Molecular and functional characterization of pituitary adenylate cyclase-activating polypeptide (PACAP-38)/vasoactive intestinal polypeptide receptors in pancreatic beta-cells and effects of PACAP-38 on components of the insulin secretory system.

It has been previously demonstrated that pituitary adenylate cyclase-activating polypeptide (PACAP) regulates insulin secretion. PACAP exerts its biological action by binding to at least three different receptor subtypes coupled to different signal transduction mechanisms. The signaling pathways underlying the insulinotropic effect of PACAP involve mainly the activation of adenylate cyclase to form cAMP, which directly and indirectly, through increased intracellular Ca2+, stimulates insulin exocytosis. In the present study we have characterized the functional and molecular expression of PACAP/vasoactive intestinal polypeptide receptors isoforms and subtypes and its isoforms in a beta-cell line and in isolated rat pancreatic islets. Although insulinoma cells express the messenger RNA encoding PAC1 (-R and -hop variants), VPAC1 and VPAC2, binding experiments indicate the preponderance of PAC1 over VPAC 1-2 receptors. We have also shown that the main signaling pathway of PACAP in beta-cells is mediated by adenylate cyclase, whereas the inositol 1,4,5-trisphosphate pathway is almost inactive. Furthermore, we have demonstrated that PACAP exerts long-term effects on beta-cells, such as transcriptional regulation of the insulin gene and genes of the glucose-sensing system (GLUT1 and hexokinase 1).

GLUT2 and glucokinase expression is coordinately regulated by sulfonylurea.

In the present study we examined the effect of sulfonylurea on the expression of the glucose transporter GLUT2 and the glucose phosphorylating enzyme Glucokinase (GK) in betaTC6-F7 cells; furthermore, we studied the modifications induced by sulfonylurea on glucose-responsiveness and -sensitivity. Results demonstrate that sulfonylurea increases GLUT2 and GK mRNA expression after 24 h in a dose-dependent manner. On the contrary, after 48 and 72 h a time-dependent reduction of both GLUT2 and GK mRNA occurs. GLUT2 and GK protein expression follow the same modifications. Therefore, GLUT2 and GK are coordinately regulated by sulfonylurea, probably by a common mechanism. Glucose-induced insulin release is increased by sulfonylurea as well as glucose sensitivity. Our study suggests that short-term effect of sulfonylurea increases while long-term effect reduces the expression of glucose sensing elements. The long-term inhibitory effect on glucose sensing elements would explain the reduced insulin secretion occurring after chronic sulfonylurea treatment.

Dose-dependent effect of octreotide on insulin secretion after OGTT in obesity.

OBJECTIVE: The present study aimed at evaluating the acute effect of increasing doses of octreotide (OCT), a long-acting somatostatin analogue, on glucose tolerance and insulin secretion. METHODS: A standard and two other oral glucose tolerance tests 30 min after subcutaneous administration of OCT were performed in randomized order in each subject. Obese subjects received 10, 25, or 50 microg of OCT; control subjects received only 10 and 25 microg. Fifteen obese and 10 control subjects were studied; all of them had a normal glucose tolerance. Plasma glucose and insulin levels were measured at times -30, 0, 30, 60, 90, 120, 150, and 180 min after the glucose tolerance test. RESULTS: The results demonstrated that, following OCT administration, both control and obese subjects developed a reduced glucose tolerance, a delayed glycemic peak, and an increase of late plasma glucose values. Fasting as well as stimulated insulin secretions were higher in obese subjects as compared with controls, and insulin secretion was inhibited in a dose-dependent manner by OCT. CONCLUSIONS: These data indicate that the action of OCT might be due to at least two different cooperative mechanisms: (1) a delayed glucose absorption, as suggested by the delay of glycemic peak, and (2) a direct or vagal-mediated effect on beta-cells, as suggested by the reduction of the area under the curve values in spite of the elevated late glycemic levels. It is noteworthy that doses of OCT as low as 10 and 25 microg are sufficient to inhibit insulin secretion both in normal and obese subjects.

The effects of pregnancy steroids on adaptation of beta cells to pregnancy involve the pancreatic glucose sensor glucokinase.

Pregnancy is associated with adaptive changes including increased number and size of beta cells and enhanced gap-junctional coupling among beta cells, increased glucose-induced insulin response and decreased glucose stimulation threshold. The role exerted by pregnancy steroids and lactogenic hormones in the development of islets upregulation during pregnancy has been widely investigated. In the present study we studied the possibility that pregnancy steroids induce functional modifications of beta cells involving the expression and function of glucokinase. Our results indicate that estradiol and progesterone do not influence significantly glucokinase mRNA expression, while they induce a dose-dependent and time-dependent increase of glucokinase activity in RIN 1046-38 cells. The increased enzymatic activity results in an increased glucose-induced insulin release. Therefore it is possible to hypothesize that pregnancy steroids influence glucokinase expression in beta cells at a post-transcriptional level and that this effect contributes to the development of hyperinsulinemia during pregnancy.

Effect of biotin on glucokinase activity, mRNA expression and insulin release in cultured beta-cells.

Biotin is known to influence hepatic glucokinase (GK) expression both at a transcriptional and at a translational level. The aim of the present paper was to investigate the effect of biotin on pancreatic GK. For this purpose, RIN1046-38 cells were cultured in the presence of different biotin concentrations for different times; there-after, GK mRNA expression, GK activity and insulin release were studied. Results demonstrated that biotin has a biphasic effect on GK mRNA expression, being stimulatory after short-term treatment and inhibitory after longterm treatment. GK activity was increased after long-term treatment. Insulin release was not affected by biotin treatment. These data suggest that biotin may influence glucose metabolism also by acting directly at the level of beta-cells.

Quantitative analysis of pancreatic glucokinase gene expression in cultured beta cells by competitive polymerase chain reaction.

Regulation of glucokinase (GK) gene expression in pancreatic beta cells has been poorly investigated, both due to low abundance of the gene and to difficulties in cells isolation. The present study describes the establishment of a competitive RT-PCR method for quantitative analysis of GK gene. The method has been applied to the analysis of GK mRNA expression RIN 1046-38 cells. We have monitored modifications of GK mRNA expression after different periods of time in culture and we have studied the effect induced by dexamethasone (DEX) treatment. We show that the method is very sensitive and requires very low amount of RNA. Data demonstrate that GK mRNA expression in RIN cells is reduced as a function of passages in culture and that the reduction is positively correlated with the decrease of insulin responsiveness observed in high passages cells. DEX treatment inhibits GK mRNA expression in RIN cells in a dose-dependent and time-dependent manner.

[Utility of electrophysiologic study using the blink reflex and brainstem evoked potentials for the evaluation of the course of uremic polyneuropathy]. / Utilità dello studio elettrofisiologico mediante blink reflex e potenziali evocati del tronco encefalico (BAEPS), per la valutazione dell'evoluzione della polineuropatia uremica.

On the present study the authors evaluate the utility of electrophysiologic examination in uraemic polyneuropathy. A group of 19 uraemic patients in chronic dialysis underwent the Blink reflex and BAEPSs study to evaluate the alterations of nervous pathways. The results obtained were compared with those of a group of 10 healthy patients comparable for age and sex. The electrophysiologic parameters have been statistically compared with the plasma levels of vit. B12. folic acid, PTH and beta-2-microglobulin. The results show a significant difference of uremic patients compared with the healthy ones for the Blink reflex (ipsilateral and contralateral R2 responses). Also BAEPSs show significant alterations in the uraemic group (latencies of the III, V components). A statistically significantly inverse correlation is present between folic acid values and blink reflex R1 and R2 responses. Therefore our study shows the existence of a combined degeneration of central and peripheral nervous pathways in chronic uraemic patients. We believe that the decrease in folic acid concentration found in our study may be one of the causes of the beginning and then of the worsening of neurologic damage.

DBI mRNA is expressed in endocrine pancreas and its post-translational product DBI(33-50) inhibits insulin release.

It has been previously demonstrated that DBI is present in endocrine pancreas and it is able to inhibit insulin release in isolated rat islets. Its mechanism of action has been investigated, demonstrating the possible involvement of cAMP and ATP-dependent K(+) channels. DB1(33-50), a post-translational product of DBI, is also able to inhibit insulin release, but its action has not been characterized. In the present study, we have investigated the presence of DBI mRNA in pancreas, islets and cultured ß cells. The possible mechanism of action of DBI(33-50) and the involvement of BZ/GABA(A) receptors has been studied.