Role of cholecystokinin in canine pancreatic exocrine response to bombesin stimulation

Pulmonary Neuroendocrine Cells and Lung Development

Pulmonary neuroendocrine cells produce bioactive peptides such as gastrin-releasing peptide (GRP) at high levels in developing fetal lung. The role of GRP and other peptides in promoting branching morphogenesis, cell proliferation, and cell differentiation during lung organogenesis is reviewed. Possible roles for bioactive peptides derived from these cells in the pathophysiology of perinatal lung disorders are discussed.

Mechanism of bombesin-induced pancreatic secretion in unanesthetized rats

It is unclear whether stimulation of pancreatic enzyme secretion by intravenously administered bombesin is a direct effect on acinar cells or is mediated by release of CCK; this distinction is important for defining the potential role of bombesin-like peptides as regulators of pancreatic secretion. The role of CCK in bombesin-induced pancreatic secretion was examined in rats using CCK radioimmunoassay and the CCK receptor antagonist L-364,718. A biphasic pancreatic response occurred to sequential doubling doses of bombesin (31 to 2000 pmol/kg/h, each for 30 min; n = 9 rats); amylase secretion increased to peak at 250 pmol/kg/h (11.5 +/- 1.7 kU/30 min; 4.2 +/- 0.6 kU/30 min, basal) and then declined to basal levels at 2000 pmol/kg/h. The ED50 dose of bombesin for stimulation was 31 pmol/kg/h, and the maximal response did not differ significantly from that to exogenous CCK-8 (10.6 +/- 1.5 kU/30 min) in the same rats. When single doses of bombesin were infused for 2 h (31, 62, 125, 250 pmol/kg/h; one dose per day; order randomized; n = 8), a similar dose-response relationship was seen, both for peak amylase response and cumulative output over basal. L-364,718 (0.5 mg/kg IV) had no effect on the pancreatic response to ED50 or maximal doses of bombesin. Neither dose of bombesin altered plasma CCK levels. In contrast, other stimulants of pancreatic secretion (food ingestion, soybean trypsin inhibitor) caused marked elevations in plasma CCK levels. These results indicate that the potent stimulation of pancreatic secretion by exogenous bombesin in rats is not mediated by CCK, similar to findings in humans.

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)

Dose-related involvement of CCK in bombesin-induced pancreatic growth

We examined the role of CCK in bombesin-induced pancreatic growth in rats using the CCK receptor antagonist L-364,718. Rats (155 +/- 1 g, 8-10 per group) received subcutaneous injections every 8 h for 5 days with bombesin (0.6, 1.7 and 5 nmol/kg) or bombesin in combination with L-364,718 (1 mg/kg). After 5 days the pancreas was removed and pancreatic weight, protein content, DNA, amylase and chymotrypsin contents were determined. Bombesin produced a significant increase (48-475%) of pancreatic weight, tissue contents of protein, DNA, amylase and chymotrypsinogen (F = 82, P less than 0.001). When a large dose of bombesin (5 nmol/kg) was combined with L-364,718 a significant inhibition (up to 70%) of all tissue parameters was observed (P less than 0.001). L-364,718 did not affect the growth response to a small dose of bombesin (0.6 nmol/kg). Plasma CCK levels 15 min after a single injection of bombesin (0.6, 1.7 and 5 nmol/kg) were significantly increased in response to the 5 nmol/kg dose (2.0 +/- 0.7 to 3.4 +/- 0.8 pM, F = 6.9, P less than 0.01). No increases of CCK plasma levels were found in response to the 0.6 and 1.7 nmol/kg doses of bombesin, corresponding to the lack of effects of L-364,718 on growth parameters at these doses. Measuring the time-course of CCK plasma levels after a single injection of 5 nmol/kg bombesin revealed an increase from basal values of 1.4 +/- 0.3 pM to maximal levels of 3.5 +/- 0.5 pM after 15 min (F = 7.1, P less than 0.001). Values returned to basal after 60 min. These results suggest that low doses of bombesin act directly at the acinar cell or through release of non-CCK growth factors whereas high doses of bombesin act in part through CCK release.

CCK receptors in release of pancreatic polypeptide (PP) in dogs

Pancreatic polypeptide (PP) is released after ingestion of protein-fat meals and following administration of some gut hormones (CCK and bombesin), but the hormonal contribution to the physiological release of PP has not been elucidated. We used specific and potent CCK-receptor antagonist, L-364,718, administered intravenously in a dose of 0.5 mumol/kg or intraduodenally in a dose of 2 mumol/kg to assess the role of CCK in the release of PP. Exogenous CCK-8 infused intravenously in gradually increasing doses (12.5-400 pmol/kg/hr) caused a dose-dependent increase in plasma PP from basal 28 +/- 4 pM to 136 +/- 18 pM, and this PP increase was completely suppressed by both intravenous and intraduodenal administration of L-364,718. Meat feeding caused a dramatic increase in plasma PP from a basal level of 26 +/- 4 pM to a peak of about 190 +/- 32 pM, and the pretreatment with intravenous or intraduodenal L-364,718 reduced this PP increase by about 60%. Duodenal perfusion with oleate (0.12-4.0 mmol/hr) or L-Trp (0.12-4.0 mmol/hr) also increased plasma PP, reaching, respectively, 180 +/- 28 pM and 76 +/- 6 pM. Pretreatment with intravenous or intraduodenal L-364,718 completely abolished the plasma PP responses to oleate and L-Trp. Bombesin (100 pmol/kg/hr) raised plasma PP to the level similar to that achieved by meat feeding and L-364,718 given intravenously or intraduodenally blocked completely these plasma PP increments.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholecystokinin receptor antagonism by peptidergic and non-peptidergic agents in rat pancreas

1. Graded doses of bombesin infused I.V. into conscious rats with chronic pancreatic fistulae induced a dose-dependent stimulation of protein secretion, similar to that obtained with caerulein. This stimulation does not appear to be mediated by cholecystokinin (CCK) receptors because peptidergic (CR-1409) and non-peptidergic (L-364718) CCK antagonists failed to affect protein secretion at a dose range which caused almost complete suppression of caerulein-induced pancreatic secretion. 2. Studies in vitro on isolated rat pancreatic acini revealed that caerulein, pentagastrin and bombesin all showed the same efficacy in their ability to stimulate amylase release. In contrast, CCK antagonists competitively inhibited amylase release induced by caerulein and pentagastrin but not by bombesin or urecholine, indicating that the latter two agents act directly on acinar cells via receptors which are separate from those involved in stimulation induced by caerulein and pentagastrin. 3. DNA synthesis, measured by the incorporation of [3H]thymidine into DNA, was significantly stimulated by caerulein, soybean trypsin inhibitor (FOY 305), pentagastrin and by bombesin in a dose-dependent manner. CCK receptor antagonists prevented stimulation of DNA synthesis induced by caerulein, FOY 305 and pentagastrin but not by bombesin. 4. This study indicates that bombesin strongly stimulates pancreatic enzyme secretion, with an efficacy similar to that of caerulein, and also exerts a potent growth-promoting action on the pancreas, both effects appearing to be mediated by mechanisms independent of the CCK receptors.

Study into the role of cholecystokinin in bombesin-stimulated pancreatic growth in rats and hamsters

The effects of cholecystokinin and bombesin on growth of the exocrine pancreas have been studied extensively in rats but not in hamsters. Since hamsters are frequently used for studying pancreatic carcinogenesis it seems highly relevant to determine the effects of these peptides on growth of the hamster pancreas as well. In order to determine whether or not bombesin stimulates pancreatic growth in hamsters and to investigate the role of cholecystokinin in mediating this effect, we conducted a 2-week experiment in which cholecystokinin and bombesin were administered to both rats and hamsters, either with or without lorglumide (CR-1409; a specific cholecystokinin receptor antagonist). Rats were included in the study for comparison. Cholecystokinin and bombesin were found to stimulate pancreatic growth and DNA synthesis in both species. Lorglumide did not significantly influence the effect of bombesin on pancreatic weight but did significantly inhibit DNA synthesis in both species. The effect of cholecystokinin on pancreatic weight was greater in hamsters than in rats. This effect was significantly inhibited by lorglumide in hamsters but not in rats, whereas a significant decrease in DNA content was attained in both species. Thus, this study shows marked differences between rats and hamsters in the pancreatic growth response to cholecystokinin and bombesin.

Influence of cholecystokinin antagonist on the effects of cholecystokinin and bombesin on azaserine-induced lesions in rat pancreas

Both cholecystokinin and bombesin have been shown to promote pancreatic carcinogenesis in the azaserine-rat model. The present study was undertaken to discriminate between the effects of cholecystokinin and bombesin and to establish the modulating properties of the specific cholecystokinin receptor antagonist CR-1409 on pancreatic carcinogenesis. After initiation with 30 mg/kg of azaserine, six groups of 15 Wistar rats were treated for 16 wk with cholecystokinin, bombesin, or gelatin (control), some in combination with CR-1409. Doses of cholecystokinin (2.5 micrograms/kg) and bombesin (10 micrograms/kg) were chosen that rendered approximately equal plasma cholecystokinin levels. Both cholecystokinin and bombesin were found to stimulate pancreatic growth, whereas CR-1409 only inhibited the growth-promoting effect of cholecystokinin significantly. Furthermore, both peptides stimulated the development of putative preneoplastic lesions, whereas CR-1409 only inhibited the effect of cholecystokinin significantly. It is concluded that (a) CR-1409 inhibits the promoting effect of cholecystokinin on pancreatic growth and azaserine-induced early pancreatic lesions and (b) the effects of bombesin cannot be fully ascribed to stimulation of the secretion of endogenous cholecystokinin.

Evidence against cholecystokinin mediation of basal and bombesin-stimulated pancreatic secretion in the rat

Studies in dogs suggest that bombesin-stimulated pancreatic exocrine function is mediated via endogenous cholecystokinin. We studied (a) the short-term effects of bombesin on pancreatic juice volume and protein output in unconscious rats and (b) whether a potent cholecystokinin-receptor antagonist, L-364,718, affects the pancreatic exocrine response to bombesin. A 4-h i.v. infusion of low-dose (0.2 nmol/kg.h) or high-dose (1.0 nmol/kg.h) bombesin elicited significant increases in pancreatic juice volume and protein output, which were unaltered by treatment with L-364,718 at a dose capable of fully suppressing cholecystokinin-octapeptide-stimulated pancreatic juice volume and protein output. We conclude that the effects of exogenously administered bombesin on the exocrine pancreas in the rat are not mediated via release of endogenous cholecystokinin.

The cholecystokinin antagonist L-364,718 inhibits the action of cholecystokinin but not bombesin on rat pancreatic secretion in vivo

The action of an antagonist of peripheral CCK receptors, L-364,718 on CCK8 and bombesin-induced pancreatic exocrine secretion was studied in anaesthetized rats. Both CCK8 and bombesin increased the rate of flow and rate of enzyme secretion from the pancreas. Intravenous L-364,718 inhibited the actions of CCK8, but not those of bombesin. It is concluded that bombesin acts on the exocrine pancreas directly rather than through the release of CCK.