The role of insulin in the interaction of secretin and cholecystokinin in exocrine secretion of the isolated perfused rat pancreas

IV Hypertonic Glucose Stimulates the Exocrine Pancreas in Rat

BACKGROUND

Pancreatic atrophy and dysfunction resulting from parenteral nutrition (PN) may be explained by several mechanisms; one of the important factors is the nutrient in the circulation, which affects the pancreatic growth and secretion. The effect of nutrients on the pancreatic exocrine still has controversies. The aim of the present study is to better understand the effect of i.v. glucose on the exocrine pancreas stimulated by cholecystokinin during the parenterally fed condition.

METHODS

Two mixed solutions consisting of 30% and 50% glucose, respectively, were used. Thirty male Sprague-Dawley rats were randomly divided into 5 groups, including a control group; a freely fed group received cholecystokinin; 2 groups were infused with 30% and 50% glucose, and a group was infused with 50% glucose with cholecystokinin. The body weight and pancreatic contents were measured after 10 days.

RESULTS

The body weight in all groups was increased but lower than in the freely fed rats. I.v. glucose caused a decrease in the pancreatic weight, the amount of the pancreatic protein and DNA, and the level of amylase but elevated the level of trypsin in all treated groups. The enzymes were elevated after infusion of cholecystokinin with glucose, but they were lower than in freely fed animals with cholecystokinin. Compared with 30% glucose, 50% glucose elevated the level of amylase but did not affect the level of trypsin.

CONCLUSIONS

I.v. glucose results in atrophy of the exocrine pancreas, elevates the amylase in pancreas, but suppresses the stimulatory effect of cholecystokinin on the exocrine pancreas.

Age-associated changes in pancreatic exocrine secretion of the isolated perfused rat pancreas

Gut functions, such as gastrointestinal motility, gastric secretion and pancreatic secretion, were reduced with age. Glucose tolerance is impaired, and the release of insulin and β-cell's sensitivity on glucose are reduced with age. However, a lot of controversial data have been reported as insulin concentrations after glucose ingestion are either higher or no different in elderly and young subjects. Thus, this study was aimed to investigate whether aging could affect pancreatic exocrine secretion and its action mechanisms. An isolated perfused rat pancreatic model was used to exclude the effects of external nerves or hormones. Pancreatic secretion was increased by CCK under 5.6 mM glucose background in the isolated perfused pancreas of young (3 months), 12 months and 18 months aged rats. There was no significant difference between young and aged rats. In 3 months old rats, CCK-stimulated pancreatic secretion was potentiated under 18 mM glucose background. However, the potentiation effects of endogenous insulin and CCK were not observed in 12 and 18 months old rats. Exogenous insulin also potentiated CCK-stimulated pancreatic secretion in 3 months old rats. Similarly, exogenous insulin failed to potentiate CCK-stimulated pancreatic secretion as that of 3 months old rats. Wet weight of pancreas and amylase content in pancreatic tissue were not changed with age. These results indicate that pancreatic exocrine secretion is reduced with age and endogenous insulin secretion and/or action is involved in this phenomenon.

Regulation of gene expression and biochemical changes in small intestine of newborn diabetic rats by exogenous ghrelin

The aim of this study was to investigate (i) the cholecystokinin, somatostatin and apelin mRNA levels, (ii) the changes in levels and localization of these peptides, (iii) relation between these peptides, (iv) antiapoptotic effects and (v) antioxidant effects of ghrelin. The rats were divided into four groups second day after birth. These groups were respectively treated with physiological saline, ghrelin (100μg/kg/day), streptozotocin (100mg/kg), ghrelin and streptozotocin. After four weeks, small intestine and blood samples were taken from rats. Cholecystokinin mRNA and peptide, somatostatin mRNA, release to duodenal lumen of apelin peptide and apelin mRNA signals decreased in ghrelin-treated diabetic rats compared to the diabetic group. There was no statistically significant difference among the four groups for somatostatin and apelin peptides. Caspase-3 signals were not observed only in diabetic group treated with ghrelin. Caspase-8 signals were increased while PCNA signals were decreased in diabetic group given ghrelin compared to diabetic group. Small intestine CAT, SOD, GP(x) and GST activities and GSH levels were decreased and LPO, PC levels were increased in diabetic rats. Administration of ghrelin to diabetic rats caused an increase in intestinal CAT, SOD, GP(x) and GST activities and GSH levels, while PC levels decreased. As a result, we observed positive changes in diabetic rats treated with ghrelin in both microscopic and biochemical studies. We can suggest that ghrelin may be an important hormone for the treatment of diabetes.

Roles of Non-cholinergic Intrapancreatic Nerves, Serotonergic Nerves, on Pancreatic Exocrine Secretion in the Isolated Perfused Rat Pancreas

It has been rereported that axons which display 5-hydroxytryptamine (5-HT) immunoreactivity are abundant in the pancreas and the majority of serotonergic axons terminate within intrapancreatic ganglia, islet and acini. This histological result strongly suggests that intrapancreatic serotonergic nerves could affect to the pancreatic endocrine and exocrine secretion. Thus, this study was aimed to investigate whether intrapancreatic serotonergic nerves could affect pancreatic exocrine secretion and an action mechanism of the intrapancreatic serotonergic nerves. The rats were anesthetized with a single injection of urethane. The median line and the abdominal aorta was carefully dissected and cannulated with PE-50 tubing just above the celiac artery, and then tightly ligated just below the superior mesenteric artery. The pancreatic duct was also cannulated with Tygon microbore tubing. With the addition of serotonin, pancreatic volume flow and amylase output were significantly inhibited electrical field stimulation (EFS). On the other hand, pancreatic volume flow and amylase output were significantly elevated in EFS with the addition of spiperone. EFS application, however, pancreatic volume flow and amylase output had no significant change in cholecystokinin (CCK) alone when serotonin was applied under a 5.6 mM glucose background. Pancreatic volume flow and amylase output under 18 mM glucose background were significantly elevated in CCK plus serotonin than in CCK alone. These data suggest that intrapancreatic serotonergic nerves play an inhibitory role in pancreatic exocrine secretion and an important role in the insulin action or release.

The islet-acinar axis of the pancreas: more than just insulin

Although the role of the islets in the regulation of acinar cell function seemed a mystery to investigators who observed their dispersion among pancreatic acini, over time an appreciation for this intricate and unique structural arrangement has developed. The last three decades have witnessed a steadily growing understanding of the interrelationship of the endocrine and the exocrine pancreas. The islet innervation and vascular anatomy have been more fully characterized and provide an appropriate background for our current understanding. The interrelationship between the endocrine and exocrine pancreas is mediated by islet-derived hormones such as insulin and somatostatin, other humoral factors including pancreastatin and ghrelin, and also neurotransmitters (nitric oxide, peptide YY, substance P, and galanin) released by the nerves innervating the pancreas. Although considerable progress has been achieved, further work is required to fully delineate the complex interplay of the numerous mechanisms involved. This review aims to provide a comprehensive update of the current literature available, bringing together data gleaned from studies addressing the actions of individual hormones, humoral factors, and neurotransmitters on the regulation of amylase secretion from the acinar cell. This comprehensive view of the islet-acinar axis of the pancreas while acknowledging the dominant role played by insulin and somatostatin on exocrine secretion sheds light on the influence of the various neuropeptides on amylase secretion.

γ-aminobutyric acid secreted from islet β-cells modulates exocrine secretion in rat pancreas

AIM: To investigate the role of endogenous γ-amino-butyric acid (GABA) in pancreatic exocrine secretion.

METHODS: The isolated, vascularly perfused rat pancreas was employed in this study to eliminate the possible influences of extrinsic nerves and hormones. Cholecystokinin (CCK; 10 pmol/L) was intra-arterially given to stimulate exocrine secretion of the pancreas.

RESULTS: Glutamine, a major precursor of GABA, which was given intra-arterially at concentrations of 1, 4 and 10 mmol/L, dose-dependently elevated the CCK-stimulated secretions of fluid and amylase in the normal pancreas. Bicuculline (10 μmol/L), a GABA_A receptor antagonist, blocked the enhancing effect of glutamine (4 mmol/L) on the CCK-stimulated exocrine secretions. Glutamine, at concentrations of 1, 4 and 10 mmol/L, dose-dependently increased the GABA concentration in portal effluent of the normal pancreas. The effects of glutamine on the CCK-stimulated exocrine secretion as well as the GABA secretion were markedly reduced in the streptozotocin-treated pancreas.

CONCLUSION: GABA could be secreted from β-cells into the islet-acinar portal system after administration of glutainine, and could enhance the CCK-stimulated exocrine secretion through GABA_A receptors. Thus, GABA in islet β-cells is a hormone modulating pancreatic exocrine secretion.

Role of endogenous insulin in pancreatic secretion in rats

BACKGROUND

Pancreatic exocrine tissue is influenced by islet hormones through the islet-acinar portal system. We investigated the role of endogenous insulin in pancreatic secretion in vivo in rats.

METHODS

Anesthetized rats were prepared by cannulation of the pancreatic duct, and pure pancreatic juice was collected. Pancreatic secretion was stimulated by intravenous infusion of cholecystokinin (CCK) (0.06 microgram/kg/h) with or without intravenous infusion of glucose (0.25 and 0.5 g/kg/h). The effect of intravenous infusion of glucose (0.5 g/kg/h) on pancreatic secretion stimulated by intraduodenal infusion of casein (400 mg/h) was also studied.

RESULTS

Intravenous infusion of glucose (0.5 g/kg/h) produced a significant elevation of plasma insulin and glucose levels. The basal pancreatic secretion was not influenced by glucose. Pancreatic secretion of juice volume, amylase, and trypsin in response to stimulation by CCK was significantly augmented by glucose (0.5 g/kg/h). Intravenous infusion of glucose also resulted in significant increases in casein-stimulated pancreatic juice volume, amylase and trypsin outputs.

CONCLUSION

Endogenously released insulin plays an important role in potentiating pancreatic secretion stimulated by exogenous CCK and intraduodenal infusion of casein.

Endogenous somatostatin inhibits interaction of insulin and cholecystokinin on exocrine secretion of isolated, perfused rat pancreas

INTRODUCTION

Although somatostatin inhibits pancreatic exocrine secretion, the inhibitory mechanism of endogenous somatostatin is not clearly understood.

AIM

To investigate the effect of endogenous somatostatin on the interaction between endogenous insulin and exogenous cholecystokinin (CCK) in exocrine secretion of the totally isolated, perfused rat pancreas.

METHODOLOGY

Endogenous releases of somatostatin and insulin were induced by 18 mM glucose. Streptozotocin (75 mg/kg) or cysteamine (300 mg/kg) was injected into rats 24 hours before the experiment to deplete insulin or somatostatin in the pancreas.

RESULTS

Glucose (18 mM) enhanced CCK (10 pM)-stimulated secretions of fluid and amylase in the normal pancreas, which was further elevated by a somatostatin antagonist. Exogenous insulin (100 nM) also enhanced CCK-stimulated secretions in the streptozotocin-treated pancreas, which was also markedly increased by the somatostatin antagonist. The glucose (18 mM)-enhanced CCK-stimulated secretions were much higher in the cysteamine-treated pancreas than in the normal pancreas, which was dose-dependently reduced by exogenous somatostatin (30, 100 pM). However, endogenous or exogenous somatostatin did not modify the pancreatic responses to CCK alone.

CONCLUSION

Endogenous somatostatin inhibits the interaction of endogenous insulin and CCK on pancreatic exocrine secretion in the rat rather than reducing the action of CCK alone or endogenous release of insulin.