Stereospecific binding of propranolol and catecholamines to the beta-adrenergic receptor

Functional and Regional Specificity of Noradrenergic Signaling for Encoding and Retrieval of Associative Recognition Memory in the Rat

Recognition of a familiar object in a novel location requires retrieval of the former object-place association and encoding of novel information. Such object-in-place (OiP) memory recruits a neural network including the hippocampus (HPC), medial prefrontal cortex (mPFC), and nucleus reuniens of the thalamus (NRe); however, the underlying cellular mechanisms are not understood. Locus ceruleus (LC) noradrenergic neurons signal novelty; thus here we focused on the contribution of LC-forebrain projections and noradrenaline (NA) receptor subtypes to OiP encoding compared with retrieval, using an arena-based OiP task in male rats. The NRe was found to receive a catecholaminergic input from LC, with the strongest innervation directed to rostral NRe. Interestingly optogenetic inactivation of the LC→NRe pathway impaired OiP retrieval but was without effect on encoding, while inactivation of the LC→HPC selectively impaired encoding. Consistent with this double dissociation, pharmacological blockade of NRe α1-adrenoreceptors selectively impaired memory retrieval, while blockade of HPC β-adrenoreceptors impaired encoding. Finally, pharmacological attenuation of noradrenergic signaling in the NRe and HPC through the infusion of the α2-adrenergic receptor agonist UK 14,304 impaired retrieval and encoding, respectively. Surprisingly, antagonism or agonism of adrenoreceptor subtypes in the mPFC had no effect on memory performance. Together these results reveal the importance of NA within the HPC and NRe for OiP, whereby selectivity of function is achieved via spatially distinct LC output projections and NA receptor subtypes consistent with a modular view of NA function. These results are also important in demonstrating the distinct neuronal mechanisms by which encoding and retrieval are achieved.

Propranolol Sensitizes Vascular Sarcoma Cells to Doxorubicin by Altering Lysosomal Drug Sequestration and Drug Efflux

Angiosarcoma is a rare cancer of blood vessel-forming cells with a high patient mortality and few treatment options. Although chemotherapy often produces initial clinical responses, outcomes remain poor, largely due to the development of drug resistance. We previously identified a subset of doxorubicin-resistant cells in human angiosarcoma and canine hemangiosarcoma cell lines that exhibit high lysosomal accumulation of doxorubicin. Hydrophobic, weak base chemotherapeutics, like doxorubicin, are known to sequester within lysosomes, promoting resistance by limiting drug accessibility to cellular targets. Drug synergy between the beta adrenergic receptor (β-AR) antagonist, propranolol, and multiple chemotherapeutics has been documented in vitro, and clinical data have corroborated the increased therapeutic potential of propranolol with chemotherapy in angiosarcoma patients. Because propranolol is also a weak base and accumulates in lysosomes, we sought to determine whether propranolol enhanced doxorubicin cytotoxicity via antagonism of β-ARs or by preventing the lysosomal accumulation of doxorubicin. β-AR-like immunoreactivities were confirmed in primary tumor tissues and cell lines; receptor function was verified by monitoring downstream signaling pathways of β-ARs in response to receptor agonists and antagonists. Mechanistically, propranolol increased cytoplasmic doxorubicin concentrations in sarcoma cells by decreasing the lysosomal accumulation and cellular efflux of this chemotherapeutic agent. Equivalent concentrations of the receptor-active S-(-) and -inactive R-(+) enantiomers of propranolol produced similar effects, supporting a β-AR-independent mechanism. Long-term exposure of hemangiosarcoma cells to propranolol expanded both lysosomal size and number, yet cells remained sensitive to doxorubicin in the presence of propranolol. In contrast, removal of propranolol increased cellular resistance to doxorubicin, underscoring lysosomal doxorubicin sequestration as a key mechanism of resistance. Our results support the repurposing of the R-(+) enantiomer of propranolol with weak base chemotherapeutics to increase cytotoxicity and reduce the development of drug-resistant cell populations without the cardiovascular and other side effects associated with antagonism of β-ARs.

Ferrous iron binding to epinephrine promotes the oxidation of iron and impedes activation of adrenergic receptors

Upon release in response to stress, epinephrine (Epi) may interact with labile iron pool in human plasma with potentially important (patho)physiological consequences. We have shown that Epi and Fe³⁺ build stable 1:1 high-spin bidentate complex at physiological pH, and that Epi does not undergo degradation in the presence of iron. However, the interactions of Epi with the more soluble Fe²⁺, and the impact of iron on biological activity of Epi are still not known. Herein we showed that Epi and Fe²⁺ build colorless complex which is stable under anaerobic conditions. In the presence of O₂, Epi promoted the oxidation of Fe²⁺ and the formation of Epi-Fe³⁺ complex. Cyclic voltammetry showed that mid-point potential of Epi-Fe²⁺ complex is very low (-582 mV vs. standard hydrogen electrode), which explains catalyzed oxidation of Fe²⁺. Next, we examined the impact of iron binding on biological performance of Epi using patch clamping in cell culture with constitutive expression of adrenergic receptors. Epi alone evoked an increase of outward currents, whereas Epi in the complex with Fe³⁺ did not. This implies that the binding of Epi to adrenergic receptors and their activation is prevented by the formation of complex with iron. Pro-oxidative activity of Epi-Fe²⁺ complex may represent a link between chronic stress and cardiovascular problems. On the other hand, labile iron could serve as a modulator of biological activity of ligands. Such interactions may be important in human pathologies that are related to iron overload or deficiency.

Emerging therapeutic approaches for canine sarcomas: Pushing the boundaries beyond the conventional

Sarcomas represent a group of genomically chaotic, highly heterogenous tumours of mesenchymal origin with variable mutational load. Conventional therapy with surgery and radiation therapy is effective for managing small, low-grade sarcomas and remains the standard therapeutic approach. For advanced, high-grade, recurrent, or metastatic sarcomas, systemic chemotherapy provides minimal benefit, therefore, there is a drive to develop novel approaches. The discovery of "Coley's toxins" in the 19th century, and their use to stimulate the immune system supported the application of unconventional therapies for the treatment of sarcomas. While promising, this initial work was abandoned and treatment paradigm and disease course of sarcomas was largely unchanged for several decades. Exciting new therapies are currently changing treatment algorithms for advanced carcinomas and melanomas, and similar approaches are being applied to advance the field of sarcoma research. Recent discoveries in subtype-specific cancer biology and the identification of distinct molecular targets have led to the development of promising targeted strategies with remarkable potential to change the landscape of sarcoma therapy in dogs. The purpose of this review article is to describe the current standard of care and limitations as well as emerging approaches for sarcoma therapy that span many of the most active paradigms in oncologic research, including immunotherapies, checkpoint inhibitors, and drugs capable of cellular metabolic reprogramming.

Identifying binding modes of two synthetic derivatives of adrenalin to the α2C-adrenoceptor by using molecular modeling; insights into the α2C-adrenoceptor activation

Although, α2C adrenergic receptor (AR) mediates a number of physiological functions in vivo and has great therapeutic potential, the absence of its crystal structure is a major difficulty in the activation mechanism studies and drug design endeavors. Here, a homology model of α2C AR has been presented by means of multiple sequence alignment. The used templates were the latest crystal structures of the other ARs (Protein Data Bank IDs: 2R4R, 2RH1, 4GPO, 3P0G, 4BVN and 4LDO) that have 38.4% identity with the query. We then conducted docking simulations to understand and analyze the binding of noradrenaline (NOR), and its derivatives, namely arachidonoyl adrenalin (AA-AD) and arachidonoyl noradrenalin (AA-NOR) to the receptor. The existence of H-bonds between the ligands and SER218 residue implies the same binding site of derivatives with respect to the NOR. AA-AD and AA-NOR bind to the receptor with the larger binding affinities. The presence of salt bridge between ARG149 and GLU377 in the free receptor, obtained from molecular dynamics studies proved that the receptor still is in its basal state before binding process take places. The activation process is characterized by increasing in the RMSD values of the backbone receptor in the bound state, increasing the RMSF of the transmembrane involved in the activation process and the disappearance of the ARG149-GLU377 salt bridge.

Entropic and enthalpic contributions to stereospecific ligand binding from enhanced sampling methods

The stereoselective binding of R- and S-propranolol to the metabolic enzyme cytochrome P450 2D6 and its mutant F483A was studied using various computational approaches. Previously reported free-energy differences from Hamiltonian replica exchange simulations, combined with thermodynamic integration, are compared to the one-step perturbation approach, combined with local-elevation enhanced sampling, and an excellent agreement between methods was obtained. Further, the free-energy differences are interpreted in terms of enthalpic and entropic contributions where it is shown that exactly compensating contributions obscure a molecular interpretation of differences in the affinity while various reduced terms allow a more detailed analysis, which agree with heuristic observations on the interactions.

Previously unsuspected widespread cellular and tissue distribution of β-adrenoceptors and its relevance to drug action

The discovery of β-adrenoceptors in previously unsuspected cell types is contributing to the rethinking of new drug targets. Recent developments in β-adrenoceptor pharmacology might have excited and surprised James Black, given his interest in developing drugs based on the selective manipulation of receptors to alter physiological responses. β-adrenoceptors continue to generate surprises at molecular and pharmacological levels that often require knowledge of receptor location to interpret. In this review, we emphasize the use of fluorescent ligands as the most selective means of demonstrating receptor localization. Fluorescent ligand binding in live tissues can provide quantitative pharmacological data, under carefully controlled conditions, relevant to other signalling parameters. Consideration of the role of β-adrenoceptors in many cell types (previously ignored) is needed to understand the actions of drugs at β-adrenoceptors throughout the body, particularly in the lung epithelium, vascular endothelium, immune cells and other 'structural' and 'restorative' cell types.

Mechanism of anemia associated with autonomic dysfunction in rats

The aim of this study was to elucidate the mechanism of anemia associated with autonomic dysfunction in rats. Using 6-hydroxydopamine (6-OHDA)-treated sympathectomized rats, changes in systolic blood pressure, plasma catecholamine levels, hemograms, erythropoietin (EPO) secretion, and beta-adrenergic receptors on erythrocytes were monitored, and compared with desipramine- and 6-OHDA-treated, and control rats. In 6-OHDA-treated rats, systolic blood pressure and plasma catecholamine levels significantly decreased from 7 days after 6-OHDA administration, returning to the control values on day 28. Hemoglobin (Hb), hematocrit (Hct) and red blood cell (RBC) levels significantly decreased from day 14 to day 28, and reached normal values after day 35, but neither corpuscular constants nor white blood cell (WBC) levels changed after anemia occurred. Administration of desipramine 1 day before 6-OHDA injection prevented anemia. EPO levels did not elevate, even after bloodletting to load anemia, and the EPO circadian rhythm was irregular in 6-OHDA-treated rats. beta-adrenergic receptors measured using 125I-cyanopindolol (CYP) significantly decreased from day 7 to day 28, and reached normal values after day 35. These results suggest that irregular EPO secretion via disordered autonomic nerves may induce anemia in patients with autonomic disorders.

From epinephrine to cyclic AMP

Binding of catecholamines to the beta-adrenergic receptor results in the activation of adenylate cyclase and the intracellular formation of adenosine 3',5'-monophosphate (cAMP). In the past 20 years the events that lead from hormone binding at the cell surface receptor site to the synthesis of cAMP at the inner layer of the membrane have been intensively studied. Signal transduction in this system involves the sequential interaction of the beta-adrenergic receptor with the guanine nucleotide-binding protein (Gs) and the adenylate cyclase catalyst (C). The mechanism of signal transduction from the receptor through Gs to C, as well as the role of the adenylate cyclase inhibitory G protein Gi, is discussed.

Reemergence of ocular dominance plasticity during recovery from the effects of propranolol infused in kitten visual cortex

We wanted to know whether ocular dominance plasticity can increase under the condition in which the number of available beta adrenoreceptors is expected to increase within kitten visual cortex. We adopted a paradigm in which monocular lid suture was carried out some time after the termination of direct infusion of the cortex with a beta adrenoreceptor antagonist. A significant change in ocular dominance was obtained as shown by a decrease in binocular cortical neurons, when time interval between the end of the d,l-propranolol infusion and the start of monocular deprivation was one week. With a 3-week interval (the longest tested), an even greater change in ocular dominance was evident. This consisted of a marked decrease in binocular neurons and a shift in ocular dominance toward the nondeprived eye. In a control study an inert stereoisomer, d-propranolol, did not block the ocular dominance shift. These results were interpreted as suggesting that the level of ocular dominance plasticity becomes high in parallel to an expected increase in availability of beta adrenoceptors for endogenous noradrenaline (NA). We next asked whether it is possible to accelerate or decelerate the naturally occurring recovery of ocular dominance plasticity. When either NA or tunicamycin (an inhibitor of protein glycosylation) was infused into the same cortical area immediately after the end of the propranolol infusion, opposite effects were observed: exogenous NA accelerated the recovery of the shift in ocular dominance and tunicamycin suppressed it. When tunicamycin infusion was delayed by one week, however, its suppressive effect was negligible. Thus, the restoration of ocular dominance plasticity seems to occur in parallel to an increase in the availability of beta adrenoreceptors for endogenous as well as exogenous NA.

Characterization and localization of (-)[125I]-cyanopindolol binding to non-beta-adrenoceptor sites in dog kidney

1. (-)[125I]-Cyanopindolol (CYP) binding to non-beta-adrenoceptor sites in dog kidney was characterized in homogenate preparations and their distribution in sections determined using autoradiography. 2. In homogenate studies, (-)[125I]-CYP bound to a single population of non-interacting sites (Bmax = 5.45, s.e.m. = 1.00 fmol/mg wet weight; nH = 0.99, s.e.m. = 0.01) with high affinity (KD = 3.84, s.e.m. = 0.76 nmol/l, n = 40. 3. In competition studies, compounds selective for alpha- and beta-adrenoceptors, muscarinic cholinoceptors and receptors for 5-HT, histamine and benzodiazepines, calcium channel antagonists, catecholamine uptake inhibitors, MAO inhibitors and adrenergic neurone blockers were ineffective at concentrations of 10 mumol/l. 4. Compounds selective for dopamine D1-receptors (fluphenazine, SCH 23390 and SK & F 82526) and D2-receptors (pimozide, domperidone, spiperone, haloperidol, sulpiride, cis- and trans-flupenthixol) competed with similar affinities (5-25 mumol/l) for (-)[125I]-CYP binding. 5. In autoradiographic studies, (-)[125I]-CYP binding to non-beta-adrenoceptor sites was localized over glomeruli, juxtaglomerular apparatus, distal tubules, blood vessels and medullary rays and tubules. 6. It is concluded that in dog kidney, (-)[125I]-CYP binds to a site closely associated with dopamine receptors.

Stereospecific antibodies to propranolol

The beta-adrenergic antagonist propranolol was activated through its side chain, coupled to bovine serum albumin, and injected into BALB/c mice. After fusion of the splenocytes from these immunized mice with the NS-1 myeloma cell line, two hybridomas, producing monoclonal anti-propranolol antibodies, were isolated. Clone P-49 was monospecific for propranolol, with a significant preference for the 1-stereoisomer, as compared to the d form. On the other hand, clone P-28 cross-reacted with alprenolol as well as some other beta-antagonists. Both classes of antibodies competed with A431 epidermoid carcinoma beta 2-adrenoceptors for the binding of [3H]propranolol. When ascites cells from clone P-28 were fixed with glutaraldehyde, the anti-propranolol monoclonal antibody became cell bound. These cell-bound P-28 antibodies bind propranolol and other beta-adrenergic ligands with a similar ranking order to the soluble monoclonal antibody. The cell-bound antibody displayed a 5-fold higher affinity towards 1-propranolol than the soluble monoclonal antibody. The practical implications of these findings are discussed.

Autoradiographic analysis of (-)-[125I]-CYP binding in mouse kidney

The distribution and binding characteristics of the radioligand (-)-[125I]-cyanopindolol (CYP) have been examined in slide mounted mouse kidney sections, using the technique of in vitro labelling and autoradiography. (-)-[125I]-CYP binding to sections was of high affinity (KD = 55.8 pmol/l, s.e.m. = 8.1, n = 4) to a single population of non-interacting sites (nH = 0.95, s.e.m. = 0.01, Bmax = 0.74 fmol/section, s.e.m. = 0.12, n = 4) and stereoselective with respect to the (-)- and (+)-isomers of both propranolol and pindolol. Autoradiographic studies showed that (-)-[125I]-CYP binding was localized to areas in the renal cortex and medulla. Both cortical and medullary binding were abolished by the inclusion of (-)-propranolol (1 mumol/l) in the incubation medium, whereas (-)-isoprenaline (200 mumol/l) selectively abolished cortical binding. Medullary binding could be prevented by the inclusion of the lipophilic compounds cinanserin (10 mumol/l), haloperidol (10 mumol/l) or phentolamine (10 mumol/l), either alone or together or by washing at 37 degrees C. These results suggest that medullary binding sites are lipid rather than receptor-related. In conclusion, in mouse kidney sections, (-)-[125I]-CYP binds to discrete areas in the cortex and medulla. Cortical binding sites have the molecular characteristics of beta-adrenoceptors while medullary binding sites are lipid-related. Caution should therefore be exercised when defining non-specific binding of lipophilic radioligands. The autoradiographic technique is useful for discriminating between receptor and non-receptor binding sites.

(+/-)[125Iodo] cyanopindolol, a new ligand for beta-adrenoceptors: identification and quantitation of subclasses of beta-adrenoceptors in guinea pig

(+/-)[125Iodo] cyanopindolol (ICYP) is a radioligand which binds with an extraordinarily high affinity and specificity to beta-adrenoceptors. In contrast to (+/-) [125Ido]-hydroxybenzylpindolol (IHYP), the new ligand has neither affinity to alpha-nor to 5-HT-receptors. The dissociation constants of ICYP for beta- adrenoceptors in various tissues range from 27 to 40 pM, thereby exceeding the affinity of IHYP by a factor of approximately 3. ICYP does not discriminate between beta 1- and beta 2-adrenoceptors. Therefore, the densities of the two receptor subtypes can be determined from competition curves of ICYP by drugs previously found to show in vitro selectivity for beta 1-adrenoceptors. The guinea pig left ventricle contains only beta 1-adrenoceptors, whereas in a lung tissue, the ratio of beta 1-to beta 2-adrenoceptors is 1 to 4. The calculated affinities of five beta 1-selective antagonists for beta 1-adrenoceptors were nearly identical in the ventricle and the lung. Kinetic studies of ICYP binding to guinea pig lung membranes indicated that the dissociation reaction consists of two components, a fast process (t 1/2 = 9 min) and a slower process (t 1/2 = 8.8 h). A mathematical treatment revealed two possibilities of interpretation: 1. Two forms of the receptor exist which are interconvertible. 2. The (+)- and (-)- enantiomers of ICYP dissociate with different rate constants. The low dissociation constant of ICYP in combination with its high specific radioactivity (2175 Ci mmole -1) allows binding studies to be carried out with small protein and ligand concentrations, e.g. 3 microgram protein per assay in guinea pig lung membranes.

Selectivity of dobutamine for adrenergic receptor subtypes: in vitro analysis by radioligand binding

The cardiovascular responses elicited by dobutamine are distinctly different from those produced by other adrenergic or dopaminergic agonists. To test the hypothesis that dobutamine could have differential affinities for adrenergic receptor subtypes, and that such subtype selectivity could be related to its relatively unique pharmacologic properties, we assessed the ability of dobutamine to displace adrenergic radioligands from membrane receptors in a number of tissues of previously characterized adrenergic receptor subtype. For beta adrenergic receptors identified by (-) [(3)H]dihydroalprenolol (DHA), dobutamine had significantly greater affinity for the beta(1) subtype (K(D) = 2.5 muM in rat heart and 2.6 muM in turkey erythrocyte) than for the beta(2) subtype (K(D) = 14.8 muM in frog heart and 25.4 muM in rat lung) (P < 0.001). For alpha adrenergic receptors, dobutamine had markedly greater affinity for the alpha(1)-subtype identified by [(3)H]prazosin (K(D) = 0.09 muM in rat heart and 0.14 muM in rabbit uterus) than for the alpha(2)-subtype identified by [(3)H]dihydroergocryptine (DHE) (K(D) = 9.3 muM in human platelet) or by [(3)H]yohimbine (K(D) = 5.7 muM in rabbit uterus) (P < 0.001). Like other beta(1)-agonists, in the absence of guanine nucleotide, dobutamine competition curves for DHA binding in rat heart demonstrated two classes of binding sites, with one site of significantly higher affinity (K(D) = 0.5 muM, P = 0.008) than the single class of binding sites (K(D) = 5.2 muM) identified in the presence of guanine nucleotide. However, unlike beta(2)- or alpha(2)-agonists, dobutamine displacement of DHA binding in rat lung or of DHE binding in human platelets demonstrated only a single class of binding sites, and guanine nucleotide had only minimal effects. We conclude that dobutamine is selective for beta(1) as opposed to beta(2), and for alpha(1) as opposed to alpha(2) adrenergic receptors. Furthermore, guanine nucleotide effects on dobutamine binding, and biochemical response data in vitro suggest that dobutamine is a beta(1)-agonist, but has little intrinsic activity at beta(2) and alpha(2)-receptors. This selectivity for adrenergic receptor subtypes may be part of the basis for dobutamine's distinctive pharmacologic properties in vivo.

Regulation of collagen production by the beta-adrenergic system

The suppression of collagen production by increasing the cyclic (c) AMP content of cultured cells was examined vis-à-vis the beta-adrenergic system. Cultured human fetal lung fibroblasts incubated for 6 h with the beta-agonists isoproterenol or epinephrine produced approximately 30% less collagen per cell than in the absence of the hormones. To demonstrate that the beta-agonists were operating by their interaction with the beta-receptor to stimulate adenylate cyclase to increase the intracellular content of cAMP, d- and l-isoproterenol were incubated separately with the cultured cells. Only l-isoproterenol increased intracellular cAMP and decreased collagen production. While 20 nM l-isoproterenol was effective, the d-isomer was ineffective even at 2muM. An increase in cAMP from 40 to 73 pmol/mg protein was effective in suppressing collagen production; increasing the cAMP content to much higher levels had little additional effect on collagen production. 3-Isobutyl-1-methylxanthine, an analog of theophylline that inhibits phosphodiesterase, potentiated the effect of isoproterenol in suppressing collagen production. Further support for the concept that isoproterenol suppressed collagen production by acting through the beta-receptor was provided by the finding that only the l-isomer of propranolol, a beta-blocker, was effective in blocking both the increase in intracellular cAMP and the suppression of collagen production caused by isoproterenol. These results demonstrate that collagen production in human fibroblasts can be regulated by the beta-adrenergic system and indicate that when the cAMP content is increased beyond a threshold value, collagen production is suppressed. Since collagen production is sensitive to the small changes of cAMP content of cells brought about by beta-stimulation in cultured cells, the results point to a possibly important mechanism for the regulation of collagen production in the body.

The beta-adrenergic receptor and its mode of coupling to adenylate cyclase

The article first includes a discussion on the classification of catecholamine receptors followed by a discussion on the binding studies of beta-receptors and their affinity labeling. Next a brief discussion on the solubilization and the current attempts to purify the receptor is presented. A large section is then devoted to the mode of coupling between beta-receptors and cyclase where much space is devoted to the role of GTP and of the membrane matrix. The review ends with a discussion on beta-receptor desensitization, supersensitivity, and the "spare receptor" concept.

Cytoskeletal constraint of the beta-adrenergic receptor in frog erythrocyte membranes

A fluorescence receptor binding assay, based upon the high-affinity beta-adrenergic receptor antagonist propranolol, is utilized to probe the microenvironment of the antagonist-receptor complex in the frog (Rana catesbeiana) erythrocyte membrane. The technique of steady-state fluorescence depolarization is applied to the propranolol-receptor complex, allowing quantitation of the rotational relaxation time of the complex. It is found that the complex is dynamically constrained at 20 degrees C. However, in the temperature range 6-10 degrees C a sharp reversible release of constraint is observed. It is further demonstrated that the addition of drugs that are known to specifically disrupt the cytoskeleton (colchicine, vincristine, and vinblastine) causes a similar but irreversible release of constraint at 20 degrees C. Cytochalasin B has a much smaller influence on the rotational mobility of the propranolol-receptor complex than do the other drugs that disrupt the cytoskeleton. Amphotericin B is without effect on the rotational constraint of the complex. Binding of the antagonist [3H]dihydroalprenolol is not influenced by colchicine. A model is proposed which postulates that cytoskeletal elements are linked to the antagonist-receptor complex. Antagonist binding does not result in cytoskeletal release, whereas agonist binding is postulated to lead to dissociation of the agonist-receptor complex from the cytoskeleton, thereby activating adenylate cyclase.

Transport of catecholamines by native and reconstituted rat heart synaptic vesicles

Highly purified "heavy" synaptic vesicles were isolated from rat heart by differential centrifugation. Because of the high intravesicular concentrations of proteins, catecholamine, and ATP, resealed vesicle ghosts were prepared and used to study the detailed kinetics of catecholamine transport. ATP stimulated the uptake of l-norepinephrine and was saturable with a Km for l-norepinephrine at 3.3 microM and 1.8 mM for ATP. The ghosts also accumulated 5-hydroxytryptamine and l-epinephrine via an ATP-dependent mechanism. Uptake was stereospecific for the l-form. A functional catecholamine transporter could be solubilized by the detergent octyl-glucoside and incorporated into phospholipid vesicles, which, after detergent removal, generated proteoliposomes that accumulated l-norepinephrine. Reserpine- and l-propranolol-sensitive accumulation against a concentration gradient is achieved by artificially creating a pH gradient across the membrane, and lends further support to the idea that at least the initial phase of catecholamine transport is driven by the trans-membrane pH gradient created by the proton-translocating ATPase.

beta 1-Adrenergic receptors in kidney tubular cell membrane in the rat

We used a beta-adrenergic antagonist, (-) 3H-dihydroalprenolol, to demonstrate binding sites in purified rat kidney preparations that consistsed of plasma membranes of cells from tubules. The tubular origin of these plasma membranes was shown by electron microscopy and Na-K-ATPase enrichment. The binding was rapid (t1/2, 78 sec) and rapidly reversible (t1/2, 48 sec). The binding sites were saturable and bound 69.8 +/- (SD) 29.1 fmoles/mg of membrane protein. The binding was stereospecific with the isomers of beta-adrenergic agonists and beta-adrenergic antagonist propranolol, the (-) isomers being about 40 times more potent than the (+) isomers incompeting for these sites. (-) 3H-dihydroalprenolol had a high affinity for the binding sites, expressed by the mean equilibrium dissociation constant (KD) (KD, 7.1 nM). The beta-adrenergic antagonist (-) propranolol also showed high affinity (KD, 62.8 nM). The order of potency for inhibition of binding by beta-adrenergic agonists was: (-) isoproterenol (KD, 0.66 microM) greater than (-) epinephrine (KD, 4.3 microM) greater than or equal to (-) norepinephrine (KD, 13.5 microM). Conclusion. The (-) 3H-dihydroalprenolol binding sites in the rat kidney tubular cell membrane are beta-adrenergic receptor of the beta1 subgroup.

Biochemical characterization of the beta-adrenergic receptor of the frog erythrocyte

The beta-adrenergic receptor which is coupled to adenylate cyclase in the frog erythrocyte plasma membrane provides a convenient model system for probing the molecular characteristics of an adenylate cyclase coupled hormone receptor. Direct radioligand binding studies with beta-adrenergic agonists and antagonists such as [3H]hydroxybenzylisoproterenol and [3H]dihydroalprenolol have shed new light on the biochemical properties of the receptor as well as on its mode of interaction with other components of the adenylate cyclase system. Agonist binding to the receptor induces a high affinity state of the receptor which can be selectively reverted to a low agonist affinity state by guanyl nucleotides. This agonist-induced high affinity state of the receptor appears to correspond to a receptor moiety which has larger apparent molecular weight and which is probably a complex of the beta-adrenergic receptor and nucleotide regulatory binding protein. Antagonists do not appear capable of inducing or stabilizing the formation of this high affinity receptor-nucleotide site complex. The beta-adrenergic receptors have been solubilized using the plant glycoside digitonin as the detergent and have been highly purified by biospecific affinity chromatography on an alprenolol-agarose affinity support. These highly purified receptor preparations retain all of the binding characteristics observed in the unpurified soluble receptor preparations. Remarkably, antibodies raised in rabbits against affinity chromatography purified preparations of the receptor, themselves bind beta-adrenergic ligands with typical beta-adrenergic specificity. Such antibodies which possess binding sites similar to those of physiological receptors provide useful model systems for further probing the molecular characteristics of beta-adrenergic binding sites.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

A comparison between propranolol and hypothermia in preventing ischemic contracture of the left ventricle (stone heart)

Ischemic contracture of the left ventricle ("stone heart") was studied utilizing a previously described stone heart model. Our studies suggest that beta-adrenergic blockade is not quantitatively as important as hypothermia in protecting ischemic myocardium. On the basis of reduced fibrillatory activity and a slight protective effect shown by electron microscopy, it would appear that combining propranolol with hypothermia may be superior to either used singly.

Adenyl cyclase

Certain hormones regulate the activity of their target cells by stimulating adenyl cyclase, which is an enzyme located within the target cell's plasma membrane. Adenyl cyclase catalyzes the formation of cyclic AMP, which is released into the cell and modulates cell functions. In this communication the characteristics of adenyl cyclase are reviewed. The coupling between hormone receptors and this enzyme is discussed as is the ability of agents such as hormones, ATP, magnesium, calcium, guanine nucleotides and prostaglandins to alter cyclase activity. Several diseases that result from derangements of the adenyl cyclase system are known and the molecular bases for these diseases are discussed in this review.

Probing of beta-adrenergic receptors by novel fluorescent beta-adrenergic blockers

The synthesis of two high-affinity fluorescent beta-adrenergic blockers is described: dl-N(1)-[2-hydroxy-3-(1-naphthyloxy)propyl]-N(2)-(9-acridyl)-1,2-propanediamine (9-aminoacridylpropanolol, 9-AAP) and dl-N-[2-hydroxy-3-(1-naphthyloxy)propyl]-N'-dansylethylenediamine (dansyl analogue of propranolol, DAPN). Both 9-AAP and DAPN inhibit competitively the l-epinephrine-dependent adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in turkey erythrocyte membranes without affecting the fluoride-stimulated adenylate cyclase activity. Similarly, 9-AAP and DAPN inhibit in a competitive manner the binding of [(125)I]-iodohydroxybenzylpindolol to these beta-adrenergic receptors. The two fluorescent beta-adrenergic blockers 9-AAP and DAPN probe specifically beta-adrenergic receptors in the central nervous system as well as in other organs when injected into rats. The fluorescence pattern can be monitored by fluorescence microscopy performed on cryostat slices of these organs. The appearance of the characteristic fluorescence pattern can be blocked in a stereospecific fashion by a prior injection of l-propranolol and not by a prior injection of d-propranolol. These compounds therefore offer a powerful means to map beta-adrenergic receptors in vivo. The stereospecific displacement of 9-AAP from the beta-adrenergic receptors of turkey erythrocyte membranes by l-propranolol and by l-epinephrine can be detected in vitro using front-face fluorescence. The potential use of these compounds to probe beta-receptors in vitro and in vivo is discussed.

Cell membrane receptors for cardiac glycosides in the heart

Cell membranes contain special binding proteins for hormones and drugs. These binding sites ("receptors") located on the outside surface are linked to or are part of an enzyme facing the inner side of the membrane and are transducing and probably amplifying the information carried by the pharmacological agent to the cell. As the first step of their action cardiac glycosides reversibly bind with high affinity to specific receptors in cardiac cells and by this inhibit the (Na+ + K+)-ATPase, which is the enzyme system responsible for the active transmembraneous transport of sodium and potassium. It is thought that the inhibition of this active cation transport precedes the positive inotropic effect. Cardio-inactive glycosides have but low affinity to this receptor and thus do inhibit the (Na+ + K+)-activated ATPase only at very high concentrations. The characterization of the cardiac glycoside-receptor interaction in the heart reveals several factors that influence the affinity of the binding sites for the glycosides and thereby determine the sensitivity to this widely used group of potent drugs.

Catecholamine binding to the beta-adrenergic receptor

The adenylate cyclase-coupled beta-adrenergic receptors of frog erythrocyte membranes have been identified by direct radioligand binding techniques using the potent catecholamine agonist (+/-)[3H]hydroxybenzylisproterenol (2-[3, 4-dihydroxyphenyl]-2-hydroxy-1', 1'-dimethyl-2'-[4-hydroxyphenyl]-diethylamine). The successful experimental conditions included the use of (i) high concentrations of catechol and ascorbic acid to suppress nonreceptor binding, (ii) a very potent radiolabeled catecholamine (10 times more potent than isoproterenol), and (iii) membranes rich in binding sites for beta-adrenergic receptors. Thus, previous problems in accomplishing successful catecholamine binding to the beta-receptors have been overcome. The binding sites identified with (+/-)[3H]hydroxybenzylisoproterenol in the erythrocyte membranes have all the characteristics expected of true beta-adrenergic receptors. These include rapidity of binding, saturability, specificity for beta-agonists and antagonists, and stereospecificity [(-)isomers more potent than (+)isomers]. Physiologically inactive compounds containing a catechol moiety do not compete for occupancy of these binding sites. Dissociation of the radiolabeled agonist from the receptors is slow and incomplete in the absence of guanine nucleotides. In the presence of nucleotide, however, dissociation is rapid and complete. beta-Adrenergic agonists and antagonists compete for the (+/-)[3H]hydroxybenzylisoproterenol binding sites in a fashion parallel to their competition for the receptors, as previously delineated with the beta-adrenergic antagonist (-)[3H]dihydroalprenolol.

Beta-adrenergic receptors in rat myocardium: direct detection by a new fluorescent beta-blocker

A new fluorescent beta-blocker, 9-amino-acridin propranolol (9-AAP), was administered i.v. to rats. Multiple fluorescent 9-AAP binding sites were observed on cardiac muscle cells in frozen sections. Intensity and density of cardiac 9-AAP fluorescence were markedly reduced following pretreatment with (+/-)- and (-)-propranolol but not with (+)-propranolol. Our findings suggest that 9-AAP may label beta-adrenergic receptor sites in rat myocardium.

Affinity label for beta-adrenergic receptor in turkey erythrocytes

The compound N-[2-hydroxy-3-(1-naphthoxy)-propyl]-N'-bromoacetylethylenediamine (NHNP-NBE) was found to label covalently the beta-adrenergic receptor in turkey erythrocytes. The compound inhibits irreversibly 1-epinephrine-dependent adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in the whole turkey erythrocyte as well as in the erythrocyte membranes possessing the beta-receptor. The affinity label blocks, also irreversibly, the specific [3H] propranolol binding, whereas other bromoacetyl compounds tested have no effect on binding, even at high concentrations, which cause enzyme inactivation. 1-Epinephrine and propranolol offer protection against the affinity label in whole turkey erythrocytes as well as in membranes prepared from these cells. The potential usefulness of an irreversible beta-antagonist is discussed.