Vasoactive peptides. State-of-the-art review

In Cerebellar Atrophy of 12-Month-Old ATM-Null Mice, Transcriptome Upregulations Concern Most Neurotransmission and Neuropeptide Pathways, While Downregulations Affect Prominently Itpr1, Usp2 and Non-Coding RNA

The autosomal recessive disorder Ataxia-Telangiectasia is caused by a dysfunction of the stress response protein, ATM. In the nucleus of proliferating cells, ATM senses DNA double-strand breaks and coordinates their repair. This role explains T-cell dysfunction and tumour risk. However, it remains unclear whether this function is relevant for postmitotic neurons and underlies cerebellar atrophy, since ATM is cytoplasmic in postmitotic neurons. Here, we used ATM-null mice that survived early immune deficits via bone-marrow transplantation, and that reached initial neurodegeneration stages at 12 months of age. Global cerebellar transcriptomics demonstrated that ATM depletion triggered upregulations in most neurotransmission and neuropeptide systems. Downregulated transcripts were found for the ATM interactome component Usp2, many non-coding RNAs, ataxia genes Itpr1, Grid2, immediate early genes and immunity factors. Allelic splice changes affected prominently the neuropeptide machinery, e.g., Oprm1. Validation experiments with stressors were performed in human neuroblastoma cells, where ATM was localised only to cytoplasm, similar to the brain. Effect confirmation in SH-SY5Y cells occurred after ATM depletion and osmotic stress better than nutrient/oxidative stress, but not after ATM kinase inhibition or DNA stressor bleomycin. Overall, we provide pioneer observations from a faithful A-T mouse model, which suggest general changes in synaptic and dense-core vesicle stress adaptation.

TSG-6 potentiates the antitissue kallikrein activity of inter-alpha-inhibitor through bikunin release

TSG-6 (the protein product of TNF-stimulated gene-6), an inflammation-associated protein, forms covalent complexes with heavy chains (HCs) from inter-alpha-inhibitor and pre-alpha-inhibitor and associates noncovalently with their common bikunin chain, potentiating the antiplasmin activity of this serine protease inhibitor. We show that TSG-6 and TSG-6.HC complexes are present in bronchoalveolar lavage fluid from patients with asthma and increase after allergen challenge. Immunodetection demonstrated elevated TSG-6 in the airway tissue and secretions of smokers. Experiments conducted in vitro with purified components revealed that bikunin.HC complexes (byproducts of TSG-6.HC formation) release bikunin. Immunoprecipitation revealed that bikunin accounts for a significant proportion of tissue kallikrein inhibition in bronchoalveolar lavage after allergen challenge but not in baseline conditions, confirming that bikunin in its free state, but not when associated with HCs, is a relevant protease inhibitor in airway secretions. In primary cultures of differentiated human airway epithelial and submucosal gland cells, TSG-6 is induced by TNF-alpha and IL-1beta, which suggests that these cells are responsible for TSG-6 release in vivo. Bikunin and HC3 (i.e., pre-alpha-inhibitor) were also induced by TNF-alpha in primary cultures. Our results suggest that TSG-6 may play an important protective role in bronchial epithelium by increasing the antiprotease screen on the airway lumen.

Mesenteric hemodynamic response to circulatory shock

PURPOSE OF REVIEW

The mesenteric hemodynamic response to circulatory shock is substantial and asymmetrical; the vasoconstrictive response disproportionately affects the mesenteric organs. The cardiac output is sustained partially, at no cost in nutrient flow to the mesenteric organs, by vasoconstriction of the mesenteric veins, resulting in the "autotransfusion" of up to 30% of the circulating blood volume into the systemic circulation.

RECENT FINDINGS

Hemorrhagic or cardiogenic shock also results in decreased perfusion pressure, prompting selective vasoconstriction of the mesenteric arterioles to maintain perfusion pressure of the vital organs, here at the selective expense of the mesenteric organs. Septic shock may be associated with increased or decreased mesenteric blood flow but is characterized by increased oxygen consumption, exceeding the capability of mesenteric oxygen delivery.

SUMMARY

The response to any of these conditions can, variably and unpredictably, cause hemorrhagic gastric stress erosions, nonocclusive mesenteric ischemia of the small bowel, ischemic colitis, ischemic hepatitis, acalculous cholecystitis, and/or ischemic pancreatitis. Injury to the mesenteric organs can also initiate the systemic inflammatory response syndrome and, consequently, multiple organ failure.

Specific angiotensin II receptor blockage improves intestinal perfusion during graded hypovolemia in pigs

OBJECTIVE

To investigate the potential of specific angiotensin II subtype 1 (AT1) receptor blockade to modify the mesenteric hemodynamic response to acute hypovolemia and retransfusion.

DESIGN

Prospective, randomized, controlled experimental study.

SETTING

University-affiliated animal research laboratory.

SUBJECTS

Fasted, anesthetized, ventilated, juvenile domestic pigs of both sexes.

INTERVENTIONS

Acute, graded hypovolemia by 20% and 40% of the total estimated blood volume followed by retransfusion in control animals (CTRL; n = 10) and animals pretreated with the AT1 receptor blocker candesartan (CAND; n = 10).

MEASUREMENTS AND MAIN RESULTS

Invasive monitoring of arterial and central venous blood pressures, cardiac output, portal venous blood flow, and jejunal mucosal blood flow. Blood gases were repeatedly analyzed to calculate oxygen delivery and consumption. Thirty minutes after each level of hypovolemia at 20% and 40%, cardiac output was decreased in CTRL animals from a baseline of 2.9 +/- 0.1 to 1.8 +/- 0.2 and 1.1 +/- 0.2 L/min, with no differences compared with CAND animals. Cardiac output was restored to 3.0 +/- 0.3 L/min 30 mins after retransfusion in CTRL animals, with no significant intergroup differences. Baseline portal venous blood flow (Q(MES)) and jejunal mucosal perfusion (PU(JEJ)) were greater in CAND animals compared with CTRL animals. During graded hypovolemia, CAND animals maintained Q(MES) and PU(JEJ) at significantly higher levels compared with CTRL animals, particularly after 40% hemorrhage (+221% and + 244%, respectively, relative to the mean values in CTRL animals). The same pattern was observed after retransfusion. Moreover, the calculated mesenteric critical oxygen delivery was significantly greater in CTRL animals (74 mL/min) compared with CAND animals (34 mL/min). No animals died in the CAND group, whereas four animals died during 40% hypovolemia or retransfusion in the CTRL group.

CONCLUSIONS

Specific AT1 blockade before acute hypovolemia significantly ameliorated mesenteric and, in particular, jejunal mucosal hypoperfusion. In addition, cardiovascular stability was improved, and mortality in conjunction with acute hypovolemia and retransfusion could be completely avoided. These findings support a fundamental role of the renin-angiotensin system in the mesenteric response to acute hypovolemia and indicate a substantial interventional potential for candesartan in conjunction with circulatory stress.

Modulation of bradykinin receptor ligand binding affinity and its coupled G-proteins by nitric oxide

To determine whether nitric oxide (NO) can modulate bradykinin (BK) signaling pathways, we treated endothelial cells with an NO donor, S-nitrosoglutathione (GSNO), to determine its effect(s) on G-proteins (Gi and Gq) that are coupled to the type II kinin (BK2) receptor. Radioligand binding assays and Western analyses showed that GSNO (10-500 microM, 0-72 h) did not alter the expression of BK2 receptor, Gi, or Gq. However, GSNO caused a 6-fold increase in basal cGMP production and decreased high affinity BK bindings sites and GTPase activity by 74 and 85%, respectively. The cGMP analogue, dibutyryl-cGMP, also inhibited BK-stimulated GTPase activity by 74% suggesting that some of the effects of NO may be mediated through activation of guanylyl cyclase. The NO synthase inhibitor, Nomega-monomethyl-L-arginine, inhibited endogenous NO synthase activity and cGMP production by 91 and 76%, respectively, but increased BK-stimulated GTPase activity by 61%. To determine which G-proteins are affected by NO, we performed GTP binding assays with [35S]GTPgammaS followed by immunoprecipitation with specific G-protein antisera. Both GSNO and dibutyryl-cGMP increased basal G-protein GTP binding activities by 18-26%. However, GSNO decreased BK-stimulated Galphai2, Galphai3, and Galphaq/11 GTP binding activity by 93, 61, and 90%, respectively, whereas epinephrine-stimulated Galphas GTP binding activity was unaffected. These results suggest that NO can modulate BK signaling pathways by selectively inhibiting G-proteins of the Gi and Gq family.

The role of neuropeptides in the regulation of adrenal vascular tone: effects of vasoactive intestinal polypeptide, substance P, neuropeptide Y, neurotensin, Met-enkephalin, and Leu-enkephalin on perfusion medium flow rate in the intact perfused rat adrenal

There is evidence that adrenal blood flow may be regulated in part by neuropeptides released from the capsular region of the adrenal gland in response to splanchnic nerve stimulation. The present study investigated the effects of various neuropeptides on the rate of perfusion medium flow through an intact in situ perfused rat adrenal preparation. Vasoactive intestinal polypeptide (VIP) had the greatest effect, causing a 136% increase in flow at the highest dose used (10 nmol in a 200 microliters bolus). Of the other peptides tested Met-enkephalin caused a 50% increase in flow, and the others (Leu-enkephalin, neurotensin and substance P) had only a minor effect, increasing perfusion medium flow rate by no more than around 35%. Neuropeptide Y, in contrast, caused a significant decrease in perfusion medium flow rate: the maximum effect was a 30% decrease with a dose of 1 nmol in a 200 microliters bolus. The significance of this observation awaits elucidation. It is clear from the actions of the neuropeptides tested that they may have a significant role in the regulation of adrenal blood flow. In view of the findings of other authors: that VIP is released in response to splanchnic nerve stimulation, and that it is specifically localised in the capsular region of the adrenal, it seems most likely that VIP is the major peptide involved in mediating the increased adrenal blood flow following splanchnic nerve stimulation.

Effect of vasoactive intestinal polypeptide on porcine gastrointestinal electrical activity

The influence of intravenous infusion of VIP, 150 and 300 pmol/kg/min, on gastrointestinal electrical activity was studied in conscious piglets with electrodes implanted in the wall of the antrum pylori, duodenum, jejunum, and ileum. Both doses resulted in a decrease in antral electrical activity. In the small intestine, only the lower dose caused a shortening of the irregular spiking activity phase in the jejunum and ileum. In the jejunum this resulted in a reduction of the MMC interval. It may be concluded that the prevailing effect of VIP is an inhibition of gastrointestinal electrical activity in the piglet.

Acute mesenteric ischemia: pathophysiology, diagnosis, and treatment

Ischemia has traditionally been viewed as arising only from abnormalities of oxygen dynamics, namely the cellular hypoxia resulting from the imbalances between oxygen supply, consumption, and demand. Recently, it has become clear that such a view is too restrictive. Hypoperfusion may be caused by both anatomic and functional impediments to either inflow or to outflow from an organ. Furthermore, the pathophysiologic consequences are likely to involve not only cellular hypoxia, but also a restricted supply of nutrients and other important molecules and an abnormal elimination of physiologic wastes such as carbon dioxide. Hence the recommendation that ischemia be defined as a dual defect of oxygen deficit and carbon dioxide excess. AMI is, therefore, a severe anatomic or functional impediment to the splanchnic circulation, resulting in a dual defect of intestinal hypoxia and cellular hypercarbia. Although the functional and structural consequences of cellular hypoxia are well known, the pathophysiology of cellular hypercarbia has only begun to be explored. AMI syndromes include three related processes: occlusive mesenteric ischemia, nonocclusive ischemia, and sepsis-induced SI. Leakage of bacteria or bacterial toxins into the circulation during mesenteric ischemia forms the basis of the systemic components of this syndrome. Striving for an earlier diagnosis, treating the systemic (septic) consequences, and taking measures to promptly restore mucosal oxygen balance through aggressive pharmacologic and appropriate surgical intervention have significantly improved the prognosis. About 80% of patients with acute arterial embolism, 60% of those with nonocclusive ischemia, and only 20% of patients with arterial thrombosis are expected to live without significant residual nutritional deficits. The cause of death is usually sepsis and multisystem organ failure, and therefore, further reductions in mortality are likely to occur with the improved prevention and treatment of sepsis.

Pathophysiology of mesenteric ischemia

Intestinal ischemia can result from a host of pathophysiologic disturbances and, in turn, may produce a variety of adverse local and systemic consequences. Mechanisms of ischemic injury and the central role of vasoconstriction are discussed.

Increased plasma levels of vasoactive intestinal polypeptide in pre-eclampsia

OBJECTIVE

To investigate the plasma vasoactive peptide (VIP) levels in pregnancies complicated by pre-eclampsia.

DESIGN

A prospective clinical study.

SETTING

University Department of Obstetrics, Tromsø, Norway.

SUBJECTS

18 women with untreated gestational proteinuric hypertension between 32 and 40 weeks gestation (13 primigravid) and 8 women with normal pregnancies of similar gestational age.

INTERVENTIONS

Fasting blood samples on two occasions, 10 min apart.

MAIN OUTCOME MEASURES

Plasma VIP measured by radioimmunoassay.

RESULTS

Mean maternal plasma VIP was 13.9 (SEM 1.7) pmol/l in those with pre-eclampsia and 4.4 (SEM 0.5) pmol/l in normal pregnancies (P less than 0.0001).

CONCLUSION

The increased levels of VIP in pre-eclampsia may represent a powerful compensatory mechanism to restore vascular perfusion of various organs, including the uterus and placenta.

The effect of oral glucose, protein, fat and water loading on blood pressure and the gastrointestinal peptides VIP and somatostatin in hypertensive elderly subjects

In elderly subjects blood pressure (BP) may fall after a meal. It can be reproduced by oral glucose, but the effect of fat and protein ingestion on postprandial BP has not been reported. Furthermore, we hypothesized that vasoactive gastrointestinal peptides are involved in this phenomenon. We studied 10 hypertensive elderly subjects (mean age 74 +/- 3 years) for the effects of oral glucose, fat, protein and water loading on BP in relation to plasma concentrations of vasoactive intestinal polypeptide (VIP), somatostatin and insulin. Glucose loading resulted in a decrease of mean arterial pressure by 14 +/- 2 mmHg (P less than or equal to 0.001). In contrast, the ingestion of fat, protein or water had no significant effect on BP. Somatostatin increased after fat and protein loading, whereas VIP increased only after fat loading. These data indicate that postprandial BP reduction in the elderly is related to glucose-related factors. The gut peptide VIP does not seem to play a role in this phenomenon.

The actions of calcitonin gene related peptide and vasoactive intestinal peptide as vasodilators in man in vivo and in vitro

1 The two peptides calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP) produced marked dilatation of the forearm vascular bed when infused via the brachial artery. 2 CGRP relaxed preconstricted segments of human radial, coronary, gastric and cerebral arteries in an endothelium dependent manner. 3 VIP relaxed human gastric and transverse cervical arteries in an endothelium dependent manner, but relaxation of the human pulmonary artery was not dependent on endothelium. 4 The characteristics of the endothelium dependent relaxation of these medium-sized muscular arteries indicated involvement of the endothelium derived relaxing factor in vitro. 5 Caution is expressed in drawing comparisons between the mechanisms involved in the in vivo and in vitro vascular responses.

Effects of indomethacin and (+/-)-propranolol on the cardiovascular and renin responses to vasoactive intestinal polypeptide (VIP) infusion in man

The mechanisms of the cardiovascular and renin responses to vasoactive intestinal polypeptide (VIP) are unclear. Rabbit studies suggest that VIP-induced tachycardia is largely beta-adrenoceptor mediated, but that the renin response may be partially prostaglandin-dependent. To examine the relative importance of prostaglandins and reflex sympathetic activation in the haemodynamic and renin responses to VIP infusion in man, we completed two randomised single-blind crossover studies in two groups of six healthy male volunteers (aged 24-35 years). We recorded the effects of indomethacin and propranolol pretreatment on VIP-related changes in heart rate (HR), blood pressure (BP), forearm vascular resistance (FVR), plasma renin activity (PRA), plasma noradrenaline (PNA) and plasma arginine vasopressin (AVP) concentrations. Intravenous VIP (calculated dose: 6 pmol kg-1 min-1) produced cutaneous flushing, increased HR and PRA, decreased FVR, but did not alter mean arterial BP or AVP levels. Indomethacin (375 mg over 3 days) lowered basal PRA and propranolol (circa 40 mg i.v. over 60 min) decreased resting HR and increased FVR. Although indomethacin and propranolol reduced the absolute rise in PRA and HR, respectively, during VIP infusion, the percentage changes were no different from control. Neither drug altered the flush response to VIP and propranolol did not affect the fall in FVR. We conclude that the measured cardiovascular responses to VIP infusion in man are probably direct and do not involve a significant contribution from reflex sympathetic stimulation, nor prostaglandin release.

Metabolism of opioid peptides by cerebral microvascular aminopeptidase M

Aminopeptidase M (EC 3.4.11.2), which can degrade low molecular weight opioid peptides, has been reported in both peripheral vasculature and in the CNS. Thus, we have studied the metabolism of opioid peptides by membrane-bound aminopeptidase M derived from cerebral microvessels of hog and rabbit. Both hog and rabbit microvessels were found to contain membrane-bound aminopeptidase M. At neutral pH, microvessels preferentially degraded low molecular weight opioid peptides by hydrolysis of the N-terminal Tyr1-Gly2 bond. Degradation was inhibited by amastatin (I50 = 0.2 microM) and bestatin (10 microM), but not by a number of other peptidase inhibitors including captopril and phosphoramidon. Rates of degradation were highest for the shorter peptides (Met5- and Leu5-enkephalin) whereas beta-endorphin was nearly completely resistant to N-terminal hydrolysis. Km values for the microvascular aminopeptidase also decreased significantly with increasing peptide length (Km = 91.3 +/- 4.9 and 28.9 +/- 3.5 microM for Met5-enkephalin and Met5-enkephalin-Arg6-Phe7, respectively). Peptides known to be present within or in close proximity to cerebral vessels (e.g., neurotensin and substance P) competitively inhibited enkephalin degradation (Ki = 20.4 +/- 2.5 and 7.9 +/- 1.6 microM, respectively). These data suggest that cerebral microvascular aminopeptidase M may play a role in vivo in modulating peptide-mediated local cerebral blood flow, and in preventing circulating enkephalins from crossing the blood-brain barrier.

Substance P distribution and effects in the canine epicardial coronary arteries

Substance P (SP), a vasoactive neuropeptide detected in animal and human hearts has been reported to increase coronary blood flow in animals. However, no data are available on SP effects on epicardial coronary arteries, the site of coronary disease. To determine the amount and distribution of SP and its action in the large coronary vessels, we studied two groups of dogs. One group was anesthetized for collecting three 1 cm segments of the circumflex coronary artery (CX) and left anterior descending artery (LAD) through a left thoracotomy. These segments represented proximal (I), middle (II), and distal (III) portions of the two arteries. Concentrations (ng/g) of SP-like immunoreactivity (SP-LI) were determined by radioimmunoassay. SP-LI was present in LAD (I: 1.17 +/- 0.20, II: 1.08 +/- 0.36, III: 1.14 +/- 0.25) and CX (I: 1.44 +/- 0.38, II: 1.51 +/- 0.47, III: 0.70 +/- 0.20). SP differences among segments of LAD and segments I and II of CX were not significant, but there was a significant difference between segment III of CX and the others. In the second group of closed chest anesthetized dogs, we examined the effects of intracoronary SP infusion before and during administration of serotonin (5HT). LAD and CX artery responses (% area change) to SP and to SP plus 5HT were examined using quantitative coronary angiography. Intracoronary 133Xe in saline provided coronary flow data. SP infusion produced significant vasodilation in segment II (15% area increase) and III (17%) during the highest dose (1 microgram/min). The three SP doses infused with 5HT (0.05 mg/min) did not produce vasodilation, although LAD segment III constriction from 5HT was abolished during the highest dose of SP infusion. The presence of SP, and its dilatory effect on the coronary arteries, suggests a role in maintaining vasodilator tone in the coronary arteries.

Action and localization of vasoactive intestinal peptide in the coronary circulation: evidence for nonadrenergic, noncholinergic coronary regulation

Vasoactive intestinal polypeptide, a neurotransmitter peptide detected in animal and human hearts, has been found in nerves of coronary arteries. To determine the amount and distribution of vasoactive intestinal polypeptide in the large coronary vessels and its possible participation in coronary vasoregulation, two groups of animals were studied. In the first group, 11 anesthetized dogs were sacrificed to collect three (1 cm) segments along the circumflex and left anterior descending coronary arteries. These segments represented proximal (I), middle (II) and distal (III) portions of the two arteries. Concentrations (ng/g) of vasoactive intestinal polypeptide-like immunoreactive substance were determined by radioimmunoassay. Vasoactive intestinal polypeptide-like immunoreactivity was present in the left anterior descending (I = 7.28 +/- 1.65, II = 3.74 +/- 0.57, III = 2.29 +/- 0.53) and circumflex (I = 4.16 +/- 1.52, II = 4.58 +/- 1.13, III = 4.00 +/- 0.81) coronary arteries. The difference in vasoactive intestinal polypeptide-like immunoreactivity among epicardial segments of the anterior descending artery was significant, but there was no significant difference among segments of the circumflex coronary artery. In the second group (eight closed chest anesthetized dogs), the effects of vasoactive intestinal polypeptide intracoronary infusion on epicardial coronary constriction were examined at rest and with the artery constricted by serotonin. Left anterior descending (segments I, II and III) artery responses (% area change) to vasoactive intestinal polypeptide and vasoactive intestinal polypeptide plus serotonin were examined using quantitative coronary angiography. Vasoactive intestinal polypeptide infusion resulted in significant vasodilation in all the segments (I, II and III) of the left anterior descending artery.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional receptors for vasoactive intestinal peptide in cultured vascular smooth muscle cells from rat aorta

Specific binding sites for vasoactive intestinal peptide (VIP), a potent vasodilatory polypeptide, and its effect on formation of intracellular cyclic AMP levels were studied in cultured vascular smooth muscle cells (VSMC) from rat aorta. Specific binding of 125I-labeled-VIP to cultured VSMCs was time- and temperature-dependent. Scatchard analysis of binding studies suggested the presence of two classes of high and low affinity binding sites for VIP; the apparent Kd and the number of maximal binding capacity were approximately 8 X 10(-9) M and 60,000 sites/cell (high-affinity sites) and approximately 4 X 10(-8) M and 140,000 sites/cell (low-affinity sites), respectively. Unlabeled VIP competitively inhibited the binding of 125I-labeled-VIP to its binding sites, whereas neither peptides structurally related to VIP, nor other vasoactive substances affected the binding. VIP stimulated formation of intracellular cyclic AMP in cultured VSMCs in a dose-dependent manner; the stimulatory effect of VIP on cyclic AMP formation was not blocked by propranolol and was additive with isoproterenol. The present study first demonstrates the presence of specific receptors for VIP in VSMCs functionally coupled to adenylate cyclase system. It is suggested that VIP exerts its vasodilatory effect through its specific receptors distinct from beta-adrenergic receptors.

Effects of vasoactive intestinal polypeptide on isolated urethral and urinary bladder smooth muscle from rabbit and man

Vasoactive intestinal polypeptide concentration-dependently inhibited the contractant responses of isolated preparations of the female rabbit bladder and urethra induced by electrical field stimulation and exogenous application of acetylcholine (bladder) and noradrenaline (urethra). The inhibition of alpha-adrenoceptor and muscarinic cholinoceptor-mediated activity in the urethra and bladder amounted to 50 to 90 per cent of induced contractions. The nonadrenergic noncholinergic contraction induced by electrical field stimulation in the urethra was reduced slightly, whereas corresponding response in the bladder was more sensitive. The maximum inhibition of both the electrically induced responses and contractions induced by exogenous noradrenaline and acetylcholine was of comparable size in the urethra and the bladder. The effects of vasoactive intestinal polypeptide seemed to be exerted postjunctionally since no significant influence of the peptide was seen on the release of 3H-noradrenaline from adrenergic nerve endings in the urethra. The effects of vasoactive intestinal polypeptide in human urethral and bladder preparations were less consistent. The noradrenaline-induced contraction in urethral preparations was inhibited by 29 +/- 9 per cent (no. = 22). The effects on electrically induced contractions in the urethra, and on responses to acetylcholine and electrical field stimulation in the bladder, were small and inconsistent. It is concluded that vasoactive intestinal polypeptide may be of importance for regulation of lower urinary tract smooth muscle activity in the rabbit. It cannot be excluded that the peptide has a modulatory role in neurotransmission in human urethral muscle. However, the present results do not support the view of vasoactive intestinal polypeptide as an inhibitor of contraction in human detrusor.

Isolation, localization, and characterization of gastrointestinal peptides

Increasing numbers of peptides have been found in the gastrointestinal tract. Some of these are localized primarily in endocrine cells in the gastrointestinal epithelium (e.g., gastrin, secretin) or pancreatic islets (e.g., insulin, glucagon), and function chiefly as circulating hormones. Other peptides, e.g., VIP, are principally neuropeptides present mainly in nerve cells, nerve fibres and nerve terminals; they function mainly as neurotransmitters or neuromodulators. The isolation of gut peptides has usually depended on the use of characteristic bioassays. More recently, some peptides have been isolated on the basis of unique terminal amino-acid composition. Certain chemical structures are associated with typical biological actions, and several "families" of related peptides are recognized.