized for hypothermia, spontaneous activity, analgesia and catalepsy in rodents is the same pharmacological receptor that inhibits adenylate cyclase in vitro. The inhibition of adenylate cyclase by the cannabinoid receptor results from an interaction with Gi, based on the biochemical kinetic properties of the response, the sensitivity to pertussis toxin ADP-ribosylation, and the thermodynamic characteristics of the response. From precedents based on studies of the well-characterized G protein coupled receptors, rhodopsin and the beta-adrenergic receptor, we can predict the tertiary structure of the cannabinoid receptor. Three sites of potential glycosylation are present on the receptor. However, treatment of N18TG2 neuroblastoma cells with tunicamycin to prevent glycosylation of newly synthesized receptors failed to alter cannabinoid-induced inhibition of cyclic AMP accumulation. The cannabinoid response was rapidly desensitized (within 1/2 h). Treatment of cells with tunicamycin failed to alter agonist-induced desensitization processes. These findings can be more veraciously interpreted as we gain a better understanding of the cellular dynamics of the cannabinoid receptor.

Immunological approaches for probing receptor structure and function

Molecular cloning has revealed the primary sequence of numerous membrane receptors, and this information catalysed two important efforts: modeling of receptor structure by hydropathy analysis and generating sequence-specific immunological probes with which these models can be tested experimentally. Craig Malbon and his colleagues outline the recent advances that illustrate how anti-peptide antibodies raised to synthetic sequences of membrane receptor have generated new information on the topology, functional domains and cellular localization of transmembrane signaling elements. They focus on two examples, the G protein-linked beta-adrenoceptor, and the nicotinic acetylcholine receptor, an intrinsic ion channel receptor. These two classes of receptor provide templates for the analysis of topographical models of membrane proteins with immunological probes, especially anti-peptide antibodies, and demonstrate how these results complement those obtained from molecular, biochemical and biophysical techniques. Although this powerful strategy is not without faults, it is likely to continue to be applied successfully to the analysis of the structure and function of receptors, ion channels and other membrane proteins.

Expression of three human beta-adrenergic-receptor subtypes in transfected Chinese hamster ovary cells

The genes coding for three pharmacologically distinct subtypes of human beta-adrenergic receptors (beta 1 AR, beta 2 AR and beta 3 AR) were transfected for expression in Chinese hamster ovary (CHO) cells. Stable cell lines expressing each receptor were analyzed by ligand binding, adenylate cyclase activation and photoaffinity labeling, and compared to beta AR subtypes observed in previously described tissues, primary cultures and transfected cell lines. Each of the three receptor subtypes displayed saturable [125I]iodocyanopindolol-binding activity. They showed the characteristic rank order of potencies for five agonists, determined by measuring the accumulation of intracellular cAMP. These recombinant cell lines express a homogeneous population of receptors and display the known pharmacological properties of beta 1 AR and beta 2 AR, in human tissues. It is therefore likely that the pattern of ligand binding and adenylate cyclase activation, mediated by the new beta 3 AR in CHO cells, also reflects the yet-undetermined pharmacological profile in humans.

The biology of beta-adrenergic receptors: analysis in human epidermoid carcinoma A431 cells

1. G-protein-linked transmembrane signaling has emerged as a major pathway for information transduction across the cell membrane. 2. In addition to photopigments that propagate the signal from light, cell-surface receptors for hormones, neurotransmitters, and autacoids propagate signals from ligand binding to membrane-bound effector units via G-proteins. 3. Biochemical and molecular features of one prominent member of these receptors, the beta-adrenergic receptor, will be highlighted in the present article. 4. The role of the human epidermoid carcinoma A431 cells as a model for the study of the structure and biology of beta-adrenergic receptors will be emphasized. 5. A model for receptor regulation, gleaned from recent advances in the biochemistry, cell and molecular biology of beta-adrenergic receptors, is discussed.

Mapping of a functional autoimmune epitope on the beta 1-adrenergic receptor in patients with idiopathic dilated cardiomyopathy

The presence and properties of serum autoantibodies against beta-adrenergic receptors in patients with idiopathic dilated cardiomyopathy were studied using synthetic peptides derived from the predicted sequences of the human beta-adrenergic receptors. Peptides corresponding to the sequences of the second extracellular loop of the human beta 1- and beta 2-adrenergic receptors were used as antigens in an enzyme immunoassay to screen sera from patients with dilated cardiomyopathy (n = 42), ischemic heart disease (n = 17), or healthy blood donors (n = 34). The sera of thirteen dilated cardiomyopathy patients, none of the ischemic heart disease patients, and four of the healthy controls monospecifically recognized the beta 1-peptide. Only affinity-purified antibodies of these patients had a inhibitory effect on radioligand binding to the beta 1 receptor of C6 rat glioma cells. They recognized the receptor protein by immunoblot and bound in situ to human myocardial tissue. We conclude that a subgroup of patients with idiopathic dilated cardiomyopathy have in their sera autoantibodies specifically directed against the second extracellular loop of the beta 1-adrenergic receptor. These antibodies could serve as a marker of an autoimmune response with physiological and/or pathological implications.

Biotechnology of beta-adrenergic receptors

The emergence of Biotechnology has provided pharmacologists with a variety of methods for investigating the structure, the function, and the regulation of membrane-bound receptors with a precision that was not imagined even five years ago. These new tools have been developed and used to analyze the known catecholamine beta 1- and beta 2-adrenergic receptors and to discover and study a new subtype, the beta 3 receptor. We review here the salient features of each of these three receptors, compare their structural and functional properties, and propose models to explain their differential regulation in time and space. A whole family of proteins has now been found to share with the beta-adrenergic receptors their most prominent features, including seven transmembrane domains and coupling with GTP-binding "G" proteins. We therefore propose that the biotechnology-based procedures developed for the beta-adrenergic receptors will be well applicable to the other members of this "R7G" family of receptors.

Localization and characterization of three different beta-adrenergic receptors expressed in Escherichia coli

After fusion with the N-proximal portion of the outer membrane protein LamB, three beta-adrenergic receptors, the human beta 1- and beta 2- and turkey beta 1-adrenergic receptor, were expressed in Escherichia coli with retention of their own specific pharmacological properties. Molecular characterization and localization of the three receptors in bacteria and comparison of the behaviour of each hybrid protein are reported. The bacteria were lysed and fractionated on a sucrose gradient. Saturable [125I]iodocyanopindolol binding activity was found associated mainly with the inner membrane fraction, suggesting that the receptor is correctly folded in this membrane. Binding activity was also found in the outer membrane fraction but varied according to the receptor type. Photoaffinity labeling experiments revealed that the receptors exhibit binding activity only after proteolytic removal of the LamB moiety from the fusion protein. The three hybrid proteins, detected in immunoblots by anti-peptide antibodies, were found mainly in the outer membrane fraction. Each of them exhibited different susceptibility to intrinsic bacterial proteolytic enzymes; sites of proteolytic cleavage were localized by the use of anti-peptide antibodies. The functional expression in E. coli of three beta-adrenergic receptors with similar structure but different amino acid sequences suggests that this expression system may be a general feature among similar receptors of the family of G-protein-coupled receptors. The level of expressed binding activity of a given receptor will be within the control of proteolytic degradation processes, depending on the primary sequence of the receptor. Constructions of new hybrid proteins, in combination with expression in protease mutants of E. coli, should help in controlling such processes.

Subtype-selective immunoprecipitation of the beta 2-adrenergic receptor

Most antibodies known to interact with beta-adrenergic receptors do not exhibit subtype selectivity, nor do they provide quantitative immunoprecipitation. A monoclonal antibody, G27.1 raised against a synthetic peptide corresponding to the C-terminus of the beta 2-adrenergic receptor of hamster, is selective for the beta 2 subtype. G27.1 provides nearly quantitative immunoprecipitation of the beta 2-adrenergic receptor from hamster lung that has been photoaffinity-labeled and solubilized with sodium dodecyl sulfate. Immunoprecipitation is completely blocked by nanomolar concentrations of the immunizing peptide. This antibody interacts with beta 2-adrenergic receptors from three rodent species, but not with those from humans. When C6 glioma cells, which contain both beta 1- and beta 2-adrenergic receptors, are photoaffinity-labeled in the absence or presence of subtype-selective antagonists, subtype-selective photoaffinity-labeling results. G27.1 can immunoprecipitate beta 2-, but not beta 1-, adrenergic receptors from these cells. Similar results were obtained following subtype-selective photoaffinity-labeling of membranes from rat cerebellum and cerebral cortex. The beta-adrenergic receptors from C6 glioma cells and rat cerebral cortex exist as a mixture of two molecular weight species. These species differ in glycosylation, as shown by endoglycosidase F digestion of crude and immunoprecipitated receptors.

Intracellular distribution of adrenoceptors in the failing human myocardium

Although it is increasingly recognized that the density of cardiac membrane-bound beta adrenoceptors declines in heart failure, the mechanisms involved are unclear. Furthermore, it is not known whether cardiac alpha-1 adrenoceptors are similarly affected. Inasmuch as agonist-induced desensitization results in translocation of adrenoceptors from the plasma membrane to an intracellular vesicular fraction, we determined the intracellular distribution of cardiac adrenoceptors in two groups: group 1 (n = 9) consisted of papillary muscles from patients with mild-to-moderate heart failure undergoing valve replacement, and group 2 (n = 8) consisted of severely failing hearts removed during orthotopic cardiac transplantation. The density of cardiac beta adrenoceptors was lower in membranes from group 2 (17.8 +/- 3.3 fmol/mg protein vs 27.8 +/- 3.7 fmol/mg in group 1; (p less than 0.01), and the percentage of beta receptors recovered in the vesicular fraction was higher in group 2 (47.1 +/- 3.3% vs 36.8 +/- 5.0% in group 1; p less than 0.01). In group 1 but not group 2 there was a significant inverse correlation (r = -0.87; p less than 0.001) between the density of membrane-bound beta receptors and the percentage of beta receptors recovered in the vesicular fraction. Alpha-1 adrenoceptors were lower in both membrane and vesicular fraction of group 2 compared to group 1; in group 2 but not group 1 there was a significant negative correlation between the density of membrane-bound alpha-1 adrenoceptors and the percentage of alpha-1 receptors in the vesicular fraction (r = -0.8; p less than 0.01). These results suggest that the regulation of alpha-1 and beta adrenoceptors differs in the failing myocardium. Furthermore, agonist-induced desensitization may play a predominant role only in mild-to-moderate heart failure.

Human beta 2-adrenergic receptors expressed in Escherichia coli membranes retain their pharmacological properties

The coding region of the gene for the human beta 2-adrenergic receptor gene was fused to the beta-galactosidase gene of the lambda gt11 expression vector. The Y1089 Escherichia coli strain was lysogenized with this modified vector and transcription of the fusion gene was induced. Expression of this transcription unit was shown by the appearance in the bacteria of proteins of molecular weight higher than that of native beta-galactosidase, which are immunoreactive with anti-beta-galactosidase antibodies. Production of beta 2-adrenergic receptors was shown by the presence, on intact bacteria, of binding sites for catecholamine agonists and antagonists possessing a typical beta 2-adrenergic pharmacological profile. Binding and photoaffinity labeling studies performed on intact E. coli and its membrane fractions showed that these binding sites are located in the inner membrane of the bacteria. Expression of pharmacologically active human beta 2-adrenergic receptors in E. coli further supports the similar transmembrane organization proposed for bacteriorhodopsin and eukaryotic membrane-embedded receptors coupled to guanine nucleotide-binding regulatory proteins. Moreover, this system should facilitate future analyses of the ligand-binding properties within this family of membrane receptors.