Identification of the neurotransmitter/neuromodulator functions of the neuropeptide gastrin-releasing peptide in the porcine antrum, using the antagonist (Leu13-psi-CH2 NH-Leu14)-bombesin

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

PACAP 1-38 as neurotransmitter in the porcine antrum

The concentration of PACAP 1-38 in porcine antrum amounted to 15.4+/-7.9 and 20.3+/-8 pmol/g tissue in the mucosal and muscular layers. PACAP immunoreactive (IR) fibres innervated the muscular (co-localised with VIP) and submucosal/mucosal layers (some co-storing VIP and CGRP) including myenteric and submucosal plexus and blood vessels. Only myenteric nerve cell bodies contained PACAP-IR (co-storing VIP). In isolated perfused antrum, vagus nerve stimulation (8 Hz) and capsaicin (10(-5) M) increased PACAP 1-38 release. PACAP 1-38 (10(-9) M) increased substance P (SP), gastrin releasing peptide (GRP) and VIP release. PACAP 1-38 (10(-8) M) inhibited gastrin secretion and stimulated somatostatin secretion and motility dose-dependently. PACAP-induced motility was strongly inhibited by the antagonist PACAP 6-38 but also by atropine and substance P-antagonists (CP99994/SR48968) but PACAP 6-38 had no effect on vagus-induced secretion or motility.

CONCLUSION

PACAP 1-38 may be involved in antral motility and secretion by interacting with cholinergic, SP-ergic, GRP-ergic and/or VIP-ergic neurones, and may also be involved in afferent reflex pathways.

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

BACKGROUND & AIMS

This study was designed to characterize [D-F(5)Phe(6)D-Ala(11)]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 microg. 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 (t(50%), 35 +/- 4 minutes), which BIM26226 inhibited significantly (t(50%), 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.

Tachykinins in the porcine pancreas: potent exocrine and endocrine effects via NK-1 receptors

The localization, release, and effects of substance P and neurokinin A were studied in the porcine pancreas and the localization of substance P immunoreactive nerve fibers was examined by immunohistochemistry. The effects of electrical vagus stimulation and capsaicin infusion on tachykinin release and the effects of substance P and neurokinin A infusion on insulin, glucagon, somatostatin, and exocrine secretion were studied using the isolated perfused porcine pancreas with intact vagal innervation. NK-1 and NK-2 receptor antagonists were used to investigate receptor involvement. Substance P immunoreactive nerve fibers were localized to islets of Langerhans, acini, ducts, and blood vessels. Vagus stimulation had no effect on substance P and neurokinin A release, whereas capsaicin infusion stimulated release of both. Substance P and neurokinin A infusion increased release of insulin, glucagon, and exocrine secretion, whereas somatostatin secretion was unaffected. The effect of substance P on insulin, glucagon, and exocrine secretion was blocked by the NK-1 receptor antagonist. The effect of electrical stimulation of vagus nerves on insulin and exocrine secretion was not influenced by tachykinin receptor antagonists. We conclude that tachykinins stimulate both endocrine and exocrine pancreatic functions through NK-1 receptors. Tachykinins are not involved in vagal regulation of pancreatic secretion in pigs but could constitute part of an alternative stimulatory system.

Glucagon-like peptide-1 inhibits gastropancreatic function by inhibiting central parasympathetic outflow

Glucagon-like peptide (GLP)-1 inhibits acid secretion and gastric emptying in humans, but the effect on acid secretion is lost after vagotomy. To elucidate the mechanism involved, we studied its effect on vagally stimulated gastropancreatic secretion and motility in urethan-anesthetized pigs with cut splanchnic nerves, in which insulin-induced hypoglycemia elicited a marked stimulation of gastropancreatic secretion and antral motility. In addition, we studied vagally stimulated motility and pancreatic secretion in isolated perfused preparations of the porcine antrum and pancreas. GLP-1 infusion (2 pmol. kg-1. min-1) strongly and significantly inhibited hypoglycemia-induced antral motility, gastric acid secretion, pancreatic bicarbonate and protein secretion, and pancreatic polypeptide (PP) secretion. GLP-1 (at 10(-10)-10(-8) mol/l) did not inhibit vagally induced antral motility, pancreatic exocrine secretion, or gastrin and PP secretion in isolated perfused antrum and pancreas. We conclude that the inhibitory effect of peripheral GLP-1 on upper gastrointestinal secretion and motility is exerted via interaction with centers in the brain or afferent neural pathways relaying to the vagal motor nuclei.

Increased sensitivity of gastrin cells to gastric distension following antral denervation in the rat

1. Secretion of the antral hormone gastrin is increased by protein in the gastric lumen and by nervous reflexes. We have examined the relative importance of luminal and neuronal mechanisms, by lesioning the antral innervation using benzalkonium chloride. 2. Benzalkonium chloride was applied to the serosa of the antrum in anaesthetized rats. In some animals, a stainless-steel cannula was also implanted in the corpus. Animals were allowed 10 days to recover. Plasma gastrin was measured by radioimmunoassay and mRNAs encoding gastrin, somatostatin and histidine decarboxylase were measured by Northern blot. 3. Antral denervation was associated with gastric retention after fasting, and elevated plasma gastrin (28.4 +/- 7 pM compared with 7.6 +/- 1.0 pM in controls). When fasted control or denervated rats were refed, plasma gastrin increased 3-fold in both cases. A gastrin-releasing peptide antagonist inhibited the post-prandial rise in plasma gastrin in control rats, but had no effect in antrally denervated rats. 4. In fasted, antrally denervated rats with a gastric fistula, basal gastric acid secretion was depressed 3-fold, and plasma gastrin concentrations were similar to controls. 5. Distension of the stomach with peptone via a barostat attached to the gastric cannula (5 cm H2O, 30 min), produced 3-fold increases in plasma gastrin in both control and denervated rats. However, distension with a non-nutrient solution at pH 6.0 had no effect in controls, but increased gastrin to a similar extent to peptone in denervated rats; distension with 50 mM HCl had no effect in either control or denervated rats. 6. Somatostatin and gastrin mRNA abundances in the antrum were depressed by about 35% by antral denervation, but somatostatin mRNA in the corpus was unchanged; GAPDH mRNA abundance was unaffected by antral denervation. 7. The data suggest that luminal nutrient releases gastrin in the rat, in vivo, via activation of antral neurons secreting gastrin-releasing peptide, and that the antral innervation normally inhibits G-cell responses to non-nutrient distension of the stomach. After antral denervation, gastric distension with a non-nutrient solution is an adequate stimulus for gastrin release.

Role of vagal fibers and bombesin/gastrin-releasing peptide-neurons in distention-induced gastrin release in rats

In the rat the exact role of vagal fibers and the interaction between the extrinsic and intrinsic neural system in distention-induced gastrin release are still a matter of debate. Accordingly, the aim of the present study was to examine the contribution of afferent and efferent vagal fibers as well as intrinsic neurons on gastrin response to gastric distention. In anesthetized rats graded gastric distention by 5, 10 and 15 ml saline for 20 min caused a significant volume-dependent increase of plasma gastrin levels by 12+/-6 pg/ml (5 ml saline, n = 8, P =0.05), 26+/-7 pg/ml (10 ml saline, n = 10, P < 0.05) and 37+/-7 pg/ml (15 ml saline, n = 8, P < 0.01 ), respectively. To examine the role of the extrinsic vagal innervation, gastrin response to distention was studied in anesthetized rats after bilateral truncal vagotomy (n = 9) or selective afferent vagotomy following pretreatment with capsaicin (n = 6). Stimulation of gastrin release by 10 ml distention in sham-operated control rats was reversed to an inhibition after truncal vagotomy (26+/-7 vs. -11+/-4 pg/ml; P<0.05) and capsaicin-treatment (37+/-18 vs. -34+/-11 pg/ml; P<0.05). A contribution of cholinergic mechanisms to this vagovagal-mediated stimulation of distention-induced gastrin release was excluded, since atropine (100 microg/kg/h; n = 8) further augmented distention-stimulated gastrin release. Since bombesin/gastrin-releasing peptide (GRP)-neurons contribute to vagally stimulated gastrin secretion, we have examined gastrin response to distention in the presence of the specific bombesin-receptor antagonist D-Phe6-BN(6-13)OMe (400 microg/kg/h: n = 10). This bombesin-antagonist completely reduced distention-stimulated gastrin release in vivo. In contrast, distention of the isolated, extrinsically denervated stomach significantly decreased gastrin release by 13+/-5 pg/min (5 ml saline, n = 8, P < 0.05), 28+/-8 pg/min (10 ml saline, n = 11, P < 0.05) and 35+/-10 pg/min (15 ml saline, n = 8, P < 0.01), respectively, without changing the activity of bombesin/GRP-neurons. Distention-induced decrease of gastrin release was attenuated to 50 percent by atropine (10(-7) M: n = 10) or tetrodotoxin (TTX) (10(-6) M; n = 10), respectively. These data demonstrate, that in anesthetized rats distention-stimulated gastrin secretion depends on the activation of a vagovagal reflex and intrinsic bombesin/GRP-neurons. In contrast distention of the isolated rat stomach inhibits gastrin release in part via intrinsic cholinergic pathways and other as yet unknown mechanisms.

Tachykinins may mediate capsaicin-induced, but not vagally induced motility in porcine antrum

Tachykinins are thought to be involved in extrinsic control of motility in the gastrointestinal tract. Using the isolated perfused porcine antrum with intact vagal innervation, we studied the effects of substance P, neurokinin A and capsaicin infusion, and electrical stimulation of the vagus nerves on antral motility without or with infusion of non-peptide antagonists for NK-1 receptors (CP96345) and NK-2 receptors (SR48968). Substance P and neurokinin A stimulated antral motility in a dose-dependent manner. The effect could be inhibited by atropine or a combination of the NK-1 and NK-2 receptor antagonists. Electrical stimulation of the vagus nerves and infusion of capsaicin (10(-5) M) stimulated antral motility. Vagally induced motility was not influenced by infusion of CP96345 and SR48968, whereas the effect of capsaicin was blocked. We conclude that tachykinins may be involved in regulation of antral motility through sensory nerves in the porcine antrum, but they do not seem to be involved in vagal regulation of antral motility.

Role of endogenous bombesin-peptides during vagal stimulation of gastric acid secretion in the rat

The stimulatory effect of exogenous bombesin and its related mammalian peptides on gastric acid secretion and gastrin release has been examined in detail, while the regulatory role of endogenously released bombesin-like peptides is largely unknown. Accordingly we have determined the effect of a specific bombesin receptor antagonist during vagal stimulation of gastric acid secretion and gastrin release. In anesthetized rats electrical stimulation of the vagal nerves (10 V, 10 Hz, 1 ms) significantly increased plasma gastrin levels by 82 +/- 11 pg/20 min (P < 0.01) and gastric acid output by 99.4 +/- 9.9 mueq/20 min (P < 0.01). Intravenous infusion of the specific bombesin receptor antagonist D-Phe6-BN(6-13)OMe (400 nmol/kg/h) significantly reduced vagally induced increase of plasma gastrin levels by 70% to 29 +/- 8 pg/20 min (P < 0.05 vs control) and vagally stimulated gastric acid output by 40% to 57.4 +/- 10.6 mueq/20 min (P < 0.05 vs control). To demonstrate that the residual gastrin and acid response is due to non-bombesinergic mechanisms and not to an inadequate dose of the receptor antagonist, the latter was tested against gastrin-releasing peptide (GRP) at the maximally effective concentration of 300 pmol/kg/h, which resulted in an even 50% higher increase of plasma gastrin levels compared to vagal stimulation. The dose of the antagonist employed (400 nmol/kg/h) was sufficient to abolish GRP-induced stimulation of gastrin and gastric acid secretion. Previously it has been postulated that endogenous bombesin-peptides can stimulate acid secretion via gastrin-independent mechanisms. To investigate this possibility further the effect of the antagonist was examined on vagally induced acid secretion while gastrin levels were restored to the range of the respective control experiments. In presence of the antagonist the infusion of gastrin-17 (15 pmol/kg/h) in addition to vagal stimulation elevated plasma gastrin to levels not different from those during vagal stimulation alone. With identical plasma gastrin levels the bombesin receptor antagonist had no effect on vagally stimulated acid secretion (86.3 +/- 10.7 mueq/20 min vs 99.4 +/- 9.9 mueq/20 min in the controls; n.s.). In conclusion, the present data demonstrate for the first time that in rats in vivo endogenous bombesin peptides contribute to vagal stimulation of gastrin release and gastric acid secretion. Furthermore, endogenous bombesin-peptides exert their action on parietal cell function via an increase of gastrin release, while non-gastrinergic mechanisms are unimportant under the experimental conditions employed.

Inhibitory effects of gastrin releasing peptide on gastric emptying in rats

Gastrin-releasing peptide (GRP) has a wide range of biological actions, including stimulation of the frequency of antral contractions and delaying gastric emptying. The present study was designed to evaluate the role of GRP in the control of gastric emptying of liquid test meals in the rat. The emptying of methyl cellulose given by gavage to fasted rats, or of saline given via the fistula to conscious gastric fistula rats was not influenced by the GRP antagonists, NC-8-89 (Leu13-psi-(CH2NH)-Leu14-bombesin) and 2258U89 ((de-NH2)Phe19, D-Ala24, D-Pro26 psi (CH2NH)Phe27(-GRP (19-27)), at 2 mg/kg, s.c. However, both antagonists (0.02, 0.2 and 2 mg/kg) reversed the inhibitory effect of HCI on gastric emptying in gastric fistula rats (P < 0.05-0.001). When peptone was administered after a preload, but not otherwise, the inhibition of emptying was also partly reversed by both antagonists at all doses used (P < 0.05-0.001). Interestingly, the delay in the emptying of hyperosmolal saline compared to saline, was enhanced at a dose of 0.2 mg/kg of both antagonists (P < 0.05 and P < 0.01). Food intake did not change significantly with the two lower doses of antagonists, but was decreased by the highest dose of NC 8-89. We conclude that GRP specifically inhibits gastric emptying of acid and peptone solutions in the conscious rat.

Effect of motilin, somatostatin and bombesin on gastroduodenal myoelectric activity in sheep

The effects of motilin, erythromycin, somatostatin and bombesin on antroduodenal myoelectric activity were investigated in conscious sheep. Myoelectric recordings were obtained from electrodes chronically implanted on the antrum and duodenal bulb. Peptides or erythromycin were infused intravenously (i.v.) during 5 min. Antagonists were injected i.v. as a bolus. Neither motilin (2.5-80 ng/kg/min) nor erythromycin (2-16 micrograms/kg/min) modified the antroduodenal myoelectric activity, although a single bolus of these compounds (250 ng/kg and 50 micrograms/kg respectively) increased the antral activity. Somatostatin at 5 ng/kg/min induced a decrease in the myoelectric activity of antrum and duodenum. However, doses of 10 to 40 ng/kg/min evoked a duodenal phase III-like activity with a subsequent quiescence period and a concomitant inhibition of the antral activity. These effects were reproduced by bombesin (2.5 to 40 ng/kg/min). Furthermore, an initial increase in the myoelectric activity and in the frequency of slow waves were recorded in the antrum when the highest doses were used. On the other hand, atropine (0.2 mg/kg) or hexamethonium (2 mg/kg) caused a long-lasting inhibition of antroduodenal myoelectric activity. These cholinergic antagonists abolished the effects induced by somatostatin (20 ng/kg/min) but not those evoked by bombesin but not motilin are putative modulators of the migrating myoelectric complex (MMC) in sheep. Moreover, a cholinergic neural pathway is involved in the somatostatin but not in the bombesin-induced effects.

Effect of endogenous opioids on vagally induced release of gastrin, somatostatin and bombesin-like immunoreactivity from the perfused rat stomach

The aim of the present study was to evaluate the effect of the opiate receptor antagonist naloxone on vagally stimulated secretion of bombesin-like immunoreactivity (BLI), somatostatin and gastrin from the isolated rat stomach, which was perfused via the celiac artery with Krebs-Ringer buffer. Vagal stimulation was performed for 10 min with 1 ms, 10 V and 2, 5, 10 or 20 Hz, respectively. In control experiments BLI release increased significantly above basal secretion during a stimulation frequency of 10 Hz (1367 +/- 357 pg/10 min; P < 0.001) and 20 Hz (996 +/- 202 pg/10 min; P < 0.01), but not at 2 and 5 Hz. In comparison to the controls naloxone (10(-6) M) significantly increased BLI secretion at 5 Hz by 573 +/- 150 pg/10 min (P < 0.05), but attenuated the BLI response to higher stimulation frequencies of 10 and 20 Hz to 284 +/- 143 pg/10 min (P < 0.001) and 490 +/- 114 pg/10 min (P < 0.01), respectively. At 2 Hz naloxone had no effect on BLI release. As shown previously the cholinergic blocker atropine (10(-7) M) induced a significant BLI release during vagal stimulation at 2 Hz (680 +/- 233 pg/10 min; P < 0.01) and 5 Hz (935 +/- 324 pg/10 min; P < 0.05), but was without effect at 10 and 20 Hz compared to the controls. The effects of the combination of naloxone and atropine were similar to naloxone and atropine alone. Naloxone had no effect on vagal or GRP-induced regulation of gastrin and somatostatin release.(ABSTRACT TRUNCATED AT 250 WORDS)

Bombesin stimulates the in vitro growth of a human gastric cancer cell line

Bombesin (BBS) and its mammalian equivalent, gastrin-releasing peptide (GRP), exhibit diverse biological functions, including that of a neurotransmitter, a regulator of gastrointestinal hormone release, and a trophic factor for various normal and neoplastic tissues. Bombesin stimulates the growth of normal cells of the stomach, pancreas, and bronchial epithelium as well as cells in breast cancer, gastrinoma, and small cell lung cancer. The purpose of this study was to determine whether BBS regulates the growth of a human gastric cancer cell line (SIIA) in vitro, and if so, to examine the mechanisms of signal-transduction that are involved. We found that BBS stimulated the growth of SIIA cells in vitro. The GRP receptor antagonists, BIM 26189 and BIM 26226, had no effect on growth of SIIA cells. Although these antagonists blocked the BBS-induced increase of [Ca2+]i, they failed to block the growth-stimulatory effect of BBS. BBS stimulated intracellular tyrosine phosphorylation of multiple proteins, with a predominant protein of apparent molecular weight of 125 kDa. Inhibition of intracellular tyrosine kinases by tyrphostin blocked the growth-stimulatory effect of BBS on SIIA cells. These results indicate that BBS exerts its trophic effect on SIIA cells through a receptor(s) linked to tyrosine kinase pathway, but not to the phospholipase C (PLC) pathway.

A novel synthetic phyllolitorin analogue [desTrp3,Leu8]phyllolitorin inhibits scratching behavior induced by neuromedin C in rats

[DesTrp3,Leu8]phyllolitorin (DTP) (pGlu-Leu-Ala-Val-Gly-Ser-Leu-Met-NH2) was synthesized as an analogue of phyllolitorins, a new member of bombesin family, and examined if it antagonized neuromedin C (NMC)-induced scratching. DTP inhibited dose-dependently the scratching behavior by NMC (1 microgram), whereas it did not alter any element of other grooming behaviors. DTP (6 micrograms) alone was found to be neither toxic nor active in inducing both scratching and grooming, which were comparable to vehicle alone. Assuming that the scratching behavior is commonly and specifically elicited by bombesin family peptides, DTP might be classified as a new type of bombesin antagonist.

Effect of bombesin antagonist D-Phe6-BN(6-13)OMe on vagally induced gastrin release from perfused rat stomach

The aim of the present study was to evaluate the effect of the bombesin antagonist D-Phe6-BN(6-13)OMe (BN-antagonist) on vagally stimulated gastrin release from the isolated rat stomach, which was perfused via the celiac artery with Krebs-Ringer buffer. Vagal stimulation was performed for 10 minutes with 1 ms, 10 V and 10 or 2 Hz, respectively. Gastrin secretion increased significantly during stimulation with 10 and 2 Hz. BN-antagonist was added to the perfusate at the concentration of 10(-6) M, which induced a significant reduction of vagally stimulated gastrin release at 10 Hz (619 +/- 65 vs. 252 +/- 62 pg/10 min, p < 0.05), but not at 2 Hz (564 +/- 117 vs. 493 +/- 113 pg/10 min, p > 0.05). In contrast, atropine (10(-7) M) reduced significantly the gastrin response at 2 Hz (270 +/- 78 pg/10 min, p < 0.01), but not at 10 Hz (446 +/- 87 pg/10 min, p > 0.05). The combination of BN-antagonist and atropine elicited an inhibition of vagally stimulated gastrin release similar to each substance when given alone. Basal gastrin release was not changed by the BN-antagonist. The present data suggest, that in the rat stomach endogenously released bombesin-related peptides contribute to the noncholinergic stimulation of gastrin release at higher stimulation frequencies (10 Hz), however, bombesin-related peptides are not involved, when lower stimulation frequencies (2 Hz) are employed. At both stimulation frequencies additional mechanisms are activated which are noncholinergic and not related to bombesin peptides.

Actions of novel bombesin receptor antagonists on pancreatic secretion in rats

Recently synthesized highly specific and potent bombesin receptor antagonists permit study of the role of endogenous bombesin-like peptides in the physiological regulation of pancreatic secretion. We now tested the action of three novel pseudononapeptide bombesin/gastrin releasing peptide (GRP) antagonists (RC-3095, RC-3100 and RC-3120) on amylase release in vitro from isolated rat pancreatic acini and on protein secretion in vivo in chronic pancreatic fistula rats. In isolated pancreatic acini, all three bombesin receptor antagonists inhibited the amylase secretion induced by bombesin by shifting to the right the amylase response to bombesin without altering the maximal response. These antagonists alos reduced concentration dependently the near-maximal amylase response to bombesin, the concentration required for 50% reduction (IC50) being about 10(-7) M for RC-3095 and RC-3100 and 10(-6) M for RC-3120. None of the bombesin/GRP antagonists used affected the amylase response to CCK, pentagastrin or urecholine. In conscious rats with a chronic pancreatic fistula, all three bombesin antagonists shifted to the right the pancreatic protein response to graded doses of bombesin without changing the maximal response. These antagonists inhibited the protein response to constant background stimulation with bombesin in a dose-dependent manner, the ID50 being about 20 nmol/kg per h for RC-3095 and RC-3100 and about 160 nmol/kg per h for RC-3120. None of the antagonists significantly affected basal pancreatic secretion or secretion induced by sham-feeding, ordinary feeding or the diversion of pancreatic juice from the duodenum. These results indicate that exogenous bombesin is a potent direct stimulant of pancreatic enzyme secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Progress in the development of potent bombesin receptor antagonists

Bombesin and the mammalian-related peptides gastrin-releasing peptide (GRP), GRP and neuromedin B have been shown to have numerous actions in the CNS, gastrointestinal tract and on growth. However, the role of the peptides in various physiological processes has remained unclear because of the lack of potent antagonists. Recent in vitro studies have described four different classes of bombesin receptor antagonist, some of which are active in the nanomolar range and in vivo. Robert Jensen and David Coy describe recent insights into peptide structural determinants of biological activity. Evidence from structure-function studies have resulted in identification of some analogues that function as potent antagonists in all systems examined. Furthermore, various subtypes of bombesin receptors can now be differentiated by these various classes of antagonist.