Vasoactive Intestinal Polypeptide or VIP

Department of Molecular & Integrative Physiology, University of Michigan, Ann Arbor, Michigan, 48109-5622

Entry Version: 

Version 1.0 (October 7th, 2020)


Williams, JA. (2020) Vasoactive Intestinal Polypeptide or VIP.
Pancreapedia: Exocrine Pancreas Knowledge Base, DOI: 10.3998/panc.2020.03

Gene Name: VIP

1. General Information

Vasoactive intestinal polypeptide (VIP) was isolated by Said and Mutt in 1970 from pig small intestine based on its vasodilating activity (64).  In their initial studies using dogs, the peptide was shown to cause hypotension, increased cardiac output, stimulation of respiration and hyperglycemia.  The purified substance was determined to be a highly basic, single chain polypeptide of 28 amino acids with sequence similarity to secretin (27 aa) and glucagon (29 aa) (53).  Said and Mutt named the peptide vasoactive intestinal polypeptide and also showed that it stimulated pancreatic bicarbonate secretion in the cat although was only 5-10% as potent as secretin (63).  These early studies also showed that injected VIP was inactivated in the liver.  VIP is well conserved evolutionarily from fish to mammals except for the guinea pig and chicken.  The sequence of cow, pig, human, goat, dog, rabbit, and rat are identical; guinea pig and chicken VIP differs in 4 amino acids (16, 17).  VIP was cloned from human neuroblastoma cells and from a human tumor of VIP producing cells (37, 67).  The mRNA sequence predicts a 170 amino acid precursor with a N-terminal signal peptide.  VIP and another potential physiologically active peptide termed PHM for its amino terminal histidine and carboxy terminal methionine.  PHM is somewhat similar in structure and actions to VIP and PHI (Peptide with amino terminal histidine and carboxy terminal isoleucine), another peptide isolated from the intestine.  VIP is released from pro-VIP by prohormone convertase yielding VIP with the carboxyl amino acid extension GKR which is further processed by a carboxypeptidase like enzyme to cleave the terminal dipeptide and then the glycine is converted to an amidated C-terminal (38).  The human gene was found to contain 7 exons spanning about 9 kB of DNA with VIP and PHM/PHI present in adjoining exons (10, 48, 67, 70).  The human VIP gene is located on chromosome 6 while rat and mouse VIP genes are on chromosome 1.

Antisera to VIP showed that it is a neuropeptide present in nerve endings and cell bodies throughout the body including the enteric nervous system, brain and pancreas (14, 20, 46).  It is abundant in gut neurons but there is no evidence for a GI endocrine cell localization.  In neurons it is synthesized by ribosomes in the cell body, undergoes axonal transport and is stored as dense core vesicles about 100 nm in diameter (57).  It can be released from nerve terminals following electrical stimulation or physiological neuronal firing and its action is terminated by enzymatic degradation.  In the peripheral nervous system, VIPergic neurons regulate blood vessels, the digestive system including smooth muscle, endocrine function, the urogenital system, the respiratory system and the immune system (20, 38). One important action in the digestive tract is its role along with nitric oxide to relax smooth muscle in the stomach and intestine particularly at sphincters (47).  Gene deletion of VIP induces a relatively mild phenotype with alterations in circadian rhythms, reproductive function, behavior, metabolism and bladder function (14).

In the CNS, VIP neurons are present in the cerebral cortex, innervate cerebral vasculature and are believed to couple neuronal activity to blood flow (30).  VIP receptors (VPAC1) are present on circular smooth muscle cells of cerebral arterioles (21).  Moreover, VIP antibodies block the increase in blood flow.  Increasing evidence suggests that VIP and VPAC receptors in the suprachiasmatic nucleus (SCN) play a role in generation of circadian rhythms (28, 29).  VIP applied to brain slices increases the firing of SCN neurons (59) and increases the expression of Per genes (54).

In the immune system, VIP neurons innervate lymphatic tissue and VIP is produced by many immune cells (1).  Immune cells also bear VPAC receptors especially VPAC1 (see below).  VIP can act as an immunomodulatory (15) and blocks inflammation, modify the Th response favoring Th2 and induce regulatory T cells (1).

The two receptors recognizing VIP with high affinity are termed VPAC1 and VPAC2 because they recognize both VIP and PACAP (Pituitary adenylyl cyclase activating peptide) (16, 28).  They and PAC1, which specifically binds PACAP and not VIP are a subfamily of Class B, G-protein coupled receptors which also includes secretin receptors.  Class B receptors have a long extracellular N-terminal which is involved in binding ligand. The three receptors were originally identified by radioligand binding, development of specific agonists and antagonists and then by molecular cloning (16, 29, 45).  Both VPAC receptors couple through the heterotrimeric G protein Gs to adenylyl cyclase and are broadly distributed throughout the body.  Functions of the receptors have been shown using mouse genetic models where knockout of VPAC1 and VPAC2 effect different physiological actions of VIP.  VPAC1 KO mice show impaired growth with intestinal obstruction and smaller pancreatic islets (19). VPAC2 KO mice show growth impairment, decreased fat mass, reduced metabolic rate and impaired circadian rhythm (5).

Endocrine tumors with islet characteristics that are most commonly located in the pancreas are associated with a syndrome of watery diarrhea and hypokalemia described initially by Werner and Morrison in 1958 (69).  The syndrome is also known as pancreatic cholera and is due to intestinal secretion that is similar to that induced by cholera toxin.  Bloom et al recognized that the patients had elevated VIP levels in plasma and in the tumors (9).  The syndrome is due to VIP binding to receptors on enterocytes, activating adenylyl cyclase and stimulating chloride rich fluid secretion (44).  The symptoms subside after removal of the tumor or can be blocked by administration of long acting somatostatin

2. VIP and the Pancreas

Purified or synthetic VIP injected as a bolus or by infusion stimulates pancreatic bicarbonate rich fluid secretion in all mammalian species tested but has only 15-20% of the maximal effect of secretin in most species.  It is a partial or lower potency agonist compared to secretin in rats (17), dogs (35, 42, 51, 63) or humans (18, 22). In cats, VIP induced a similar maximal flow to secretin but required much higher concentrations (41, 60).  In these in vivo studies there was no increase in digestive enzyme secretion in response to VIP.  The effects on fluid and bicarbonate secretion are presumably mediated by the pancreatic ducts.  At high concentrations VIP also stimulated insulin secretion and VPAC receptors are known to be present in islet beta cells (6).  VIP stimulates insulin secretion in a glucose dependent manner and also stimulates glucagon secretion (72).

This lack of an effect or low potency of VIP administered in vivo is probably due to the fact that VIP is now known to be a neuropeptide and not a GI hormone (20, 38).  VIP is present in a variety of pre- and post- ganglionic autonomic neurons and in the pancreas is localized in neurons innervating both exocrine and endocrine cell in a variety of vertebrate species (3, 7, 32, 34, 46, 49, 66).  VIP is also found in nerve fibers which innervate blood vessels as well as secretory cells (2).  There is longstanding evidence for a non-cholinergic, non-adrenergic innervation of the exocrine pancreas which is very strong in the pig (31).  Both in vivo and in a perfused pancreas preparation, vagal electrical stimulation enhances pancreatic fluid and bicarbonate secretion, releases VIP and insulin into the blood and reduces vascular resistance (23, 34, 39, 71).  Most of the secretory effects are blocked by VIP antisera (34).  PHI (peptide histidine isoleucine) an intestinal GI hormone which is analogous to PHM in the human VIP precursor also stimulate pancreatic fluid and bicarbonate secretion although less potent than VIP (33).  Neural VIP is probably not as important for fluid secretion in other species (71) but contributes to the increase in pancreatic blood flow in dogs (8, 35, 43) and rats (6).  VIP is often present in postganglionic parasympathetic neurons along with acetylcholine.  While this two transmitter paradigm is best studied in salivary gland innervation. it probably also occurs in the pancreas.

VIP action has been extensively studied on isolated pancreatic acini where its main observable action is to stimulate secretion of amylase and other stored digestive enzymes.  These studies use acinar cells as a model cell for studying VIP action.  Early studies showed that VIP was a full agonist on amylase secretion from guinea pig isolated cells and acini (25, 27, 56, 65) and induced a partial response from rat and mouse acini (11, 13, 65).  VIP was found to increase cyclic AMP in acini and to synergize with stimuli that acted through an increase in intracellular Ca2+ such as CCK or cholinergic agonists.  These effects are similar to studies using secretin, cholera toxin and dibutyryl cAMP.  Thus the species dependence reflects the ability of cyclic AMP to act in a particular species (26).  The action of VIP is now known to be mediated by binding to VPAC receptors and both VPAC1 and VPAC2 have been identified in rodent acini at the RNA level by Northern blotting and by Southern blotting after RT-PCR.  Most (85-90%) of the physiological effects on cAMP formation and amylase secretion are believed to be mediated by VPAC1 (36) as shown using subtype specific ligands and substantiated by means of specific receptor KO mice.

In addition to amylase release, isolated acini have been used to show other actions of VIP mostly related to signaling initiated by cAMP.  VIP increased adenylyl cyclase activity in guinea pig and rat isolated acini and calf pancreas membrane fractions (52), increased cAMP levels (25, 68) and the activity of PKA (39, 62).  In mouse acini and ducts, the effect of VIP was mediated by the AC6 isoform of adenylyl cyclase. The cAMP produced activates PKA and leads to the phosphorylation of specific proteins on serine and threonine residues (12, 68).  Of these proteins two have been identified as ribosomal protein S6 and CREB.  In addition to PKA, cyclic AMP activates EPAC (exchange protein directly activated by cAMP) which is involved in the activation of the small G protein Rap1 (61) and of another protein kinase, PAK4 (58).  An additional effect of VIP was to increase the tyrosine phosphorylation of proteins of 115 and 130 kDa similar to the effects of forskolin and dibutyryl cAMP (50).

Isolated pancreatic duct fragments have also been used to show actions of VIP and secretin on pancreatic duct cells.  VIP increased cAMP levels in rat and mouse duct fragments (24, 62).  However, in another study VIP had no effect on fluid secretion by isolated rat duct fragments (4).  VIP was, however, shown to depolarize the basolateral membrane of dissected rat ducts (55).

VIP is also known to reduce the severity of experimental pancreatitis in mice (40).  VPAC1 agonists also reduced the increase in cytokines including IL-6 and TNF-α.

3. Tools for the Study of VIP

a. Peptides

VIP peptides are sold by Phoenix Pharmaceutical, Peptides International, and Antibodies online.

b. Antibodies

Mouse and rabbit antibodies to VIP are available from Abcam that are primarily designed and validated for immunofluorescence and immunohistochemistry.  Antibodies are also available from Santa Cruz, Sigma and Thermo Fisher.

4. References

  1. Abad C, and Tan YV. Immunomodulatory Roles of PACAP and VIP: Lessons from Knockout Mice. J Mol Neurosci 66: 102-113, 2018. PMID: 30105629.
  2. Adeghate E. Distribution of calcitonin-gene-related peptide, neuropeptide-Y, vasoactive intestinal polypeptide, cholecystokinin-8, substance P and islet peptides in the pancreas of normal and diabetic rats. Neuropeptides 33: 227-235, 1999. PMID: 10657496.
  3. Arciszewski MB, and Zacharko-Siembida A. A co-localization study on the ovine pancreas innervation. Ann Anat 189: 157-167, 2007. PMID: 17419548.
  4. Ashton N, Argent BE, and Green R. Effect of vasoactive intestinal peptide, bombesin and substance P on fluid secretion by isolated rat pancreatic ducts. J Physiol 427: 471-482, 1990. PMID: 1698981.
  5. Asnicar MA, Koster A, Heiman ML, Tinsley F, Smith DP, Galbreath E, Fox N, Ma YL, Blum WF, and Hsiung HM. Vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating peptide receptor 2 deficiency in mice results in growth retardation and increased basal metabolic rate. Endocrinology 143: 3994-4006, 2002. PMID: 12239111.
  6. Bertrand G, Puech R, Maisonnasse Y, Bockaert J, and Loubatieres-Mariani MM. Comparative effects of PACAP and VIP on pancreatic endocrine secretions and vascular resistance in rat. Br J Pharmacol 117: 764-770, 1996. PMID: 8646426.
  7. Bishop AE, Polak JM, Green IC, Bryant MG, and Bloom SR. The location of VIP in the pancreas of man and rat. Diabetologia 18: 73-78, 1980. PMID: 6988269.
  8. Blitz W, Beijer HJ, and Charbon GA. Cholinergic and pancreatic actions of purified porcine and synthetic vasoactive intestinal peptide (VIP). Arch Int Pharmacodyn Ther 277: 66-76, 1985. PMID: 4062433.
  9. Bloom SR, Polak JM, and Pearse AG. Vasoactive intestinal peptide and watery-diarrhoea syndrome. Lancet 2: 14-16, 1973. PMID: 4123289.
  10. Bodner M, Fridkin M, and Gozes I. Coding sequences for vasoactive intestinal peptide and PHM-27 peptide are located on two adjacent exons in the human genome. Proc Natl Acad Sci U S A 82: 3548-3551, 1985. PMID: 2987932.
  11. Burnham DB, McChesney DJ, Thurston KC, and Williams JA. Interaction of cholecystokinin and vasoactive intestinal polypeptide on function of mouse pancreatic acini in vitro. J Physiol 349: 475-482, 1984. PMID: 6204039.
  12. Burnham DB, Sung CK, Munowitz P, and Williams JA. Regulation of protein phosphorylation in pancreatic acini by cyclic AMP-mediated secretagogues: interaction with carbamylcholine. Biochim Biophys Acta 969: 33-39, 1988. PMID: 2450590.
  13. Collen MJ, Sutliff VE, Pan GZ, and Gardner JD. Postreceptor modulation of action of VIP and secretin on pancreatic enzyme secretion by secretagogues that mobilize cellular calcium. Am J Physiol 242: G423-428, 1982. PMID: 6175231.
  14. Colwell CS, Michel S, Itri J, Rodriguez W, Tam J, Lelievre V, Hu Z, Liu X, and Waschek JA. Disrupted circadian rhythms in VIP- and PHI-deficient mice. Am J Physiol Regul Integr Comp Physiol 285: R939-949, 2003. PMID: 12855416.
  15. Delgado M, Pozo D, and Ganea D. The significance of vasoactive intestinal peptide in immunomodulation. Pharmacol Rev 56: 249-290, 2004. PMID: 15169929.
  16. Dickson L, and Finlayson K. VPAC and PAC receptors: From ligands to function. Pharmacol Ther 121: 294-316, 2009. PMID: 19109992.
  17. Dimaline R, and Dockray GJ. Potent stimulation of the avian exocrine pancreas by porcine and chicken vasoactive intestinal peptide. J Physiol 294: 153-163, 1979. PMID: 512939.
  18. Domschke S, Domschke W, Rosch W, Konturek SJ, Sprugel W, Mitznegg P, Wunsch E, and Demling L. Vasoactive intestinal peptide: a secretin-like partial agonist for pancreatic secretion in man. Gastroenterology 73: 478-480, 1977. PMID: 892345.
  19. Fabricius D, Karacay B, Shutt D, Leverich W, Schafer B, Takle E, Thedens D, Khanna G, Raikwar S, Yang B, Desmond ME, and O'Dorisio MS. Characterization of intestinal and pancreatic dysfunction in VPAC1-null mutant mouse. Pancreas 40: 861-871, 2011. PMID: 21697765.
  20. Fahrenkrug J. Transmitter role of vasoactive intestinal peptide. Pharmacol Toxicol 72: 354-363, 1993. PMID: 8103215.
  21. Fahrenkrug J, Hannibal J, Tams J, and Georg B. Immunohistochemical localization of the VIP1 receptor (VPAC1R) in rat cerebral blood vessels: relation to PACAP and VIP containing nerves. J Cereb Blood Flow Metab 20: 1205-1214, 2000. PMID: 10950381.
  22. Fahrenkrug J, Pedersen JH, Yamashita Y, Ottesen B, Hokfelt T, and Lundberg JM. Occurrence of VIP and peptide HM in human pancreas and their influence on pancreatic endocrine secretion in man. Regul Pept 18: 51-61, 1987. PMID: 3306816.
  23. Fahrenkrug J, Schaffalitzky de Muckadell OB, Holst JJ, and Jensen SL. Vasoactive intestinal polypeptide in vagally mediated pancreatic secretion of fluid and HCO3. Am J Physiol 237: E535-540, 1979. PMID: 517650.
  24. Folsch UR, Fischer H, Soling HD, and Creutzfeldt W. Effects of gastrointestinal hormones and carbamylcholine on cAMP accumulation in isolated pancreatic duct fragments from the rat. Digestion 20: 277-292, 1980. PMID: 6248407.
  25. Gardner JD, Conlon TP, and Adams TD. Cyclic AMP in pancreatic acinar cells: effects of gastrointestinal hormones. Gastroenterology 70: 29-35, 1976. PMID: 173615.
  26. Gardner JD, and Jensen RT. Receptors and cell activation associated with pancreatic enzyme secretion. Annu Rev Physiol 48: 103-117, 1986. PMID: 2423019.
  27. Gardner JD, Korman LY, Walker MD, and Sutliff VE. Effects of inhibitors of cyclic nucleotide phosphodiesterase on the actions of vasoactive intestinal peptide and secretin on pancreatic acini. Am J Physiol 242: G547-551, 1982. PMID: 6178297.
  28. Harmar AJ, Fahrenkrug J, Gozes I, Laburthe M, May V, Pisegna JR, Vaudry D, Vaudry H, Waschek JA, and Said SI. Pharmacology and functions of receptors for vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide: IUPHAR review 1. Br J Pharmacol 166: 4-17, 2012. PMID: 22289055.
  29. Harmar AJ, Marston HM, Shen S, Spratt C, West KM, Sheward WJ, Morrison CF, Dorin JR, Piggins HD, Reubi JC, Kelly JS, Maywood ES, and Hastings MH. The VPAC(2) receptor is essential for circadian function in the mouse suprachiasmatic nuclei. Cell 109: 497-508, 2002. PMID: 12086606.
  30. Heistad DD, Marcus ML, Said SI, and Gross PM. Effect of acetylcholine and vasoactive intestinal peptide on cerebral blood flow. Am J Physiol 239: H73-80, 1980. PMID: 7396021.
  31. Hickson JC. The secretion of pancreatic juice in response to stimulation of the vagus nerves in the pig. J Physiol 206: 275-297, 1970. PMID: 5498476.
  32. Holmgren S, and Olsson C. Autonomic control of glands and secretion: a comparative view. Auton Neurosci 165: 102-112, 2011. PMID: 21130712.
  33. Holst JJ, Fahrenkrug J, Knuhtsen S, Jensen SL, Nielsen OV, Lundberg JM, and Hokfelt T. VIP and PHI in the pig pancreas: coexistence, corelease, and cooperative effects. Am J Physiol 252: G182-189, 1987. PMID: 3548423.
  34. Holst JJ, Fahrenkrug J, Knuhtsen S, Jensen SL, Poulsen SS, and Nielsen OV. Vasoactive intestinal polypeptide (VIP) in the pig pancreas: role of VIPergic nerves in control of fluid and bicarbonate secretion. Regul Pept 8: 245-259, 1984. PMID: 6379759.
  35. Ito O, Naruse S, Kitagawa M, Ishiguro H, Ko S, Nakajima M, and Hayakawa T. The effect of VIP/PACAP family of peptides on pancreatic blood flow and secretion in conscious dogs. Regul Pept 78: 105-112, 1998. PMID: 9879753.
  36. Ito T, Hou W, Katsuno T, Igarashi H, Pradhan TK, Mantey SA, Coy DH, and Jensen RT. Rat and guinea pig pancreatic acini possess both VIP(1) and VIP(2) receptors, which mediate enzyme secretion. Am J Physiol Gastrointest Liver Physiol 278: G64-74, 2000. PMID: 10644563.
  37. Itoh N, Obata K, Yanaihara N, and Okamoto H. Human preprovasoactive intestinal polypeptide contains a novel PHI-27-like peptide, PHM-27. Nature 304: 547-549, 1983. PMID: 6571696.
  38. Iwasaki M, Akiba Y, and Kaunitz JD. Recent advances in vasoactive intestinal peptide physiology and pathophysiology: focus on the gastrointestinal system. F1000Res 8: 2019. PMID: 31559013.
  39. Jensen RT, and Gardner JD. Cyclic nucleotide-dependent protein kinase activity in acinar cells from guinea pig pancreas. Gastroenterology 75: 806-816, 1978. PMID: 212341.
  40. Kojima M, Ito T, Oono T, Hisano T, Igarashi H, Arita Y, Kawabe K, Coy DH, Jensen RT, and Nawata H. VIP attenuation of the severity of experimental pancreatitis is due to VPAC1 receptor-mediated inhibition of cytokine production. Pancreas 30: 62-70, 2005. PMID:15632701.
  41. Konturek SJ, Domschke S, Domschke W, Wunsch E, and Demling L. Comparison of pancreatic responses to portal and systemic secretin and VIP in cats. Am J Physiol 232: E156-158, 1977. PMID: 842623.
  42. Konturek SJ, Thor P, Dembinski A, and Krol R. Comparison of secretin and vasoactive intestinal peptide on pancreatic secretion in dogs. Gastroenterology 68: 1527-1535, 1975. PMID: 1132634.
  43. Konturek SJ, Yanaihara N, Pawlik W, Jaworek J, and Szewczyk K. Comparison of helodermin, VIP and PHI in pancreatic secretion and blood flow in dogs. Regul Pept 24: 155-166, 1989. PMID: 2922493.
  44. Krejs GJ. VIPoma syndrome. Am J Med 82: 37-48, 1987. PMID: 3035922.
  45. Laburthe M, Couvineau A, and Tan V. Class II G protein-coupled receptors for VIP and PACAP: structure, models of activation and pharmacology. Peptides 28: 1631-1639, 2007. PMID: 17574305.
  46. Larsson LI, Fahrenkrug J, Schaffalitzky De Muckadell O, Sundler F, Hakanson R, and Rehfeld JR. Localization of vasoactive intestinal polypeptide (VIP) to central and peripheral neurons. Proc Natl Acad Sci U S A 73: 3197-3200, 1976. PMID: 787988.
  47. Li CG, and Rand MJ. Nitric oxide and vasoactive intestinal polypeptide mediate non-adrenergic, non-cholinergic inhibitory transmission to smooth muscle of the rat gastric fundus. Eur J Pharmacol 191: 303-309, 1990. PMID: 1964906.
  48. Linder S, Barkhem T, Norberg A, Persson H, Schalling M, Hokfelt T, and Magnusson G. Structure and expression of the gene encoding the vasoactive intestinal peptide precursor. Proc Natl Acad Sci U S A 84: 605-609, 1987. PMID: 3025882.
  49. Love JA, and Szebeni K. Morphology and histochemistry of the rabbit pancreatic innervation. Pancreas 18: 53-64, 1999. PMID: 9888661.
  50. Lutz MP, Piiper A, Gaisano HY, Stryjek-Kaminska D, Zeuzem S, and Adler G. Protein tyrosine phosphorylation in pancreatic acini: differential effects of VIP and CCK. Am J Physiol 273: G1226-1232, 1997. PMID: 9435547.
  51. Makhlouf GM, Yau WM, Zfass AM, Said SI, and Bodanszky M. Comparative effects of synthetic and natural vasoactive intestinal peptide on pancreatic and biliary secretion and on glucose and insulin blood levels in the dog. Scand J Gastroenterol 13: 759-765, 1978. PMID: 694415.
  52. Meuth-Metzinger VL, Philouze-Rome V, Metzinger L, Gespach C, and Guilloteau P. Differential activation of adenylate cyclase by secretin and VIP receptors in the calf pancreas. Pancreas 31: 174-181, 2005. PMID: 16025005.
  53. Mutt V, and Said SI. Structure of the porcine vasoactive intestinal octacosapeptide. The amino-acid sequence. Use of kallikrein in its determination. Eur J Biochem 42: 581-589, 1974. PMID: 4829446.
  54. Nielsen HS, Hannibal J, and Fahrenkrug J. Vasoactive intestinal polypeptide induces per1 and per2 gene expression in the rat suprachiasmatic nucleus late at night. Eur J Neurosci 15: 570-574, 2002. PMID: 11876785.
  55. Pahl C, and Novak I. Effect of vasoactive intestinal peptide, carbachol and other agonists on the membrane voltage of pancreatic duct cells. Pflugers Arch 424: 315-320, 1993. PMID: 8414920.
  56. Peikin SR, Rottman AJ, Batzri S, and Gardner JD. Kinetics of amylase release by dispersed acini prepared from guinea pig pancreas. Am J Physiol 235: E743-749, 1978. PMID: 736135.
  57. Probert L, De Mey J, and Polak JM. Distinct subpopulations of enteric p-type neurones contain substance P and vasoactive intestinal polypeptide. Nature 294: 470-471, 1981. PMID: 6171734.
  58. Ramos-Alvarez I, Lee L, and Jensen RT. Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na(+),K(+)-ATPase in pancreatic acinar cells. Am J Physiol Gastrointest Liver Physiol 316: G263-G277, 2019.  PMID: 30520694.
  59. Reed HE, Cutler DJ, Brown TM, Brown J, Coen CW, and Piggins HD. Effects of vasoactive intestinal polypeptide on neurones of the rat suprachiasmatic nuclei in vitro. J Neuroendocrinol 14: 639-646, 2002. PMID: 12153466.
  60. Roche C, Vagne M, Scarpignato C, and Mutt V. Comparative effects of vasoactive intestinal peptide and secretin on exocrine pancreatic secretion in the cat. Regul Pept 12: 125-132, 1985. PMID: 2999882.
  61. Sabbatini ME, Chen X, Ernst SA, and Williams JA. Rap1 activation plays a regulatory role in pancreatic amylase secretion. J Biol Chem 283: 23884-23894, 2008. PMID: 18577515.
  62. Sabbatini ME, D'Alecy L, Lentz SI, Tang T, and Williams JA. Adenylyl cyclase 6 mediates the action of cyclic AMP-dependent secretagogues in mouse pancreatic exocrine cells via protein kinase A pathway activation. J Physiol 591: 3693-3707, 2013. PMID: 23753526.
  63. Said SI, and Mutt V. Isolation from porcine-intestinal wall of a vasoactive octacosapeptide related to secretin and to glucagon. Eur J Biochem 28: 199-204, 1972. PMID: 5069712.
  64. Said SI, and Mutt V. Polypeptide with broad biological activity: isolation from small intestine. Science 169: 1217-1218, 1970. PMID: 5450698.
  65. Singh M. Role of cyclic adenosine monophosphate in amylase release from dissociated rat pancreatic acini. J Physiol 331: 547-555, 1982. PMID: 6185668.
  66. Sundler F, Alumets J, Hakanson R, Fahrenkrug J, and Schaffalitzky de Muckadell O. Peptidergic (VIP) nerves in the pancreas. Histochemistry 55: 173-176, 1978. PMID: 632129.
  67. Tsukada T, Horovitch SJ, Montminy MR, Mandel G, and Goodman RH. Structure of the human vasoactive intestinal polypeptide gene. DNA 4: 293-300, 1985. PMID: 3899557.
  68. Vandermeers A, Vandermeers-Piret MC, Rathe J, Dehaye JP, Winand J, and Christophe J. Phosphorylation of 3 particulate proteins in rat pancreatic acini in response to vasoactive intestinal peptide (VIP), secretin and cholecystokinin (CCK-8). Peptides 5: 359-365, 1984. PMID: 6089135.
  69. Verner JV, and Morrison AB. Islet cell tumor and a syndrome of refractory watery diarrhea and hypokalemia. Am J Med 25: 374-380, 1958. PMID: 13571250.
  70. Waschek JA, Bravo DT, Sena M, Casillas R, Rodriguez W, Nguyen T, and Colburn S. Targeting of embryonic and postnatal autonomic and enteric neurons with a vasoactive intestinal peptide transgene. J Neurochem 73: 1739-1748, 1999. PMID: 10501223.
  71. Wheeler S, Eardley JE, McNulty KF, Sutcliffe CP, and Morrison JD. An investigation into the relative merits of pituitary adenylate cyclase-activating polypeptide (PACAP-27) and vasoactive intestinal polypeptide as vagal neuro-transmitters in exocrine pancreas of rats. Exp Physiol 82: 729-747, 1997. PMID: 9257115.
  72. Winzell MS, and Ahren B. Role of VIP and PACAP in islet function. Peptides 28: 1805-1813, 2007. PMID: 17559974.