Low Affinity Binding Component Research Articles

Oligomerization Study of Human Organic Anion Transporting Polypeptide 1B1.

Organic anion-transporting polypeptides play important roles in the uptake of various endogenous and exogenous compounds. It has been proposed that OATP family members, as membrane proteins, may form oligomers. However, oligomerization status of OATPs is still largely unclear. In the present study, HEK293 cells stably expressing OATP1B1 were generated to investigate the oligomerization status of the transporter. Chemical cross-linking and coimmunoprecipitation experiments revealed that OATP1B1 may form homo-oligomers, possibly through disulfide bonds. When wild-type OATP1B1 was coexpressed with a loss-of-function mutant W258A, cells showed reduced uptake of prototypic substrate estrone-3-sulfate (ES). Interestingly, such a coexpression did not affect OATP1B1 transport activity of high concentrations ES, implicating that oligomerization status may affect only the high affinity component of ES. OATP1B1 possesses three GXXXG motifs that have been associated with protein dimerization in other membrane proteins. When glycine residues were replaced with alanine, G219A and G393A showed drastically reduced uptake function. Further studies revealed that G219A has a similar association capability to that of the wild-type, while mutation at Gly393 may affect oligomerization status of the transporter. Kinetic analysis showed that both G219A and G393A have a dramatically reduced Vmax for ES uptake. Km of G219A was increased while that of G393A exhibited a decreased value for high affinity component of ES binding. Our studies demonstrated that OATP1B1 may function as oligomers in the high affinity site of ES while acting as monomers for the low affinity binding component of the substrate.

Nanomolar concentrations of cocaine enhance D2-like agonist-induced inhibition of the K+-evoked [3H]-dopamine efflux from rat striatal synaptosomes: a novel action of cocaine

Previous studies have indicated that cocaine binding sites contain both high- and low-affinity binding components and have actions not related to dopamine uptake inhibition. Therefore, it has been studied if concentrations of cocaine in the range of 0.1-100 nM can affect not only dopamine uptake but also the quinpirole-induced inhibition of the K(+)-evoked [(3)H]-dopamine efflux from rat striatal synaptosomes. It was found that quinpirole-induced inhibition of K(+)-evoked [(3)H]-dopamine efflux was significantly enhanced by cocaine at 1 and 10 nM but not at 0.1 nM with cocaine alone being inactive and 1 nM cocaine lacking effects on [(3)H]-dopamine uptake in rat striatal synaptosomes. The results indicate the existence of a novel allosteric agonist action of cocaine in low concentrations, not affecting dopamine uptake, at striatal D(2) autoreceptors modulating striatal dopamine transmission.

Separate Functions for the Two Modules of the Membrane-proximal Cytokine Binding Domain of Glycoprotein 190, the Leukemia Inhibitory Factor Low Affinity Receptor, in Ligand Binding and Receptor Activation

The receptor for the cytokine leukemia inhibitory factor (LIF) associates the low affinity binding component gp190 and the high affinity converter gp130. Both are members of the hematopoietic receptors family characterized by the cytokine receptor homology (CRH) domain, which consists of two barrel-like modules of around 100 amino acids each. The gp190 is among the very few members of this large family to contain two CRH domains. The membrane-distal one (herein called D1) is followed by an immunoglobulin-like domain, a membrane-proximal CRH domain called D2, and three type III fibronectin-like repeats. A minimal D1IgD2 fragment is required for binding LIF. By using transmembrane forms of deletion mutants in gp190 ectodomain, we demonstrated that removal of D1 led to spontaneous activation of the receptor and that this property was devoted to a peptidic sequence localized within the last 42 amino acids of the carboxyl-terminal module of D2. By using soluble forms of deletion mutants made by progressive truncations from the end of the D1IgD2 fragment, we demonstrated that the carboxyl-terminal module of D2 was dispensable for LIF binding and that the correct conformation of the D1Ig fragment required a full amino-terminal module of D2. Therefore, the two constitutive modules of the membrane-proximal CRH domain D2 of gp190 fulfill two distinct roles in gp190 function, i.e. in stabilizing the conformation of gp190 allowing LIF binding and in activating the receptor.

Identification of Agonistic and Antagonistic Antibodies against gp190, the Leukemia Inhibitory Factor Receptor, Reveals Distinct Roles for Its Two Cytokine-binding Domains

The receptor for the cytokine leukemia inhibitory factor (LIF) associates the low affinity binding component gp190 and the high affinity converter gp130, both of which are members of the family of hematopoietic receptors characterized by the cytokine receptor homology (CRH) domain. The gp190 is among the very few members of this large family to contain two CRH domains. The membrane-distal one (herein called D1) is followed by an Ig-like domain, a membrane-proximal CRH domain called D2, and three type III fibronectin repeats. We raised a series of monoclonal antibodies specific for the human gp190. Among them was the blocking antibody 1C7, which was directed against the D1Ig region and which impaired the binding of LIF to gp190. Another blocking antibody, called 12D3, was directed against domain D2 and interfered with the reconstitution of the high affinity receptor complex, independently of the interaction between LIF and gp190. The blocking effect of these two antibodies concerned four cytokines known to use gp190, i.e. LIF, oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1. Among 23 antibodies tested alone or in combination (two anti-D2 and 21 anti-D1Ig), only the mixture of the two anti-D2 antibodies displayed agonistic activity in the absence of the cytokine. Taken together, these results demonstrate that the two CRH domains of gp190 play different functions in ligand binding and receptor activation.

Interaction of a recombinant form of apolipoprotein[a] with human fibroblasts and with the human hepatoma cell line HepG2

We have studied the binding, uptake, and degradation of a recombinant form of apolipoprotein[a] (r-apo[a]) using a cultured cell model. In HepG2 cells and in human fibroblasts, r-apo[a] complexed with low density lipoprotein(LDL) is bound and internalized via high affinity (Kd = 10 nM) receptors; in both cell types, low affinity (Kd = 200-300 nM) sites also mediate free apo[a] uptake. Using competition studies, we found that the high affinity binding component corresponds to the LDL receptor. Involvement of the LDL receptor in r-apo[a] uptake by fibroblasts was confirmed using fibroblasts derived from an individual homozygous for familial hypercholesterolemia; in contrast to normal fibroblasts, these cells lacked the high affinity r-apo[a] binding component. Cell association of 125I-labeled r-apo[a] was increased and decreased concomitantly with the up- and down-regulation of the LDL receptor in response to a number of compounds. The addition of alpha 2-macroglobulin as well as treatment with heparinase, chondroitinase ABC, and sodium chlorate did not decrease total specific binding of r-apo[a], suggesting that neither the low density lipoprotein receptor-related protein nor cell surface proteoglycans are involved in r-apo[a] clearance. The low affinity binding component present in both fibroblasts and HepG2 cells likely corresponds to the plasminogen receptor, as binding of r-apo[a] to these sites was specifically decreased by the addition of plasminogen or the lysine analogue epsilon-aminocaproic acid, but not by the addition of tissue-type plasminogen activator. Heparin abolished uptake of r-apo[a] by the LDL receptor component only; this indicates that apo[a] must be associated with LDL to be cleared by this receptor. In contrast, free apo[a] can be effectively cleared by the plasminogen receptor which may represent a significant route of clearance for free apo[a] in vivo.

Characterization of [3H]MK-801 binding to N-methyl-D-aspartate receptors in cultured rat cerebellar granule neurons and involvement in glutamate-mediated toxicity.

The conditions required for growth and survival of cerebellar granule neurons in vitro are known to alter the developmental regulation of NMDA receptor subunit mRNA. In the present report, we have examined the functional and pharmacological characteristics of NMDA receptors on cerebellar granule neurons at 12 days in culture (12 DIC). Under open-channel conditions in extensively washed membranes, [3H]MK-801 labeled a uniform population of sites (Kd = 3.2 +/- 0.3 nM) in a saturable manner (Bmax = 416 +/- 18 fmol/mg); however, biexponential association and dissociation kinetics indicated the possible existence of at least two NMDA receptor populations that differ in pharmacological properties. The kinetically derived equilibrium dissociation constants for the high- and low-affinity binding components were 0.56 and 771 nM, respectively. The equilibrium competition analysis of MK-801 and other channel-blocking compounds as displacers of [3H]MK-801 revealed the presence of high- and low-affinity binding sites with relative apportionments of 70% and 30%, respectively. The rank-order potency profile of competitor binding at the high-affinity site was (+)-MK-801 > TCP > dextrorphan > dextromethorphan > (+)-ketamine. When tested for the ability to protect 12 DIC cerebellar granule neurons from acute glutamate-induced toxicity, the neuroprotective rank-order potency of these compounds was MK-801 > TCP > dextrorphan > (+)-ketamine > dextromethorphan, which correlated significantly with the high-affinity competition binding profile and thus established the role of NMDA receptors in glutamate toxicity. The findings of these experiments indicate that NMDA receptors on 12 DIC cerebellar granule neurons are a heterogenous population that functionally mediate glutamate-induced neurotoxicity. The heterogenous [3H]MK-801 binding sites may represent NMDA receptor channels composed of different subunits.

Mapping dopamine transporters in the human brain with novel selective cocaine analog [125I]RTI-121.

The novel cocaine analog RTI-121 [3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid isopropyl ester] was evaluated as a probe for the in vitro labeling and localization of the dopamine transporter in the human brain. Saturation binding experiments conducted in sucrose-phosphate buffer (10 mM sodium phosphate, pH 7.4, 0.32 M sucrose) revealed high- and low-affinity binding components with affinity values (KD) of 0.25 +/- 0.04 and 4.9 +/- 1.6 nM (mean +/- SE) and densities (Bmax) of 56.8 +/- 13.8 and 147.7 +/- 23.4 pmol/g tissue, respectively. In contrast, when saturation binding experiments were performed in phosphate-buffered saline (10 mM Na2HPO4, 1.8 mM KH2PO4, 136 mM NaCl, 2.8 mM KCl, 10 mM NaI, pH 7.4), a 9-fold decrease in the density of the low-affinity component was noted, suggesting that the low-affinity RTI-121 binding site is associated with the region of the transporter involved in the ionic dependence of substrate recognition and/or uptake. The rank order of potency for inhibition of [125I]RTI-121 binding to human caudate membranes demonstrates that the radioligand selectively labels the dopamine transporter (GBR 12909 > RTI-121 > mazindol > nomifensine > (-) cocaine > desipramine > citalopram). Autoradiographic mapping of [125I]RTI-121 revealed very high densities of cocaine recognition sites over areas known to be rich in dopaminergic innervation, including the caudate, putamen, and nucleus accumbens. Moderate densities were also observed over the substantia nigra and the ventral tegmental area. Low-to-background labeling of [125]RTI-121 was seen throughout the cerebral cortex, amygdaloid nuclei, globus pallidus, and thalamus. In comparison with the autoradiographic distribution of the cocaine analogs [3H]WIN 35,428 (or CFT) and [125I]RTI-55 (or beta-CIT), the labeling pattern for [125I]RTI-121 was more restricted. These studies demonstrate that [125I]RTI-121 labels dopamine-rich brain regions with greater selectivity than other currently available cocaine analogs, which makes it a potentially superior imaging probe for mapping the dopamine transporter in the human brain.

Nitromethyleneimidazolidine Radioligand ([ 3H]NMI): High Affinity and Cooperative Binding for House Fly Acetylcholine Receptor

1-[ N-(2-Chloro-5-thiazolylmethyl)]-2-nitromethylene-imidazolidine (NMI) is a very potent insecticide and is 6-fold more effective than imidacloprid (IMI) in displacing [ 3H]IMI from its binding site in the house fly acetylcholine (ACh) receptor (AChR). NMI differs from IMI in two isosteric replacements, i.e., 2-chloro-5-thiazolyl (CT) for 6-chioro-3-pyridinyl (CP) and nitromethylene for nitroimine. The CP and CT moieties in this series confer almost equivalent potency and binding properties allowing intercomparisons based on the nitromethylene and nitroimine substituents. [ 3H]NMI (55 Ci/mmol) was prepared from 2-chloro-5-(carbethoxy)thiazole by reducing with lithium aluminum tritide to the alcohol which was converted to the chloromethyl derivative and then coupled with ethylenediamine followed by reaction with 1,1-bis(methylthio)-2-nitroethylene. Binding parameters in house fly head membranes treated with Triton X-100 are very similar for [ 3H]NMI and [ 3H]IMI, each with a single saturable specific binding site of K d = 1.2 n M and B max = 853-897 fmol/mg protein, and there are also similar initial rates of association and dissociation for the two radioligands. However, there is a significant difference in the Hill coefficient with 1.4 ± 0.06 for NMI and 1.0 ± 0.1 for IMI. Without Triton X-100 treatment, there are both low and high affinity binding components for [ 3H]IMI but only a low affinity one for [ 3H]NMI. Competing ligands are less effective at displacing [ 3H]NMI than [ 3H]IMI, e.g., 9-fold for ACh (with paraoxon to inhibit acetylcholinesterase), 40-fold for carbachol, and 2- to 6-fold for the nicotinic agents (−)-nicotine and α-bungarotoxin. The enhanced insecticidal activity and receptor potency of NMI compared with IMI may be associated with its higher apparent cooperativity facilitating disruption of the AChR.

Kinetic studies of killer toxin K1 binding to yeast cells indicate two receptor populations.

A recently described new method for determination of killer toxin activity was used for kinetic measurements of K1 toxin binding. The cells of the killer sensitive strain Saccharomyces cerevisiae S6 were shown to carry two classes of toxin binding sites differing widely in their half-saturation constants and maximum binding rates. The low-affinity and high-velocity binding component (KT1 = 2.6 x 10(9) L.U./ml, Vmax1 = 0.19 s-1) probably reflects diffusion-limited binding to cell wall receptors; the high-affinity and low-velocity component (KT2 = 3.2 x 10(7) L.U./ml, Vmax2 = 0.03 s-1) presumably indicates the binding of the toxin to plasma membrane receptors. Adsorption of most of the killer toxin K1 to the surface of sensitive cells occurred within 1 min and was virtually complete within 5 min. The amount of toxin that saturated practically all cell receptors was about 600 lethal units (L.U.) per cell of S. cerevisiae S6.

Characterization of calcium binding to brain spectrin.

Brain spectrin alpha and beta chains bind 45Ca2+, as shown by the calcium overlay method. Flow dialysis measurements revealed eight high affinity binding sites/tetramer that comprise two binding components (determined by nonlinear regression analysis). The first component has one or two sites (kd = 2-30 x 10(-8) M), depending on the ionic strength of the binding buffer, with the remaining high affinity sites in the second component (kd = 1-3 x 10(-6) M). In addition, there is a variable, low affinity binding component (n = 100-400, kd = 1-2 x 10(-4) M). Magnesium inhibits calcium binding to the low affinity sites with a K1 = 1.21 mM. Proteolytic fragments from trypsin or chymotrypsin digests of brain spectrin bind 45Ca2+ if they include alpha domain IV, alpha domain III, or the amino-terminal half of the beta chain (but more than 25 kDa from the amino-terminal). These data suggest that calcium ions bind with high affinity to the putative EF-hands in alpha domain IV and to one site in the amino-terminal half of the beta chain that is associated with alpha domain IV in the native dimer. The localization is consistent with a direct calcium modulation of the spectrin-actin-protein 4.1 interaction. In addition, there appears to be one high affinity site near the hypersensitive region of alpha brain spectrin. All four proposed binding sites occur near probable calmodulin-binding or calcium-dependent protease cleavage sites.

High and low affinity receptors for human interleukin for DA cells/leukemia inhibitory factor on human cells. Molecular characterization and cellular distribution.

Radioiodinated recombinant human interleukin DA (HILDA)/leukemia inhibitory factor (LIF) purified from conditioned medium of Chinese hamster ovary transfected cells enabled the identification of specific receptor sites on a variety of human cell types. Using low concentrations (up to 500 pM) of the ligand iodinated at a high specific radioactivity, high affinity receptors (equilibrium dissociation constant Kd in the range of 30-100 pM) were first demonstrated. They were expressed at low levels by human peripheral blood monocytes but not by lymphocytes, NK cells, granulocytes, and platelets. The myelomonocytic cell line THP1 as well as the T lymphoma cell line HSB2 and the lymphoblastoid B cell line DAB were also receptor-negative. In contrast, most of the non-lymphoid tumoral cell lines tested, including melanomas, neuroblastomas, and carcinomas, expressed high affinity HILDA/LIF receptors at variable levels (Bmax from 20 to 600 sites/cell). The kinetics of HILDA/LIF high affinity binding to the choriocarcinoma JAR cell line were characterized at 4 degrees C with association and dissociation rate constants of k1 = 2.2 10(9) M-1 min-1 and k-1 = 0.0084 min-1, respectively, corresponding to a steady-state dissociation constant k1/k-1 = 3.8 pM. The subsequent use of higher concentrations of HILDA/LIF labeled at a lower specific radioactivity enabled the identification of a low affinity component on several cell lines (Kd in the range of 1-4 nM; Bmax from 1,000 to 5,000 sites/cell). On JAR cells, this low affinity component was characterized by association and dissociation rate constants at 4 degrees C of k1 = 7.3 10(7) M-1 min-1 and k-1 = 0.19 min-1, respectively (k-1/k1 = 2.6 nM). Affinity cross-linking of HILDA/LIF to JAR cells showed two cross-linked species under both reducing and nonreducing conditions corresponding to receptor species of 120 and 250 kDa, respectively. Whereas both bands had similar intensities under high affinity conditions, the higher band predominated under low affinity conditions. Our data suggest that the 250-kDa chain could constitute the low affinity binding component whereas the association of both 250- and 120-Da subunits would form the high affinity structure.

Determination of the structural domain of ApoAI recognized by high density lipoprotein receptors.

There is good evidence that high density lipoprotein (HDL) interacts with high affinity sites present on hepatocytes. The precise nature of the ligand recognized by putative HDL receptors remains controversial, although there is a consensus that apolipoprotein AI (apoAI) is involved. This suggestion would be strengthened if a biologically active site demonstrating a high affinity for the receptor could be isolated. Cyanogen bromide fragments (CF) of apoAI (CF1-CF4) were complexed with phospholipid, and their ability to associate with the receptor was compared in various binding studies. Careful analysis of the concentration-dependent association of 125I-labeled dimyristoyl phosphatidylcholine (DMPC) recombinants to rat liver plasma membranes revealed high and low affinity binding components. As all DMPC recombinants displayed the low affinity binding component, it was postulated that this interaction was independent of the protein present in the particle and may well represent a lipid-lipid or lipid-protein association with the membranes. Only 125I-labeled CF4.DMPC displayed a high affinity binding component with similar Kd and Bmax (8 x 10(-9) M, 1.6 x 10(-12) mol/mg plasma membrane protein) to that of 125I-labeled AI.DMPC (7 x 10(-9), 1.4 x 10(-12) mol/mg plasma membrane protein). Similarly, egg yolk phosphatidylcholine complexes containing CF4 (CF4.egg PC) showed higher affinity binding than CF1-egg yolk phosphatidylcholine complexes confirming the results obtained with DMPC complexes. Furthermore, ligand blotting studies showed that only 125I-labeled CF4.DMPC associated specifically with HB1 and HB2, two HDL binding proteins recently identified in rat liver plasma membranes. We conclude that a region within the carboxyl-terminus of apoAI is responsible for the interaction with putative HDL receptors present in rat liver plasma membranes.

Characterization of axonally transported [125I]PYY binding sites in rat vagus nerve

Quantitative receptor autoradiography was used to characterize [125I]PYY binding to slide-mounted sections of rat cervical vagus nerve which had been ligated 24 h prior to extraction. Saturation and competitive binding characteristics were determined for the accumulation of binding sites detected in the segment of the nerve proximal to the ligature. High and low affinity binding components were resolved (Kd = 93 pM and 2.1 nM). The rank order of potency for competing ligands was PYY greater than NPY much greater than PYY13-36 much greater than [Pro34]NPY suggesting that vagal PYY/NPY binding sites are PYY-preferring and of the Y2 type.

Modulation of Opioid Receptor Binding by Cis and Trans Fatty Acids

In synaptosomal brain membranes, the addition of oleic acid (cis), elaidic acid (trans), and the cis and trans isomers of vaccenic acid, at a concentration of 0.87 mumol of lipid/mg of protein, strongly reduced the Bmax and, to a lesser degree, the binding affinity of the mu-selective opioid [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol ([3H]DAMGO). At comparable membrane content, the cis isomers of the fatty acids were more potent than their trans counterparts in inhibiting ligand binding and in decreasing membrane microviscosity, both at the membrane surface and in the core. However, trans-vacenic acid affected opioid receptor binding in spite of just marginally altering membrane microviscosity. If the receptors were uncoupled from guanine nucleotide regulatory protein, an altered inhibition profile was obtained: the impairment of KD by the fatty acids was enhanced and that of Bmax reduced. Receptor interaction of the delta-opioid [3H](D-Pen2,D-Pen5)enkephalin was modulated by lipids to a greater extent than that of [3H]DAMGO: saturable binding was abolished by both oleic and elaidic acids. The binding of [3H]naltrexone was less susceptible to inhibition by the fatty acids, particularly in the presence of sodium. In the absence of this cation, however, cis-vaccenic acid abolished the low-affinity binding component of [3H]naltrexone. These findings support the membrane model of opioid receptor sequestration depicting different ionic environments for the mu- and delta-binding sites. The results of this work show distinct modulation of different types and molecular states of opioid receptor by fatty acids through mechanisms involving membrane fluidity and specific interactions with membrane constituents.

Separation of different receptor-mediated effects of a prostaglandin H 2 analogue (U46619) on human platelets by means of human granulocytic elastase and chymotrypsin

Previous investigations indicated two classes of thromboxane A 2/prostaglandin H 2 (TXA 2/PGH 2) receptors on human platelets and suggested that shape change and myosin light chain phosphorylation correlated with the occupancy of high affinity receptors while serotonin release was related to a putative low affinity binding component (Morinelli TA et al., Am J Physiol 253: H1035–H1043, 1987). The current study shows that chymotrypsin destroyed three receptor-mediated responses of platelets to U46619 (a TXA 2/PGH 2 agonist), i.e. shape change, myosin light chain phosphorylation and serotonin release. Human granulocyte elastase selectively inactivated platelet ability to release serotonin following stimulation with U46619, but it did not affect significantly shape change and myosin light chain phosphorylation. In conclusion, it is possible to separate different receptor-mediated effects of U46619 on human platelets by means of human granulocytic elastase and chymotrypsin.

β-VLDL and acetylated-LDL binding to pigeon monocyte macrophages

Blood-derived monocytes are an important source of foam cells in atherosclerotic lesions of White Carneau pigeons. Based upon studies with cultured blood monocytes (monocyte macrophages) and peritoneal macrophages from a variety of mammalian species, it has been proposed that these cells become loaded with cholesteryl esters through the uptake of lipoproteins including, β-migrating very low density lipoproteins (β-VLDL) and low density lipoproteins that have been chemically modified in a manner analogous to experimental acetylation (Ac-LDL). The purpose of this study was to determine whether similar mechanisms functioned in pigeon monocyte macrophages. Radioiodinated pigeon β-VLDL and Ac-LDL were incubated with White Carneau pigeon monocyte macrophages that had been maintained in culture for 7 days. Scatchard analysis of the specific binding data revealed the presence of specific and saturable receptors for both β-VLDL and Ac-LDL. β-VLDL receptors had both low and high affinity binding components, whereas Ac-LDL receptors displayed a single class of high affinity binding sites. β-VLDL binding remained relatively constant from 3 to 10 days in culture while Ac-LDL binding increased with time in culture. Competition studies demonstrated a high degree of binding specificity for 125I-Ac-LDL, but less for 125I-β-VLDL. Binding of 125I-β-VLDL was not competed for by Ac-LDL, but was by β-VLDL and by low density lipoproteins from both normal and hypercholesterolemic pigeons. Following binding of β-VLDL and Ac-LDL, the lipoproteins were rapidly internalized and degraded. Although the majority of degradation was secondary to internalization by the monocyte macrophages, approx. 5% of the degradation resulted from enzymatic activity in the culture medium, presumably due to secretion of proteolytic enzymes by the cells. As measured by esterification of [ 14C]oleate to cholesterol, it was shown that the cholesterol liberated from the degradation of both β-VLDL and Ac-LDL stimulated cholesteryl ester synthesis in pigeon monocyte macrophages. These studies confirm the existence of specific β-VLDL and Ac-LDL receptors on the surface of pigeon monocyte macrophages which facilitate both internalization of the lipoproteins and subsequent stimulation of cholesteryl ester synthesis. This is the first demonstration of β-VLDL and Ac-LDL receptors on monocyte macrophages from an avian species, and the findings support the potential role for the receptor-mediated uptake of a variety of abnormal lipoproteins in the formation of monocyte-derived foam cells in the arterial wall of White Carneau pigeons during the development of atherosclerosis.

Characteristics and displaceability of neurotensin binding sites in the rat cerebral cortex and corpus striatum

1. Neurotensin binding to synaptosomes isolated from the rat cerebral cortex and corpus striatum reached saturation after 4 min of incubation. 2. The rates of binding were found to be higher in the corpus striatum than in the cerebral cortex. 3. Concentration study revealed the presence of two different and independent classes of neurotensin binding sites with a negative cooperative interaction within each class of binding sites. 4. At saturation levels, the low affinity binding component was found to have a flat regional difference, while the high affinity binding component showed regional differences in terms of maximal binding capacity, suggesting a higher number of high affinity binding sites in the striatum than in the cortex. 5. In the competition related experiments, dopamine was found to displace around 70% of neurotensin binding sites in the corpus striatum and the cerebral cortex. 5. Substance P displaced around 50% of [ 3H]neurotensin in the cerebral cortex. Whereas, 50% and 70% displacement were observed in the striatum at high and low affinity binding sites, respectively. 6. These results show the overlap in the binding sites of neurotensin, dopamine and substance P.

Nerve growth factor receptors on cultured rat Schwann cells

Neonatal rat Schwann cells were grown in tissue culture and assayed for NGF receptors with time in culture. NGF receptor levels on freshly prepared Schwann cells (day 0) were low but increased dramatically during the first week in culture. Characterization of 125I-NGF binding to resuspended cells grown for 4 d in culture revealed that binding was not saturable at high ligand concentrations (50-70 nM) and that a high-capacity, low-affinity NGF binding component existed on these cells as compared to PC12 cells. The monoclonal antibody, 192-IgG, which recognizes the rat NGF receptor, was used as an immunohistochemical tool to verify the presence of NGF receptors on the cultured rat Schwann cells. In contrast to radiolabeled NGF, 125I-192-IgG demonstrated saturable binding to Schwann cells in suspension, with Kd and Bmax values of 4 nM and 115 fmol/10(6) cells, respectively. Schwann cells showed no evidence of slow dissociation or internalization of NGF binding at any of several NGF concentrations. 192-IgG was used to immunoprecipitate 125I-NGF chemically crosslinked to cell membranes. SDS-PAGE and subsequent autoradiography of the immunoprecipitated NGF receptors revealed that 2 species of NGF receptors were precipitated from Schwann cells and PC12 cells. In PC12 cells, 2 bands with molecular weights of 90 and 210 kDa were identified. The Schwann cell NGF receptor species migrated slower on the gels, with apparent molecular weights of 95 and 220 kDa. Further analysis of glial cell NGF receptors showed that Schwann cells isolated from the vagus nerve of neonatal rats also expressed NGF receptors in culture; however, astrocytes cultured from neonatal rat cerebral cortex, cultured under conditions reported here, were devoid of detectable NGF receptors. These results show that NGF receptor levels on Schwann cells increase with time in culture, and this resembles what is observed in Schwann cells in vivo when adult peripheral nerve is injured. The data are discussed in terms of a supportive role for the Schwann cell in facilitating peripheral nerve development and regeneration.

Pharmacological specificity of some psychotomimetic and antipsychotic agents for the sigma and PCP binding sites

The pharmacological specificity of representative psychotomimetic agents such as phencyclidine (PCP) analogs, opiate benzomorphans and several antipsychotic agents was assessed for the sigma and PCP binding sites. In a series of binding experiments, in rat brain membranes, sigma and PCP binding sites were labeled with [ 3H]-1-[1- (3-hydroxyphenyl) cyclohexyl] piperidine ([ 3H]PCP-3-OH), (+)[ 3H]-N-allylnormetazocine [(+)[ 3H]SKF 10047] and (+)[ 3H]-3-[3-hydroxy-phenyl]-N-(1-propyl)piperidine [(+)[ 3H]-3-PPP]. PCP analogs inhibit potently high affinity [ 3H]PCP-3-OH binding and (+)[ 3H]SKF 10047 binding, moderately the low affinity binding component of [ 3H]PCP-3-OH and very weakly (+)[ 3H]-3-PPP binding. (+)SKF 10047 and cyclazocine are potent to moderate inhibitors of (+)[ 3H]SKF 10047, high affinity [ 3H]PCP-3-OH and (+)[ 3H]-3-PPP binding, but extremely weak inhibitors of low affinity [ 3H]PCP-3-OH binding. The antipsychotic agents display high affinity for (+)[ 3H]-3-PPP binding sites, moderate affinity for (+)[ 3H]SKF 10047 sites and have no effect on either the high or low affinity [ 3H]PCP-3-OH binding. The present data further support the existence of multiple sigma and PCP binding sites.

Temperature-dependence and heterogeneity of muscarinic agonist and antagonist binding

The binding parameters of muscarinic agonists and antagonists in rat central (brain, cerebellum and striatum) and peripheral (heart and lung) tissues were determined at 2° and 37° from competitive binding experiments with ( 3H)-quinuclidinylbenzylate (QNB). Muscarinic ligands binding affinities for cerebellum, heart and lung were tightly correlated. These tissues were also characterized by their low concentration of muscarinic receptors with high affinity for agonists. In contrast, brain and striatum contained a higher concentration of muscarinic receptors with a lower affinity for agonists. The affinity of QNB was lower at 2° whereas that of other ligands was higher. The temperature-dependent shifts of competition curves differed from tissue to tissue and from compound to compound. The shifts were highest with gallamine and carbachol. The binding isotherms of muscarinic ligands were tentatively studied with a two-site binding model. The percentage of high- and low-affinity binding components of agonists differed with the compound. Guanine nucleotides and the temperature increase lowered agonist affinity without changing the proportions of the high- and low-affinity binding components. These results corroborate that the binding heterogeneity of muscarinic ligands does not depend only on the presence of two distinct receptors. Neither guanine nucleotides nor temperature changes allow conversion between the different putative conformational states of the muscarinic receptors.