Islet cannabinoid receptors: cellular distribution and biological function.

OBJECTIVES: This study aimed to determine the cellular distribution of islet cannabinoid receptors (CBs) and their involvement in the development of metabolic and hormonal changes in rats fed a fructose-rich diet (F). METHODS: In normal rat islets, we determined CBs (immunofluorescence and retrotranscription-polymerase chain reaction) and glucose-stimulated insulin secretion (GSIS) of isolated islets incubated with the CB1 antagonist rimonabant (R) and/or different CBs agonists. In 3-week F-fed rats, we determined the in vivo effect of R on serum glucose, triglyceride, and insulin levels; homeostasis model assessment for insulin resistance, GSIS, and CBs and insulin receptor substrate gene expression levels (real-time polymerase chain reaction). RESULTS: Cannabinoid receptors appeared exclusively in islet α cells. Whereas different CB agonists enhanced GSIS in normal rat islets, R did not affect it. F rats had higher serum triglyceride and insulin levels and homeostasis model assessment for insulin resistance than control rats; these alterations were prevented by R coadministration. Although R did not correct the increased GSIS observed in F islets, it modulated CBs and insulin receptor substrate gene expression. CONCLUSIONS: Islet CBs would exert an important modulatory role in metabolic homeostasis. Administration of R and F affected islet CB expression and prevented the development of F-induced metabolic impairment. Selective islet CB1 blockers could be useful to prevent/treat the alterations induced by the intake of unbalanced/unhealthy diets.

Changes in islet plasma membrane calcium-ATPase activity and isoform expression induced by insulin resistance.

We studied the effect of insulin resistance (IR) induced by administration of a fructose-rich diet (FRD) to normal Wistar rats for 21days, upon islet plasma membrane calcium ATPases (PMCAs) and insulin secretion. FRD rats showed significantly higher triglyceride and insulin levels, insulin:glucose ratio and HOMA-IR index than controls. FRD islets released significantly more insulin in response to glucose and showed (a) marked changes in PMCA isoform protein content (decreased PMCA 2 and increased PMCA 3), (b) a decrease in total PMCAs activity, and (c) higher levels of cytosolic calcium [Ca(2+)](i). The lower PMCAs activity with the resultant increase in [Ca(2+)](i) would favor the compensatory greater release of insulin necessary to cope with the IR state present in FRD rats and to maintain normal glucose homeostasis. Thus, changes in PMCAs activity and isoform expression play a modulatory role upon insulin secretion during long-term adaptation to an increased hormone demand.

Islet neogenesis-associated protein pentadecapeptide (INGAP-PP): mechanisms involved in its effect upon beta-cell mass and function.

The effect of islet neogenesis-associated protein pentadecapeptide (INGAP-PP) administration to normal male hamsters upon serum glucose and triglyceride levels, beta-cell mass and function was studied. INGAP-PP (500 mug) or saline was injected twice daily during 10 days. Both groups showed comparable body weight, serum glucose and triglyceride levels. INGAP-PP treated animals had significantly higher HOMA-IR and HOMA-beta and their islets released more insulin in response to glucose; they had lower islet DNA content, significantly increased number of islets/unit area, beta-cell replication rate and mass, cells co-expressing Pdx-1/INGAP and islets in contact with ducts, and decreased beta-cell apoptosis rate. The percentage of cells expressing Pdx-1 alone or together with INGAP or insulin increased significantly in ducts. These animals also showed a significantly higher concentration of Pdx-1 and Ngn-3 mRNA and a lower number of INGAP-positive cells. In conclusion, INGAP-PP promoted a controlled and functionally active increase of beta-cell mass; our data demonstrate for the first time the mechanism responsible for such changes; that Ngn-3 would be involved in INGAP-PP-induced neogenesis; and the existence of a negative feedback loop with endogenous INGAP-producing cells. Accordingly, INGAP-PP could be used to induce these effects in people with or at risk of developing diabetes.

Quinolizidine alkaloids isolated from Lupinus species enhance insulin secretion.

We have analyzed the effect of quinolizidine alkaloids from Lupinus species upon insulin secretion. Isolated normal rat islets were incubated with 3.3, 8.3, and 16.7 mM glucose, in the presence or absence of different concentrations of lupanine (0.05, 0.5, and 1.0 mM), 13-alpha-OH lupanine, 17-oxo-lupanine, and 2-thionosparteine. Insulin release was measured by radioimmunoassay. While 2-thionosparteine enhanced insulin secretion at all glucose concentrations, lupanine did at 8.3 and 16.7 mM, and 13-alpha-OH lupanine or 17-oxo-lupanine only at 16.7 mM glucose. Diazoxide (0.1 mM) decreased the effect of all alkaloids, without suppressing it completely. Consequently, blockage of beta-cell K(ATP)-sensitive channels is at least one of the mechanisms involved in the enhancing secretagogue effects of quinolizidine alkaloids. The fact that 13-alpha-OH lupanine and 17-oxo-lupanine only exert their secretagogue effect at high glucose concentrations could be of additional value when considering their potential use in the treatment of type 2 diabetes.