Functional and immunological evidence for stable association of solubilized vasoactive-intestinal-peptide receptor and stimulatory guanine-nucleotide-binding protein from rat liver

Expression of vasoactive intestinal peptide receptor messenger RNA in the hypothalamus and pituitary throughout the turkey reproductive cycle

Vasoactive intestinal peptide (VIP) has been implicated in the regulation of avian reproductive activity and appears to act at the level of the hypothalamus and pituitary. This in situ hybridization histochemistry study describes the distribution of VIP receptor mRNA expression in the hypothalamus and the pituitary of reproductively active (laying) and quiescent (nonphotostimulated, incubating, and photorefractory) female turkeys and characterizes the differences observed in VIP receptor gene expression. VIP receptor mRNA, while expressed throughout the hypothalamus, was specifically expressed in areas known to contain GnRH-I neurons in the chicken, i.e., the lateral septum, medial preoptic area, anterior hypothalamus, and paraventricular nucleus. Significant differences in VIP receptor mRNA expression between different reproductive states was observed only within the infundibular nuclear complex. VIP receptor mRNA was markedly less in nonphotostimulated and photorefractory hens as compared with laying and incubating hens. The most dense VIP receptor mRNA was found in the anterior pituitary, where it was 2.4- and 3.0-fold greater in laying and incubating hens, respectively, as compared with that in nonphotostimulated ones. Hens that stopped incubating and became photorefractory displayed pituitary VIP receptor mRNA levels similar to those of nonphotostimulated birds. The changes in pituitary VIP receptor mRNA expression were positively correlated with known changes in pituitary prolactin (PRL) mRNA expression and PRL content and release. These findings indicate that the variations in PRL secretion observed across the turkey reproductive cycle are, in part, regulated by changes in VIP receptors at the pituitary level.

Molecular pharmacology and structure of VPAC Receptors for VIP and PACAP

VIP and PACAP are two prominent neuropeptides which share two common G protein-coupled receptors VPAC1 and VPAC2 while PACAP has an additional specific receptor PAC1. This paper reviews the present knowledge regarding three aspects of VPAC receptors including: (i). receptor specificity towards natural VIP-related peptides and pharmacology of synthetic agonists or antagonists; (ii). receptor signaling; (iii). molecular basis of ligand-receptor interaction as determined by site-directed mutagenesis, construction of receptor chimeras and structural modeling.

The polypeptide PHI discriminates a GTP-insensitive form of VIP receptor in liver membranes

In early reports on 125I-VIP binding experiments in liver membranes, it has been proposed that, the VIP binding sites were partially sensitive to GTP. Here we confirm that the VIP binding sites of chicken liver membranes consisted mainly in bivalent VIP/PACAP receptors and that about 50% of the 125I-VIP binding capacity was not affected by the GTP analogue GppNHp. Part of these bivalent receptors also appeared to represent PHI binding sites. In GppNHp-treated membranes, the GTP-insensitive VIP binding sites displayed a 17-fold higher relative affinity than in control membranes for the VIP analogue PHI. Such data suggested that GTP-insensitive VIP receptors may correspond to a subclass of high-affinity PHI receptors. Cross-linking of 125 I-VIP or 125 I-PHI to their receptors, revealed 2 components of 48 and 60 kDa. The radiolabelling of the 60 kDa component was strongly affected by increasing concentrations of the GTP analogue but was modestly abolished by an excess of PHI. Conversely, the radiolabelling of the 48 kDa molecular form was not affected by the GTP analogue but was efficiently abolished by increasing concentrations of PHI. Taken together, the data suggest that the 48 kDa component expressed in chicken liver membranes display the properties of a GTP-insensitive VIP/PHI receptor that can be pharmacologically discriminated from the GTP-sensitive 60 kDa form, through its much higher affinity for PHI.

VIP activates G(s) and G(i3) in rat alveolar macrophages and G(s) in HEK293 cells transfected with the human VPAC(1) receptor

We have characterized vasoactive intestinal peptide (VIP) receptor/G-protein coupling in rat alveolar macrophage (AM) membranes and find that pertussis toxin treatment and antisera against G(alphai3) and G(alphas) reduce high-affinity (125)I-VIP binding, indicating that both G(alphas) and G(alphai3) couple to the VIP-receptor. The predominant VIP-receptor subtype in AM is VPAC(1) and we examined the G-protein interactions of the human VPAC(1) that had been transfected into HEK293 cells. VPAC(1) has a molecular mass of 56 kDa; GTP analogs reduced (125)I-VIP binding to this protein demonstrating that high-affinity binding of VIP to the receptor requires coupling to G-protein. Functional VIP/VPAC(1)/G-protein complexes were captured by covalent cross-linking and analyzed by Western blotting. The transfected human VPAC(1) receptor in HEK293 was found to be coupled to G(alphas) but not G(alphai) or G(alphaq). Furthermore, pertussis toxin treatment had no effect on VPAC(1)/G-protein coupling in these cells. These observations suggest that the G-proteins activated by VPAC(1) may be dependent upon species and cell type.

Distribution of VIP mRNA and two distinct VIP binding sites in the developing rat brain: relation to ontogenic events

The peptide neurotransmitter vasoactive intestinal peptide (VIP) has neurotrophic properties and influences neurobehavioral development. To assess the role of VIP during neural ontogeny, the present work traces the development of VIP mRNA with in situ hybridization and VIP receptors with in vitro autoradiography in rat central nervous system (CNS) from embryonic day 14 (E14) to the adult. VIP mRNA was not evident in the CNS until birth. Postnatally, it was expressed in several distinct brain regions, but its distribution bore little relation to that of VIP receptors. VIP receptors were present and expressed changing patterns of distribution throughout CNS development. The changing patterns were the result of 1) the transient appearance of GTP-insensitive VIP receptors in several regions undergoing mitosis or glial fasciculation and 2) the transient appearance of GTP-sensitive VIP receptors homogeneously distributed throughout the CNS during the first 2 postnatal weeks, the period of the brain growth spurt. At E14-16 VIP binding was dense throughout the brainstem and spinal cord, but limited in the rest of the brain. From E19 to postnatal day 14 (P14), while VIP binding was higher in germinal zones, it tended to be uniformly dense throughout the remainder of the brain. By P21 the adult pattern began to emerge; VIP binding was unevenly distributed and was related to specific cytoarchitectural sites. Since the expression of VIP in the CNS is limited to postnatal development but VIP receptors are abundant prenatally, we suggest that extraembryonic VIP may act upon prenatal VIP receptors to regulate ontogenic events in the brain.

Characterization of vasoactive intestinal peptide receptors in human liver

The stoichiometric, pharmacological and molecular properties of vasoactive intestinal peptide (VIP) receptors have been analyzed in human liver membranes and compared in parallel with those in rat liver membranes. The binding of [125I]VIP was rapid, saturable and specific. The stoichiometric data indicated the presence of two classes of binding sites in both human and rat liver membranes with Kd values of 0.22 (human) and 0.20 (rat) nM for the high-affinity site, and 27.3 (human) and 3.6 (rat) nM for the low-affinity site. Tracer binding was displaced by structurally related peptides with an order of potency: VIP = PACAP-27 > helodermin > secretin in human liver, and VIP = PACAP-27 = helodermin > secretin in rat liver. GTP inhibited [125I]VIP binding in a dose-dependent manner suggesting the involvement of a G protein in the signal transduction pathway. Cross-linking experiments revealed an apparent molecular mass for the VIP-receptor complex that was 67,500 +/- 2700 and 50,500 +/- 900 in human and rat preparations, respectively. VIP receptors were functional, since VIP stimulated adenylyl cyclase activity in a dose dependent manner with similar efficacy but different potency in human (ED50 = 1.2 nM) and rat (ED50 = 5.8 nM) liver membranes.

Molecular cloning and functional characterization of a human liver vasoactive intestinal peptide receptor

We have isolated a cDNA from a human liver library which is 2349 base pairs in length and encodes a near-full length seven transmembrane receptor (432 amino acids), 85% homologous to the amino acid sequence for the rat vasoactive intestinal peptide (VIP) receptor. Northern blot analysis identifies a major species at 3.3 kb in lung, and to a lesser extent in brain, heart and liver. In order to confirm the identity of this human clone, double-stranded oligonucleotides encoding the signal peptide of the rat VIP receptor were constructed by polymerase chain reaction and attached to the 5' end of the human clone. COS cells transiently transfected with this human VIP receptor chimera, express a single binding site for 125I-VIP with a Kd of 9.2 +/- 2 nM. Related peptides displace 125I-VIP with a relative potency of VIP = PACAP > helodermin >> PHM > secretin, which is similar to the binding profile seen in human tissues. This human chimeric receptor is functionally coupled to the stimulation of adenylyl cyclase in transfected COS cells, as evidenced by a dose-dependent increase in intracellular cAMP accumulation. These studies indicate that this cDNA encodes a human liver VIP receptor which is functionally coupled to the activation of adenylyl cyclase.

Solubilization of binding sites for IAPP and CGRP from rat lung

Functional binding sites for [125I]IAPP and [125I]CGRP were solubilized from rat lung membranes with CHAPSO (10 mM). Rat IAPP had a higher affinity (Ki = 22.9 nM) for [125I]IAPP binding and rat alpha CGRP (Ki = 0.904 nM) had a higher affinity for [125I]CGRP binding over related peptides. [125I]IAPP binding was unaffected by GTP gamma S, but [125I]CGRP binding was 50% inhibited, indicating solubilization of a G-protein-receptor complex for CGRP but not IAPP binding. Wheat germ agglutinin affinity columns gave a 25-fold purification of IAPP binding sites, but no CGRP binding sites were eluted from the column, indicating different patterns of glycosylation of the two sites.

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

The VIP2 receptor: molecular characterisation of a cDNA encoding a novel receptor for vasoactive intestinal peptide

We have cloned and sequenced a cDNA (RPR4) encoding a new member of the secretin/calcitonin/parathyroid hormone (PTH) receptor family. RPR4 was identified by PCR of rat pituitary cDNA, and a full-length clone was isolated from a rat olfactory bulb cDNA library. When RPR4 was functionally expressed in COS 7 cells, cyclic adenosine monophosphate (cAMP) production was stimulated by vasoactive intestinal peptide (VIP), pituitary adenylate cyclase activating peptides (PACAP-38 and PACAP-27) and helodermin, with equal potency. Peptide histidine isoleucine (PHI) and rat growth hormone releasing hormone (rGHRH) also stimulated cAMP production at lower potency. This suggests that RPR4 encodes a novel VIP receptor which we have designated the VIP2 receptor. In situ hybridisation showed that mRNA for this receptor was present mainly in the thalamus, hippocampus and in the suprachiasmatic nucleus.

Transmitter role of vasoactive intestinal peptide

Vasoactive intestinal polypeptide (VIP) is a 28 amino acid with a wide-spread neuronal localization. VIP fulfils many of the classical criteria for neurotransmission. In the cerebral cortex bipolar VIP neurones are involved in the coupling between energy metabolism, blood flow and neuronal activity. Furthermore, VIP in the brain plays a role in circadian rhythms and melatonin and pituitary hormone secretion. In the peripheral nervous system VIP is the transmitter of a number of non-cholinergic, non-adrenergic autonomic events. Thus, the peptide is involved in the control of smooth muscle tone and motility, blood flow and secretion in the digestive tract, respiratory tract and urogenital tract. The effects of VIP are mediated by a specific membrane-bound receptor linked to adenylate cyclase via a stimulatory G-protein. It is likely that impairment of VIP nerves is involved in some autonomic dysfunctions, an example being male impotence which is successfully treated with VIP injections.

Glycosylation of VIP receptors: a molecular basis for receptor heterogeneity

Apparent molecular weights of VIP-binding proteins differ greatly according to species and to tissue. In this study, we used plasma membranes from various species (human, rat, pig) and tissues (melanoma, intestine, liver), which display major 125I-VIP-labeled components with molecular weights ranging from M(r) = 51,800 to 66,800. With the exception of porcine receptor, the various VIP receptors had similar apparent molecular weights after removal of their N-linked carbohydrates. In addition to differences in the amount of asparagine-linked glycans, our results also revealed differences in the composition of the oligosaccharide chains, which can also account for the heterogeneity in the molecular weights of the VIP receptor.

Simultaneous solubilization of high-affinity receptors for VIP and glucagon and of a low-affinity binding protein for VIP, shown to be identical to calmodulin

Anion-exchange chromatography of solubilized pig liver cell membranes on DEAE-Sepharose gave a fraction with high affinity binding proteins for VIP and glucagon distinct from each other. Scatchard analysis indicated the presence of one binding site for VIP (Kd 1.5 +/- 0.6 nM and Bmax 1.3 +/- 0.4 pmol/mg). The order of potency for VIP-related peptides to inhibit [125I]VIP binding was: VIP > peptide histidine isoleucine amide (PHI) > rat growth hormone releasing factor (rGRF) > secretin. GTP-gamma-S inhibited [125I]VIP binding and reduced the affinity of VIP binding sites to 6.5 nM. In the same isolated fraction, [125I]glucagon binding was displaced by glucagon preferentially to oxyntomodulin, and GTP did not affect this [125I]glucagon binding. Scatchard analysis indicated the presence of one binding site for glucagon (Kd 0.08 +/- 0.03 nM and Bmax 0.31 +/- 0.01 pmol/mg). A low-affinity VIP binding protein (IC50 0.7 microM) was detected in a fraction eluting later and exhibited a peptide specificity: rGRF > VIP > VIP(10-28) > secretin > PHI. This rGRF-preferring protein (18 kDa) was purified and had a partial amino-acid sequence identical to that of calmodulin. Its [125I]VIP binding was competitively inhibited by VIP and calmidazolium in a manner similar to that for pig brain calmodulin. Thus we have co-solubilized VIP and glucagon high affinity receptors from pig liver cell membranes and separated them from VIP-binding calmodulin.

Solubilization and characterization of motilin-receptor complexes from rabbit antral smooth muscle tissue

In the present study, the motilin receptor was characterized by enzymatic digestion studies and by solubilization of the motilin-receptor complex from prelabeled membranes using the anionic detergent cholic acid. Motilin binding was significantly decreased by preincubation of membranes of rabbit antral tissue with trypsin, phospholipase A2, C, D, dithiothreitol and 2-mercaptoethanol but not by neuraminidase and beta-galactosidase. Treatment of prelabeled membranes with 1% cholic acid resulted in solubilization of 24 +/- 5% of the proteins and 65 +/- 3% of the radioactivity. The latter was for 77 +/- 4% due to the presence of the motilin-receptor complex as estimated with PEG-precipitation. Upon gel-filtration on Superose 6 the complex partially dissociated but 43 +/- 3% eluted with macromolecular components in the void volume. This peak was not detected when membranes were first incubated with unlabeled motilin. Further disaggregation was accomplished by the addition of 0.5 M NaCl to the elution buffer. The chromatographic profile then showed a peak of about 370 kDa and a second one of 100 kDa. The latter value probably reflects the molecular mass of a single 125I-motilin-receptor-complex.

Guanine nucleotide regulation of VIP binding to rat peritoneal macrophage membranes

In the present study, we have examined the effect of guanine nucleotides on VIP binding to rat peritoneal macrophage membranes. Both guanosine 5'-triphosphate (GTP) and its nonhydrolizable analog guanosine 5'-beta, Y-imidotriphosphate [Gpp(NH)p] inhibited, in a dose-dependent manner, the VIP binding to its specific binding sites. Half-maximal inhibition (IC50) was observed at 5.4 +/- 0.5 microM GTP. The inhibitory effect of GTP was due to an increase of the dissociation rate of peptide bound to membranes. The specificity of the binding inhibition was assessed from the lack of action of the other nucleotides tested. These results directly suggest the coupling of VIP binding sites with guanine nucleotide binding proteins in rat peritoneal macrophage membranes.

Molecular properties of the vasoactive intestinal peptide receptor in aorta and other tissues

The molecular weight of the vasoactive intestinal peptide (VIP) receptor was assessed in bovine aorta, and rat liver, lung, and brain by covalent cross-linking and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The receptor in all four tissues was found to be a single polypeptide of approximate M(r) 54,000, contradicting previous claims for substantial heterogeneity in the molecular weight of this receptor. Guanine nucleotides inhibit cross-linking of 125I-VIP to its receptor, and cross-linking with ethylene glycolbis(succinimidylsuccinate) provides further evidence for complex formation between VIP, its receptor and a guanine nucleotide-binding regulatory protein (G-protein). The precise mechanism of receptor-G-protein coupling may differ between the aorta and other tissues.

Vasoactive intestinal peptide receptors in rat liver after partial hepatectomy

We describe the status of vasoactive intestinal peptide (VIP) receptors in regenerating liver. VIP-stimulated adenylate cyclase activity was markedly decreased in proliferating liver 3 days after partial (70%) hepatectomy. This was associated with a reduced efficacy of VIP (53% compared with controls), with no change in the potency of the peptide (ED50 0.8 nM). In contrast, forskolin- and guanosine 5'-[beta gamma-imido]triphosphate (Gpp[NH]p)-stimulated enzyme activities were not decreased after hepatectomy. The expression of Gs protein subunits (alpha and beta) was studied by cholera toxin-catalysed ADP ribosylation of alpha s and by immunoblotting of alpha s and beta subunits. Both subunits were increased in regenerating liver, further suggesting that the decreased response to VIP was not related to a decreased expression of Gs proteins. In fact, the reduced adenylate cyclase response to VIP in regenerating liver was associated with quantitative and structural changes in VIP receptors. Equilibrium binding data obtained with 125I-VIP indicated the presence of two classes of binding sites, the Kds of which were not altered after hepatectomy. In contrast, changes in binding capacity (Bmax.) were as follows: 0.11 +/- 0.01 and 0.05 +/- 0.01 pmol/mg of protein for high-affinity sites in control and hepatectomized rats respectively; and 2.3 +/- 0.2 and 0.65 +/- 0.03 pmol/mg of protein for low-affinity sites in control and hepatectomized rats respectively. Moreover, affinity labelling experiments showed that the M(r) value of 125I-VIP-receptor complexes was higher in regenerating liver than in quiescent hepatocytes, e.g. 58,000 and 53,000 respectively. It is concluded that VIP receptors are altered in regenerating liver, resulting in a decreased response of adenylate cyclase to the neuropeptide.

Gs and Gi protein subunits during cell differentiation in intestinal crypt-villus axis: regulation at the mRNA level

The expression of subunits of the guanine nucleotide regulatory protein that mediates hormonal stimulation of adenylyl cyclase (Gs) and of the guanine nucleotide regulatory protein that mediates hormonal inhibition of adenylyl cyclase (Gi) was studied during cell migration and differentiation in the rat small intestine crypt-villus axis. Proliferative crypt cells were separated from nonproliferative mature villus cells and the following data were obtained: 1) alpha s subunits were more abundant in crypt cells than in villus cells as evidenced by cholera toxin-catalyzed [32P]NAD ribosylation and Western blotting of this relative molecular weight (M(r)) 42,000 protein; 2) alpha i2- and alpha i3-subunits (M(r) 40,000 and M(r) 41,000, respectively) were preferentially expressed in villus cells as evidenced by pertussis toxin-catalyzed [32P]NAD ribosylation and Western blotting (alpha i1-subunit was not detectable in intestinal epithelium by using these techniques); 3) Western blotting showed a higher expression of the common beta- (M(r) 36,000) subunit of G proteins in villus cells than in crypt cells; and 4) Northern blot analysis using an alpha s-subunit oligonucleotide probe showed a 1.9-kb mRNA that was more abundant in crypt cells than in villus cells. In contrast, alpha i2- and alpha i3-mRNA species (2.3 and 3.5 kb, respectively), analyzed by using specific cDNA probes, were much more abundant in villus cells than in crypt cells. Finally, two beta-subunit mRNA species of 3.3 and 1.8 kb were detectable in intestinal epithelial cells and were more abundant in villus cells than in crypt cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of guanine nucleotide-sensitive and guanine nucleotide-insensitive vasoactive intestinal peptide receptors in rat brain

Based on the ability of guanine nucleotides to inhibit the binding of vasoactive intestinal peptide to its receptors, a guanosine 5'-triphosphate analog, guanylyl-imidodiphosphate, was used to differentiate two subtypes (or different functional states of a single subtype) of vasoactive intestinal peptide receptor in brain with in vitro autoradiography. In most brain regions, guanylyl-imidodiphosphate reduced vasoactive intestinal peptide binding between 40 and 60%. However, in the supraoptic nucleus, locus coeruleus, interpeduncular nucleus, facial nucleus, olfactory tubercle and periventricular hypothalamic nucleus, 80% or more of vasoactive intestinal peptide binding was inhibited. In other brain regions, including the medial geniculate, olfactory bulbs, and ventral thalamic nuclei, guanylyl-imidodiphosphate had little effect on vasoactive intestinal peptide binding. In liver, lung and intestine it also partly inhibited vasoactive intestinal peptide binding. Electrophoretic analysis of vasoactive intestinal peptide, covalently cross-linked to its receptors in brain membranes, revealed a pair of bands between 44,000 and 52,000 mol. wt, a component at 64,000 mol. wt and another at 92,000 mol. wt. All were displaceable with vasoactive intestinal peptide but guanylyl-imidodiphosphate displaced only the 64,000 mol. wt band suggesting that the GTP-sensitive vasoactive intestinal peptide receptor seen in brain sections has a molecular weight of about 61,000. The differential sensitivity to guanylyl-imidodiphosphate suggests the existence of at least two vasoactive intestinal peptide receptor subtypes in brain, with distinct regional distribution, and may reflect differential coupling to second messenger systems.

Mechanisms of action of cyclosporin A on islet alpha- and beta-cells. Effects on cAMP- and calcium-dependent pathways

The involvement of cAMP- and calcium-dependent pathways on the inhibitory effect of CsA (0.5 micrograms/ml) on insulin and glucagon release was studied in collagenase-isolated islets. CsA suppressed by 50% the release of insulin in pertussis toxin treated islets stimulated by 20 mM D-glucose. CsA blocked glucagon and insulin release induced by 0.2 mM IBMX (80% and 50% respectively). Similarly it inhibited glucagon and insulin release induced by 1 microM A23187 (53% and 40% respectively). CsA also abolished 0.1 microM glucagon-induced insulin release and 10 ng/ml VIP-induced glucagon release (70% and 38% respectively). The glucagon response to 2 mM D-glucose and to 10 mM arginine was decreased 25% and 45% respectively by CsA. The inhibitory effect of 0.1 microM somatostatin on insulin release was significantly abolished by CsA (p less than 0.001 vs control). On the other hand 1 microM forskolin induced insulin and glucagon release was not modified by CsA. Rats treated with CsA (10 mg/kg body wt) during 10 days showed hyperglycaemia, hypoglucagonemia and higher contents of pancreatic glucagon. It is concluded that CsA affects alpha- and beta-cell function, in vivo and in vitro, acting through calcium and cAMP-dependent pathways. This latter pathway involves the Ca(2+)-calmodulin dependent phosphodiesterase and the regulatory proteins Gs and Gi.

VIP receptors from porcine liver: high yield solubilization in a GTP-insensitive form

Vasoactive intestinal peptide (VIP) receptors were solubilized from porcine liver membranes using CHAPS. The binding of 125I-VIP to solubilized receptors was reversible, saturable and specific. Scatchard analysis indicated the presence of one binding site with a Kd of 6.5 +/- 0.3 nM and a Bmax of 1.20 +/- 0.15 pmol/mg protein. Solubilized and membrane-bound receptors displayed the same pharmacological profile since VIP and VIP-related peptides inhibited 125I-VIP binding to both receptor preparations with the same rank order of potency e.g. VIP greater than helodermin greater than rat GRF greater than rat PHI greater than secretin greater than human GRF. GTP inhibited 125I-VIP binding to membrane-bound receptors but not to solubilized receptors supporting functional uncoupling of VIP receptor and G protein during solubilization. Affinity labeling of solubilized and membrane-bound VIP receptors with 125I-VIP revealed the presence of a single molecular component with Mr 55,000 in both cases. It is concluded that VIP receptors from porcine liver can be solubilized with a good yield, in a GTP-insentive, G protein-free form. This represents a major advance towards the purification of VIP receptors.