V. Schusdziarra

Release of bombesin-like immunoreactivity from the isolated perfused rat stomach

In the present study the release of bombesin-like immunoreactivity (BLI), somatostatin and gastrin was determined form the isolated perfused rat stomach. Gastric inhibitory polypeptide (GIP, 2 X 10(-9) M) had no effect on BLI while stimulating somatostatin and gastrin release. In these experiments the luminal pH of the stomach was kept at pH 7. Reduction of the luminal pH to 2 resulted in an inhibition of BLI secretion by GIP while gastrin release was abolished and somatostatin remained unaffected compared to luminal pH 7. Acetylcholine (10(-6) and 2 X 10(-6) M) elicited a dose-dependent stimulation of BLI secretion while gastrin was stimulated and somatostatin secretion suppressed independent of the administered dose. The present data demonstrate that release of bombesin-like immunoreactivity can be modulated by intestinal hormones and neurotransmitters and is integrated into the complex system of gastrointestinal neuroendocrine regulation.

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

Recently we have shown the release of bombesin-like immunoreactivity (BLI) from the isolated perfused rat stomach. In these experiments we have shown that BLI secretion is stimulated by acetylcholine. Gastric inhibitory peptide (GIP) exerts an inhibitory effect which is dependent on the intraluminal pH. The present study was designed to examine further the exact cholinergic mechanisms and to study the interaction between cholinergic and histaminergic mechanisms as well as the effect of the intraluminal pH. Acetylcholine elicited a dose-dependent increase in BLI and gastrin secretion (10(-6) M and 2 X 10(-6)M), whereas somatostatin release was suppressed at luminal pH 7. Blockade of muscarinic cholinergic receptors by atropine (10(-5)M) and nicotinic cholinergic receptors by hexamethonium (10(-5) M) abolished the effect of acetylcholine on all three peptides. Reduction of the intraluminal pH to 2 also abolished acetylcholine-induced stimulation of BLI and gastrin secretion and the inhibition of somatostatin secretion. Changes of intraluminal pH per se had no effect on the secretion of either peptide. Somatostatin (10(-7) M) reduced both BLI and gastrin secretion during stimulation with acetylcholine. The addition of the H2-receptor antagonist cimetidine (10(-5) M) abolished the effect of both doses of acetylcholine on BLI and somatostatin secretion and also the effect of the lower dose of acetylcholine (10(-6) M) on gastrin secretion during luminal pH 7. At luminal pH 2 cimetidine did not alter BLI and somatostatin secretion in response to acetylcholine, however, gastrin release was augmented in the presence of cimetidine. These data demonstrate that the effect of acetylcholine on BLI, gastrin, and somatostatin secretion is mediated by muscarinic and nicotinic cholinergic receptors and also by histamine H2-receptors. Somatostatin inhibits cholinergically induced BLI secretion. The cholinergic effects on BLI, somatostatin and gastrin secretion are abolished during an acidic intragastric pH. In this isolated perfused rat stomach model the inhibitory effect of intraluminal acid on gastrin secretion is, at least in part, mediated by H2-receptors. This suggests that the secretion of bombesin, a potential peptidergic neurotransmitter is modulated by neural, endocrine and local tissue factors and also by alterations of intragastric pH.

Effect of naloxone on pancreatic and gastric endocrine function in response to carbohydrate and fat-rich test meals

The present study was designed to determine the effect of naloxone, a specific opiate receptor antagonist, on postprandial levels of insulin, glucagon, pancreatic polypeptide (PP), somatostatin-like immunoreactivity (SLI) and gastrin in response to carbohydrate and fat-rich test meals in a group of 6 healthy volunteers. The addition of naloxone to a meal consisting of 50 g sucrose dissolved in 200 ml water augmented the rise of plasma insulin levels significantly during the first 30 min after its ingestion and reduced the rise in plasma insulin and pancreatic polypeptide and elevated glucagon levels during the last 30 min of the experimental period. When sucrose was dissolved in 200 ml cream the addition of naloxone augmented the postprandial rise of insulin levels between 15 and 60 min after ingestion of the meal and elicited an increase of plasma SLI and PP levels throughout the entire experimental period which indicates that post-prandial levels of insulin, glucagon, PP and SLI are modulated via endogenous opiate receptors during the ingestion of carbohydrate and fat test meals and that this effect depends on the composition of the ingested nutrients. These data raise the possibility that endogenous opiates participate in the regulation of postprandial insulin, glucagon, somatostatin and pancreatic polypeptide release not only in certain disease states as demonstrated recently for insulin secretion in type II diabetes mellitus but endogenous opiates may also be of importance under physiological conditions.

Carbohydrates modulate opiate receptor mediated mechanisms during postprandial endocrine function.

The present study was designed to determine the role of carbohydrates during naloxone-induced opiate receptor blockade upon the postprandial rise of plasma somatostatin (SLI), insulin and pancreatic polypeptide (PP) levels in response to protein and fat test meals in conscious dogs. Test meals consisting of 50 g liver extract + 50 g sucrose or 50 g corn oil + 50 g sucrose dissolved in 300 ml water were instilled intragastrically, respectively. Additionally, liver extract and fat meals were given with a concomitant intravenous infusion of glucose. To all test meals either naloxone (4 mg) or saline was added. The addition of sucrose to liver extract or the infusion of i.v. glucose during the liver meal abolished the inhibitory effect of naloxone on the rise of postprandial somatostatin levels which has been described recently. The addition of carbohydrate either orally or intravenously to the fat meal resulted in an even stimulatory effect of naloxone upon the rise of postprandial somatostatin levels. Insulin levels were not changed during liver extract + sucrose or i.v. glucose, respectively. When sucrose or i.v. glucose was administered together with the fat meal the addition of naloxone augmented postprandial insulin secretion. Pancreatic polypeptide (PP) release was augmented during the combination of sucrose or i.v. glucose with the fat and liver meal when naloxone was present in the meals. The present data demonstrate that the addition of carbohydrates either orally or intravenously to fat and protein meals modulates the effect of endogenous opiates in the regulation of postprandial somatostatin, insulin and pancreatic polypeptide release in dogs in a way that carbohydrates induce inhibitory mechanisms that are mediated via endogenous opiate receptors.

Effect of physiological increments of blood glucose on plasma somatostatin and pancreatic polypeptide levels in dogs

The present study was designed to determine the effects of physiological increments of plasma glucose levels upon basal and stimulated plasma somatostatin and pancreatic polypeptide levels. In seven conscious dogs the elevation of plasma glucose levels by 30-40 mg/dl did not change basal somatostatin and pancreatic polypeptide levels. During stimulation of these two hormones by acetylcholine and the octapeptide of cholecystokinin intravenous infusion of glucose elicited a significant decrease of somatostatin levels by 30 pg/ml and of pancreatic polypeptide levels by 300 pg/ml. The present data demonstrate that a physiological elevation of plasma glucose levels inhibits stimulated but not basal somatostatin and pancreatic polypeptide levels which may be of importance for nutrient entry and metabolism.

EVIDENCE FOR A ROLE OF ENDOGENOUS OPIATES IN POSTPRANDIAL SOMATOSTATIN RELEASE

Abstract Previously, we have demonstrated the effects of exogenously administered opiates on somatostatin release in dogs and therefore the present study was designed to determine the effect of endogenous opiates via naloxone-induced opiate receptor blockade on somatostatin release. Additionally, plasma insulin and pancreatic polypeptide (PP) levels were determined in response to intragastrically instilled protein, carbohydrate and fat test meals in a group of eight conscious dogs. To all test meals either naloxone (4 mg) or saline was added. The rise of plasma somatostatin levels in response to liver extract, sucrose and fat was attenuated significantly by naloxone. Naloxone had no effect on the rise of postprandial plasma insulin and PP levels. The present data demonstrate that endogenous opiates have a stimulatory effect on postprandial somatostatin release in dogs which indicates a tight interaction that might be of relevance for nutrient homeostasis.

Endogenous opioids modulate the effect of cholecystokinin on insulin release in dogs.

Recently we have demonstrated in dogs and man that endogenous opioids participate in the regulation of pancreatic endocrine function following the ingestion of a meal. Since intestinal hormones such as cholecystokinin (CCK) are also released by the presence of nutrients in the gastrointestinal tract and participate in the postprandial stimulation of pancreatic endocrine function, an interaction between CCK and endogenous opioids seems possible. The present study was designed to examine this further. In a group of 8 conscious dogs the octapeptide of CCK was infused intravenously in its sulfated (CCK-8S) or nonsulfated (CCK-8NS) form and in addition the tetrapeptide of CCK (CCK-4) was given at increasing infusion rates of 50, 200 and 500 pmol/kg . h, respectively. The experiments were performed during a background infusion of saline to assess the effect on basal insulin and during a background infusion of glucose (0.2 g/min) to determine the effects on stimulated insulin release. The effect of endogenous opioids was examined by addition of the opiate-receptor antagonist naloxone. The studies demonstrate that in the basal state CCK-8S has no stimulatory effect on insulin secretion unless naloxone is added indicating that endogenous opioids help to prevent insulin secretion in the absence of elevated glucose levels. During i.v. glucose naloxone reduced the stimulatory effect of CCK-8S at 50 and 200 pmol/kg . h and that of CCK-4 at 50 pmol/kg . h. Infusion of CCK-8S and CCK-4 at 500 pmol/kg . h had no effect on glucose-stimulated insulin levels, however, the addition of naloxone elicited a significant stimulatory effect. These data demonstrate stimulatory as well as inhibitory effects of endogenous opioids depending on the dose of CCK-8 and -4. CCK-8NS reduced glucose-stimulated insulin release already at the lowest dose of 50 pmol/kg . h. This was reversed to a stimulatory effect with the addition of naloxone. These data demonstrate that the interaction between CCK-8 and -4 and endogenous opioids on prestimulated insulin secretion is much more dependent on the dose of CCK - low doses induce stimulatory and high doses inhibitory mechanisms via endogenous opioids. In view of previous in vitro and in vivo studies with exogenously infused opiate-active compounds it might be speculated that increasing doses of CCK elicit a parellel increase in the release of endogenous opioids which might be responsible for some but certainly not all of the effects observed recently for the action of naloxone in the post-prandial state.

Cholinergic mechanisms in intestinal phase insulin secretion in rats

The present study was designed to examine the role of neural cholinergic mechanisms in intestinal phase insulin secretion during increasing intraduodenal instillation rates of the test meals. In groups of 12 anesthetized rats arterial insulin levels rose significantly in relation to the increase of the rate at which the liver extract/sucrose was instilled intraduodenally (0.15, 0.5 and 1.5 ml/min). The injection of atropine (10 micrograms/kg) 5 min prior to the intraduodenal infusion of the test meals abolished this rate-dependent augmentation of insulin levels completely. Similarly, no effect of increasing intraduodenal infusion rates of the meal was observed in islet-transplanted rats. These data demonstrate that neural - at least in part muscarinic cholinergic - mechanisms participate in intestinal phase insulin secretion of rats supporting previous observations about the importance of neural factors in the regulation of postprandial insulin release.

Effect of histamine H2-receptor stimulation on postprandial pancreatic and gastric endocrine function in dogs

SummaryThe effect of histamine H2-receptor stimulation via the infusion of impromidine was assessed with regard to postprandial plasma insulin, pancreatic polypeptide (PP), somatostatin, and gastrin levels. The effect of impromidine was assessed in the postprandial state during a liver extract/sucrose test meal which had a buffer capacity to maintain the intragastric pH at a constant level for the time impromidine was infused. Postprandial plasma insulin and gastrin levels were not changed by impromidine (10µg/kg·h−1). Plasma somatostatin levels rose significantly, whereas the postprandial increase of plasma PP levels was attenuated. The effects on somatostatin and PP were antagonized by the infusion of cimetidine, a specific histamine H2-receptor blocker.In conclusion the present data demonstrate that in the postprandial state activation of H2-receptors stimulates somatostatin and inhibits PP release while insulin and gastrin release are not affected.

Modulation of motilin-induced somatostatin release in dogs by naloxone

Somatostatin release in dogs is modulated by exogenous and endogenous opioids. Since postprandial somatostatin secretion is in part due to the stimulatory effect of postprandially activated gastrointestinal hormones as well as endogenous opioids, it was of interest to determine the interaction between motilin, a known stimulus of somatostatin release, and endogenous opioids with regard to activation of D-cell function. In a group of eight conscious dogs the infusion of synthetic porcine motilin at doses of 0.05, 0.25 and 0.5 micrograms/kg X hr elicited a significant increase of peripheral vein plasma somatostatin-like immunoreactivity (SLI), confirming previously reported data. The additional infusion of the opiate receptor antagonist naloxone attenuated this SLI response, suggesting that endogenous opioids participate in motilin-induced SLI release. Since previous studies have shown that the interaction between endogenous opioids and postprandial somatostatin secretion is modified by elevated plasma glucose levels, the experiments were repeated during an IV glucose (0.2 g/min) background infusion increasing circulating glucose levels by 20-30 mg/dl. During IV glucose, the SLI response to motilin was almost abolished. In this group the addition of naloxone restored the SLI response, indicating that the inhibitory effect of elevated glucose on D-cell function is, at least in part, mediated by endogenous opioids. These data suggest that motilin has to be considered as one regulatory factor which participates in the previously observed interaction between glucose and endogenous opioids during postprandial SLI release.