Vasoactive intestinal polypeptide and pituitary adenylate cyclase activating polypeptide: effects on insulin release in isolated mouse islets in relation to metabolic status and age.

Obesity and development of the metabolic syndrome is related to an increased parasympathetic tone and hyperinsulinemia. We have now studied the effects of age and metabolic status on glucose-induced insulin release stimulated by the neuropeptides vasoactive intestinal polypeptide (VIP; 10 nM) and pituitary adenylate cyclase activating polypeptide (PACAP; 10 nM), that are constituents of the parasympathetic nerves in the islets, and the cholinergic agonists acetylcholine (ACh; 10 microM) and carbachol (10 microM), in isolated islets from female obese ob/ob mice and lean mice. Both VIP and PACAP enhanced insulin secretion in islets from 4-week-old hyperglycemic ob/ob mice. VIP did not increase 11.1 mM glucose-induced insulin release in islets from 4-week-old lean normoglycemic mice and neither did PACAP in the absence of bicarbonate. The neuropeptides increased insulin release in islets from 9 to 10-month-old mice but VIP and PACAP had no effect in islets from very old mice. ACh had no effect in islets from 9 to 10-months and older ob/ob mice in the absence of bicarbonate. The combination of VIP and cholinergic agonists had an additive effect in islets from ob/ob mice, and PACAP combined with carbachol potentiated insulin release in islets from 4-week-old lean mice. VIP increased early phase insulin release in perifused islets from young mice. A higher concentration of theophylline was needed to potentiate glucose-induced insulin release in islets from young lean mice than in islets from old lean mice and ob/ob mice. The present results demonstrate age-related dynamics in the effects of neuropeptides affecting cAMP in pancreatic islets. We suggest that VIP and PACAP contribute to the developing metabolic syndrome in ob/ob mice by aggravating hyperinsulinemia.

Expression of the NK-1 receptor on islet cells and invading immune cells in the non-obese diabetic mouse.

The underlying mechanistic causes of immune cell infiltration in the islets of Langerhans and beta cell failure in the non-obese diabetic (NOD) mouse is still to be completely revealed. Substance P (SP) is a substance known to have pro-inflammatory, endocrine, neuromodulatory and trophic effects, and its preferred receptor, the neurokinin receptor 1 (NK-1 R), is reported to be involved in extravasation of granulocytes and in inflammation and tissue derangement. Therefore, we have investigated the expression of NK-1 R during development of insulitis in the NOD mouse. We show that the magnitude of immunoreactivity scoring NK-1 R expression in the islets was increased in the 12-week-old NOD mouse. Expression of NK-1 R co-localized with expression of glucagon. In line with this expression pattern, we did not detect any effect of SP on glucose-induced insulin release. NK-1 R expression was particularly observed in islet cells in association with the clusters of immune cells. Expression of NK-1 R was also demonstrated in a fraction of the infiltrating B and T lymphocytes, as well as on infiltrating macrophages and dendritic cells. The observations show that the level of NK-1 R expression is increased in 12-week-old NOD mice, being correlated with the occurrence of islet mononuclear infiltration. Our data suggest that SP may act as a chemoattractant, contributing to the pathogenic mononuclear infiltration process in the NOD mouse. On the whole, the observations suggest that SP and the NK-1 R to certain extents are involved in the changes that occur during the development of insulitis in the NOD mouse.

Remodeling of the innervation of pancreatic islets accompanies insulitis preceding onset of diabetes in the NOD mouse.

The innervation of the islets of Langerhans may constitute a first target for the autoimmunity that develops in type 1 diabetes. Here, we report the occurrence of a decrease in general innervation within the islets in the nonobese diabetic (NOD) mouse, and the establishment of strands of Schwann cells, as detected via p75 and S-100 immunoreactivity (IR), and varicose nerve fibers expressing tyrosine kinase A (TrkA) in association with the immune cells. The findings suggest that there are marked attempts for neurotrophins to promote nerve ingrowth and survival for islet tissue and that remodeling of innervation occurs in the continuation of the insulitis process preceding the onset of type 1 diabetes.

Effects of cholinergic m-receptor agonists on insulin release in islets from obese and lean mice of different ages: the importance of bicarbonate.

OBJECTIVES: Decreased beta-cell function is often observed in older individuals and may predispose to the development of type 2 diabetes. We have studied the age-related effects of M-receptor agonism on insulin release in islets isolated from female ob/ ob and lean mice. METHODS: Islets were challenged with 11.1 or 16.7 mmol/L glucose in media with HCO3/CO2 (KRBH) or without (KRH). RESULTS: Acetylcholine (ACh) (10 micromol/L) increased glucose-induced insulin release in islets from 4- to 5-week-old ob/ob mice both in KRBH and KRH. In islets from 9- to 13-month-old ob/ob mice, 10 micromol/L ACh and 10 micromol/L carbachol enhanced insulin release in KRBH but not in KRH. ACh increased insulin release in islets from 4- to 5-week-old and 16-month-old lean mice incubated in KRH but not in islets from 24-month-old lean mice. The Na/H exchange inhibitor dimethylamiloride (100 micromol/L) did not affect insulin release stimulated by M-receptor agonists. Carbachol did not enhance glucose-induced insulin secretion in islets from 9- to 10-month-old ob/ob mice in the presence of low extracellular Na concentration. ACh stimulated cytoplasmic Ca mobilization in islets from 9- to 10-month-old mice also when bicarbonate was omitted. The results suggest that cholinergic signal transduction involving extracellular bicarbonate and Na is reduced with age in mouse pancreatic islets. CONCLUSION: Chronic hyperglycemia may add to the age-related decrease in M-receptor-mediated insulin release by affecting the buffering capacity of the islets through mechanisms other than amiloride-sensitive proton exchange.

Effects of glass ionomers and dental resin composites on viability of beta-cells and insulin release in isolated islets of Langerhans.

Information on the biocompatibility of glass ionomers and resin composites is sparse. To extend the scale of biological testing we evaluated the influence of those materials on insulin secretion at whole organ level in vitro. The effects on insulin secretion of three glass ionomers and two resin composites, aged for 1 week, were studied in isolated mouse islets of Langerhans at basal (5.5mM) and at stimulatory (11.1mM) D-glucose concentrations. In addition, viability of single mouse beta-cells was evaluated. The effect of glass ionomer specimens aged for 1 and 4 months on insulin secretion at 11.1mM D-glucose was also studied. None of the materials affected the viability of the beta-cells. At 5.5mM D-glucose none of the materials affected the insulin secretion. At 11.1mM D-glucose, the glass ionomers only decreased the secretion and glass ionomers aged for 1 month still decreased insulin release whereas after 4 months ageing only one of the glass ionomers affected the release. The result shows a dynamic effect on insulin release of the elements and/or compounds released from the specimens.

Ryanodine receptors of pancreatic beta-cells mediate a distinct context-dependent signal for insulin secretion.

The ryanodine (RY) receptors in beta-cells amplify signals by Ca2+-induced Ca2+ release (CICR). The role of CICR in insulin secretion remains unclear in spite of the fact that caffeine is known to stimulate secretion. This effect of caffeine is attributed solely to the inhibition of cAMP-phosphodiesterases (cAMP-PDEs). We demonstrate that stimulation of insulin secretion by caffeine is due to a sensitization of the RY receptors. The dose-response relationship of caffeine-induced inhibition of cAMP-PDEs was not correlated with the stimulation of insulin secretion. Sensitization of the RY receptors stimulated insulin secretion in a context-dependent manner, that is, only in the presence of a high concentration of glucose. This effect of caffeine depended on an increase in [Ca2+]i. Confocal images of beta-cells demonstrated an increase in [Ca2+]i induced by caffeine but not by forskolin. 9-Methyl-7-bromoeudistomin D (MBED), which sensitizes RY receptors, did not inhibit cAMP-PDEs, but it stimulated secretion in a glucose-dependent manner. The stimulation of secretion by caffeine and MBED involved both the first and the second phases of secretion. We conclude that the RY receptors of beta-cells mediate a distinct glucose-dependent signal for insulin secretion and may be a target for developing drugs that will stimulate insulin secretion only in a glucose-dependent manner.

Effects of dental materials on insulin release from isolated islets of Langerhans.

OBJECTIVES: To investigate the possibility of using a whole organ model for evaluating the biological effects of dental restoration materials in vitro. METHODS: The effect on insulin release of isolated Langerhans islets of a series of dental materials was examined. The islets were incubated for 1h with extracts obtained from various dental materials and insulin was assayed radioimmunologically with crystalline mouse insulin. The results were analysed statistically using analysis of variance (ANOVA) and Scheffe's test at a significance level of p < 0.05. RESULTS: One dental ceramic, Vita VMK 95, significantly (p < 0.01) decreased the insulin release, whereas another dental ceramic, Empress (p < 0.01), a partly re-cast high-noble gold alloy (p < 0.001), a modified high-noble gold alloy (p < 0,05), and unalloyed copper (p < 0.001) significantly increased the release of insulin. SIGNIFICANCE: The results demonstrate a new instance of examining the biological effects of dental restoration materials. The method provides information about the effect of different materials on organ level in vitro that can complement other in vitro tests.