[The vasoactive intestinal polypeptide (VIP). Its role in the physiology of the digestive system]

The literature on the possible implications of the Vasoactive Intestinal Polypeptide (VIP) in the digestive processes is widely and critically reviewed. Special attention is directed to the likely function of the polypeptide as a neurotransmitter in the gastroenteropancreatic system.

Effect of vasoactive intestinal polypeptide on cerebral blood flow in the goat

The effect of vasoactive intestinal polypeptide (VIP) on the cerebral blood flow was investigated in the goat. An electromagnetic flow probe was placed around the internal maxillary artery for continuous measurement of ipsilateral blood flow. Intraarterial injection of VIP resulted in a dose-dependent increase in the cerebral blood flow. The effect was not antagonized by any of the antagonists atropine, propranolol, phentolamine and naloxone administered intraarterially 1 min before VIP. It is discussed that VIP may play a physiological role in the local blood flow regulation in the CNS.

[PRF activity of VIP in vitro (author's transl)]

The effect of VIP on prolactin secretion from incubated rat hemipituitaries was characterized. Under these conditions, the secretion of GH, LH, FSH, ACTH was not affected, indicating that the effect of VIP is hormone specific. The stimulation of prolactin was dose-dependent, with an apparent affinity of VIP of 10.9 +/- 3.1 nM and a maximal stimulation of 57.7 +/- 4.2%. Secretin, a structurally related peptide, was also active at higher concentrations, whereas another partial analogue, glucagon, was ineffective. Furthermore, VIP does not act through pituitary DA receptors since alpha-flupentixol, a potent dopaminergic antagonist, does not block the stimulation of prolactin secretion by VIP. In addition, stimulation by VIP and TRH was additive. Naloxone and met-enkephalin were ineffective on the VIP effect on prolactin release. In contrast, SRIF seems to inhibit the VIP stimulation of prolactin release. Our data suggest that VIP, which was found in the hypothalamo-hypophyseal blood at concentrations of the same order of magnitude as that found to stimulate PRL in vitro, could be a physiological PRF.

Effects of vasoactive intestinal polypeptide (VIP), secretin and gastrin on insulin secretion in the mouse

The in vivo effects of vasoactive intestinal polypeptide (VIP), secretin and two different molecular forms of gastrin, gastrin 17 and pentagastrin, on basal and stimulated insulin secretion have been investigated in the mouse. All these peptides induced a moderate dose-dependent increase in basal insulin secretion. The different polypeptides showed complex effects on insulin release stimulated by glucose, the cholinergic agonist carbachol or the beta-adrenergic agonist L-isopropylnoradrenaline (L-IPNA), these effects being dependent on the nature of the secretagogue. VIP and secretin both potentiated glucose-induced insulin release. Secretin inhibited insulin secretion induced by carbachol and L-IPNA, whereas VIP potentiated L-IPNA-induced insulin secretion and had no influence on the effect of carbachol. Gastrin 17 and pentagastrin did not affect glucose- or carbachol-induced insulin release, whereas they inhibited L-IPNA-induced insulin secretion. The results suggest that VIP, secretin and gastrin display their effects on insulin secretion through different mechanisms. The results indirectly suggest the existence of separate insulin secretory pathways which operate differently, or at least partly differently, after glucose stimulation, cholinergic stimulation, and beta-adrenergic stimulation.

Distribution and motor effect of VIP in female genital tract

The distribution and concentration of vasoactive intestinal polypeptide (VIP) in different parts of the female genital organs from various species (cat, goat, pig, rabbit, rat, and guinea pig) were studied by means of immunohistochemistry and immunochemistry. In addition, the effect of VIP on the mechanical activity of uterine muscle was investigated in vitro. VIP immunoreactive nerve fibers innervating vessels and smooth muscle cells were fairly numerous in the genital organs of the cat and goat and less numerous in the pig, rabbit, rat, and guinea pig. In all species studied, VIP-containing nerves were most numerous in the cervix. The tissue concentration of VIP measured by immunochemistry was in agreement with the immunohistochemical findings. VIP inhibited the mechanical uterine activity in the cat, goat, pig, and rabbit, whereas only minimal effects were obtained in the rat and guinea pig. This difference between the species corresponded to the number of VIP-containing nerves in the uterine muscle. The data support the hypothesis that VIP may play a physiological role in the local nervous control of the uterine mechanical activity.

Vasoactive intestinal peptide: a possible transmitter of nonadrenergic relaxation of guinea pig airways

Vasoactive intestinal peptide, a smooth-muscle relaxant neuropeptide with neurotransmitter properties, was relaxed during electrical field stimulation of guinea pig trachea. The amount released correlated with the degree of relaxation, and the release was blocked by tetrodotoxin. Prior incubation of the trachea with antiserum to vasoactive intestinal peptide reduced the relaxation. Thus vasoactive intestinal peptide may mediate the nonadrenergic relaxation of tracheal smooth muscle.

[Physiology and disease of the endocrine function of the pancreas (author's transl)]

Qualitative and quantitative immunocytochemistry, electronmicroscopy and radio-immuno-assays led to the discovery of 5 pancreatic polypeptide hormones under physiological conditions. The active endocrine cells and the produced hormones are termed A, B, D, D1, and PP cell and glucagon, insulin, somatostatin, vasoactive intestinal polypeptide (VIP) and pancreatic polypeptide (PP) respectively. Beside the physiology of secretion and action a survey of pathological conditions in the paediatric age group is given. Insulin is the most important of pancreatic hormones in childhood. Therefore diagnosis and treatment of hyperinsulinism are described in extension.

Neuroendocrinology and brain peptides

To date about thirty peptides--low-molecular-weight, single-chain amino acid compounds--are known to be distributed widely in the central nervous system within selective neuron pathways. These findings, combined with a large body of neuropharmacological, behavioral, and electrophysiological data, open new horizons in neurobiology, force a reexamination of old and accepted hypotheses, and hold important implications for the clinician. There is evidence that substance P and the opioid peptides play a major role in the pain pathway, particularly at the level of the spinal cord. Available evidence also implicates vasoactive intestinal polypeptide in the control of cerebral circulation, cholecystokinin in the regulation of appetite, and vasopressin and adrenocorticotropic hormone in memory. Many questions, however, remain. For most peptides there is little information on mechanisms of biosynthesis, release, interaction with receptors, and termination of biological effect. Another important question is the interaction of peptides with other neurotransmitters. The evidence that both "classic" neurotransmitters and peptides can be found in the same neuronal necessitates reformulation of Dale's "one neuron, one neurotransmitter" hypothesis. It may be that a single cell, while containing different classes of neurotransmitter, will contain only one member of any particular class. It is not too early to speculate on the role of the numerous and diverse peptides in neuronal tissue and on the implications of peptide abnormalities in a variety of neurological diseases. The answers to these and other questions pose a fascinating challenge to neurobiologist and clinician alike.

VIP as a possible neurotransmitter of non-cholinergic non-adrenergic inhibitory neurones

Many peripheral autonomic nerves are neither cholinergic nor adrenergic. Such nerves are widely distributed in the gastrointestinal, urogenital and respiratory tracts, and in blood vessels. The nature of their neurotransmitter is not known. We have previously reported that vasoactive intestinal polypeptide (VIP) is a potent inhibitor of opossum lower oesophageal sphincter (LOS) and that its inhibitory effect is exerted directly on the sphincter muscle. Subsequent studies have confirmed the inhibitory effect of VIP on LOS in other species. Recently, very high tissue levels of VIP have been reported in the LOS and other gastrointestinal sphincters. Furthermore, VIP has been localized to intramural neurones and is released upon electrical stimulation of the vagus nerve. We report here that immunoantagonism of VIP with a high-titre antiserum antagonized inhibitory neuromuscular transmission in the LOS. These findings provide evidence of a role for VIP as an inhibitory neurotransmitter.

Relations between neuropeptides and gut hormones

Certain peptides have a dual distribution in brain and in neurons or endocrine cells, or both, in the gut. Substance P, neurotensin and somatostatin have been isolated from brain and gut (or pancreas); cholecystokinin has been isolated from intestine as peptides of 33 and 39 residues and their active C-terminal octapeptide has been isolated from brain, which suggests differences between biosynthetic processing pathways for cholecystokinin in neurons and those in endocrine cells. Identification of other peptides (vasoactive intestinal peptide, bombesin, enkephalin etc.) rests in part on immunochemical evidence and needs to be confirmed by isolation. The combined evidence of immunochemical and physiological studies suggests that gut target cells can receive peptides in at last four ways: (1) as hormones in the blood; (2) from intrinsic neurons in the enteric plexuses; (3) from extrinsic nerves like the vagus; (4) by local (paracrine) release from mucosal endocrine-like cells, possibly by basal axon-like processes.

Evidence against vasoactive intestinal polypeptide being the non-adrenergic, non-cholinergic inhibitory transmitter released from nerves supplying the smooth muscle of the guinea-pig taenia coli

Vasoactive intestinal polypeptide (VIP), located within intrinsic neurones of the guinea-pig taenia coli, has been proposed as a neurotransmitter which may mediate certain non-adrenergic, non-cholinergic effects within the gastrointestinal tract. The relaxation of the taenia coli produced by exogenous VIP has a longer latency and time to maximum than the relaxation which is obtained during electrical stimulation of the non-adrenergic, non-cholinergic nerves. The responses to exogenous VIP (0.03-1 microM) were abolished by the proteolytic enzyme alpha-chymotrypsin (1 U/ml), whereas there were no statistically significant changes in the inhibitory responses to intramural nerve stimulation (0.1-5 Hz) or exogenous ATP (0.03-100 microM), which closely mimics the nerve-mediated response. Apamin is a neurotoxin which prevents the increase in K+ conductance associated with the inhibitory junction potential produced by stimulation of the non-adrenergic, non-cholinergic inhibitory nerves. In the presence of apamin (0.005-1 microM) the responses to intramural nerve stimulation and exogenous ATP were significantly antagonised, and often converted to contractile responses. The VIP-induced relaxations during apamin treatment were not significantly decreased. These results suggest that VIP is unlikely to be the transmitter released from the non-adrenergic, non-cholinergic nerves of the guinea-pig taenia coli, and are consistent with the view that these nerves are purinergic in nature.

Pathophysiology of gastrointestinal hormones. Implications for paediatrics

Gastrointestinal hormones (GI hormones) have received growing interest in endocrinology, gastroenterology and neuroendocrinology. Because of new methodological techniques, they can be measured in plasma and therefore be related to different pathophysiological conditions. In childhood, our present knowledge is as yet limited to the physiological rôle of gastrin at different ages and in some diseases (gastrinoma; Verner-Morrison syndrome) caused by humoral dysfunction. The present review relates the clinical important GI hormones to chemically classified families. The diagnostic value of determining endogenous hormone concentration in plasma and the validity of function tests carried out by administration of exogenous hormones are pointed out. Particular emphasis is given to the trophic action of GI hormones in the development and function of the gastrointestinal tract during childhood. More speculatively, GI hormones are involved in the complex function of the central nervous system, thus making food intake a trophotropic action in a broader sense.

Brain peptides as neurotransmitters

Numerous peptides appear to be neurotransmitter candidates in the brain. Some, such as the opioid peptide enkephalins, neurotensin, and substance P, were first isolaterd from the brain. Peptides, such as cholecystokinin and vasoactive intestinal polypeptide, were known as intestinal hormones and later recognized as brain constituents. Certain hypothalamic-releasing hormones, pituitary peptides, and blood-derived peptides like angiotensin II and bradykinin, may also be central neurotransmitters. The diversity of localization of these peptides throughout the brain implies a multiplicity of potential roles.

Vasoactive intestinal polypeptide (VIP) inhibits prostaglandin-F2 alpha-induced activity of the rabbit myometrium

Seven female rabbits ahd myometrial autografts transplanted into ear-chambers. The electrical activity of the graft was recorded and mechanical activity observed. Vasoactive intestinal polypeptide (VIP) dose-dependently inhibited myometrial activity induced by prostaglandin-F2 alpha. This evidence and the occurrence of "VIPergic" nerve fibres, which seem to innervate the myometrial cells, suggest that VIP may play a physiological role in the local control of the myometrial activity.

Vasoactive intestinal polypeptide in cholinergic neurons of exocrine glands: functional significance of coexisting transmitters for vasodilation and secretion

By a combination of the indirect immunofluorescence technique with acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) staining, it was shown that vasoactive intestinal polypeptide (VIP) is present in cholinergic (acetylcholinesterase-rich) neurons involved in control of secretion and vasodilation in exocrine glands of cat. The submandibular salivary gland was used as a functional model. Preganglionic nerve stimulation induced an atropine-resistant, hexamethonium-sensitive vasodilation and release of VIP into the venous outflow from the gland and an atropine- and hexamethonium-sensitive secretion. Infusion of VIP antiserum reduced both the vasodilation and secretion. Infusion of VIP caused vasodilation only, whereas acetylcholine caused both vasodilation and secretion. Simultaneous infusion of VIP and acetylcholine in low doses resulted in a marked potentiation of both vasodilation and secretion. The present morphological and functional data support the following hypothesis for regulation of vasodilation and secretion in exocrine glands. Preganglionic cholinergic nerves activate, via nicotinic receptors, postganglionic neurons, causing concomitant release from the same nerve endings of two coexisting putative transmitters, acetylcholine and VIP. Acetylcholine produces mainly secretion by a muscarinic action and VIP causes mainly vasodilation, but the two substances seem to cooperate directly or indirectly in both types of response. Thus, the coexistence of two putative neurotransmitters, VIP and acetylcholine, in one neuron may explain the dual effector response (i.e., the cholinergic secretion and the atropine-resistant vasodilation) caused by nerve stimulation in exocrine glands.

Vasoactive intestinal polypeptide nerves in the human female genital tract

VIP, a recently recognized neuropeptide, has been demonstrated by immunohistochemistry in nerves of the human female genital organs. Such nerves were most numerous in the isthmic part of the fallopian tube and in the cervix. They were found in smooth muscles, around blood vessels, and beneath the epithelium. Immunoreactive VIP nerve cell bodies were found in paracervical tissue, suggesting a local origin of genital VIP nerves. Besides the well-known adrenergic and cholinergic nerves, peptidergic nerves may constitute a new component in the autonomic nervous system.

In vitro stimulation of prolactin release by vasoactive intestinal peptide

The effect of vasoactive intestinal peptide (VIP) on anterior pituitary hormone release was examined in a variety of in vitro preparations. Synthetic VIP was capable of stimulating increased prolactin (PRL) release from male rat hemipituitaries in doses as low as 10-9M only when the enzyme inhibitor bacitracin was present in the incubation medium. Natural porcine VIP was similarly capable of stimulating PRL release, but only at higher doses (10-6M). Additionally, synthetic VIP was capable of stimulating PRL release from dispersed anterior pituitary cells harvested from adult male and lactating female rats and from an enriched population of lactotrophs obtained by unit gravity sedimentation of similar dispersed cells from infantile female rats. No effect of VIP on luteinizing hormone, growth hormone or thyroid stimulating hormone release was seen. These findings taken in concert with the presence of VIP in the hypothalamus, pituitary and hypophyseal portal plasma of the rat suggest a physiological role for VIP in the control of PRL secretion.

The peptidergic neural control of the feline pylorus

By immunohistochemistry the three neuropeptides enkephalin, substance P, and VIP were all localized to numerous nerve fibers in the smooth muscle layer and the myenteric plexus of the feline pylorus. The motor activity of the pylorus was studied by recording changes in the volume flow of body-warm saline through the sphincter at constant perfusion pressure in anesthetized cats. Efferent electric stimulation of the cervical vagi (8 V; 5 ms; 8 Hz) elicited a noncholinergic, nonadrenergic pyloric contraction, which was blocked by infusion of the opiate receptor antagonist naloxone. The peptides were injected into a regional gastric artery to the pylorus antrum. Enkephalin and substance P in micronanomolar doses resulted in a prompt contraction of 3 - 5 min duration. Equimolar doses of naloxone blocked the enkephalin-induced contraction, and atropine blocked the SP contraction. On the other hand, VIP caused a prompt dilatation, which could not be blocked in this study. Altogether the findings suggest a peptidergic motor control of the feline pylorus using several transmission mechanisms.

Projections of intestinal neurons showing immunoreactivity for vasoactive intestinal polypeptide are consistent with these neurons being the enteric inhibitory neurons

Experiments were performed to determine if the distribution of vasoactive intestinal peptide(VIP)-like immunoreactivity in nerve cell bodies and axons of the myenteric plexus and circular muscle of the small intestine is consistent with VIP being the transmitter of enteric inhibitory neurons. Immunoreactivity for VIP was found in nerve cell bodies of the myenteric plexus and in axons within the myenteric plexus and circular muscle. When the axons in the myenteric plexus were interrupted, there was accumulation of material showing reactivity for VIP on the oral side, indicating that the neurons project in an anal direction. The VIP-like immunoreactivity in axons which supply the circular muscle disappeared after a myectomy in which the overlying myenteric plexus was removed, but remained intact when extrinsic nerves were served. The projections of VIP neurons from the myenteric plexus to the circular muscle correspond to the expected projections of enteric inhibitory neurons determined by functional studies.

Localization and possible function of peptidergic neurons and their interactions with central catecholamine neurons, and the central actions of gut hormones

The localization of various neuropeptides is described in the gut and in the hypothalamus in the rat. Evidence is given for the presence of material resembling corticotropin-like intermediate peptide in arcuate and periarcuate neurons, projecting to various hypothalamic nuclei, limbic areas and the thalamus. beta-Endorphin and glucagon decrease dopamine turnover in the median eminence, while secretin increases dopamine turnover and vasoactive intestinal polypeptide (VIP) has no effect. beta-Endorphin, VIP, secretin, and glucagon all produce discrete changes in norepinephrine turnover in various hypothalamic nuclei. Mainly increases of norepinephrine turnover were observed. These catecholamine turnover changes appear to cause changes in the secretion of prolactin and growth hormone. The results therefore indicate that gut hormones and opioid peptides may act directly on the hypothalamus on specific types of receptors to participate in the control of hypothalamic functions such as control of hormone secretion from the anterior pituitary and of food intake. It seems possible that gastrointestinal peptides released from the gastrointestinal tract into the circulation under certain circumstances could reach the hypothalamus and modulate its activity via the above-mentioned mechanisms. It may therefore be speculated that disturbances in gastrointestinal functions could lead to pathological changes in food intake via modulation of hypothalamic activity.

Evolutionary relationships of the gut hormones

Peptides identical or related to mammalian gut hormones occur widely, not just in gut endocrine cells but also in central or peripheral nerves, amphibian skin glands, and a variety of invertebrate tissues. The dual distribution in brain and gut was probably already established early in the vertebrate line; representatives of the oldest vertebrate group, the cyclostomes, have cholecystokinin-like factors in gut endocrine cells and in brain. The related sequences of certain gut peptides, notably gastrin and cholecystokinin (CCK), and secretin, glucagon, vasoactive intestinal polypeptide (VIP), and gastric inhibitory peptide (GIP), indicate evolution from common ancestral molecules by gene duplication and divergence. Functionally important residues are conserved. Thus the COOH-terminal pentapeptide common to gastrin and CCK also contains their minimal active fragment. There are also evolutionary changes at the level of the target organ receptor mechanisms: these are also evolutionary changes at the level of the target organ receptor mechanisms; these are illustrated by evidence suggesting that secretin regulates the flow of pancreatic juice in mammals whereas the structurally related peptide VIP has a similar role in birds.