Endogenous Gastrin-releasing Peptide Research Articles

Central ventilatory and cardiovascular actions of trout gastrin-releasing peptide (GRP) in the unanesthetized trout

SummaryGastrin-releasing peptide (GRP), a neuropeptide initially isolated from porcine stomach, shares sequence similarity with bombesin. GRP and its receptors are present in the brains and peripheral tissues of several species of teleost fish, but little is known about the ventilatory and cardiovascular effects of this peptide in these vertebrates. The goal of this study was to compare the central and peripheral actions of picomolar doses of trout GRP on ventilatory and cardiovascular variables in the unanesthetized rainbow trout. Compared to vehicle, intracerebroventricular (ICV) injection of GRP (1–50 pmol) significantly elevated the ventilation rate (ƒV) and the ventilation amplitude (VAMP), and consequently the total ventilation (VTOT). The maximum hyperventilatory effect of GRP (VTOT: +225%), observed at a dose of 50 pmol, was mostly due to its stimulatory action on VAMP (+170%) rather than ƒV (+20%). In addition, ICV GRP (50 pmol) produced a significant increase in mean dorsal aortic blood pressure (PDA) (+35%) and in heart rate (ƒH) (+25%). Intra-arterial injections of GRP (5–100 pmol) were without sustained effect on the ventilatory variables but produced sporadic and transient increases in ventilatory movement at doses of 50 and 100 pmol. At these doses, GRP elevated PDA by +20% but only the 50 pmol dose significantly increased HR (+15%). In conclusion, our study suggests that endogenous GRP within the brain of the trout may act as a potent neurotransmitter and/or neuromodulator in the regulation of cardio-ventilatory functions. In the periphery, endogenous GRP may act as locally-acting and/or circulating neurohormone with an involvement in vasoregulatory mechanisms.

New molecular targets for treatment of peptic ulcer disease.

Most patients with peptic ulcer disease are currently treated with proton pump inhibitors or histamine H(2) receptor antagonists. The long-term use of these compounds has been associated with two potential problems. Firstly, proton pump inhibitors may induce enterochromaffin-like (ECL) cell hyperplasia. Secondly, ulcers may relapse despite maintenance therapy with histamine H(2) antagonists. This has been the rationale for the development of new antisecretory agents, including antagonists against gastrin and gastrin releasing peptide (GRP), as well as ligands to histamine H(3) receptors. Several potent, high affinity cholecystokinin (CCK)-2 receptor antagonists have recently been identified such as L-365260, YM-022, RP-73870, S-0509, spiroglumide and itriglumide (CR-2945). Current data suggest that they all have antisecretory and anti-ulcer effects. In addition to reducing acid production, CCK-2 receptor antagonists may possibly also accelerate gastric emptying, a combination of functions which could potentially be beneficial in patients with functional dyspepsia. Receptors for bombesin and its mammalian counterpart GRP have been localised in the brain, spinal cord and enteric nerve fibres of the gut as well as on secretory cells and smooth muscle cells of the intestinal tract. Current data clearly indicate that endogenous GRP is involved in the regulation of basal and postprandial acid secretion. However, at this stage it is not clear whether GRP agonists or GRP antagonists can be developed into useful drugs. The peptide has a wide range of biological effects and it is likely that analogues of GRP or antagonists of the peptide affect not only gastric acid secretion but also induce considerable side effects. Histamine plays a central role in the stimulation of acid secretion. After their detection in the brain, H(3) receptors have been identified in a variety of tissues including perivascular nerve terminals, enteric ganglia of the ileum and lung, and ECL cells. Despite many studies, the role of H(3) receptors in the regulation of gastric acid secretion is still unclear. Controversial data have been presented, and study results largely depend on the species and experimental models. It seems unlikely that proton pump inhibitors or H(2) receptor antagonists will be replaced in the near future by new antisecretory agents. The current shortcomings of the new compounds include mainly their reduced clinical effectiveness and pharmacological limitations. However, the development of these new antisecretory compounds provides interesting tools to assess the physiological and pharmacological role of different receptors within the gastrointestinal tract. The use of CCK-2 receptor antagonists in patients with functional dyspepsia and Zollinger-Ellison syndrome should be examined in randomised, controlled trials.

Gastric secretion

Overlapping neural, hormonal, and paracrine pathways finely regulate gastric acid secretion. In rats and guinea pigs, most of the intrinsic neural innervation to the gastric mucosa originates in the myenteric plexus. In contrast, human stomachs have a clearly defined submucosal plexus that contains a variety of transmitters including nitric oxide, vasoactive intestinal peptide (VIP), gastrin-releasing peptide (GRP), substance P, and calcitonin gene-related peptide (CGRP). Although GRP is known to participate in meal-stimulated acid secretion by releasing gastrin in a variety of laboratory animals, recent studies were unable to demonstrate a role for endogenous GRP in meal-stimulated gastrin secretion in humans. Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin-glucagon-VIP family, has been localized to gastric mucosal neurons and may participate in vagally mediated acid secretion. Two novel peptides, ghrelin and leptin, have been localized to the stomach. Peripheral administration of ghrelin stimulates and of leptin inhibits acid secretion. The binding of secretagogues to parietal cells generates changes in second messengers that regulate the translocation and activation of the proton pump, HK-ATPase. In resting cells, HK-ATPase is contained within cytoplasmic tubulovesicles in an inactive form. At stimulation, the tubulovesicles fuse with the apical canaliculi and the HK-ATPase is incorporated into the apical membrane where it actively pumps H ions in exchange for K. Acute infection with Helicobacter pylori results in hypochlorhydria, whereas chronic infection can cause either hypo- or hyperchlorhydria, depending on the distribution of the infection and the degree of corpus gastritis. Recent studies suggest that inflammatory cytokines, produced in response to the organism, can play a role in the perturbations in acid and gastrin secretion induced by H. pylori.

Gastrin-releasing peptide mediated regulation of 5-HT neuronal activity in the hypothalamic paraventricular nucleus under basal and restraint stress conditions

The purpose of this study was to examine the gastrin-releasing peptide (GRP) mediated regulation of 5-HT neuronal activity in the paraventricular nucleus of the hypothalamus under basal and restraint stress conditions. Intracerebroventricular (icv) administration of GRP (1, 10, 100 ng/rat) increased 5-HIAA concentrations in the paraventricular nucleus (PVN) of the hypothalamus, but was without effect in the accumbens, suprachiasmatic and arcuate nuclei. Administration of (Leu 13-ψ-CH 2NH-Leu 14) Bombesin (10, 100 and 1000 ng/rat; icv), a GRP antagonist, had no effect by itself on PVN serotonergic activity; however, a dose of 1 μg/rat of this compound, completely blocked the increase of 5-HIAA concentrations induced by GRP (10 ng). Restraint stress increased serotonergic activity -as shown by an elevation of 5-HIAA in the PVN- as well as plasma ACTH and corticosterone. This stress-induced activation of both the serotonergic neurons and the hypothalamus–pituitary–adrenal axis was blocked by CRF and GRP antagonists. Interestingly, when the activation of hypothalamic 5-HT neurons was induced by GRP administration, α-helical (9-41) CRF was ineffective. These data suggest that GRP, by acting on GRP receptors but not via CRF receptors, increases 5-HT neuronal activity in the PVN. In turn, it appears that endogenous GRP and CRF receptor ligands are both simultaneously involved in the regulation of the increase in 5-HT neuronal activity, ACTH and corticosterone secretion, under stress conditions.

Regulation of gastric function by endogenous gastrin releasing peptide in humans: studies with a specific gastrin releasing peptide receptor antagonist

BACKGROUND AND AIMSThe main goal of our study was to characterise the activity of BIM26226 as a peripheral gastrin releasing peptide (GRP) receptor antagonist in healthy human subjects and to...

Blockade of GRP receptors inhibits gastric emptying and gallbladder contraction but accelerates small intestinal transit

Background & Aims: This study was designed to characterize [D-F5Phe6D-Ala11]Bn(6-13)OMe (BIM26226) as a gastrin-releasing peptide (GRP)-preferring bombesin receptor antagonist and to determine whether GRP physiologically regulates gastrointestinal motility. Intravenous BIM26226 (5–500 μg · kg−1 · h−1) inhibits GRP-induced gallbladder contraction and plasma cholecystokinin (CCK) release in a dose-dependent fashion. Methods: Gastric emptying and small bowel transit of a solid meal were quantified using scintigraphy. Meal-stimulated gallbladder contraction was measured by sonography in a 2-period crossover design. Results: Intravenous BIM26226 potently inhibited gastric lag time (114 ± 7 vs. 41 ± 6 minutes [control]) and gastric emptying rate (0.11 ± 0.02%/min vs. 0.26 ± 0.04%/min [control]), whereas concomitant infusion of BIM26226 accelerated small bowel transit time (153 ± 41 vs. 262 ± 20 minutes [control]). A continuous liquid meal perfusion into the duodenum induced complete gallbladder contraction (t50%, 35 ± 4 minutes), which BIM26226 inhibited significantly (t50%, 64 ± 8 minutes). BIM26226 did not alter plasma CCK response, indicating that circulating CCK did not mediate these effects. Conclusions: These data show that BIM26226 is a potent antagonist of exogenous and endogenous GRP and suggest that GRP is a major physiologic regulator of gastric emptying, small bowel transit, and gallbladder contraction.GASTROENTEROLOGY 2001;120:361-368

Impact of Gastrin-Releasing Peptide on Intestinal Microcirculation after Ischemia-Reperfusion in Rats

We investigated the effect of gastrin-releasing peptide (GRP) and its antagonist RC-3095 on intestinal microcirculation after ischemia-reperfusion. Intestinal ischemia was induced in female Wistar rats by occlusion of the superior mesenteric artery for 40 min. Ten minutes prior to reperfusion, infusion of GRP or RC-3095 was started. A jejunal segment was exteriorized and the microhemodynamics of the mucosa and submucosa were examined by intravital microscopy and compared both with normal and ischemic controls (without application of the regulatory peptide). Ischemia-reperfusion significantly decreased functional capillary density from 891.2 ± 14.1 to 398.3 ± 11.4 cm^–1. Capillary red blood cell velocity was reduced from 0.46 ± 0.01 to 0.37 ± 0.01 mm/s (p < 0.05). Furthermore, both sticking and rolling of leukocytes were enhanced. 3.4 ± 1.1% of the villi were not perfused at all. GRP infusion reversed the microcirculatory ischemia-reperfusion injury by increasing functional capillary density to 669.8 ± 8.3 cm^–1 and red blood cell velocity to 0.62 ± 0.01 mm/s (p < 0.05). In addition, application of GRP resulted in a complete absence of stasis (0%) in the villi. Leukocyte-endothelium adherence remained unchanged when compared to the ischemic controls. In contrast, application of RC-3095 caused an aggravation of microcirculatory disturbances demonstrated by a markedly increased number of non-perfused villi (42.5 ± 4.2%; p < 0.05 vs. ischemic controls) and a significantly reduced functional capillary density (346.2 ± 8.4 cm^–1, p < 0.05 vs. ischemic controls). In addition, RC-3095 led to an increased permanent leukocyte adherence in postcapillary venules whereas rolling was significantly reduced when compared to normal controls. We conclude that GRP in pharmacological doses has a protective effect on intestinal microcirculation during reperfusion. Furthermore, these data suggest that endogenous GRP may play a decisive role in the maintenance of microvascular integrity during reperfusion.

Role of Corticotropin-Releasing Hormone in Gastrin-Releasing Peptide-Mediated Regulation of Corticotropin and Corticosterone Secretion in Male Rats

Gastrin-releasing peptide (GRP) exerts several functions within the hypothalamus and may be involved in the regulation of pituitary hormone secretion. The purpose of this study was to investigate the central effect of GRP on hypothalamic-pituitary-adrenal axis activity in the male rat. Intracerebroventricular (i.c.v.) but not intravenous administration of GRP (1, 10, 100 ng/rat) increased plasma ACTH and corticosterone concentrations in a dose-dependent manner. The highest dose (100 ng/rat) of GRP increased plasma ACTH and corticosterone 4- and 14-fold, respectively. This increase peaked at 30–60 min after i.c.v. injection, decreased gradually and returned to baseline levels 240 min after GRP administration. The i.c.v. administration of (Leu¹³-ψ-CH₂NH-Leu¹⁴) bombesin, a competitive and specific GRP receptor antagonist, had no effect on ACTH and corticosterone secretion; however, a dose of 1 µg/rat completely blocked the increase of both hormones induced by GRP (10 ng). By using α-helical (9–41) corticotropin-releasing factor (CRF), a competitive antagonist of CRF, the role of CRF on GRP-induced ACTH and corticosterone secretion was also explored. α-Helical (9–41) CRF (10 µg/rat) blocked the increase in ACTH and corticosterone secretion induced by GRP (10 ng). The results obtained in this study suggest that GRP increases the secretion of ACTH and corticosterone in the plasma by acting centrally on GRP receptors, and that endogenous GRP receptor ligands do not tonically regulate ACTH and corticosterone secretion. Furthermore, the hypothalamus-pituitary-adrenal-activating effects induced by GRP appear to be mediated, at least in part, by CRF.

FOETAL METABOLISM, PLACENTAL TRANSFER AND ORIGIN OF GASTRIN RELEASING PEPTIDE IN THE SHEEP

1. Plasma gastrin releasing peptide (GRP) is elevated in the foetal and maternal circulations of pregnant sheep. To determine the mechanisms for this increase the synthesis, secretion rate, metabolism and placental transfer of GRP were measured. 2. Foetal metabolic clearance rate of GRP was significantly increased (P < 0.05) compared to the non-pregnant ewe (19.9 +/- 2.6 (s.e.m.) and 11.8 +/- 2.0 mL/min per kg, respectively). Production rate of GRP in the foetus was four-fold higher than in the non-pregnant ewe reflecting the combination of the increased basal concentration and metabolic clearance rate in the foetus. 3. Infused GRP did not cross the placenta. However, endogenous GRP was higher in the umbilical vein than in the umbilical artery, suggesting a uteroplacental origin for some of the GRP in the foetal circulation. 4. Gastrin releasing peptide mRNA was synthesized in the pregnant endometrium with lower amounts found in the pregnant myometrium. No GRP mRNA was detected in the amnion or chorioallantois. 5. The results show that the previously reported increase in foetal concentration of GRP is from foetal and uteroplacental sources and is not a result of immaturity of clearance mechanisms but rather from an increased production of GRP. With the demonstration that the uteroplacental unit synthesizes and stores GRP, additional studies on the regulation of GRP production from these sources were warranted.

Evidence that endogenous GRP in rat stomach mediates omeprazole-induced hypergastrinemia.

To investigate the physiological role of endogenous gastrin-releasing peptide (GRP) in regulating the release of gastrin, we evaluated the response of intragastric pH, gastrin, and GRP after omeprazole treatment in rats. A significant elevation of the plasma level of GRP (P < 0.01) and a significant reduction of the antral content of GRP (P < 0.05) were observed after the administration of 100 mg/kg omeprazole. The antral content of GRP was significantly decreased 12 h after omeprazole administration and thereafter gradually returned to control levels. Peak values for intragastric pH and plasma GRP were observed 3 h after omeprazole administration and before the peak of serum gastrin. The bombesin antagonist [D-Phe6]-bombesin-(6,13)-methyl ester significantly inhibited gastrin release after omeprazole treatment (P < 0.05). These observations indicate that omeprazole-induced inhibition of acid secretion stimulates the release of GRP and suggest that the secretion of GRP induced by omeprazole may stimulate the secretion of gastrin, at least in the early phase.

Prefeeding potentiates anorectic actions of neuromedin B and gastrin releasing peptide

We report that the anorectic potency of neuromedin B (NMB) and gastrin-releasing peptide (GRP) is potentiated using a prefeed paradigm. During the light phase, adult male Sprague-Dawley rats (groups of n = 6) were given 30-min (pre-feed) access to a liquid diet. Thirty min later rats were injected IP with GRP or NMB (0, 8, 16, 32 or 64 μg kg −1) and given further access to the liquid diet. In prefed (PF) rats, high doses of NMB (32 and 64 μg kg −1) delayed the onset of eating in the test by > 10 min, significantly suppressing 30-min intake. The same doses of NMB in non prefed (NPF) rats were ineffective. Weak effects of GRP in NPF rats to reduce meal size were also enhanced under PF conditions. Exogenous GRP and NMB, in particular, appear to interact with processes stimulated by the postingestive actions of food, supporting a role for peripheral, endogenous GRP and NMB in the development of satiation and the maintenance of postprandial satiety, respectively.

Vagal regulation of GRP, gastric somatostatin, and gastrin secretion in vitro.

The effect of electrical stimulation of the vagus nerves on the release of immunoreactive gastrin-releasing peptide (GRP), gastrin, and somatostatin was investigated using the isolated perfused rat stomach. Electrical stimulation (10 Hz, 1 ms duration, 10 V) of the peripheral end of the subdiaphragmatic vagal trunks produced a significant increase in both GRP and gastrin but a decrease in somatostatin. The infusion of atropine sulfate at a concentration of 10(-5) M augmented GRP release and reversed the decrease in somatostatin release in response to vagal stimulation to an increase above basal levels. However, the gastrin response to vagal stimulation was not affected by atropine. The infusion of hexamethonium bromide at a concentration of 10(-4) M significantly suppressed GRP release but did not affect gastrin secretion in response to vagal stimulation. On the other hand, the somatostatin response to vagal stimulation was completely abolished by hexamethonium. These findings lead us to conclude that the intramural GRP neurons might play an important role in the regulation of gastrin as well as somatostatin secretion and that somatostatin secretion may be controlled not only by a cholinergic inhibitory neuron but also by a noncholinergic, e.g., peptidergic stimulatory neuron, both of which may be regulated through preganglionic vagal fibers via nicotinic receptors. In addition, because the infusion of 10(-7) M GRP suppressed the somatostatin secretion, we suggest that either GRP should be excluded from the list of candidates for the noncholinergic stimulatory neurotransmitter for somatostatin secretion or that there are different mechanisms of action for endogenous and exogenous GRP.