Modification of ovine opsin with the photosensitive hydrophobic probe 1-azido-4-[125I]iodobenzene. Labelling of the chromophore-attachment domain.

The hydrophobic photosensitive probe 1-azido-4-[125I]iodobenzene (AIB) partitioned preferentially into photoreceptor disc membranes and, upon u.v. irradiation, became covalently bound to opsin and phospholipid. The labelling of both protein and phospholipid was linearly related to AIB concentration. The amount of probe incorporated into protein was not significantly different when membranes were irradiated at -100 degrees, 4 degrees or 25 degrees C, but irreversible aggregation of monomeric opsin was dramatically reduced by performing the photolysis at -100 degrees C. Labelling of opsin after irradiation at -100 degrees or 4 degrees was not significantly reduced by the presence of lysine in the aqueous buffer, indicating that significant amounts of reactive species did not enter the aqueous phase. The incorporation into phospholipid, unlike that into opsin, decreased as the temperature of irradiation increased. Some labelling of opsin occurred on incubation with pre-photoactivated AIB, indicating that reaction may also occur with reactive species of longer lifetimes than the nitrene. Proteolysis of labelled opsin with Staphylococcus aureus V8 proteinase yielded two radiolabelled membrane-bound fragments. The location of the modified sites (cysteine, tryptophan, tyrosine, lysine and histidine residues: all nucleophiles) in the smaller fragment was entirely consistent with putative models for the protein derived from other studies.     

2[125I]iodomelatonin binding sites in spleens of guinea pigs.

2-[125I]Iodomelatonin was found to bind specifically to the membrane preparations of the spleens of guinea pigs with high affinity. The binding was rapid, stable, saturable and reversible. Scatchard analysis of the binding assays revealed an equilibrium dissociation constant (Kd) of 49.8 +/- 4.12 pmol/l and binding site density (Bmax) of 0.69 +/- 0.082 fmol/mg protein at mid-light (n = 10). There was no significant change in the Kd (41.8 +/- 3.16 pmol/l) or the Bmax (0.58 +/- 0.070 fmol/mg protein) at mid-dark (n = 10). Kinetic analysis showed a Kd of 23.13 +/- 4.81 pmol/l (mean +/- SE, n = 4), in agreement to that derived from the saturation studies. The 2-[125I]iodomelatonin binding sites have the following order of potency: 2-iodomelatonin greater than melatonin greater than 6-chloromelatonin much greater than N-acetylserotonin, 6-hydroxymelatonin greater than 5-methoxytryptamine, 5 methoxytryptophol greater than serotonin, 5-methoxyindole-3-acetic acid greater than 5-hydroxytryptophol, 3-acetylindole, 1-acetylindole-3-carboxyaldehyde, L-tryptophan greater than tryptamine, 5-hydroxyindole-3-acetic acid. Differential centrifugation studies showed that the binding sites are localized mainly in the nuclear fraction (65.5%), the rest are distributed in the microsomal fraction (17.4%), mitochondrial fraction (14.7%) and cytosolic fraction (0.3%). The demonstration of 2-[125I]iodomelatonin binding sites in the spleen suggests the presence of melatonin receptors and a direct mechanism of action of melatonin on the immune system.     

Autoradiographic localization of [125I]-C-ANP compared to [125I]-ANP in rat tissue.

Whole-body autoradiography demonstrated the different distribution of [125I]-C-ANP and [125I]-ANP to rat tissues. Highest enrichment of radioactivity of both labelled peptides was found in the kidney. In some organs we found remarkable differences between [125I]-ANP and [125I]-C-ANP. In the kidney cortex, especially in the glomeruli, as well as in the endocardium, the zona glomerulosa and the medulla of the adrenal gland, where high levels of radioactivity after [125I]-ANP administration were detected, no or just few radioactivity was found after administration of [125I]-C-ANP. On the other hand in the kidney papilla and the outer subcortical medulla, characteristic blackening was found after [125I]-C-ANP administration. Those differences might be important for the understanding of pharmacological actions of ANP analogues.     

Autoradiographic localizatio of [125I]-C-ANP compared to [125I]-ANP in rat tissue

Summary Whole-body autoradiography demonstrated the different distribution of [¹²⁵I]-C-ANP and [¹²⁵I]-ANP to rat tissues. Highest enrichment of radioactivity of both labelled peptides was found in the kidney. In some organs we found remarkable differences between [¹²⁵I]-ANP and [¹²⁵I]-C-ANP. In the kidney cortex, especially in the glomeruli, as well as in the endocardium, the zona glomerulosa and the medulla of the adrenal gland, where high levels of radioactivity after [¹²⁵I]-ANP administration were detected, no or just few radioactivity was found after administration of [¹²⁵I]-C-ANP. On the other hand in the kidney papilla and the outer subcortical medulla, characteristic blackening was found after [¹²⁵I]-C-ANP administration. Those differences might be important for the understanding of pharmacological actions of ANP analogues.This work is part of the doctoral thesis of Frank Heidemann to be presented at the Ludwig-Maximilians-Universität München, FRG     

Identification of [I]endothelin binding sites in human coronary tissue

In vitro receptor autoradiography has been used to study the distribution of [¹²⁵I]endothelin binding sites in human coronary tissue from patients undergoing cardiac transplantation. Dense binding of [¹²⁵I]endothelin was associated with the smooth muscle of epicardial coronary arteries as well as to perivascular regions of these vessels. Binding was also associated with the ventricular myocardium. There was an increased binding of [¹²⁵I]endothelin to atheromatous tissue, both coronary arteries and vein graft.

The [¹²⁵I]endothelin binding sites identified using in vitro autoradiography are likely to be functionally relevant since endothelin causes a concentration-dependent contraction of segments of human epicardial coronary arteries in vitro and also has positive inotropic activity on isolated human cardiomyocytes.

The presence of specific binding sites for [¹²⁵I]endothelin on coronary tissue and the increased binding in atheromatous tissue suggest that endothelin is a peptide which may play a role in the maintenance of vascular tone and/or the pathogenesis of ischaemic heart disease.     

HPLC-purified 2-[125I]iodomelatonin labels multiple binding sites in hamster brain

Binding of 2-[125I]iodomelatonin in hamster brain synaptosomal membranes at 0 degrees C is rapid, saturable, reversible and sensitive to heat and trypsin treatment. Computer resolution of curvilinear Scatchard plots yielded high- and low-affinity components as follows: Kd1 = 0.32 +/- 0.14 nM, Bmax1 = 5.6 +/- 1.7 fmol/mg protein and Kd2 = 10.5 +/- 3.2 nM, Bmax2 = 123 +/- 33 fmol/mg protein (n = 3). Competition experiments indicated that 2-iodomelatonin and prazosin are the most potent inhibitors of high-affinity binding. Unlike prazosin, several alpha-adrenergic agents and various neurotransmitters were ineffective. These findings suggest that prazosin may be a potent antagonist at a unique, non-alpha-adrenergic, high-affinity binding site for melatonin.     

Treatment of neuroblastoma with [125I]metaiodobenzylguanidine.

To find a treatment that may be effective against micrometastases of advanced, stage III or IV neuroblastoma, [125I]metaiodobenzylguanidine (125I-MIBG) was used in a phase I toxicity trial. In seven patients, thrombocytopenia was encountered with absorbed whole body doses of 85-135 rad from 125I-MIBG, but the dosimetry was imprecise in predicting bone marrow injury. Three patients survived for over one year, results that may indicate efficacy of 125I-MIBG therapy.     

Comparative affinities of adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) for [125I] AM and [125I] CGRP specific binding sites in porcine tissues.

We have investigated the binding characteristics of rat [125I] adrenomedullin (AM) and human [125I] calcitonin gene-related peptide (CGRP) to membranes prepared from a number of porcine tissues including atrium, ventricle, lung, spleen, liver, renal cortex and medulla. These membranes displayed specific, high affinity binding for [125I] rat AM and [125I] human CGRP. Porcine lung displayed the highest density of binding sites for radiolabeled AM and CGRP followed by porcine renal cortex. Competition experiments performed with [125I] rat AM indicated that the rank order of potencies of various peptides for inhibiting [125I] rat AM binding to various tissues were rat AM > or = human AM > or = human AM(22-52) > h alpha CGRP > or = h alpha CGRP(8-37) > sCT except spleen, atrium, renal cortex and renal medulla where rAM and hAM were 20-300 fold more potent than hAM (22-52). When the same experiments were performed using [125I] h alpha CGRP as the radioligand, the rank order potencies for various peptides were rAM = hAM > h alpha CGRP > h alpha CGRP(8-37) in most of the tissues except in spleen and liver where h alpha CGRP was the most potent ligand. In lung, h alpha CGRP was almost as potent as rAM and hAM in displacing [125I] h alpha CGRP binding. These data suggest the existence of distinct CGRP and AM specific binding sites in contrast to previous reports that showed that both peptides interact differently in rat tissues.     

Renal adrenergic receptors: localization of [125I]prazosin binding sites along the microdissected rat nephron.

Norepinephrine stimulates renal tubular sodium reabsorption, probably through an alpha 1-adrenoceptor-mediated mechanism. Although the distribution of alpha 1-adrenoceptors in the kidney has been studied with autoradiography, the precise location of these receptors in isolated nephron segments is unclear. Using a microassay we determined the specific binding of [125I]iodoarylazidoprazosin ([125I]prazosin), a high specific radioactivity analog of the selective alpha 1-antagonist prazosin, to microdissected glomeruli and tubule segments. Specific binding of [125I]prazosin (3 nM) in the proximal convoluted tubule was time- and concentration-dependent, saturable, and reversible. In this segment the apparent KD by association and dissociation rate constants of [125I]prazosin binding was 0.47 nM, and the maximum receptor density was approximately 0.19 fmol/mm, or 720 fmol/mg protein. Binding specificity was verified in competition studies with excess (3 microM) unlabeled prazosin and probes for alpha 2- (yohimbine), beta- (propranolol), dopamine1- (SCH23390), and dopamine2- (S-sulpiride) receptors. [125I]Prazosin binding was inhibited significantly only by unlabeled prazosin. Mapping of prazosin binding along the nephron revealed that the highest density was in the proximal convoluted tubule, followed by the proximal straight tubule. Lesser binding was found in the thick ascending limb and in the distal convoluted tubule, whereas in the cortical and outer medullary collecting duct and in glomeruli, binding was not significantly different from zero.(ABSTRACT TRUNCATED AT 250 WORDS)     

125I]-galanin binding sites in the human nodose ganglion

The cell bodies of centrally-projecting vagal afferent neurons are contained in the inferior vagal (nodose) ganglion. Although binding sites for a number of different neuropeptides/modulators have been detected in the human nodose ganglion, the presence of galanin binding sites has not been reported. In vitro receptor autoradiography using [125I]-galanin enabled visualisation of binding sites for galanin in the human nodose ganglion. The presence of such binding sites suggests a potential role for galanin in the neuromodulation of vagal transmission in humans.     

Characterization of the [125I]endomorphin-2 binding sites in the MCF7 breast cancer cell line

In the present study, the expression of the micro-opioid receptor on protein level has been demonstrated in MCF7 breast cancer cells. Binding of the [125I]-labeled micro-opioid receptor selective ligand endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) was examined in vitro using a cross-linking assay followed by a Western blot technique. The radioactive complex had a molecular weight of about 65 kDa and was detectable by anti-micro-opioid receptor antibody, indicating the presence of micro-opioid receptors in MCF7 cell membranes. Characterization of endomorphin-2 binding to the membranes obtained from MCF7 cells was performed. Cold saturation experiments with [125I]endomorphin-2 showed biphasic binding curves in Scatchard coordinates. One component represents a high affinity and low capacity, and the other low affinity and higher capacity binding sites. The obtained Bmax values for [125I]endomorphin-2 binding to MCF7 membranes were much higher than those obtained for mouse brain. Pharmacological characterization of the [125I]endomorphin-2 binding sites was made using endomorphin-2 and two other micro selective ligands, morphiceptin, and [D-1-Nal3]morphiceptin on MCF7 cell membrane preparations and whole MCF7 cells. In both cases, the rank order of potency was [D-1-Nal3]morphiceptin>endomorphin-2>morphiceptin, but in case of whole MCF7 cells the IC50 values were about 40 times higher.     

Identification of alpha 1 adrenergic receptors in rabbit aorta with [125I] BE2254

Alpha-adrenergic receptors may play an important role in regulating vascular tone and reactivity. To study alpha-adrenergic receptors in blood vessels, we have developed a method to characterize and quantitate alpha-adrenergic receptors in a particulate fraction of individual rabbit aortas using the high specific activity alpha antagonist [125I] BE2254. [125I] BE2254 specifically labels a single class of binding sites with a dissociation constant of 286 pM and a maximal binding capacity of 16.7 fmoles/mg protein. Catecholamines compete for [125I] BE2254 binding stereospecifically and with the characteristic alpha-adrenergic potency series of (-)epinephrine greater than or equal to (-)norepinephrine much greater than (-)isoproterenol. The alpha 1-selective antagonist prazosin (KD = 0.7 nM) is much more potent in competing for [125I] BE2254 binding than is the alpha 2-selective antagonist yohimbine (KD = 1000 nM), which suggests that the alpha adrenergic receptor identified is predominantly of the alpha 1 subtype. Also, the dissociation constants from these binding studies were in good agreement with those reported in rabbit aorta from classical pharmacological experiments where contraction was found to be mediated via alpha 1 receptors. This extension of radioligand binding techniques to individual rabbit aortas should simplify the study of vascular alpha adrenergic receptor regulation, and provide a basis for broadening the understanding of vascular alpha adrenergic receptors.     

Quantitative in vitro autoradiographic characterization of [125I]angiotensin III binding sites in rat adrenal gland

A single class of high-affinity binding sites for [125I]angiotensin III and [125I]angiotensin II were found in rat adrenal medulla and zona glomerulosa by quantitative autoradiography. In the medulla, Kd were 1.46 and 1.16 nM, and Bmax 1700 and 1700 fmol/mg protein, for [125I]angiotensin II and [125I]angiotensin III, respectively. In the zona glomerulosa, Kd were 0.86 and 0.90 nM, and Bmax 790 and 560 fmol/mg protein, for [125I]angiotensin II and [125I]angiotensin III, respectively. Unlabeled angiotensin III and angiotensin II displaced [125I]angiotensin III with similar potency in both adrenal zona glomerulosa and medulla. Our findings suggest that angiotensin III and angiotensin II might share the same binding sites in adrenal gland and support the hypothesis of a role for angiotensin III in the adrenal medulla and zona glomerulosa.     

Purification of [125I]-vasoactive intestinal peptide by reverse-phase HPLC

VIP was labeled with sodium ¹²⁵iodide, and ¹²⁵I-VIP was purified by reverse-phase high performance liquid chromatography. Optimal separations of ¹²⁵I-VIP and unlabeled VIP were obtained using two C₁₈-Novapak columns in series and a gradient of acetonitrile in triethylamine phosphate for elution. The specific activity of the ¹²⁵I-VIP was 1.99±0.21 Ci/μmole, approaching the maximum specific activity of monoiodinated VIP (2.26 Ci/μmole). Radioimmunoassay and radioreceptorassay for VIP were more sensitive (2.6-fold, and 2.5-fold, respectively) using ¹²⁵I-VIP purified by HPLC compared to ¹²⁵I-VIP obtained from an open-end cellulose column. These results demonstrate the advantage of preparing purified ¹²⁵I-VIP by HPLC for the accurate assay of VIP and VIP-receptors in tissues and biological fluids.     

Relationship between [125I]RTI-55-labeled cocaine binding sites and the serotonin transporter in rat placenta

We investigated the characteristics of cocainelike binding sitesin rat placenta using[¹²⁵I]RTI-55.[³H]paroxetine bindingand immunocytochemical staining for serotonin [5-hydroxytryptamine (5-HT)] and for the 5-HT transporterwere also used to obtain evidence for rat placental 5-HT uptake.[¹²⁵I]RTI-55saturation analyses with membranes from normal gestational day 20 placentas yielded curvilinear Scatchard plots that were resolved intohigh- and low-affinity components (mean dissociation constants of 0.29 and 7.9 nM, respectively). Drug competition studies with variousmonoamine uptake inhibitors gave rise to complex multiphasicdisplacement curves, although the results obtained with the selective5-HT uptake inhibitor citalopram suggest that the 5-HT transporter isan important component of placental high-affinity[¹²⁵I]RTI-55 binding.The presence of a rat placental 5-HT uptake system was additionallysupported by the[³H]paroxetine bindingexperiments and by the presence throughout the placenta ofimmunoreactivity for 5-HT and the 5-HT transporter. Immunostaining withboth antibodies was most intense in the junctional zone, whereas thedensity of[¹²⁵I]RTI-55 bindingsites was greater in the placental labyrinth. This discrepancy may bedue to the fact that[¹²⁵I]RTI-55 appearsto be labeling additional cellular components besides the 5-HTtransporter. The presence of cocaine- and antidepressant-sensitive 5-HTtransporters in the placenta has important implications for thepossible effects of these compounds on pregnancy and fetal development.

     

Characterization of picomolar affinity binding sites for [125I]-human calcitonin gene-related peptide in rat brain and heart

We have characterized picomolar affinity binding sites for human calcitonin gene-related peptide (CGRP) in rat brain and heart (atria and ventricle) membranes. By saturation analysis, apparent dissociation constant (KD) values of high affinity sites for [125I]-human CGRP are 9 approximately 15 pM (brain), 34 pM (ventricle) and 85 pM (atria). Low affinity sites with KD values of about 50 nM are found in rat brain and ventricle, but not in atria. Human and rat CGRP potently inhibited [125I]-human CGRP binding to these high affinity sites with apparent inhibition constant (Ki) values comparable to their KD values. Salmon calcitonin marginally inhibited these binding with Ki values between 0.1 microM and 1 microM. Extremely potent cardiovascular and gastrointestinal actions of CGRP might be mediated through CGRP binding sites with picomolar affinity which are similar to those we characterized in this study.     

A reversible radioligand specific for the ETB receptor: [125I]Tyr13-Suc-[Glu9,Ala11,15]-endothelin-1(8- 21), [125I]IRL 1620.

Suc-[Glu9,Ala11,15]-endothelin(ET)-1(8-21), IRL 1620, is a linear ET-analog specific for the ET-isopeptide-nonselective ETB receptor. The radio-iodinated analog, [125I]IRL 1620, showed a single class of saturable binding to the ETB receptors in porcine lung membranes with a Kd of 18 pM and a Bmax of 930 fmol/mg protein, which are almost comparable to the values obtained with [125I]ET-3 (6 pM and 900 fmol/mg protein). In competitive binding assays with [125I]IRL 1620, unlabeled ET-1, ET-3, IRL 1620 and [monoiodo-Tyr13]-IRL 1620 showed almost identical displacement curves with Ki of 8 to 16 pM. However, [125I]IRL 1620 was dissociated from the binding sites by addition of an excess amount (100 nM) of any of these unlabeled peptides, each with the same t1/2 of 100 min. This was in marked contrast to [125I]ET-3 which was hardly dissociated from the binding sites.