Peptide YY receptors in the proximal tubule PKSV-PCT cell line derived from transgenic mice. Relation with cell growth

Receptors for peptide YY (PYY) were identified in the PKSV-PCT renal proximal tubule cell line, derived from transgenic mice (SV40 large T antigen under the control of the rat L-type pyruvate kinase 5'-regulatory sequence). Binding of [125I-Tyr36]monoiodo-PYY ([125I] PYY to cell was specific, saturable, and reversible. The order of potency for peptides for inhibiting [125I]PYY binding was: PYY > neuropeptide Y (NPY) = PYY (13-36) >> pancreatic polypeptide. A single class of receptors was observed with a Kd of 0.37 +/- 0.05 nM and a Bmax of 103 +/- 10 fmol/mg protein. After cross-linking, electrophoresis of covalent [125I]PYY-receptor complexes revealed a single band of M(r) 50,000. PYY receptors were exclusively present at the basolateral membrane surface of polarized cells and were coupled negatively to adenylylcyclase by a pertussis toxin-sensitive G protein. PKSV-PCT cell growth and T antigen expression could be modulated by D-glucose in the medium. PYY receptors were exclusively expressed in proliferative cells cultured in the presence of D-glucose. PYY receptors disappeared in the absence of D-glucose and were expressed again when proliferation was activated by reintroduction of D-glucose. PYY stimulated cell growth (17-26% increase) and promoted [methyl-3H]thymidine incorporation into DNA (64% increase; ED50 = 5 nM PYY) of cells grown in D-glucose-enriched medium. This latter effect of PYY was largely reversed by pretreatment of cells with pertussis toxin. These findings suggest that PYY receptors play a role in epithelial cell growth.

Distribution of PYY receptors in human fat cells: an antilipolytic system alongside the alpha 2-adrenergic system

The antilipolytic effect of peptide YY (PYY) and neuropeptide Y has recently been shown in human adipocytes. PYY receptors were investigated in three human adipose deposits. A greater number of 125I-labeled PYY binding sites was found in femoral adipocyte membranes (maximal binding = 40 +/- 4 fmol/mg protein; dissociation constant = 0.3 +/- 0.1 nM) when compared with mammary and pericolonic adipose tissue. PYY receptors, like alpha 2-adrenergic sites, were largely expressed in femoral fat cells. Such a distribution was not specific either to inhibitory or to stimulating adenylyl cyclase systems since adenosine A1 and beta-adrenergic receptors were more numerous in pericolonic adipocytes. On isolated adipocytes, PYY (10(-7) M) inhibited lipolysis by 58 +/- 2% in femoral and 14 +/- 4% in pericolonic fat cells; epinephrine had the following similar response: 62 +/- 5 and 26 +/- 8%, respectively. A close relationship between the number of alpha 2-sites and PYY sites and the antilipolytic effects initiated by PYY and an alpha 2-agonist was observed. No significant differences were noted in the amount of Gi proteins in femoral and pericolonic adipocyte membranes.

Characterization of peptide YY receptors in rabbit colonic mucosa

Quantitative receptor autoradiography localized a high-affinity binding site for 125I-peptide YY to the mucosa in rabbit distal colon. Scatchard binding analysis revealed a single-affinity binding site (KD = 0.29 nM) with binding specificity similar to other peptide YY-preferring receptors (peptide YY > or = neuropeptide Y >> pancreatic polypeptide). Radioligand binding studies using colonic mucosal membranes confirmed high-affinity peptide YY binding sites (KD = 0.26 nM) with time, temperature, and protein dependence, as well as saturability characteristic of receptor-ligand binding. Selective peptide analogues showed a subpopulation of these 125I-peptide YY binding sites to resemble the Y1-type neuropeptide Y receptor. Peptide YY may exert local antisecretory effects on the colonic epithelium via these binding sites.

An erythromycin derivative, EM-523, induces motilin-like gastrointestinal motility in dogs

The effect of an erythromycin derivative, EM-523, on gastrointestinal motility was investigated in conscious dogs and compared with that of motilin cisapride, trimebutine and metoclopramide. In the fasting state, EM-523 given i.v. or i.d. at 3 micrograms/kg or more induced contractions in the stomach that migrated along the small intestine. The pattern of the contractions was very similar to that induced by motilin. In the digestive state, EM-523 increased the amplitude of gastric contractions. Cisapride and metoclopramide increased gastrointestinal motility both in the fasting and digestive states; however, their contractile pattern was different from that of EM-523. Trimebutine did not induce gastric motility in the fasting state but rather decreased gastric motility in the digestive state. The contractions induced by EM-523 and motilin were inhibited by atropine but were not affected by naloxone, suggesting that the cholinergic pathway is important in the exertion of their action. These results indicate that EM-523 mimics motilin in stimulating gastrointestinal motility and that this agent may be useful treat gastrointestinal disorders such as gastric stasis, gastroesophageal reflux, and postoperative ileus, and so forth.

The vasoactive intestinal peptide receptor on intact human colonic adenocarcinoma cells (HT29-D4). Evidence for its glycoprotein nature

We have previously shown that the mono [125I]iodinated vasoactive intestinal peptide (125I-VIP) could be covalently cross-linked on intact colonic adenocarcinoma cells (HT29). A major Mr 67,000 and a minor Mr 120,000 cross-linked polypeptides have been characterized [Muller, Luis, Fantini, Abadie, Giannellini, Marvaldi & Pichon (1985) Eur. J. Biochem. 151, 411-417]. The glycoprotein nature of these species was investigated using endo-beta-acetylglucosaminidase F (Endo F) treatment, enzymic and chemical desialylation and wheat germ agglutinin (WGA)-Sepharose affinity chromatography. Affinity-labelled VIP-binding proteins solubilized by Nonidet P-40 bound to WGA-Sepharose and could be eluted specifically with N-acetyl-D-glucosamine. Treatment with Endo F resulted in an increased electrophoretic mobility of both polypeptides. The major and the minor VIP-binding proteins were converted respectively into Mr 47,000 and 100,000 species, indicating removal of 20 kDa of N-linked oligosaccharides. Deglycosylation with trifluoromethanesulphonic acid also led to a 20 kDa loss in mass of the Mr 67,000 component, indicating the absence of additional O-linked sugars on this polypeptide. The presence of sialic acid on the major VIP-binding protein was demonstrated after treatment of intact cells with neuraminidase or by chemical desialylation with hydrochloric acid. We conclude from this study that the VIP receptor from intact HT29-D4 cells is a glycoprotein with N-linked oligosaccharide side chains containing sialic acid.

Do secretin and vasoactive intestinal peptide have independent receptors on striatal neurons and glial cells in primary cultures?

Vasoactive intestinal peptide (VIP) and secretin are two related peptides that activate adenylate cyclase on membranes of striatal neurons and glial cells from embryonic mouse brain grown in primary culture. On the two cell types, the maximal activation that could be induced by secretin was only 40% above basal activity, which represented less than 15% of the maximal effect obtainable with VIP. From competition experiments performed on glial cells and the neuroblastoma X glioma hybrid, NG 108-15, a cell line known to possess both VIP and secretin sensitive-adenylate cyclase, we demonstrate that secretin does not activate VIP receptors. Furthermore, secretin has an apparent high affinity (EC50 10(-8) M) for its receptors on striatal neurons and NG 108-15 whereas an apparent low affinity (EC50 7 X 10(-6) M) was found on striatal glial cells. This suggests the existence of either two distinct secretin receptors or a desensitized form.

Actions of vasoactive intestinal peptide and secretin on chief cells prepared from guinea pig stomach

Vasoactive intestinal peptide and secretin increased cellular cAMP and pepsinogen secretion in dispersed chief cells from guinea pig gastric mucosa. With each peptide there was a close correlation between the dose-response curve for changes in cellular cAMP and that for changes in pepsinogen secretion. Vasoactive intestinal peptide-(10-28) and secretin-(5-27) had no agonist activity and antagonized the actions of vasoactive intestinal peptide and secretin on cellular cAMP and pepsinogen secretion. Studies of binding of 125I-vasoactive intestinal peptide and of 125I-secretin indicated that gastric chief cells possess four classes of binding sites for vasoactive intestinal peptide and secretin and that occupation of two of these classes of binding sites correlates with the abilities of vasoactive intestinal peptide and secretin to increase cellular cAMP and pepsinogen secretion. What function, if any, is mediated by occupation by the other two classes of binding sites remains to be determined.

Intestinal mucosa is a target tissue for pancreatic polypeptide

Studies were carried out to identify mammalian tissues capable of specifically binding mammalian pancreatic polypeptide (PP). Bovine PP (bPP) radiolabeled with 125I was purified by HPLC to yield [125I]iodo-(Tyr-27) bPP. The label was injected into three pairs of fasted littermate dogs and allowed to circulate for 5 min. One of the dogs was a control which received an excess of unlabeled porcine PP to provide competition for receptor binding. Unbound bPP was removed by perfusion with Krebs-Ringer bicarbonate and the tissue fixed in situ with Karnovsky's fixative. Tissue samples from various organs were removed, weighed, and counted. The entire gastrointestinal tract demonstrated high levels of 125I after injection of the labeled peptide. The duodenum, jejunum, ileum, and colon were the only tissues to exhibit specific binding of bPP. These tissues (mucosal and muscle layers) from experimental animals exhibited 31-76% higher binding than the corresponding tissues from the control animals. Sections of the gastrointestinal tract were scraped to separate the mucosal layer from the underlying muscle layer. The mucosal layer of the duodenum, jejunum, and ileum exhibited 145-162% increases in binding compared to the control animals. The muscle layer of these tissues demonstrated no significant increase. These findings demonstrate that mucosal layer of the small intestine is a target tissue for mammalian PP.

Effector mechanisms of peptides of the VIP family

The present review is focused on the exocrine pancreas and liver where the only known effector mechanism of VIP is the activation of adenylate cyclase in plasma membranes. A two-state model of activation-deactivation of the enzyme visualizes the participation of VIP receptors and Ns, the guanyl nucleotide stimulatory protein of adenylate cyclase. In the rat pancreas, VIP and GRF receptors are indistinguishable and disulfide bridges influence their functional integrity. The antagonism of VIP and somatostatin perhaps requires, at the adenylate cyclase level, the contribution of Ni, the guanyl nucleotide inhibitory protein. The potentiation of VIP by various stimulants acting on Ca2+ movements may rely on later events, e.g., on a concerted activation of protein kinases. When comparing quantitatively peptide binding to receptors with adenylate cyclase activation, cyclic AMP levels and amylase secretion, a tool is at hand to tailor synthetic agonists and antagonists of VIP, with appropriate changes in the N-terminal moiety of the peptide (a good agonist allows efficient coupling of receptors to the adenylate cyclase system). Apart from stimulus-secretion coupling, VIP may influence protein synthesis in the rat pancreas, through the phosphorylation of ribosomal protein S6, and may alter the activity of the endoplasmic reticulum via the phosphorylation of Mr = 21 kDa and Mr = 25 kDa proteins. In rat liver membranes, high affinity VIP receptors are specifically labelled with 125I-helodermin and are coupled to adenylate cyclase (at variance with low affinity VIP receptors). These receptors are highly responsive to divalent cations and to guanyl nucleotides.

Functional receptors for VIP, GIP, glucagon-29 and -37 in the HGT-1 human gastric cancer cell line

Three separate sets of receptors sensitive to VIP, GIP and pancreatic/entero-glucagons, have been characterized in HGT-1 cells. The order of relative potencies of VIP receptor agonists was VIP greater than rh GRF-43, rh GRF-29 greater than PHI greater than hp GRF-40, secretin. G-37 was about 4 times less potent than G-29 in HGT-1 cells (G-29 greater than G-37), whereas it was about 20 times more potent than G-29 in rat fundic glands (G-37 greater than G-29). Adenylate cyclase in HGT-1 cells was stimulated by VIP, G-29, G-37 and GIP, over a concentration from 3.16 X 10(-9) to 3.16 X 10(-7) M GIP. The experimental data: (1) support the enterogastrone activity of GIP, via adenylate cyclase activation and somatostatin release by gastric D cells; (2) demonstrate that HGT-1 cells originating from a human fundic tumor are sensitive to the glucagon-like peptides G-29 and -37, as rat fundic glands; (3) indicate that the pharmacological properties of the VIP receptor in this human gastric cell line are similar to those characterized in normal human gastric glands.

Secretin receptor activity in rat gastric glands. Binding studies, cAMP generation and pharmacology

We measured 125I-secretin binding to membranes prepared from rat fundic glands and compared the abilities of natural and synthetic secretin (SN) analogs to inhibit 125I-secretin binding and to activate the cAMP generating system in glandular and subcellular preparations from the fundus and antrum. The natural peptides structurally related to porcine secretin (pSN) included: chicken secretin (cSN), vasoactive intestinal peptide (VIP), porcine peptide with N-terminal histidine and C-terminal isoleucine amide (PHI), helodermin, growth hormone releasing factors isolated from the rat hypothalamus (rhGRF-43, rhGRF-29) or from a human pancreatic tumour (hpGRF-40). These peptides inhibited the binding of 125I-secretin to rat fundic membranes: pSN greater than cSN greater than PHI, VIP and activated the cAMP generating system in fundic glands, according to the following order of potency; pSN greater than cSN greater than PHI, VIP greater than rhGRF-29 greater than rhGRF-43. Porcine peptide with N-terminal tyrosine and C-terminal tyrosine (PYY), GIP, SOM and hpGRF-40 were inactive. Structural requirements for secretin receptor activity were evaluated with four synthetic secretin analogs corresponding to porcine secretin substituted at the N-terminal end by sequence portion of VIP, GIP, GLU and SOM: Ala4-Val5-SN(VIP-SN); Tyr1-Ala2-Glu3-SN (GIP-SN); Gln3-SN (GLU-SN) and Phe1-Phe1-Trp3-Lys4-SN (SOM-SN). The relative potencies of the analogs in fundic and antral preparations were: pSN greater than VIP-SN greater than VIP, GIP-SN greater than GLU-SN greater than SOM-SN for 125I-secretin displacement and cAMP production (glandular cAMP generation and adenylate cyclase activation).(ABSTRACT TRUNCATED AT 250 WORDS)

Structural requirements for gastric inhibitory polypeptide (GIP) receptor binding and stimulation of insulin release

The effect of bovine GIP 1-42 and several of its fragments in competing with the binding of 125I-GIP to beta-cell plasma membranes from transplantable hamster insulinoma, and in stimulating insulin release from the isolated perfused rat pancreas, was investigated. Our results, in association with the results of previous studies, indicate that the sequence 17-38 is necessary for receptor binding and biological activity of GIP. By contrast, the N-terminal portion of GIP can be removed without seriously impairing the activity of the molecule.

Failure of secretin to stimulate gastrin release and adenylate cyclase activity in gastrinoma in vitro

It has been hypothesized that secretin may act directly on gastrinoma through the adenylate cyclase system to cause stimulation of gastrin release. We studied gastrinoma cells in vitro to determine whether secretin would stimulate gastrin release directly and whether the gastrinoma cell membrane had a functional secretin receptor adenylate cyclase system. Fresh tumor was prepared in cell suspensions containing 1.5 X 10(6) viable cells and incubated for 2 hours with either 2 mM CaCl2 alone (control) or 2 mM CaCL2 and 0.025 U/ml secretin. The gastrin content of the cells in each incubation chamber and the medium were determined by radioimmunoassay and results were expressed as mean gastrin pg/microgram protein +/- SD. Under basal conditions the cellular gastrin content was 39.9 +/- 6.4 (control) compared with 16.7 +/- 2.1 (secretin). After 2 hours of incubation, cellular gastrin content increased in both groups: 68.5 +/- 11.9 (control) to 68.3 +/- 5.5 (secretin). However, the percent of gastrin released into the medium during incubation decreased by one half in both groups (control 37.3% +/- 4.0% to 22.2% +/- 3.0%; secretin 42.8% +/- 7.0% to 18.9% +/- 1.8%). Adenylate cyclase activity was assessed by measuring cAMP generation in fresh-frozen gastrinoma and cultured gastrinoma cell membranes. Isoproterenol (10(-5) M), PGE1 (10(-4) M), and GppNHp (guanine nucleotide) (10(-5) M) caused fivefold to 25-fold increases in cAMP generation. Secretin did not stimulate adenylate cyclase activity above basal (21.73 +/- 4.07 and 2.29 +/- 1.2 pmol cAMP/mg protein/min) for frozen and cultured gastrinoma, respectively. Secretin failed to stimulate gastrin release and adenylate cyclase in vitro. This suggests that secretin-stimulated gastrin release in vivo may not be due to a direct effect of secretin on the gastrinoma.

Functional GIP receptors in a hamster pancreatic beta cell line, In 111: specific binding and biological effects

Specific binding sites for GIP have been characterized in a insulin-secreting pancreatic tumor cell line, In 111. The specific binding of 125I-GIP is time, temperature and cells concentration dependent. Under steady state conditions (2 hours at 13 degrees C) specific binding of 125I-GIP (0.3 nM) is competitively inhibited by increasing concentrations of native GIP from 10(-10) to 10(-6) M. Scatchard analysis reveals the presence of two types of sites: a high affinity (KD = 7 nM)/low capacity (3000 sites/cell) site and a low affinity (KD = 800 nM)/high capacity (150,000 sites/cell) site. No other peptide structurally related or not to GIP, interacts with GIP receptors. GIP (10(-10) to 10(-6) M) is able to potently stimulate insulin release in In 111 cells. At 37 degrees C, the stimulation is rapid and reaches a maximum from 30 minutes of incubation. Half-maximal stimulation is elicited by 10 nM GIP and maximal effect reaches 3 times the basal level of insulin release. Concomitantly, GIP (10(-10) - 10(-6) M) increases the basal cyclic AMP level in the cells. Half-maximal stimulation is observed in the presence of 30 nM GIP, maximal stimulation induced by 10(-6) M peptide increases up to 4 times the basal cyclic AMP production. In conclusion, our data provide the first description of a functional GIP receptor in an insulin-secreting pancreatic beta cell.

The GIP receptor on pancreatic beta cell tumor: molecular identification by covalent cross-linking

125I-GIP binds reversibly to a high affinity binding site in crude plasma membranes prepared from a hamster pancreatic beta cell tumor. The treatment of labeled membranes with the cross-linker dithiobis (succinimidylpropionate) prevents, to a greater extent, the rapid dissociation of 125I-GIP-membrane complexes which is observed when 10(-6) M native GIP is added. Polyacrylamide gel electrophoresis of membrane proteins reveals a major 125I-GIP-protein complex of Mr 64,000. This labeling decreases when increasing concentrations (10(-9) -10(-6)M) of native GIP are added but is not altered by other peptide hormones (tested at 10(-6)M) including glucagon, VIP and insulin. The Mr 64,000 complex is not observed in tissues which have no specific binding sites for GIP such as intestinal epithelium. Assuming one molecule of 125I-GIP is bound per molecule of protein, one protein with Mr 59,000 is identified as the specific GIP binding site.

Importance of disulfide bonds in receptors for vasoactive intestinal peptide and secretin in rat pancreatic plasma membranes

Vasoactive intestinal peptide (VIP), secretin, and C-terminal octapeptide of cholecystokinin (CCK-8) receptors were identified in rat pancreatic plasma membranes by the ability of these peptides to stimulate adenylate cyclase activity. The membrane preparation procedure was conducted through a series of steps including discontinuous sucrose density gradient fractionation. 5 mM beta-mercaptoethanol was added stepwise. Membrane preparations obtained stepwise were preincubated for 10 min at 25 degrees C in the presence of various concentrations of beta-mercaptoethanol or dithiothreitol before assaying adenylate cyclase. The use of the reducing agents exerted no effect on p[NH]ppG-, NaF-, and CCK-8- stimulated activities. By contrast, stimulation of adenylate cyclase by low VIP concentrations was specifically altered when beta-mercaptoethanol was used during tissue homogeneization at 5 degrees C. In addition, both VIP and secretin responses were highly sensitive towards a preincubation of 10 min at 25 degrees C in the presence of dithiothreitol. These results were likely to reflect alterations at the receptor level. 125I-VIP binding was, indeed, reduced after dithiothreitol preincubation, low concentrations of the thiol reagent decreasing the apparent number of high-affinity VIP receptors and higher dithiothreitol concentrations reducing the affinity of VIP receptors.

Receptors for vasoactive intestinal peptide and secretin on rat pancreatic acini

In dispersed acini from rat pancreas, binding of 125I-labeled vasoactive intestinal peptide and 125I-labeled secretin was relatively rapid, reversible, saturable, and temperature dependent. The rate of dissociation of bound 125I-labeled peptide was not a function of the concentration of free vasoactive intestinal peptide or secretin, indicating that the apparent affinities of these labeled peptides for their binding sites do not depend on the extent of receptor occupation. Four classes of receptors are required to account for the actions of vasoactive intestinal peptide and secretin on enzyme secretion, cellular cAMP, and binding of 125I-vasoactive intestinal peptide and 125I-secretin. One class has a high affinity for vasoactive intestinal peptide, and occupation of this class of receptors causes increased cellular cAMP and stimulation of amylase secretion. A second class has a low affinity for vasoactive intestinal peptide and for secretin, and occupation of these receptors does not cause changes in cAMP or amylase secretion. A third class of receptors has a high affinity for secretin, and occupation of these receptors causes increased cAMP and stimulation of amylase secretion. A fourth class of receptors has a low affinity for secretin, and occupation of these receptors causes stimulation of amylase secretion by a non-cAMP-mediated mechanism.

Secretin receptors on neuroblastoma cell membranes: characterization of 125I-labeled secretin binding and association with adenylate cyclase

Secretin, a gut-brain peptide, elicited cyclic AMP production in a clone of neuroblastoma cells derived from the C1300 mouse tumor. Adenylate cyclase (EC 4.6.1.1) in plasma membranes from these cells was stimulated by secretin greater than vasoactive intestinal peptide greater than peptide histidine isoleucine amide, but not by the related peptides glucagon, gastric inhibitory polypeptide, or human growth hormone releasing factor. Hill coefficients for stimulation approximated one and the response to submaximal peptide concentrations was additive, as expected for hormones competing for a single receptor associated with the enzyme. Binding of 125I-labeled secretin to the neuroblastoma plasma membranes was saturable, time-dependent, and reversible. The KD determined from kinetic and equilibrium binding studies approximated 1 nM. The binding site displayed marked ligand specificity that paralleled that for stimulation of adenylate cyclase. The secretin receptor was regulated by guanine nucleotides, with guanosine 5'-(beta, gamma-imino)-triphosphate being the most potent to accelerate the rate of dissociation of bound secretin. These findings demonstrate the functional association of the secretin receptor with adenylate cyclase in neuronally derived cells.

Heart receptors for VIP, PHI and secretin are able to activate adenylate cyclase and to mediate inotropic and chronotropic effects. Species variations and physiopathology

We have assessed the presence of VIP/PHI/secretin receptors in heart by: (1) testing the ability of the corresponding peptides to activate adenylate cyclase in cardiac membranes from rat, dog, Cynomolgus monkey and man, and (2) examining the ability of the same peptides to exert inotropic and chronotropic effects on heart preparations from rat and Cynomolgus monkey in vitro. Based on their affinity for natural peptides and synthetic analogs, two types of VIP/PHI/secretin receptors were characterized: the relatively nonspecific "secretin/VIP receptor" of rat heart (that is "secretin-preferring" only in that secretin was more efficient than VIP in stimulating adenylate cyclase), and the "VIP/PHI-preferring" receptor of man, monkey and dog heart. Four physiopathological situations affecting secretin/VIP receptors in rat heart were explored: In male rats from the Okamoto strain and the Lyon strain, two strains presenting spontaneous hypertension, heart membranes exhibited a markedly decreased response of adenylate cyclase to secretin/VIP, with lesser alterations in the responses to isoproterenol and glucagon. This impairment developed in parallel with the occurrence of hypertension and was reproduced in normotensive rats submitted to chronic isoproterenol treatment (but not in Goldblatt hypertensive rats). These findings are consistent with a hyperactivity of norepinephrine pathways in spontaneously hypertensive rats, leading to a reduced number of cardiac post-junctional secretin/VIP receptors bound to adenylate cyclase. Heart membranes from genetically obese (fa/fa) Zucker rats also exhibited severely decreased responses to secretin/VIP with lesser alterations in the responses to glucagon and isoproterenol. These anomalies were specific for the heart, and developed in concomitance with obesity. The first anomaly could not be corrected by severe food restriction. Secretin stimulation of heart adenylate cyclase was also selectively altered in streptozotocin-diabetic rats. Thus, two types of diabetic cardiomyopathy were characterized by a severe local alteration of secretin/VIP receptors coupled to adenylate cyclase. Hypothyroidism, provoked in rat by thyroidectomy or propylthiouracil treatment, again induced a marked decrease in secretin-stimulated cardiac adenylate cyclase activity. In rat papillary muscle electrically stimulated in vitro, secretin exerted a positive inotropic effect. This effect was reduced in obese (fa/fa) Zucker rats. In rat right atrium, secretin also exerted a positive chronotropic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of N-terminal iodination on the biological, immunological and receptor binding properties of secretion. A role for the alpha-amino group of histidine in stabilizing hormone-receptor interactions

Both radiotrace labeled and high specific activity 125I-labeled derivatives of secretin were prepared by direction iodination of the histidyl residue with chloramine T [( 125I]secretin) and by conjugation of a preiodinated Bolton-Hunter group (iodo-3-(4-hydroxyphenyl)propionate) to the free alpha-amino group at the N-terminus [( 125I]BH-secretin). Following purification, the biological, immunological and receptor binding properties of both secretin derivatives were compared. [125I]secretin and [125I]BH-secretin were equally effective in a sensitive radioimmunoassay that detected secretin and secretin (5-27) but not CCK-8, VIP and glucagon. Although both derivatives retained 60% of the biological potency of secretin as measured by cAMP accumulation in pancreatic acinar cells, only the directly iodinated peptide [( 125I]secretin) could be used to characterize specific binding sites on rat pancreatic membranes. The N-terminal blocked derivative [( 125I]BH-secretin) in contrast dissociated rapidly from pancreatic membranes reflecting an unstable hormone-receptor complex. These results suggest that a free alpha-amino group at the N-terminus may be essential for an optimal interaction of secretin with its pancreatic receptor.

Cerebellar binding of avian pancreatic polypeptide

Studies were carried out to determine the regional central nervous system and species specificity of the previously observed [125I]iodoavian pancreatic polypeptide ( [125I]iodo-APP) specific binding to chick brain membranes. The avian species examined were chicken, pigeon, duck, quail, chukar, and pheasant. In all species, the vast majority (greater than 90%) of APP binding was localized to the area of the cerebellum; other brain regions specifically bound small amounts of APP. Cerebellar hemisphere (folia) regions may have greater specific binding capacities than deep cerebellar nuclei, although all avian cerebellar preparations exhibited affinities for APP on the order of 10(-10) M and binding capacities from approximately 0.2-1.5 pmol/mg protein for the high affinity sites. The measured affinity for binding of APP to these cerebellar binding sites is consistent with normal plasma concentrations (3-6 ng/ml) of APP in all Aves examined. Mammalian (rat and beef) brain membranes, regardless of topographical region, showed low specific binding of [125I]iodo-APP and [125I]iodobovine PP. Preliminary experiments indicate that APP is neither contained in nor released from avian central nervous system synaptosomal elements.

Interaction of Gila monster venom with secretin receptors in rat pancreatic membranes

The stimulatory effect of Gila monster venom on adenylate cyclase activity in rat pancreatic membranes was compared to that of porcine secretin and porcine VIP. The maximal effect exerted by the venom was identical to that of VIP but significantly lower than that of secretin. The effect of Gila monster venom could, however, be attributed to its interaction with secretin receptors rather than with VIP receptors, at variance with its previously described action on guinea pig pancreatic acini. Adenylate cyclase activation by both Gila monster venom and secretin in rat pancreatic membranes was, indeed: (1) dose-dependently inhibited by two secretin fragments secretin-(4-27) and secretin-(7-27), and (2) more severely depressed than VIP stimulation, after pretreating pancreatic membranes with dithiothreitol (DTT).

The adenylate cyclase activity in heart membranes from normotensive and spontaneously hypertensive rats, after chemical sympathectomy, suggests the presence of presynaptic secretin receptors

Normotensive (WKY) and spontaneously hypertensive (SHR) male adult rats were sacrificed 2 and 3 weeks after 6-hydroxydopamine treatment. Untreated WKY and SHR rats served as controls. In rat heart membranes from WKY rats, 6-hydroxydopamine treatment increased guanosine 5'-O-(2-3-imido)-triphosphate (Gpp(NH)p)-, NaF-, D,L-isoproterenol- and glucagon-stimulated adenylate cyclase activities by 18-38% while secretin stimulation was unaffected. In heart membranes from SHR rats, Gpp(NH)p, NaF, D,L-isoproterenol, or glucagon stimulation of the enzyme was similarly increased by 14-38% whilst the low secretin responsiveness which is characteristic of these animals decreased even further (by 24-47%). These results are consistent with: (1) an up regulation of postsynaptic beta-adrenergic receptors coupled to adenylate cyclase after degeneration of adrenergic nerves, and (2) a differential response of secretin receptors coupled to adenylate cyclase in the two strains of rats: there was no change in WKY rats and a decreased response in SHR rats. The possible presence and contribution of presynaptic secretin cardiac receptors is considered.