Modulation of acetylcholine-induced secretion of gastric bombesin-like immunoreactivity by cholinergic and histamine H2-receptors, somatostatin and intragastric pH

Release of bombesin-like immunoreactivity from synaptosomal membranes isolated from the rat ileum

In the enteric nervous system, direct effects on peptidergic neurotransmitter release are difficult to assess since the neuronal network predisposes to numerous interactions between the various transmitter systems. The aim of the present study was to examine the release of bombesin-like immunoreactivity from isolated nerve synapses of the enteric nervous system. Enriched synaptosomal fractions were obtained by using homogenized tissue from rat ileum, which was subjected to various steps of differential and sucrose density centrifugation. Specific binding of [3H]saxitoxin served as a marker for neuronal membranes. For comparison, the content of bombesin-like immunoreactivity was determined. Both the enriched synaptosomal fraction (mitochondrial fraction II or P2) and the purified synaptosomal fraction (F2), obtained after discontinuous sucrose density centrifugation, showed substantial enrichment of the neuronal marker [3H]saxitoxin and bombesin-like immunoreactivity. The basal release of bombesin-like immunoreactivity was 52 +/- 17 pg/mg (100%). KCl-evoked depolarization (65 mM) significantly stimulated the release of bombesin-like immunoreactivity to 142.2% (P < 0.05, n = 17). The release was abolished in Ca(2+)-free medium. Stimulation of the release of bombesin-like immunoreactivity was also observed in the presence of the Ca2+ ionophore A-23187 (10(-6) M: 129%, P < 0.05, n = 17), supporting the role of Ca2+ in the release process. Cholinergic stimulation with carbachol elicited a significant dose-dependent release of bombesin-like immunoreactivity (10(-8) M: 106%, 10(-7) M: 175%, P < 0.05, 10(-6) M: 156%, P < 0.05, 10(-5) M: 115%, n = 14), which was reduced by atropine (10(-6) M: 99%, P < 0.01, n = 14). The basal value was 67 +/- 9 pg/mg (100%). The different effects of the muscarinic M1 receptor antagonist pirenzepine, which stimulated release of bombesin-like immunoreactivity in combination with carbachol 10(-6) M (10(-6) M: 123%, n = 10), and of the muscarinic M2 receptor antagonist AFDX 116, which attenuated release of bombesin-like immunoreactivity evoked by carbachol (10(-5) M: 66%, P < 0.01, 10(-6) M: 88%, n = 10), strongly suggest modulation of the release of bombesin-like immunoreactivity at the presynaptic receptor site through an excitatory muscarinic M2 receptor. The basal value was 46 +/- 9 pg/mg (100%). In summary, bombesin-like immunoreactivity can be released from these synaptosomes by both depolarization with KCl in a Ca(2+)-dependent manner and by cholinergic stimulation. The synaptosomes of intrinsic nerves of the gut offer an approach to study the release of neuropeptides and neurotransmitters at the subcellular level independent of the ganglionic network.

The effect of glucose and insulin on vagally induced gastrin, bombesin-like immunoreactivity and somatostatin secretion from the perfused rat stomach

Previous studies in the isolated perfused rat stomach have shown that elevated glucose and insulin concentrations modulate BLI and somatostatin release during arterially administered peptidergic stimuli. In the present study the effect of elevated levels of glucose or insulin was examined on vagally induced changes of gastrin, somatostatin and BLI secretion. The lumen of the stomach was perfused with saline pH 7 or pH 2. Vagal stimulation (5 Hz, 1 msec, 10V) increased gastrin and BLI secretion and inhibited somatostatin release. The increase of the perfusate glucose concentration from 100 mg/dl to 150 or 300 mg/dl or the addition of insulin (100 microU/ml) augmented vagally stimulated gastrin release at luminal pH 7 but not pH2. Vagally induced inhibition of somatostatin was attenuated by both concentrations of glucose at either luminal pH while insulin had no effect. BLI secretion was affected neither by elevated glucose nor by insulin. On the other hand, the noncholinergic component of vagally induced BLI secretion in the presence of atropine was augmented by insulin. These data demonstrate that glucose and insulin can modulate vagally activated gastric neuroendocrine functions which could be of relevance during the ingestion of carbohydrate containing meals.

Vagally induced release of gastrin, somatostatin and bombesin-like immunoreactivity from perfused rat stomach. Effect of stimulation frequency and cholinergic mechanisms

The isolated stomach of rats was vascularly perfused to measure the secretion of gastrin, somatostatin (SLI) and bombesin-like immunoreactivity (BLI). The gastric lumen was perfused with saline pH 7 or pH 2, and electrical vagal stimulation was performed with 1 ms, 10 V and 2, 5 or 10 Hz, respectively. Atropine was added in concentrations of 10(-9) or 10(-7) M to evaluate the role of cholinergic mechanisms. In control experiments, vagal stimulation during luminal pH 2 elicited a significant increase of BLI secretion only at 10 Hz but not at 2 and 5 Hz. Somatostatin release was inhibited independent of the stimulation frequency employed. Gastrin secretion at 2 Hz was twice the secretion rates observed at 5 and 10 Hz, respectively. At luminal pH 7 BLI rose significantly at 5 and 10 Hz. SLI secretion was decreased by all frequencies. Gastrin secretion at 2 and 5 Hz was twice as high as during stimulation with 10 Hz. Atropine at doses of 10(-9), 10(-8), 10(-7) and 10(-6) M had no effect on basal secretion of BLI, SLI and gastrin. At luminal pH 2, atropine increased dose-dependently the BLI response at 2 and 5 but not at 10 Hz. The decrease of SLI during 2 and 5 Hz but not 10 Hz was abolished by atropine 10(-9) M. SLI was reversed to stimulation during atropine 10(-7) M at all frequencies. The rise of gastrin at 2 Hz was reduced by 50%. At luminal pH 7, atropine had comparable effects with a few differences: the BLI response at 10 Hz was augmented and the gastrin response to 2 and 5 Hz was reduced. In conclusion the present data demonstrate a frequency and pH-dependent stimulation of BLI and gastrin release. The stimulation of BLI is predominantly due to atropine-insensitive mechanisms while muscarinic cholinergic mechanisms exert an inhibitory effect on BLI release during lower stimulation frequencies (2 and 5 Hz) independent of the intragastric pH and also during higher frequencies at neutral pH. Both, atropine sensitive and insensitive mechanisms are activated frequency dependent. The atropine-sensitive cholinergic mechanisms but not the noncholinergic mechanisms involved in regulation of G-cell function are pH and frequency dependent. Somatostatin is regulated largely independent of stimulation frequency and pH by at least two pathways involving cholinergic mechanisms of different sensitivity to atropine. These data suggest a highly differentiated regulation of BLI, gastrin and SLI secretion and the interaction between these systems awaits further elucidation.

The effects of substance P, histamine and histamine antagonists on somatostatin and gastrin release from the isolated perfused rat stomach

Secretion of somatostatin-like immunoreactivity (SLI) from the isolated perfused rat stomach has been shown to be inhibited by substance P. The present study was initiated to examine the possibility that this action of substance P was mediated via release of histamine. Substance P (1 microM) reduced basal secretion of SLI in agreement with earlier studies. Neither pyrilamine nor cimetidine influenced this action. Basal immunoreactive gastrin (IRG) secretion was unaffected by substance P. Addition of pyrilamine during substance P perfusion increased IRG secretion whereas addition of cimetidine resulted in a delayed decrease on removal of both compounds. Histamine (1 and 10 microM) increased SLI secretion and reduced IRG secretion. Pyrilamine increased and cimetidine decreased IRG secretion but neither drug influenced SLI secretion. Pyrilamine had no effect on histamine-stimulated SLI secretion but inhibition of IRG secretion by histamine was converted to stimulation. Cimetidine potentiated histamine stimulation of SLI secretion and inhibition of IRG secretion.

IN CONCLUSION

(1) substance P inhibition of SLI secretion is unlikely to be mediated via release of histamine. (2) The gastrin cell appears to have both H1- and H2-receptors which mediate opposite actions but H1-receptor-mediated inhibition is predominant. (3) Histamine weakly stimulates SLI secretion but there may be both inhibitory and stimulatory pathways acting via H2- and H1-receptors, respectively.

Chronic bombesin treatment increased the [3H]spiperone binding, glutamate decarboxylase and choline acetyltransferase activity in the rat brain

The effects of chronic bombesin (BBS) on [3H]spiperone (SPD) binding activity, choline acetyltransferase (ChAT), acetylcholinesterase (AChE) and glutamate decarboxylase (GAD) were investigated in the rat brain corpus striatum (CS). The chronic i.p. administration of BBS to rats increased: (1) the specific [3H]SPD binding to the striatal Pm (plasma membrane) (16%, P less than 0.03 and 34%, P less than 0.008 at 5 micrograms/kg respectively), (2) the specific GAD activity in the CS by 52% (5 micrograms/kg, n.s.) and 46% (10 micrograms/kg, P less than 0.05) respectively, (3) the specific ChAT activity in the CS by 54% (10 micrograms/kg, P less than 0.002), and (4) the specific AChE activity by 23% (10 micrograms/kg, P less than 0.02) after 14 days. It increased only: (1) the specific [3H]SPD binding by 29% (P less than 0.001, at 10 micrograms/kg) and (2) the specific GAD activity by 23% (P less than 0.015, 10 micrograms/kg), after 7 days. Neither ChAT nor AChE activity was affected after 7 days treatment of BBS at 10 micrograms/kg. In vitro study showed that BBS at 0.2 microM did not affect any of the neurochemical parameters examined in the CS. Thus, the changes in brain chemistry caused by chronic BBS were not due to direct effects of BBS but may be mediated through its metabolites or CCK release. Data indicate that the central effects of peripherally administered BBS are dependent on both the duration and the dosage of the drug treatment and that the dopaminergic and GABAergic systems seem to be more vulnerable to chronic BBS than the cholinergic system in the rat brain CS.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of histamine H2-receptor stimulation on bombesin- and peptone-stimulated gastrin release in man

This study was undertaken to determine whether histamine H2-receptors are involved in the regulation of gastrin secretion in man. Since previous studies on the effect of histamine H2-receptor blockade on gastrin release are conflicting, we have studied the effect of histamine infusion (130 nmol/kg/hr) with simultaneous H1-receptor blockade on gastrin release in healthy male subjects. Intragastric pH was maintained at 4.5 by continuous intragastric titration during all studies. Histamine did not affect gastrin release stimulated by infusion of bombesin (90 pmol/kg/hr) or by a peptone meal. Integrated gastrin secretion during bombesin plus histamine was 767 +/- 151 pmol X min/liter (+/- SEM), compared to 757 +/- 144 pmol X min/liter during bombesin plus saline (not significant), whereas integrated meal-stimulated gastrin release was 1666 +/- 456 pmol X min/liter during histamine and 1856 +/- 492 pmol X min/liter during saline. It is concluded that histamine H2-receptors do not seem to be involved in the regulation of gastrin secretion in man.

Effect of insulin on secretion of bombesin-like immunoreactivity and gastrin from the isolated rat stomach in response to acetylcholine, VIP and leucine-enkephalin

Bombesin-like immunoreactivity (BLI), a putative peptidergic neurotransmitter of the gastrointestinal intrinsic nervous system is released from the isolated perfused rat stomach in response to the classical neurotransmitter acetylcholine and in response to other putative peptidergic neurotransmitters such as vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) or growth hormone releasing factor (GRF). The secretion of BLI is modulated not only by gastric factors such as the intragastric pH but also by changes of perfusate glucose concentrations indicating that alterations of carbohydrate metabolism might have an effect on gastric neuroendocrine regulation. Since previous studies have shown that insulin, the major regulatory hormone of glucose metabolism, reduces gastric somatostatin and glucagon secretion it was of interest to determine the effect of insulin on gastric BLI and gastrin secretion. The experiments were performed in the isolated perfused rat stomach model. The addition of porcine insulin to the perfusate at concentrations of 50 and 100 microU/ml had no effect on basal BLI and gastrin secretion. The infusion of acetylcholine (2 X 10(-6)M and 4 X 10(-6)M) elicited a stimulation of BLI and gastrin secretion which was not altered by the addition of insulin (100 microU/ml). On the other hand, significant effects of insulin were observed during administration of the two putative peptidergic neurotransmitters VIP and leu-enkephalin. The infusion of VIP at 10(-11)M and 10(-8)M had no effect on BLI and gastrin secretion in the absence of insulin, however, with the addition of insulin (100 microU/ml) the higher dose of VIP (10(-8)M) elicited a significant stimulation of BLI secretion while both doses of VIP (10(-11)M and 10(-8)M) significantly increased gastrin release. Similar to VIP the infusion of leu-enkephalin at doses of 10(-9)M and 10(-6)M had no effect on BLI and gastrin secretion in the absence of insulin. When insulin was added to the perfusate both doses of leu-enkephalin elicited a significant stimulation of BLI secretion while gastrin remained unchanged. The addition of the specific opiate receptor antagonist naloxone (10(-5)M) did not block the effect of leu-enkephalin in the presence of insulin. In addition the effect of naloxone was also examined during cholinergic stimulation. The addition of naloxone (10(-5)M) during the infusion of acetylcholine abolished the stimulatory effect on BLI secretion in the absence of insulin, whereas in the presence of insulin naloxone did not alter cholinergically-induced BLI secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of vasoactive intestinal peptide, peptide histidine isoleucine and growth hormone-releasing factor-40 on bombesin-like immunoreactivity, somatostatin and gastrin release from the perfused rat stomach

Bombesin-like immunoreactivity (BLI) has been demonstrated in neurons of the gastrointestinal tract and gastric BLI secretion can be demonstrated in response to the classical neurotransmitter acetylcholine. Since structurally related peptides VIP, PHI and GRF have to be considered as peptidergic neurotransmitters it was of interest to determine their effect on gastric BLI secretion. Additionally, somatostatin (SLI) and gastrin secretion was examined. The isolated stomach of overnight fasted rats was perfused with Krebs-Ringer buffer via the celiac artery and the effluent was collected via the portal vein. The gastric lumen was perfused with isotonic saline at pH7 or pH2. All four peptides were tested at a dose of 10(-11) M and 10(-8) M at both pH levels and in addition the effect of VIP and PHI was examined at 10(-14) M and 10(-12) M during luminal pH2. At luminal pH7 VIP and PHI stimulated SLI release at 10(-8) M but had no effect on BLI or gastrin secretion. rGRF and hpGRF were both ineffective on SLI and gastrin release while rGRF inhibited and hpGRF stimulated BLI secretion. This effect was not dose related. At luminal pH2 all four peptides stimulated BLI secretion. Stimulation by PHI was already observed at a dose of 10(-14) M while VIP elicited a stimulatory effect at 10(-12) M. PHI at the two lowest concentrations of 10(-14) and 10(-12) M elicited a stimulation of SLI and gastrin release while the same doses of VIP and the higher doses of all four peptides had no effect on SLI and gastrin secretion at an acidic intraluminal pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological and pathophysiological aspects of somatostatin

Somatostatin is found in the D-cells of organs that are exclusively responsible for the digestion, absorption, and metabolism of ingested nutrients. D-cells apparently release their secretory products both into the interstitial space (paracrine action) and into the circulation (endocrine action). Ingestion of all three basic nutrients--fat, carbohydrate, and particularly protein--elicits a significant increase in peripheral vein plasma somatostatin levels in dogs and humans. Acidification of a meal stimulates somatostatin release in dogs. Vagal, cholinergic, and adrenergic mechanisms exert a species-dependent effect on somatostatin release. Gut hormones also participate in the regulation of postprandial somatostatin release, and endogenous opioids have an effect that depends on the composition of the meal. Stimulation of postprandial somatostatin release by H2-receptor agonists and prostaglandins has been reported. Insulin inhibits and glucagon stimulates somatostatin release. Elevated levels of circulating glucose reduce the somatostatin response, an effect that cannot be entirely explained by the parallel augmentation of insulin secretion. Circulating nutrients also modify the effect of gut hormones on D-cell function. The physiological action of somatostatin is an inhibitory effect on virtually all gastrointestinal and pancreatic exocrine and endocrine functions. Secretory and/or motor activities are attenuated, thereby preventing an exaggerated and overshooting response. Alterations of tissue somatostatin content and plasma somatostatin levels have been observed in obesity and suggest that somatostatin deficiency may be a pathogenic factor. The observed changes of somatostatin may be secondary to alterations of other functions; nevertheless, hyposomatostatinaemia might facilitate nutrient assimilation.

Somatostatin: historical aspects

Somatostatin, in essence an almost universal chalone, initially described as a 14 amino-acid-long peptide that inhibits growth hormone (GH) release, has been shown to be one of a family of related peptides, ubiquitous in distribution and versatile as a paracrine factor with a potentially important role in the regulation of gut, pancreatic, and nervous system function, in addition to its well-recognized influence on the pituitary secretion of GH and thyroid-stimulating hormone. With the development of new super agonists, it has become possible to manipulate the endocrine milieu, to modify gut, pancreatic, and pituitary function, and, in the case of several diseases such as acromegaly and intractable diarrhoea, to make a significant advance in therapy.

Gastrointestinal somatostatin: distribution, secretion and physiological significance

Somatostatin-like immunoreactivity (SLI) has been found throughout the gastrointestinal tract in all species examined. In the stomach it is mainly present in endocrine-type D-cells whereas in the intestine there is also an extensive distribution in enteric neurones. In all regions of the gastrointestinal tract multiple forms of somatostatin exist. A precursor (prosomatostatin) has been partially sequenced, three forms with 20 (SS-20), 25 (SS-25) and 28 (SS-28) amino acids completely sequenced, and somatostatin-14 (SS-14) demonstrated by radioimmunoassay. Both SS-14 and SS-28 exert a wide range of actions on the gastrointestinal tract and there is strong supportive evidence for a role in the regulation of gastric acid and gastrin secretion, gastrointestinal motility and intestinal transport. Both in vivo and in vitro studies on the secretion of gastric SLI into the vasculature have shown that nutrients initiate the process but that subsequent events are regulated by a complex interplay between hormonal and neuronal pathways. GIP is one of the most potent hormonal secretagogues. In the stomach, acetylcholine, opioid peptides and substance P are probably involved in parasympathetic inhibitory pathways and gastrin releasing peptide in stimulatory pathways. The sympathetic nerves are also stimulatory. Regulation of secretion of intestinal SLI has not been so extensively studied. Although SLI is also found in the gastrointestinal lumen the significance is unclear. Despite these advances the exact route of delivery of somatostatin to its target organs is uncertain and paracrine, endocrine and neural pathways may all be involved.

Intravenous histamine reduces bombesin-stimulated gastrin release in dogs

The effect of histamine on gastrin release was studied in 7 conscious mongrel dogs with chronic gastric and duodenal fistulas. Histamine-2 HCl was infused in doses of 0 (control), 20, 40, 80, and 160 micrograms/kg per h for 2 h on separate days. During the second hour, bombesin 500 ng/kg per h was infused intravenously. Intragastric pH was constantly kept at 2.5 by intragastric titration during each test. Leakage of gastric contents into the duodenum was prevented by a prepyloric balloon passed retrograde through a duodenal fistula. Gastrin release, as expressed by the integrated response during the last 50 min of the bombesin infusion was significantly (P less than 0.05) decreased by all doses of histamine, compared to control. The infusion doses of histamine studied, 20, 40, 80, and 160 micrograms/kg per h reduced bombesin-stimulated gastrin release 16%, 19%, 19%, and 30%, respectively. This effect was blocked by a histamine H-2 but not an H-1 receptor antagonist. We conclude that by an H-2 mechanism, exogenous histamine reduces bombesin-stimulated gastrin release in dog.