Glucose stimulates somatostatin secretion in pancreatic δ-cells by cAMP-dependent intracellular Ca2+ release.

Somatostatin secretion from pancreatic islet δ-cells is stimulated by elevated glucose levels, but the underlying mechanisms have only partially been elucidated. Here we show that glucose-induced somatostatin secretion (GISS) involves both membrane potential-dependent and -independent pathways. Although glucose-induced electrical activity triggers somatostatin release, the sugar also stimulates GISS via a cAMP-dependent stimulation of CICR and exocytosis of somatostatin. The latter effect is more quantitatively important and in mouse islets depolarized by 70 mM extracellular K+ , increasing glucose from 1 mM to 20 mM produced an ∼3.5-fold stimulation of somatostatin secretion, an effect that was mimicked by the application of the adenylyl cyclase activator forskolin. Inhibiting cAMP-dependent pathways with PKI or ESI-05, which inhibit PKA and exchange protein directly activated by cAMP 2 (Epac2), respectively, reduced glucose/forskolin-induced somatostatin secretion. Ryanodine produced a similar effect that was not additive to that of the PKA or Epac2 inhibitors. Intracellular application of cAMP produced a concentration-dependent stimulation of somatostatin exocytosis and elevation of cytoplasmic Ca2+ ([Ca2+]i). Both effects were inhibited by ESI-05 and thapsigargin (an inhibitor of SERCA). By contrast, inhibition of PKA suppressed δ-cell exocytosis without affecting [Ca2+]i Simultaneous recordings of electrical activity and [Ca2+]i in δ-cells expressing the genetically encoded Ca2+ indicator GCaMP3 revealed that the majority of glucose-induced [Ca2+]i spikes did not correlate with δ-cell electrical activity but instead reflected Ca2+ release from the ER. These spontaneous [Ca2+]i spikes are resistant to PKI but sensitive to ESI-05 or thapsigargin. We propose that cAMP links an increase in plasma glucose to stimulation of somatostatin secretion by promoting CICR, thus evoking exocytosis of somatostatin-containing secretory vesicles in the δ-cell.

Conditional hypovascularization and hypoxia in islets do not overtly influence adult ß-cell mass or function.

It is generally accepted that vascularization and oxygenation of pancreatic islets are essential for the maintenance of an optimal ß-cell mass and function and that signaling by vascular endothelial growth factor (VEGF) is crucial for pancreas development, insulin gene expression/secretion, and (compensatory) ß-cell proliferation. A novel mouse model was designed to allow conditional production of human sFlt1 by ß-cells in order to trap VEGF and study the effect of time-dependent inhibition of VEGF signaling on adult ß-cell fate and metabolism. Secretion of sFlt1 by adult ß-cells resulted in a rapid regression of blood vessels and hypoxia within the islets. Besides blunted insulin release, ß-cells displayed a remarkable capacity for coping with these presumed unfavorable conditions: even after prolonged periods of blood vessel ablation, basal and stimulated blood glucose levels were only slightly increased, while ß-cell proliferation and mass remained unaffected. Moreover, ablation of blood vessels did not prevent ß-cell generation after severe pancreas injury by partial pancreatic duct ligation or partial pancreatectomy. Our data thus argue against a major role of blood vessels to preserve adult ß-cell generation and function, restricting their importance to facilitating rapid and adequate insulin delivery.

KATP channel-deficient pancreatic beta-cells are streptozotocin resistant because of lower GLUT2 activity.

In wild-type mice, a single injection of streptozotocin (STZ, 200 mg/kg body wt) caused within 4 days severe hyperglycemia, hypoinsulinemia, significant glucose intolerance, loss of body weight, and the disappearance of pancreatic beta-cells. However, in ATP-sensitive K(+) channel (K(ATP) channel)-deficient mice (Kir6.2(-/-) mice), STZ had none of these effects. Exposing isolated pancreatic islets to STZ caused severe damage in wild-type but not in Kir6.2(-/-) islets. Following a single injection, plasma STZ levels were slightly less in Kir6.2(-/-) mice than in wild-type mice. Despite the difference in plasma STZ, wild-type and Kir6.2(-/-) liver accumulated the same amount of STZ, whereas Kir6.2(-/-) pancreas accumulated 4.1-fold less STZ than wild-type pancreas. Kir6.2(-/-) isolated pancreatic islets also transported less glucose than wild-type ones. Quantification of glucose transporter 2 (GLUT2) protein content by Western blot using an antibody with an epitope in the extracellular loop showed no significant difference in GLUT2 content between wild-type and Kir6.2(-/-) pancreatic islets. However, visualization by immunofluorescence with the same antibody gave rise to 32% less fluorescence in Kir6.2(-/-) pancreatic islets. The fluorescence intensity using another antibody, with an epitope in the COOH terminus, was 5.6 times less in Kir6.2(-/-) than in wild-type pancreatic islets. We conclude that 1) Kir6.2(-/-) mice are STZ resistant because of a decrease in STZ transport by GLUT2 in pancreatic beta-cells and 2) the decreased transport is due to a downregulation of GLUT2 activity involving an effect at the COOH terminus.