Roles of cholecystokinin receptor phosphorylation in agonist-stimulated desensitization of pancreatic acinar cells and receptor-bearing Chinese hamster ovary cholecystokinin receptor cells

Receptor phosphorylation has been implicated in desensitization responses to some agonist ligands, in which receptors may become uncoupled from G proteins and move into cellular compartments inaccessible to hydrophilic ligands. Understanding of the linkage between these processes, however, has come largely from recombinant receptor-bearing cell systems with consensus sites of kinase action mutagenized. We recently established methodology permitting direct assessment of sites of phosphorylation of the cholecystokinin receptor (CCKR) in its native milieu in the pancreatic acinar cell and in a Chinese hamster ovary (CHO)-CCKR cell line (1, 2). Although CCK binding leads to phosphorylation of serine residues within the third intracellular loop of the receptor in both cell types, there are clear differences in the time course of phosphorylation, in the balance of action of kinases and a receptor phosphatase, and in a few of the distinct sites phosphorylated. In this work, we have directly assessed the inositol 1,4,5-triphosphate responses to CCK and desensitization of these responses in both cells. CHO cell lines expressing receptor mutants with protein kinase C consensus sites modified were also studied. CCK-stimulated inositol 1,4,5-triphosphate responses in both cells expressing wild-type receptors were rapidly and completely desensitized, associated with the onset of receptor phosphorylation. However, despite maintenance of the phosphorylated state of the receptor in the CHO-CCKR cell and its dephosphorylation returning the receptor to its basal state in the acinar cell, desensitization continued to be present in both. Mutagenesis of Ser260 and Ser264 to alanines individually reduced receptor phosphorylation by approximately 50%, whereas the dual mutant completely eliminated agonist-stimulated phosphorylation. Because other sites of phosphorylation were still intact in this construct, this raises the possibility of hierarchical phosphorylation with these two sites key in making other sites accessible to kinases. Constructs modifying Ser264 delayed the onset of desensitization, whereas all constructs proceeded to achieve complete desensitization by 10 min. Receptor internalization occurred independent of its phosphorylation state in the CHO cell lines, explaining the desensitization observed. In the acinar cell in which the receptor remains on the cell surface after agonist occupation, we postulate that receptor insulation achieves similar uncoupling from G protein association as is achieved by receptor phosphorylation early after agonist occupation.

Assessing parenting competency in individuals with severe mental illness: a comprehensive service

Determining the parenting capabilities of individuals with severe mental disorders who are alleged perpetrators of child abuse or neglect is a profoundly difficult task. This article discusses the methodological shortcomings of some widely used assessment strategies and outlines the components of a comprehensive parenting competency evaluation for individuals with severe mental illness. Procedures identifying both risk factors associated with abuse or neglect and protective influences against child maltreatment are summarized. These procedures are illustrated by describing a Chicago-based parenting assessment team for parents with severe mental disorders.

Molecular basis and species specificity of high affinity binding of vasoactive intestinal polypeptide by the rat secretin receptor

The affinity and specificity of the binding interaction between ligands and their receptors are key for appropriate hormonal regulation of target tissues. However, it is now apparent that vasoactive intestinal polypeptide (VIP) binds to the rat secretin receptor with similar affinity to that for its natural ligand, secretin (Holtmann et al., 1995). In this report, we establish that this is not a characteristic of the human secretin receptor, and use rat-human secretin receptor chimeras, site mutants and truncated receptor constructs to establish the molecular basis for this unusual binding interaction. Of note, isolated N-terminal domains of the rat secretin and the VIP receptors are capable of high affinity binding of VIP. In the recently recognized secretin family of receptors, this domain has six conserved cysteine residues and disulfide bonds that are likely important to achieve the complex conformation critical for this binding. A single acidic residue (Asp98) present in the rat secretin receptor appears to be critical, because a site-mutant changing this to the polar, but uncharged residue present in that position in the human receptor (Asn) eliminates the high affinity binding of VIP. Of interest, a previously identified critical basic residue in VIP (Lys15) provides a candidate for charge-pairing with this residue, potentially aligning the peptide ligand in a nonproductive orientation within this receptor.

Role of receptor phosphorylation in desensitization and internalization of the secretin receptor

The secretin receptor is prototypic of a recently described family of G protein-coupled receptors. We recently demonstrated its phosphorylation in response to agonist stimulation and elimination of this covalent modification by C-terminal truncation (F. Ozcelebi et al. (1995) Mol. Pharmacol. 48, 818-824). Here, we explore the functional impact of receptor phosphorylation and structural determinants for desensitization by comparing receptor behavior after agonist exposure in cell lines expressing wild-type and truncated receptor. To characterize receptor internalization, a novel fluorescent full agonist, [rat secretin-27]-Gly-rhodamine, was developed, which bound specifically and with high affinity. Both receptor constructs bound secretin normally, leading to normal G protein coupling and cAMP accumulation and prompt receptor internalization. Exposure to 10 nM secretin for 5 min or 12 h prior to washing and restimulation with a full range of concentrations demonstrated absent cAMP responses in wild-type receptor-bearing cells and responses 25 to 30% of control and shifted 1 order of magnitude to the right in the truncated receptor-bearing cells. Thus, the major mechanism of desensitization was phosphorylation-independent receptor internalization. Phosphorylation was associated with a distinct process that likely represents interference with G protein coupling, manifest as a reduced rate of cAMP stimulation. Thus, dual distinct mechanisms of desensitization exist in the secretin receptor family that should help protect receptor-bearing cells from overstimulation.

Development of magnetic beads for rapid and efficient metal-chelate affinity purifications

Metal-chelate affinity chromatography offers multiple advantages for protein purification, yet existing resins make its applications to sparse, hydrophobic, or particularly labile proteins and peptides quite difficult. In this work, we have developed a simple method to covalently modify commercially available superparamagnetic beads with a six-carbon spacer and nitrilotriacetic acid to provide a novel resin for extremely rapid and efficient metal chelate affinity purifications. Further, the small size and surface chemistry of these beads provide clear improvement in applicability to small scale purifications with reduced nonspecific adsorption. These advantages have been demonstrated relative to a commercially available nickel resin.

Relationship between native and recombinant cholecystokinin receptors: role of differential glycosylation

In an attempt to establish the relationship between the protein encoded by the recently cloned type A cholecystokinin (CCK) receptor cDNA and the two distinct plasmalemmal proteins on the rat pancreatic acinar cell that were previously described as candidates to represent this receptor, we have established a Chinese hamster ovary (CHO) cell line stably expressing large amounts of this recombinant protein and have used biochemical methods to characterize it directly. Upon affinity labeling, this protein migrated faster on a sodium dodecyl sulfate-polyacrylamide gel than the M(r) 85,000-95,000 molecule previously felt to represent the best candidate. However, deglycosylation with endoglycosidase F demonstrated that it had the same size core protein as that candidate, and this identification was further supported by protease peptide mapping. We postulated that the structural differences between the recombinant and the native proteins related to differences in glycosylation. Consistent with this, lectin-binding experiments demonstrated that both represented complex glycoproteins but that only the native receptor-bound Ulex europeus agglutinin I. Since this lectin binds to fucose residues that are added late in glycoprotein biosynthesis, it is possible that the distinct processing observed affected only that step. In spite of this structural difference, the type A CCK receptor-bearing CHO cell CCK receptor was functionally indistinguishable from the native acinar cell receptor. This included its ability to initiate signaling cascades, its sensitivity to stable GTP analogues, and its binding affinities for agonists and antagonists. The fidelity of this receptor expression system, while representing a 25-fold increase in receptor density over the native pancreatic acinar cell, should provide an ideal substrate for the examination of structure-function relationships within this molecule.

Multiple extracellular loop domains contribute critical determinants for agonist binding and activation of the secretin receptor

Distinct themes exist for ligand-binding domains of G protein-coupled receptors. The secretin receptor is prototypic of a recently described family in this superfamily which binds moderate-sized peptides possessing a diffuse pharmacophore. We recently demonstrated the importance of the N terminus and first loop of this receptor for secretin binding (Holtmann, M. H., Hadac, E. M., and Miller, L. J. (1995) J. Biol. Chem. 270:14394-14398). Here, we extend those findings to define another receptor domain important for agonist recognition and to focus on critical determinants within each of these domains. Extending the secretin-vasoactive intestinal polypeptide (VIP) chimeric receptor approach, we confirmed and refined the critical importance of the N terminus and the need to complement this with other domains of the secretin receptor. There was redundancy in the complementary determinants required, with the second extracellular loop able to compensate for the absence of the first loop. The first 10 residues of the N terminus of the secretin receptor were critical. Sequential segmental and site replacements permitted focusing on the His189-Lys190 sequence at the C terminus of the first extracellular loop, and on four residues (Phe257, Leu258, Asn260, and Thr261) in the N-terminal half of the second loop as providing critical determinants. All receptor constructs which expressed sensitive cAMP responses to secretin (EC50 <5 nM) bound this peptide with high affinity. Of note, one construct dissociated high affinity binding of secretin from its biological responsiveness, providing a clue to the conformational "switch" that activates this receptor.

Sexuality, pregnancy, and childrearing among women with schizophrenia-spectrum disorders

OBJECTIVE

This study compared sexuality, reproduction, and childrearing characteristics of women with schizophrenia-spectrum disorders with those of women without serious mental illness.

METHODS

A semistructured interview was given to 46 women meeting Research Diagnostic Criteria for schizophrenia or schizoaffective disorder and to 50 control subjects without major mental illness who were matched for age, race, education, employment status, and religion.

RESULTS

Compared with the control subjects, the women with schizophrenic disorders had more lifetime sexual partners, were less likely to have a current partner, and were more likely to have been raped and to have engaged in prostitution. Despite being at high risk for HIV infection, as a group they were less likely to have been tested for HIV. They reported wanting sex less often than did control subjects and rated their physical and emotional satisfaction with sex lower. They had fewer planned pregnancies, more unwanted pregnancies, and more abortions and were more often victims of violence during pregnancy. They were more likely to have lost custody of children and to report that they were unable to meet their children's basic needs and less likely to have another caregiver helping them raise their children. Both groups reported high rates of substance abuse during pregnancy.

CONCLUSIONS

Health care delivery systems could better meet the needs of women with severe mental illness by providing social skills training, family planning, and more consistent screening for pregnancy, HIV, and battering. In addition, barriers to care for pregnant women with severe mental illness and substance abuse should be reduced, and parenting training should be incorporated into psychosocial rehabilitation programs for mentally ill parents.

Phosphorylation of cholecystokinin receptors expressed on Chinese hamster ovary cells. Similarities and differences relative to native pancreatic acinar cell receptors

Phosphorylation of G protein-coupled receptors is an established mechanism for desensitization in response to agonist stimulation. We previously reported phosphorylation of the pancreatic acinar cell cholecystokinin (CCK) receptor and the establishment of two-dimensional phosphopeptide mapping of its sites of phosphorylation (Ozcelebi, F., and Miller, L. J. (1995) J. Biol. Chem. 270, 3435-3441). Here, we use similar techniques to map sites of phosphorylation of the same receptor expressed on a stable receptor-bearing Chinese hamster ovary (CHO)-CCKR cell line. Like the native cell, the CHO-CCKR cell receptor was phosphorylated in response to agonist stimulation in a concentration-dependent manner; however, the time course was quite different. CHO-CCKR cell receptor phosphorylation increased progressively to a plateau after 15 min, while in the acinar cell it peaks within 2 min and returns to baseline over this interval. There were distinct qualitative and quantitative differences in the sites of phosphorylation of the two receptor systems. One site previously attributed to action of a staurosporine-insensitive kinase in the acinar cell was absent in the CHO-CCKR cell. Site-directed mutagenesis was utilized to eliminate predicted sites of protein kinase C action, but only two of four such sites affected the phosphopeptide map of this receptor. Chemical and radiochemical sequencing were performed on these and other phosphopeptides which were present in both the CHO-CCKR cells and agonist-stimulated pancreatic acinar cells to provide direct evidence for the phosphorylation sites actually utilized. Thus, these data support the usefulness and limitations of a model cell system in studying receptor phosphorylation and desensitization.

Acute manic episodes in pregnancy

A 33-year-old pregnant woman at 26 weeks gestation, who had a history of bipolar mood disorder, type I, was admitted to the hospital for hypomania and poorly controlled diabetes mellitus. The patient had had her first episode of affective illness at age 28, after the birth of her second child. After an initial postpartum depression, she had cycled into a manic state. She had subsequently been hospitalized seven times for acute mania. A combination of valproate and chlorpromazine had proven effective in managing most of her manic episodes, while her two most severe episodes had been successfully managed with bilateral ECT.

Qualitative changes in hallucinations

OBJECTIVE

This study examined which characteristics of hallucinations are most likely to change after inpatient treatment.

METHOD

Fifty hallucinating psychiatric inpatients were given semistructured interviews shortly after admission and again shortly before discharge to elicit detailed descriptions of 12 characteristics of hallucinations.

RESULTS

Twenty-eight (56%) of the 50 patients continued to report hallucinations after inpatient treatment. Posttreatment hallucinations differed significantly from pretreatment hallucinations in that they were less frequent, less intense, and less likely to prompt overt behavioral responses. Other characteristics of hallucinations remained relatively stable.

CONCLUSIONS

Even when treatment does not eliminate hallucinations, it may alter them significantly. Outcome criteria that consider only the presence or absence of hallucinations may miss important changes in the nature of the symptom. This demonstrates that there is no simple "on-off switch" for hallucinations and supports multifactorial theories of the etiology of hallucinations.

Discovery of 1,5-benzodiazepines with peripheral cholecystokinin (CCK-A) receptor agonist activity. 1. Optimization of the agonist "trigger"

Directed screening of compounds selected from the Glaxo registry file for contractile activity on the isolated guinea pig gallbladder (GPGB) identified a series of 1,5-benzodiazepines with peripheral cholecystokinin (CCK) receptor agonist activity. Agonist efficacy within this series was modulated by variation of substituents on the N1-anilinoacetamide moiety. Remarkably, a single methyl group confers agonist activity, with an N-isopropyl substituent providing optimal efficacy. Hydrophilic substituents on the anilino nitrogen abolish agonist activity or produce antagonists of CCK. In contrast, hydrophilic electron-donating groups at the para-position of the anilino ring enhance or maintain in vitro and in vivo agonist activity. Despite decreased affinity for the human CCK-A receptor, relative to CCK-8, some of these compounds are equipotent to CCK as anorectic agents in rats following intraperitoneal administration.

Effect of ethanol on cholecystokinin-stimulated zymogen conversion in pancreatic acinar cells

Exocrine pancreatic zymogens are proteolytically processed to active forms after they are secreted into the small intestine. However, intracellular conversion of zymogens to active forms can be stimulated by treating pancreatic acinar cells with high doses of cholecystokinin (0.1 microM) or carbamylcholine (0.1 mM). The high doses of cholecystokinin are unlikely to be achieved physiologically. The ability of ethanol to sensitize the acinar cell to zymogen conversion Induced by cholecystokinin or carbamylcholine was examined. Ethanol (10-200 mM) had no effect alone or when combined with carbamylcholine. However, ethanol (25 mM) added with low-dose cholecystokinin (0.1 nM) generated zymogen conversion that was 1) sixfold higher than cholecystokinin alone and 2) equivalent to that generated by highdose cholecystokinin (10 microM). The ability of ethanol to enhance cholecystokinin-induced zymogen conversion was dependent on the dose of ethanol and the duration of ethanol treatment. The cholecystokinin receptor antagonist, L-364,718, blocked the conversion stimulated by the addition of ethanol with cholecystokinin. This effect of ethanol did not change the affinity or number of cholecystokinin receptors, suggesting an effect more distal in the stimulus-activation cascade. These findings demonstrate that ethanol selectively sensitizes the pancreatic acinar cell to cholecystokinin-stimulated zymogen proteolysis.

The integrity of the cholecystokinin receptor gene in gallbladder disease and obesity

Cholesterol gallstone disease and obesity are common and often associated disorders that could be affected by dysfunction of the receptor for cholecystokinin (CCK). Extending earlier studies that identified a defect at the level of receptor-G protein coupling in cholesterol gallstone disease, we characterized the primary structure of the gallbladder CCK receptor in patients undergoing a cholecystectomy. Represented were patients with cholesterol gallstones, as well as controls with pigment gallstones or without gallbladder disease. Both groups were composed of the range of body habitus from lean to morbidly obese. No evidence of any sequence mutation or polymorphism in the CCK receptor gene was found in any patient. This should lead future investigations of the pathogenesis of these problems toward the possible contribution of the plasmalemmal environment in affecting the association between normal receptors and G proteins.

Isolation and characterization of rat cholangiocyte vesicles enriched in apical or basolateral plasma membrane domains

Cholangiocytes, the epithelial cells that line intrahepatic bile ducts, are composed of plasma membranes with discrete apical (lumenal) and basolateral domains. While these domains are thought to contain different transporters, exchangers, channels, and receptors, no methodology currently exists for the isolation of these different membrane compartments. Thus, our aim was to develop a technique to isolated plasma membranes from cholangiocytes enriched in apical or basolateral domains. We isolate a cholangiocyte-enriched population of cells from rats 3 weeks after bile duct ligation (BDL), a maneuver which stimulates selective cholangiocyte proliferation. Using isopycnic centrifugation on linear sucrose gradients, we prepared a mixed cholangiocyte plasma membrane (MCPM) fraction from which we further generated separate apical and basolateral cholangiocyte plasma membrane (ACPM and BCPM, respectively). We characterized these fractions by specific marker enzyme assays, transmission electron microscopy (TEM), lipid analysis, anisotropy measurements, one- and two-dimensional gel electrophoresis, and quantitative immunoblots of the cystic fibrosis transmembrane conductance regulator (CFTR). Marker enzyme assays and TEM revealed that the MCPM fraction was essentially devoid of other organelles but was enriched approximately 70-fold in phosphodiesterase I, a general plasma membrane marker; the ACPM and BCPM were appropriately enriched in the respective apical and basolateral markers. TEM of ACPM and BCPM revealed homogeneous preparations of vesiculated membranes without contamination by other organelles. Lipid analysis, one- and two-dimensional gel electrophoresis, CFTR immunoblots, and anisotropy measurements showed unequivocal differences in lipid and protein composition and in fluidity between the ACPM and BCPM domains.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-stimulated phosphorylation of the carboxyl-terminal tail of the secretin receptor

The secretin receptor belongs to a recently recognized family of G protein-coupled receptors that lack the sequence motifs typical of the beta-adrenergic receptor family. Because our understanding of the regulatory mechanisms for these receptors is largely based on the latter group, we have begun to explore these mechanisms in the secretin receptor. In the present study, we focused on receptor phosphorylation, a key mechanism of receptor desensitization. Secretin receptor phosphorylation was demonstrated in intact transiently transfected COS cells and a stable receptor-bearing Chinese hamster ovary cell line in response to stimulation with native agonist. Secretin phosphoreceptor migrated on a sodium dodecyl sulfate-polyacrylamide gel at M(r) 57,000-62,000 in its native state and at M(r) 42,000 after deglycosylation, similar to the receptor that had been affinity-labeled with 125I-[Tyr10,p-NO2-Phe22]-secretin-27. Phosphorylation occurred rapidly in a secretagogue concentration-dependent manner, with 0.1 microM secretin eliciting a 7.2-fold increase in phosphorylation after 2 min. One-dimensional phosphopeptide mapping after cyanogen bromide cleavage revealed a single band of M(r) 9400, corresponding in size to the carboxyl-terminal tail domain. This identification was confirmed with a truncation mutant in which potential sites of phosphorylation in the tail were eliminated and no agonist-stimulated phosphorylation was observed. Phosphoamino acid analysis of the secretin phosphoreceptor demonstrated predominance of phosphothreonine over phosphoserine (3.2:1), with no phosphotyrosine observed. Three distinct carboxyl-terminal truncation mutants were constructed to each eliminate a subset of potential phosphorylation sites, and differential levels of phosphorylation were observed. Appropriate biosynthetic processing, expression on the cell surface, and signaling for each of these constructs were ensured by demonstration of ligand binding and cAMP responsiveness. Thus, receptors in the recently described secretin receptor family are phosphorylated in response to agonist stimulation in a manner analogous to the beta-adrenergic receptor, likely representing an important molecular mechanism for receptor desensitization.

Abnormal processing of the human cholecystokinin receptor gene in association with gallstones and obesity

BACKGROUND & AIMS

Cholesterol gallstone disease and obesity are often associated and share the potential, yet unreported, common etiology of cholecystokinin (CCK) dysfunction. While cloning the human CCK-A receptor complementary DNA (cDNA), we found predominance of a 262-base pair coding region deletion in a cDNA library prepared from a patient with this phenotype. The aim of this study was to determine the abundance, functional significance, and mechanism for generating this gene product.

METHODS

Relative abundance of CCK receptor gene products was determined using polymerase chain reaction and hybridization analysis. Constructs were expressed in COS cells and studied for radioligand binding and intracellular calcium responses. A human genomic clone for this receptor was sequenced, and the critical regions were compared with those of the patient.

RESULTS

Ninety-three percent of the patient's CCK receptor transcripts contained the 262-base pair deletion, whereas only 1.5% +/- 0.9% of control patients had the deletion. This encoded a receptor that did not bind or signal. The deletion corresponded with the third exon; however, this sequence and flanking introns were normal in the patient.

CONCLUSIONS

Abnormality of processing an apparently normal CCK receptor gene yields the predominant product with an absent third exon and encoding a nonfunctional receptor, probably reflecting a defective trans-acting splicing factor. An atypical lariat region in the third intron may explain the presence of small amounts of this product in control patients.

Insulation of a G protein-coupled receptor on the plasmalemmal surface of the pancreatic acinar cell

Receptor desensitization is a key process for the protection of the cell from continuous or repeated exposure to high concentrations of an agonist. Well-established mechanisms for desensitization of guanine nucleotide-binding protein (G protein)-coupled receptors include phosphorylation, sequestration/internalization, and down-regulation. In this work, we have examined some mechanisms for desensitization of the cholecystokinin (CCK) receptor which is native to the pancreatic acinar cell, and have found the predominant mechanism to be distinct from these recognized processes. Upon fluorescent agonist occupancy of the native receptor, it becomes "insulated" from the effects of acid washing and becomes immobilized on the surface of the plasma membrane in a time- and temperature-dependent manner. This localization was assessed by ultrastructural studies using a colloidal gold conjugate of CCK, and lateral mobility of the receptor was assessed using fluorescence recovery after photobleaching. Of note, recent application of the same morphologic techniques to a CCK receptor-bearing Chinese hamster ovary cell line demonstrated prominent internalization via the clathrin-dependent endocytic pathway, as well as entry into caveolae (Roettger, B.F., R.U. Rentsch, D. Pinon, E. Holicky, E. Hadac, J.M. Larkin, and L.J. Miller, 1995, J. Cell Biol. 128: 1029-1041). These organelles are not observed to represent prominent compartments for the same receptor to traverse in the acinar cell, although fluorescent insulin is clearly internalized in these cells via receptor-mediated endocytosis. In this work, the rate of lateral mobility of the CCK receptor is observed to be similar in both cell types (1-3 x 10(-10) cm2/s), while the fate of the agonist-occupied receptor is quite distinct in each cell. This supports the unique nature of desensitization processes which occur in a cell-specific manner. A plasmalemmal site of insulation of this important receptor on the pancreatic acinar cell could be particularly effective to protect the cell from processes which might initiate pancreatitis, while providing for the rapid resensitization of this receptor to ensure appropriate pancreatic secretion to aid in nutrient assimilation for the organism.

Critical contributions of amino-terminal extracellular domains in agonist binding and activation of secretin and vasoactive intestinal polypeptide receptors. Studies of chimeric receptors

Secretin and vasoactive intestinal polypeptide (VIP) receptors are closely related G protein-coupled receptors in a recently described family possessing a large amino-terminal ectodomain. We postulated that this domain might be critical for agonist recognition and therefore constructed a series of six chimeric receptors, exchanging the amino terminus, the first extracellular loop, or both in secretin and VIP receptors. Constructs were expressed in COS cells and characterized by cAMP generation and binding of both secretin and VIP radio-ligands. Wild type receptors demonstrated high affinity binding of respective ligands (IC50 values (in nM): at the secretin receptor: 2.2 for secretin, > 1000 for VIP; at the VIP receptor: 2.2 for VIP, > 1000 for secretin) and appropriately sensitive and selective biological responses (EC50 values (in nM): at the secretin receptor: 1.5 for secretin, 127 for VIP; at the VIP receptor: 1.0 for VIP, 273 for secretin). Replacement of the secretin receptor amino terminus with that of the VIP receptor resulted in biological responsiveness typical of the VIP receptor (EC50 = 120 nM for secretin, 1.7 nM for VIP). The converse was not true, with this domain of the secretin receptor not able to provide the same response when incorporated into the VIP receptor (EC50 = 50 nM for VIP, 30 nM for secretin). The addition of both the first loop and the amino terminus of the secretin receptor was effective in yielding a secretin receptor-like response (EC50 = 2.0 nM for secretin, 47 nM for VIP). All chimeric constructs expressing selectivity for secretin-stimulated activity bound this hormone with high affinity (IC50 = 0.2-2.2 nM); however, there was divergence between VIP binding and biological activity. Thus, the amino terminus of secretin and VIP receptors plays a key role in agonist recognition and responsiveness, with the first loop playing a critical complementary role for the secretin receptor.