Structural requirements for galanin interaction with receptors from pancreatic beta cells and from brain tissue of the rat

The binding activity of several galanin fragments and analogs was measured on specific receptors present in rat brain and the rat pancreatic beta cell line Rin m 5F. In both tissues it was observed that: 1) galanin(3-29), galanin(10-29) and [Ile2]-galanin were ineffective for inhibiting [125I] galanin binding and 2) active peptides had the following rank order of potency: galanin(1-29) greater than [Ac-Trp2]-galanin(2-29) greater than galanin(2-29) greater than galanin(1-15) greater than [Phe2]-galanin greater than [Tyr2]-galanin. It was concluded that the N-terminal portion of galanin is very important for interaction with central or peripheral receptors. The aromatic amino acid in position 2 (Trp in native galanin) plays a crucial role.     

Galanin binding sites in rat gastric and jejunal smooth muscle membrane preparations.

Receptors for galanin in membranes from the rat gastric and jejunal smooth muscle were studied using [125I] radioiodinated synthetic porcine galanin. Specific binding was time and temperature dependent. At 32 degrees C radioligand was degraded in the presence of smooth muscle membranes in a time-dependent manner. At optimal experimental conditions, the equilibrium binding analyses showed the presence of a single population of high affinity binding sites in both the rat stomach and jejunum (Kd value of 2.77 +/- 0.78 nM and 4.93 +/- 1.74 nM for stomach and jejunal smooth muscle membranes, respectively). The concentration of the high affinity binding sites was 58.19 +/- 11.04 and 32.36 +/- 5.68 fmol/mg protein, for gastric and jejunal preparations, respectively. Specific binding was completely inhibited by 10(-6) M of nonradioactive galanin; was 75% blocked by 1 microM of galanin(9-29); it was 10% blocked by 1 microM of galanin(15-29). Galanin(1-15) at a concentration of 1 microM was ineffective for inhibiting [125I]galanin binding. Deletion of four C-terminal amino acid residues from galanin(9-29) to give galanin(9-25) also resulted in almost complete loss of affinity. Radioiodinated galanin and N-terminally deleted fragments had receptor binding potency in the following order: galanin(1-29) greater than galanin(9-29) greater than galanin(15-29). We conclude that the C-terminal part of the galanin chain is important for the rat gastric and jejunal smooth muscle membrane receptor recognition and binding and that N-terminal amino acid sequences are probably not so important, since galanin(1-15) was not active but galanin(9-29) retained most of the receptor binding activity.     

Galanin receptor and its ligands in the rat hippocampus

Receptors for the 29-amino-acid peptide, galanin, in membranes from the rat ventral hippocampus were examined using chloramine-T-iodinated porcine galanin as ligand. The equilibrium binding of 125I-galanin showed the presence of a high-affinity binding site (Kd = 1.91 +/- 0.40 nM). The concentration of the high-affinity-binding sites was 107 +/- 15 fmol/mg membrane protein. The on rate constant was estimated to be 2.6 +/- 0.1 M-1 min-1 at 37 degrees C. The affinity of rat galanin (differing in three amino acid residues from the porcine protein) was equal to that of porcine galanin. The 125I--galanin-binding site is a trypsin-sensitive membrane protein, which is heat-denaturated at 60 degrees C within 5 min. The effect of GTP and its analogs and of pertussis-toxin-catalyzed ADP-ribosylation on the binding of 125I-galanin suggest that the galanin receptor is coupled to an inhibitory G protein (Gi protein). 127I-galanin was shown to be a ligand with affinity equal to that of galanin in displacing 125I-galanin. The 125I-galanin-binding site in the ventral hippocampus recognizes as a ligand the tryptic fragments 1-20 and 21-29 of rat galanin and the synthetic fragments 12-29, 18-29 and 21-29 of porcine galanin. None of these afforded full inhibition of the binding of fragment 1-29 of 125I-galanin at a concentration of 1 microM.     

Corticotropin-releasing factor (CRF) receptors in intermediate lobe of the pituitary: biochemical characterization and autoradiographic localization

CRF receptors were characterized using radioligand binding and chemical affinity cross-linking techniques and localized using autoradiographic techniques in porcine, bovine and rat pituitaries. The binding of 125I-[Tyr0]-ovine CRF (125I-oCRF) to porcine anterior and neurointermediate lobe membranes was saturable and of high affinity with comparable KD values (200-600 pM) and receptor densities (100-200 fmoles/mg protein). The pharmacological rank order of potencies for various analogs and fragments of CRF in inhibiting 125I-oCRF binding in neurointermediate lobe was characteristic of the well-established CRF receptor in anterior pituitary. Furthermore, the binding of 125I-oCRF to both anterior and neurointermediate lobes of the pituitary was guanine nucleotide-sensitive. Affinity cross-linking studies revealed that the molecular weight of the CRF binding protein in rat intermediate lobe was identical to that in rat anterior lobe (Mr = 75,000). While the CRF binding protein in the anterior lobes of porcine and bovine pituitaries had identical molecular weights to CRF receptors in rat pituitary (Mr = 75,000), the molecular weight of the CRF binding protein in porcine and bovine intermediate lobe was slightly higher (Mr = 78,000). Pituitary autoradiograms from the three species showed specific binding sites for 125I-oCRF in anterior and intermediate lobes, with none being apparent in the posterior pituitary. The identification of CRF receptors in the intermediate lobe with comparable characteristics to those previously identified in the anterior pituitary substantiate further the physiological role of CRF in regulating intermediate lobe hormone secretion.     

Discrimination of galanin receptor subtypes in RINm5F cells by structurally different galanin radioligands

Galanin (GAL) is a biologically active peptide that is involved in a variety of physiological functions. The purpose of this study was to evaluate whether porcine and rat galanin radioligands could be used as probes to discriminate GAL receptors (GALR) subtypes using a cell line, RINm5F, that express multiple GALR subtypes. Data from parallel equilibrium binding experiments using the same RINm5F membrane homogenates reveal that [125I]pGAL labels 20% more GALRs with a 2-fold lower affinity than those values identified when using [125I]rGAL. Competition studies using various GAL peptides showed different rank order of potencies depending on the radioligand used. Preincubation of RINm5F membranes with GppNHp, a non-hydrolizable GTP analog, prior to radioligand labeling suggests that a portion of GALRs is precoupled to G proteins. In addition, receptors labeled by [125I]rGAL appear more sensitive to GppNHp-induced uncoupling of G proteins than those labeled by [125I]pGAL. In conclusion, our data suggest that pGAL and rGAL radioligands define different pharmacological profiles of GALRs, and hence, these ligands can be used as pharmacological tools to discriminate GALR subtypes. Additionally, our data suggests that GALRs exist in a precoupled state with their respective G-proteins prior to interaction with the agonist.     

Evidence that vascular actions of PHI are mediated by a VIP-preferring receptor

Vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are homologous neuropeptides which share vasodilatory properties. This paper addresses the question of whether PHI exerts its vascular action via a receptor distinct from that for VIP. Radioligand binding experiments were done using [Tyr(125I)10]VIP, [Tyr(125I)22]porcine PHI, [Tyr(125I)10]rat PHI and arterial preparations from rat, bovine and porcine species. The radioiodination of rat PHI by the lactoperoxidase-glucose oxidase method and analysis of the structure of the major radiolabeled derivatives were described. All the receptor binding experiments identified a VIP-preferring receptor irrespective of which radioligand or arterial preparation was utilized. VIP and PHI peptides demonstrated cross-desensitization in studies of relaxation of porcine coronary arterial strips in vitro. The present results favor the conclusion that the vascular actions of the PHI peptides are best explained by binding to a VIP-preferring receptor.     

Characterization of the high-affinity receptors on Swiss 3T3 cells which mediate the binding, internalization and degradation of the mitogenic peptide bombesin.

Bombesin and bombesin-related peptides such as gastrin-releasing peptide (GRP) stimulate DNA synthesis and proliferation of Swiss 3T3 cells in culture. We have used 125I-labelled [Tyr4]bombesin and 125I-labelled GRP to characterize and identify the receptors for these peptides on Swiss 3T3 cells. The binding of 125I-[Tyr4]bombesin, which retained full biological activity, was maximal between 20 and 30 min incubation at 37 degrees C, after which continued incubation led to a decline in cell-associated radioactivity. This decline was markedly slowed by the presence of lysosomal enzyme inhibitors. Specificity of the binding site was indicated by the competitive inhibition of binding by bombesin-related peptides, but not by unrelated peptides and growth factors. Scatchard analysis of binding data indicated a single class of high-affinity receptors. The calculated value for the dissociation constant (Kd) was 2.1 nM and each cell possesses approx. 240,000 receptors. Because [Tyr4]bombesin has no free amino group, 125I-GRP was used in chemical cross-linking studies. When disuccinimidyl suberate was used to covalently couple 125I-GRP to the cells, two major radiolabelled complexes were detected with molecular masses of approx. 80,000-85,000 and 140,000. The binding of 125I-[Tyr4]bombesin to the cells was pH-dependent with maximal binding at pH 6.5-7.5 and effectively no specific binding at pH values below 4.5. At 37 degrees C, cell-associated 125I-[Tyr4]bombesin quickly became resistant to removal by acidic buffers, suggesting its rapid transfer to an intracellular compartment. However, pre-incubation with unlabelled [Tyr4]bombesin did not induce down-regulation of bombesin receptors as measured by the subsequent binding of 125I-[Tyr4]bombesin. In contrast with the Swiss 3T3 cells, specific binding of 125I-[Tyr4]bombesin was not detectable in two cell lines which are biologically unresponsive to bombesin-related peptides.     

Mechanisms of ligand binding to the parathyroid hormone (PTH)/PTH-related protein receptor: selectivity of a modified PTH(1-15) radioligand for GalphaS-coupled receptor conformations

Mechanisms of ligand binding to the PTH/PTHrP receptor (PTHR) were explored using PTH fragment analogs as radioligands in binding assays. In particular, the modified amino-terminal fragment analog, (125)I-[Aib(1,3),Nle8,Gln10,homoarginine11,Ala12,Trp14,Tyr15]rPTH(1-15)NH2, (125)I-[Aib(1,3),M]PTH(1-15), was used as a radioligand that we hypothesized to bind solely to the juxtamembrane (J) portion of the PTHR containing the extracellular loops and transmembrane helices. We also employed (125)I-PTH(1-34) as a radioligand that binds to both the amino-terminal extracellular (N) and J domains of the PTHR. Binding was examined in membranes derived from cells expressing either wild-type or mutant PTHRs. We found that the binding of (125)I-[Aib(1,3),M]PTH(1-15) to the wild-type PTHR was strongly (approximately 90%) inhibited by guanosine 5'-O-(3-thio)triphosphate (GTPgammaS), whereas the binding of (125)I-PTH(1-34) was only mildly (approximately 25%) inhibited by GTPgammaS. Of these two radioligands, only (125)I-[Aib(1,3),M]PTH(1-15) bound to PTHR-delNt, which lacks most of the receptor's N domain, and again this binding was strongly inhibited by GTPgammaS. Binding of (125)I-[Aib(1,3),M]PTH(1-15) to the constitutively active receptor, PTHR-H223R, was only mildly (approximately 20%) inhibited by GTPgammaS, as was the binding of (125)I-PTH(1-34). In membranes prepared from cells lacking Galpha(S) via knockout mutation of Gnas, no binding of (125)I-[Aib(1,3),M]PTH(1-15) was observed, but binding of (125)I-[Aib(1,3),M]PTH(1-15) was recovered by virally transducing the cells to heterologously express Galpha(S). (125)I-PTH(1-34) bound to the membranes with or without Galpha(S). The overall findings confirm the hypothesis that (125)I-[Aib(1,3),M]PTH(1-15) binds solely to the J domain of the PTHR. They further show that this binding is strongly dependent on coupling of the receptor to Galpha(S)-containing heterotrimeric G proteins, whereas the binding of (125)I-PTH(1-34) can occur in the absence of such coupling. Thus, (125)I-[Aib(1,3),M]PTH(1-15) appears to function as a selective probe of Galpha(S)-coupled, active-state PTHR conformations.     

Calcium-dependence of somatostatin binding to receptors

Binding of 125I-[Tyr11] somatostatin (S14) and 125I-[Tyr1]S14 has been studied in pancreatic acini and cerebral cortex. Ca2+-dependence of somatostatin binding to receptors was observed only with the highly non degradable iodinated analog 125I-[Tyr11] somatostatin but not with 125I-[Tyr1] somatostatin. The inhibitory activity of S14 on secretin-stimulated cAMP cellular content was decreased when Ca2+ concentration in the medium was reduced to 30 nM, indicating that the Ca2+ dependence of somatostatin binding to receptors is also present with the native peptide.     

Structural requirements for galanin action in the guinea-pig ileum.

Galanin fragments and galanin analogues were tested on neurally evoked muscle contractions in guinea-pig ileum in vitro. Galanin fragments inhibited the neurally evoked circular muscle contractions with the following order of potency: Galanin(1-29), galanin(2-29), galanin(1-15). In contrast, galanin(3-29), galanin(10-29), galanin(21-29), [D-Trp2]galanin, [Phe2]galanin and [Tyr2]galanin were ineffective. Galanin(1-29), galanin(2-29) and galanin(1-15) did not affect the neurally evoked longitudinal muscle contractions. These results indicate that (1) the two N-terminal amino acid residues of the galanin molecule are essential for the inhibitory action of galanin on neurally-evoked circular muscle contraction and (2) for the full potency also the C-terminal end is required.     

Solubilization and molecular characterization of active galanin receptors from rat brain.

Galanin receptors were solubilized from rat brain using the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS). Binding of 125I-galanin to the soluble fraction was time- and temperature-dependent, saturable, and reversible. Scatchard analysis of binding data indicated that the soluble extract contained a single class of galanin binding sites with a Kd of 0.8 nM and a Bmax of 26 fmol/mg of protein. Unlabeled galanin and its fragments galanin(2-29) and galanin(1-15) antagonized the binding of 125I-galanin to CHAPS-solubilized extracts with relative potencies similar to those observed with membrane receptors. Galanin(3-29) was found inactive. Binding of 125I-galanin to CHAPS extracts was inhibited by guanine nucleotides with the following rank order of potency: GMP-P-(NH)P greater than GTP greater than GDP. Molecular analysis of the soluble galanin receptor by covalent cross-linking of 125I-galanin to CHAPS extracts using disuccinimidyl tartrate and further identification on SDS-PAGE indicated that the soluble galanin binding site behaves as a protein of Mr 54,000. After incubation of CHAPS extracts with 125I-galanin, gel filtration on Sephacryl S-300 followed by ultracentrifugation on sucrose density gradient revealed a binding component with the following hydrodynamic parameters: Stokes radius, 5 nm; s20,w, 4.5 S; Mr, 98,000; frictional ratio, 1.6. GMP-P(NH)P treatment of CHAPS extracts gave rise to a molecular form with the following characteristics: Stokes radius, 4 nm; s20,w, 3.3 S; Mr, 57,000; frictional ratio, 1.4.(ABSTRACT TRUNCATED AT 250 WORDS)     

New specific radioligand for one subpopulation of brain somatostatin receptors

Cyclic octapeptide analogues of somatostatin (SS) like SMS 201-995 [H-(D) Phe-Cys-Phe-(D) Trp-Lys-Thr-Cys-Thr(ol)] or its Tyr3-derivative 204-090, displaced [125I-Tyr11]-SS 100% from pancreatic membranes but only 62-75% from brain membranes; the remaining sites were displaced by SS. These data indicate that some mini-somatostatins bind to a subpopulation of SS receptors in rat brain. The iodinated Tyr3-derivative (125I-204-090) can be considered a selective radioligand for one rat brain SS receptor subpopulation: It shows saturable and high affinity binding (KD = 0.29 nM; Bmax = 350 fmoles/mg protein) to rat cortex. The pharmacological properties of 125I-204-090 binding sites are similar to those of [125I-Tyr11]-SS sites. Distribution of these sites correspond to SS receptor-rich areas such as cortex, hippocampus, striatum, pituitary, pancreatic beta-cell. SS as well as SMS 201-995 bind to these sites with high affinity. The stability and high specific vs non-specific binding ratio makes 204-090 a radioligand of choice to measure this SS receptor subpopulation in CNS but also the SS receptors in pituitary and pancreas.     

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.     

Different ionic requirements for somatostatin receptor subpopulations in the brain

Two populations of brain somatostatin (SS) receptors, one with high affinity for the somatostatin octapeptide analogue SMS 201-995 (SS1 type) and one poorly sensitive to this analogue (SS2 type) have been characterised in regard to their ionic requirements using two radioligands, the iodinated Tyr3 derivative of the octapeptide SS analog SMS 201-995 and the iodinated [Tyr11]-SS. Specific binding of 125I-[Tyr11]-SS to rat cortex membrane homogenates can be increased by approximately 180% in presence of 5 mM Mg2+. The increase in number of binding sites seen by Mg2+ is not accompanied by a marked increase in affinity for SS but for SMS 201-995: the low affinity binding for SMS 201-995 seen in absence of Mg2+ is replaced in part by higher affinity binding in presence of these ions. SMS 201-995 sensitive SS1 receptor subpopulation measured with 125I-204-090, a specific ligand for SS1 subpopulation, is massively increased in presence of Mg2+. However, SMS 201-995 insensitive SS2 receptor population measured with 125I-[Tyr11]-SS in presence of excess SMS 201-995 is unchanged in presence of Mg2+. The Mg2+-dependency can also be observed with autoradiography for extra cortical, i.e. hippocampal, brain SS receptors. 120 mM Na+ does not affect the total brain SS receptor population, but reduces the specific binding of SS1 receptors and increases that of SS2 receptors. Therefore, the rat brain, in particular the cortex, possesses a SMS 201-995-sensitive, Mg2+-dependent SS receptor subpopulation (SS1) as well as a SMS 201-995-insensitive, Mg2+-independent SS population (SS2).     

Solubilization and characterization of guinea-pig pancreatic somatostatin receptors

The solubilization of somatostatin receptors from guinea-pig pancreas by different non-denaturing detergents was investigated after stabilization of the receptors by prior binding of 125I-[Tyr11]somatostatin or its analogue 125I-[Leu8,DTrp22,Tyr25]somatostatin 28, to pancreatic plasma membranes. The somatostatin-receptor complexes were solubilized in a high yield by Zwittergent 3-14 (3-[tetradecyldimethylammonio]-1-propanesulfonate), a zwitterionic detergent. Other detergents, digitonin, Triton X-100, Chaps (3-[cholamidopropyldimethylammonio]-1-propanesulfonate) and octyl beta-D-glycopyranoside, achieved only partial solubilization. The recovery of receptor complexes was increased by glycerol. In order to characterize solubilized somatostatin-receptor complexes, membranes receptors were covalently labelled using N-5-azido-2-nitrobenzoyloxysuccinimide as cross-linking reagent before solubilization. Gel filtration chromatography analysis resulted in the identification of a major protein component of apparent Mr = 93,000 which interacted with the two radioligands. In addition, a similar component of Mr = 88,000 was characterized after analysis by SDS-PAGE of membrane receptors covalently cross-linked with 125I-[Leu8,DTrp22,Tyr25]somatostatin 28 by different heterobifunctional reagents: N-5-azido-2-nitrobenzoyloxysuccinimide, N-hydroxysuccinimidyl 4-azidobenzoate, N-succinimidyl 6-(4'-azido-2'-nitrophenylamino)hexanoate. Optimal cross-linking results were obtained with N-5-azido-2-nitrobenzoyloxysuccinimide. The solubilized somatostatin-receptor complex was adsorbed to wheat-germ agglutinin-agarose column and eluted by specific sugars. We concluded that the guinea-pig pancreatic somatostatin receptor in the membrane and in the non-denaturing detergent solution behaves as a protein monomer of apparent Mr approximately 85,000-90,000. The somatostatin receptor is a glycoprotein which contains complex-type carbohydrate chains.     

Interaction of human parathyroid hormone-related peptide with parathyroid hormone receptors in clonal rat osteosarcoma cells

Synthetic peptides corresponding to the amino-terminal region of the human parathyroid hormone-related peptide (hPTHrp) were used to characterize the interaction of hPTHrp with parathyroid hormone (PTH) receptors in clonal rat osteosarcoma cells (ROS 17/2.8). Both hPTHrp-(1-34) and [Tyr40]hPTHrp-(1-40) showed full agonist activity in stimulating cyclic AMP accumulation in ROS cells; human PTHrp-(1-34) was approximately 2.5-fold as potent as hPTH-(1-34). Both [Tyr-40]hPTHrp-(3-40) and hPTH-(3-34) inhibited the cyclic AMP increase induced by either hPTHrp or PTH with parallel dose-inhibition curves. Binding to intact ROS cells of a 125I-labeled [Tyr40]hPTHrp-(1-40) (125I-[Tyr40]hPTHrp-(1-40)) which retains full biological activity was time- and temperature-dependent and reversible. Binding of 125I-[Tyr40]hPTHrp-(1-40) and 125I-labeled [Nle8, Nle18, Tyr34]bovine PTH-(1-34)NH2 to ROS cells was competed for, to the same extent and with the comparable potency, by either unlabeled hPTHrp or PTH peptides. The binding capacity and affinity of receptors in ROS cells were strikingly similar for hPTHrp and PTH. Affinity cross-linking with either radioligand resulted in high affinity, specific labeling of an apparently identical macromolecule centering at Mr = 80,000, which was detected in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in both reducing and nonreducing conditions. The data indicate that hPTHrp and PTH, their amino-terminal fragments at least, interact with the identical receptors with regard to affinity, capacity, specificity, and physicochemical characteristics in osteoblastic ROS 17/2.8 cells.     

Bombesin binding and biological effects on pancreatic acinar AR42J cells

The effects of bombesin on amylase release and the receptor binding of 125I-[Tyr4]bombesin in the rat pancreatic acinar carcinoma cell line AR42J were examined. Bombesin-like peptides stimulated amylase release from AR42J cells in a dose-dependent manner; a maximal 2-fold stimulation occurred at a bombesin concentration of 300 pM. Binding of 125I-[Tyr4]-bombesin to AR42J cells was specific, saturable and temperature dependent. The relative potencies with which various structurally related peptides stimulated amylase release correlated well with their relative abilities to compete for the bombesin receptor.     

Rapid Sequestration and Degradation of Somatostatin Analogues by Isolated Brain Microvessels

Somatostatin (SRIF) is a putative peptide neurotransmitter that may interact with brain capillaries following neurosecretion of the peptide. The present studies investigate the binding and metabolism of SRIF analogues in isolated bovine brain microvessels. 125I-[Tyr1]SRIF was rapidly degraded by capillary aminopeptidase with a half-time of approximately 3 min at 23 degrees C. The microvessel aminopeptidase had a low affinity and high capacity for the peptide, Km = 76 microM and Vmax = 74 nmol min-1 mgp-1. 125I-[Tyr11]SRIF was converted to free iodotyrosine at a much slower rate, presumably by a lower-activity endopeptidase. 125I-[Try11]SRIF was rapidly bound by microvessels, whereas another basic peptide, [Tyr8]bradykinin, or an acidic peptide, CCK8, or a neutral peptide, leucine enkephalin, were bound to a considerably less extent. The binding of 125I-[Tyr11]SRIF to the capillaries was nonsaturable up to a concentration of 1 microgram/ml of unlabeled peptide, and the binding reaction was extremely rapid, reaching equilibrium within 5 s at either 0 degrees C or 37 degrees C. Approximately 20% of the SRIF bound by the microvessels was resistant to acid wash and presumably represented internalized peptide. In addition, the 125I-[Tyr11]SRIF bound rapidly to the endothelial cytoskeleton remaining after a 1% Triton X-100 extraction of the microvessels. The peptide-cytoskeletal binding reaction was nonsaturable up to 1 microgram/ml of unlabeled [Tyr11]SRIF, but it was inhibited by 0.5% polylysine or 0.8 M KCl and was stimulated by 1 mM dithiothreiotol. These studies suggest that brain microvessels rapidly sequester and degrade SRIF analogues and that this may represent one mechanism for rapid inactivation of the neuropeptides subsequent to neurosecretion.     

Evidence for a single class of somatostatin receptors in ground squirrel cerebral cortex

In the present study we characterized high-affinity somatostatin (SRIF) binding sites (Kd = 2.06 +/- 0.32 nM and Bmax = 295 +/- 28 fmol/mg protein) in cerebral cortex membrane preparations of European ground squirrel using 125I-[Tyr0-D-Trp8]-SRIF14 as a radioligand. The inhibition of radioligand specific binding by SRIF14, as well as by its agonists (SRIF28, Tyr0-D-Trp8-SRIF14, SMS 201 995) was complete and monophasic, thus revealing a single population of somatostatinergic binding sites. Radioautographic analysis of 125I-[Tyr0-D-Trp8]-SRIF14 labeled brain sections confirmed the results of our biochemical study. The homogeneity of SRIF binding sites in the ground squirrel neocortex was not dependent on the animal's life-cycle phase.     

Delta 6- and delta 12-desaturase activities and phosphatidic acid formation in microsomal preparations from the developing cotyledons of common borage (Borago officinalis).

The somatostatin receptors on rat pancreatic acinar membranes were demonstrated by use of a radioiodinated (125I-) analogue of somatostatin (SMS 204-090 or [Tyr3]SMS). The tracer was found to bind to the receptor with a Kd of 58 pM. The number of sites detected by this tracer (4.7 pmol/mg of protein) was 5-10 times higher than the number of sites previously found with other tracers. Since the level of non-specific binding was also very low as compared with findings with other tracers, 125I-204-090 might be of interest in future attempts to characterize the somatostatin receptors in the pancreas. The prelabelled membranes were solubilized with 1% CHAPS, and the solubilized complexes were found to adsorb to wheat-germ-agglutinin-coupled agarose, from which they could be eluted with 4 mM-triacetylchitotriose. The complexes within this eluate were shown by gel filtration on Trisacryl GF-2000 to have an Mr of about 400,000. The dissociation of the complexes was augmented both within the membranes as well as in the solubilized state by incubation with the GTP analogue guanosine 5'-[gamma-thio]triphosphate, indicating that the complexes are probably functionally linked to a guanine-nucleotide-binding regulatory protein. After SDS/slab-gel electrophoresis and autoradiography of cross-linked complexes after treatment with the heterobifunctional reagent N-5-azido-2-nitrobenzoyloxysuccinimide, a broad band occurred at approximately Mr 90,000 both in the membranes and in the eluates of complexes after lectin-adsorption chromatography. We conclude that the augmentation of the number of detectable sites for binding of somatostatin, as well as the very low level of non-specific binding obtained by the use of 125I-[Tyr3]SMS as tracer, has made it possible for us to demonstrate the solubilization of the somatostatin receptor in conjunction with its ligand and a GTP-binding regulatory protein, and we have succeeded in cross-linking 125I-[Tyr3]SMS to a binding subunit of Mr 90,000 in the membranes and in demonstrating the presence of the same labelled binding subunit within complexes solubilized and chromatographed on a lectin column before cross-linking.