Joseph R. Pisegna

Pharmacology and functions of receptors for vasoactive intestinal peptide and pituitary adenylate cyclase‐activating polypeptide: IUPHAR Review 1

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase‐activating polypeptide (PACAP) are members of a superfamily of structurally related peptide hormones that includes glucagon, glucagon‐like peptides, secretin, gastric inhibitory peptide (GIP) and growth hormone‐releasing hormone (GHRH). VIP and PACAP exert their actions through three GPCRs – PAC1, VPAC1 and VPAC2– belonging to class B (also referred to as class II, or secretin receptor‐like GPCRs). This family comprises receptors for all peptides structurally related to VIP and PACAP, and also receptors for parathyroid hormone, corticotropin‐releasing factor, calcitonin and related peptides. PAC1 receptors are selective for PACAP, whereas VPAC1 and VPAC2 respond to both VIP and PACAP with high affinity. VIP and PACAP play diverse and important roles in the CNS, with functions in the control of circadian rhythms, learning and memory, anxiety and responses to stress and brain injury. Recent genetic studies also implicate the VPAC2 receptor in susceptibility to schizophrenia and the PAC1 receptor in post‐traumatic stress disorder. In the periphery, VIP and PACAP play important roles in the control of immunity and inflammation, the control of pancreatic insulin secretion, the release of catecholamines from the adrenal medulla and as co‐transmitters in autonomic and sensory neurons. This article, written by members of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC‐IUPHAR) subcommittee on receptors for VIP and PACAP, confirms the existing nomenclature for these receptors and reviews our current understanding of their structure, pharmacology and functions and their likely physiological roles in health and disease. More detailed information has been incorporated into newly revised pages in the IUPHAR database (http://www.iuphar‐db.org/DATABASE/FamilyMenuForward?familyId=67).

PACAP type I receptor activation regulates ECL cells and gastric acid secretion

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in gastric nerves, and PACAP receptors (PAC1) are found on gastric enterochromaffin-like (ECL) cells. Expression of PAC1 splice variants in purified ECL cells was determined by RT-PCR. PACAP effects on ECL cells were analyzed by video imaging of [Ca(2+)](i) and histamine release; its effects on gastric glands were examined by confocal microscopy of [Ca(2+)](i) in ECL and parietal cells. PACAP action on D cells was measured by [Ca(2+)](i) and radioimmunoassay. PACAP effects on acid secretion were determined in fistula rats with or without neutralizing anti-somatostatin antibodies. All splice variants of PAC1 were found, but vasoactive intestinal polypeptide (VIP) receptor (VPAC) products were absent. PACAP-27 and -38 dose-dependently raise [Ca(2+)](i) in ECL cells, and stimulated histamine release. VIP had a much lower affinity, which demonstrates the presence of PAC1 but not VPAC. PACAP elevated [Ca(2+)](i) in ECL and parietal cells of superfused gastric glands, but only the parietal cell signal was inhibited by ranitidine, showing the absence of PAC1 on parietal cells, and demonstrating functional coupling between the cell types. PACAP and VIP stimulated calcium signaling and somatostatin release from D cells with almost equal efficacy. Acid secretion was stimulated after intravenous injection of PACAP into rats treated with somatostatin antibody. PACAP is a candidate as a mediator of neural regulation of acid secretion.

Adenoma detection rate is necessary but insufficient for distinguishing high versus low endoscopist performance

BACKGROUNDnEndoscopist quality is benchmarked by the adenoma detection rate (ADR)-the proportion of cases with 1 or more adenomas removed. However, the ADR rewards the same credit for 1 versus more than 1 adenoma.nnnOBJECTIVEnWe evaluated whether 2 endoscopist groups could have a similar ADR but detect significantly different total adenomas.nnnDESIGNnWe retrospectively measured the ADR and multiple measures of total adenoma yield, including a metric called ADR-Plus, the mean number of incremental adenomas after the first. We plotted ADR versus ADR-Plus to create 4 adenoma detection patterns: (1) optimal (↑ADR/↑ADR-Plus); (2) one and done (↑ADR/↓ADR-Plus); (3) all or none (↓ADR/↑ADR-Plus); (4) none and done (↓ADR/↓ADR-Plus).nnnSETTINGnTertiary-care teaching hospital and 3 nonteaching facilities servicing the same patient pool.nnnPATIENTSnA total of 3318 VA patients who underwent screening between 2005 and 2009.nnnMAIN OUTCOME MEASUREMENTSnADR, mean total adenomas detected, advanced adenomas detected, ADR-Plus.nnnRESULTSnThe ADR was 28.8% and 25.7% in the teaching (n = 1218) and nonteaching groups (n = 2100), respectively (P = .052). Although ADRs were relatively similar, the teaching site achieved 23.5%, 28.7%, and 29.5% higher mean total adenomas, advanced adenomas, and ADR-Plus versus nonteaching sites (P < .001). By coupling ADR with ADR-Plus, we identified more teaching endoscopists as optimal (57.1% vs 8.3%; P = .02), and more nonteaching endoscopists in the none and done category (42% vs 0%; P = .047).nnnLIMITATIONSnExternal generalizability, nonrandomized study.nnnCONCLUSIONnWe found minimal ADR differences between the 2 endoscopist groups, but substantial differences in total adenomas; the ADR missed this difference. Coupling the ADR with other total adenoma metrics (eg, ADR-Plus) provides a more comprehensive assessment of adenoma clearance; implementing both would better distinguish high- from low-performing endoscopists.

Oral and intravenous dosage forms of pantoprazole are equivalent in their ability to suppress gastric acid secretion in patients with gastroesophageal reflux disease

OBJECTIVE:The aim of this study was to assess the ability of pantoprazole to maintain gastric acid suppression in patients with gastroesophageal reflux disease who are switched from an oral (p.o.) to an intravenous (i.v.) dosage form.METHODS:A total of 65 patients with gastroesophageal reflux disease were administered either 40 or 20 mg of p.o. pantoprazole daily for 10 days, then were switched to either a matching dose of i.v. pantoprazole or to placebo for 7 days. Acid output (basal and maximal) was measured at the end of the p.o. treatment period and on the first and last days of i.v. therapy. In the primary efficacy analysis, the acid output values at the end of the p.o. pantoprazole treatment were compared with those at the end of the i.v. treatment. Safety was monitored by periodic vital sign measurements, clinical laboratory evaluations, ophthalmic examinations, electrocardiograms, and reports of adverse events. The data were tested by an analysis of covariance and by Wilcoxon signed rank and t tests.RESULTS:Maximal acid output (mean ± SD) in the 40 mg and 20 mg pantoprazole group after p.o. treatment was 6.5 ± 5.6 mEq/h and 14.5 ± 15.5 mEq/h, respectively; whereas, at the end of the i.v. treatment period, the values were 6.6 ± 6.3 mEq/h and 11.1 ± 10.2 mEq/h, respectively. In patients given i.v. placebo, acid output was significantly (p < 0.05) increased to 29.2 ± 13.0 mEq/h by day 7. Both p.o. and i.v. pantoprazole dosage forms had similar favorable safety and tolerability profiles.CONCLUSIONS:The p.o. and i.v. formulations of pantoprazole (40 and 20 mg) are equivalent in their ability to suppress gastric acid output. The i.v. form of pantoprazole offers an alternative for gastroesophageal reflux disease patients who are unable to take the p.o. formulation.

Effects of esomeprazole on acid output in patients with Zollinger-Ellison syndrome or idiopathic gastric acid hypersecretion.

OBJECTIVES:To evaluate the efficacy and safety of oral esomeprazole in the control of gastric acid hypersecretion in patients with hypersecretory states.METHODS:In this 12-month, open-label, multicenter study, acid output (AO) was evaluated at baseline, day 10, and months 3, 6, and 12. The starting dose of esomeprazole was 40 mg or 80 mg twice daily. On day 10, patients with controlled AO were maintained on the same dose, while those with uncontrolled AO had their doses increased (maximum dose 240 mg/day) until control was attained. Esophagogastroduodenoscopy (EGD) was performed at baseline and at 6 and 12 months. Safety and tolerability were assessed throughout the study by EGD, gastric analysis, and adverse events.RESULTS:Twenty-one patients (19 with Zollinger-Ellison syndrome [ZES], 2 with idiopathic gastric acid hypersecretion [IGH]) completed the study. Of the 20 patients with controlled AO at day 10, 18 (90%) had sustained AO control for the rest of the study. At 12 months, AO was controlled in 14 of 16 patients receiving esomeprazole 40 mg twice daily, in all 4 patients receiving esomeprazole 80 mg twice daily, and in the 1 patient receiving esomeprazole 80 mg 3 times daily. At 6 and 12 months, no patient had endoscopic evidence of mucosal disease. Esomeprazole was well tolerated; 1 patient had a serious adverse event (hypomagnesemia) attributed to treatment that resolved with magnesium supplementation during continued treatment.CONCLUSION:Esomeprazole in appropriately titrated doses controls AO over 12 months in patients with hypersecretory states and is well tolerated.

Expression of pituitary adenylate cyclase-activating polypeptide and PACAP type 1 receptor in the rat gastric and colonic myenteric neurons.

Pituitary adenylate cyclase-activating polypeptide (PACAP) is known to regulate gastric acid secretion and intestinal motility. In the present study, the pattern of distribution of PACAP and PACAP type 1 receptor (PAC1) immunoreactivities were examined in the rat stomach and distal colon using a specific polyclonal antibody raised against rat/human PAC1. Western blot of the membrane preparations of NIH/3T3 cells transfected with the human PAC1 obtained by using rabbit polyclonal anti-PAC1 antibody showed a protein band with a molecular mass of approximately 50 kDa. NIH/3T3 cells transfected with the human PAC1 and incubated with the anti-PAC1 antibody displayed surface cell-type immunoreactivity, which was internalized following ligand exposure. In gastric or colonic longitudinal muscle/myenteric plexus (LMMP) whole mount preparations as well as cryostat sections, PACAP immunoreactivity was observed in cell bodies within the myenteric ganglia and nerve fibers in the muscle layers and mucosa. PAC1 immunoreactivity was confined mainly on the surface of the nerve cells. PACAP and PAC1 immunoreactivities showed a similar pattern of distribution in gastric and colonic tissues. Adjacent sections or LMMP whole mount preparations labeled with protein gene product 9.5 (PGP 9.5) revealed the neuronal identity of myenteric cells bearing PAC1. The neuronal localization of PACAP and PAC1 receptors supports their role in the neural regulation of gastric acid secretion and gastrointestinal motor function.

Identification of an essential amino acid motif within the C terminus of the pituitary adenylate cyclase-activating polypeptide type I receptor that is critical for signal transduction but not for receptor internalization.

The pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 (PAC1) receptor is a G protein-coupled receptor and class II receptor member. The receptor domains critical for signaling are unknown. To explore the role of the C terminus, truncations of 63 residues (Tr406), 53 residues (Tr416), 49 residues (Tr420), 44 residues (Tr424), and 37 residues (Tr433) were constructed and expressed in NIH/3T3 cells, and immunofluorescence, radioligand binding, adenylyl cyclase (AC) and phospholipase C (PLC) assays were performed.125I-PACAP-27 binding (K d = 0.6–1.5 nm) for the Tr406 and Tr433 were similar to wild type Hop and Null splice variants (K d = ∼1.1 nm). Although internalization of ligand for both the Tr406 and Tr433 mutants was reduced to 50–60% at 60 min compared with 76–87% for WT, loss of G protein coupling did not account for differences in internalization. Despite similar binding properties Tr406 and Tr416 mutants showed no AC or PLC response. Addition of 14 amino acids distal to HopTr406 resulted in normal AC and PLC responses. Site-directed mutagenesis indicated that Arg416 and Ser417 are essential for G protein activation. The proximal C terminus mediates signal transduction, and the distal is involved with internalization. Two residues within the C terminus, Arg416 and Ser417 conserved among class II receptors are the likely sites for G protein coupling.

PACAP (6–38) is a PACAP receptor antagonist for breast cancer cells

The effects of pituitary adenylate cyclase activating polypeptide (PACAP) analogs were investigated using breast cancer cells. 125I–PACAP–27 bound with high affinity (Kd=5u2009nM) to T47D cells (Bmaxu2009=u200929,000 per cell). Specific 125I–PACAP–27 binding was inhibited half maximally by PACAP–27, PACAP–38, PACAP(6–38) and PACAP(28–38) with IC50 values of 8, 17, 750 and >3000u2009nM, respectively. By RT–PCR, PACAP receptor mRNA was present in MCF–7 and T47D cell lines. Polyclonal antibodies to a PACAP receptor fragment (A–8–C) were elicited. The antibodies were affinity purified, recognized a 60–kDa protein by western blot, and stained malignant cells in breast cancer biopsy specimens by immunohistochemistry. PACAP–27 elevated the cAMP in T47D cells and the increase in cAMP caused by PACAP was inhibited by PACAP(6–38). PACAP–27 stimulated c–fos mRNA in T47D cells and the increase in c-fos gene expression caused by PACAP was reversed by PACAP (6–38). PACAP (6–38) inhibited colony formation using a soft agar assay and inhibited breast cancer xenograft growth in nude mice. These data suggest that PACAP (6–38) functions as a breast cancer PACAP receptor antagonist.

The Pituitary Adenylate Cyclase Activating Polypeptide Type 1 Receptor (PAC1‐R) Is Expressed on Gastric ECL Cells: Evidence by Immunocytochemistry and RT‐PCR

Abstract: The current study was undertaken to determine the presence and distribution of PAC1‐Rs within the gastric mucosa. Polyclonal antibodies to the carboxyl terminus of the rat PAC1‐R were generated and shown to be specific against the PAC1‐R expressed in NIH 3T3 cells. Western blot analysis using isolated (∼85% pure) ECL cell membranes identified a 48 kD protein consistent with the calculated molecular mass of the cloned PAC1‐R. RT/PCR performed using specific primers for the PAC1‐R confirmed the presence of splice variants of the rat PAC1‐R, but not VPAC1‐R or VPAC2‐R. These data provide the first direct evidence for the existence of functional PACAP Type I receptors on ECL cells of the gastric mucosa and suggest a potential role for PACAP in the stimulation of gastric acid secretion and in the regulation of the growth of ECL cells.

Replacement of oral proton pump inhibitors with intravenous pantoprazole to effectively control gastric acid hypersecretion in patients with Zollinger-Ellison syndrome

OBJECTIVES:In patients with Zollinger-Ellison syndrome (ZES) or other conditions requiring oral doses of proton pump inhibitors, it frequently becomes necessary to use parenterally administered gastric acid inhibitors. However, i.v. histamine-2 receptor antagonists are not effective at usual doses and lose their effectiveness because of tachyphlaxis. With the approval in the United States of i.v. pantoprazole, a substituted benzimidazole available in i.v. formulation, it will become possible to acutely manage gastric acid secretion in the acute care setting of a hospital. This study was developed to monitor the safety and establish the efficacy of i.v. pantoprazole as an alternative to oral proton pump inhibitors for the control of gastric acid hypersecretion in patients with ZES.METHODS:The efficacy of replacing oral PPI therapy with i.v. pantoprazole was evaluated in 14 ZES patients. After study enrollment, patients taking their current doses of oral PPI (omeprazole or lansoprazole) were switched to pantoprazole i.v. for 6 days during an 8-day inpatient period in the clinical research center. Effective control was defined as an acid output (AO) of <10 mEq/h (<5 mEq/h in patients with prior gastric acid-reducing surgery).RESULTS:The mean age of the 14 patients enrolled in the study was 52.4 yr (range = 38–67). Mean basal AO was 0.55 ± 0.32 mEq/h and mean fasting gastrin was 1089 pg/ml (range = 36–3720). Four patients were also diagnosed with the multiple endocrine neoplasia type I syndrome, nine were male, and two had previously undergone acid-reducing surgery. Before study enrollment, gastric acid hypersecretion was controlled in nine of 14 patients with omeprazole (20–200 mg daily) and five of 14 with lansoprazole (30–210 mg daily). In the oral phase of the study all patients had adequate control of gastric acid secretion, with a mean AO of 0.55 ± 0.32 mEq/h (mean ± SEM). Thereafter, 80 mg of i.v. pantoprazole was administered b.i.d. for 7 days by a brief (15 min) infusion and the dose was titrated upward to a predetermined maximum of 240 mg/24 h to control AO. A dose of 80 mg b.i.d. of i.v. pantoprazole controlled AO in 13 of 14 of the patients (93%) for the duration of the study (p > 0.05 compared to baseline values for all timepoints). One sporadic ZES patient (oral control value = 0.65 mEq/h on 100 mg of omeprazole b.i.d. p.o.) was not controlled with 80 mg of i.v. pantoprazole b.i.d. and dosage was titrated upward to 120 mg b.i.d. after day 2.CONCLUSIONS:There were no serious adverse events observed. Intravenous pantoprazole provides gastric acid secretory control that is equivalent to the acid suppression observed with oral proton pump inhibitors. Most ZES patients (93%) maintained effective control of AO previously established with oral PPIs when switched to 80 mg of i.v. pantoprazole b.i.d.; however, for difficult-to-control patients, doses >80 mg b.i.d. may be required.