The autoregulation of insulin secretion in the isolated pancreatic islets of lean (obOb) and obese-hyperglycemic (obob) mice

High Seeding Density Induces Local Hypoxia and Triggers a Proinflammatory Response in Isolated Human Islets

Hypoxia is the main threat to morphological and functional integrity of isolated pancreatic islets. Lack of oxygen seems to be of particular importance for functionality of encapsulated islets. The present study was initiated as an experimental model for the environment experienced by human islets in a confined space present during culture, shipment, and in an implanted macrodevice. Quadruplicate aliquots of isolated human islets (n = 12) were cultured for 24 h at 37°C under normoxic conditions using 24-well plates equipped with 8-µm pore size filter inserts and filled with islet aliquots adjusted to obtain a seeding density of 75, 150, 300, or 600 IEQ/cm(2). After culture viability, glucose-stimulated insulin release, DNA content as well as Bax and Bcl-2 gene expression were measured. Culture supernatants were collected to determine production of VEGF and MCP-1. Viability correlated inversely with IEQ seeding density (r = -0.71, p < 0.001), while the correlation of VEGF and MCP-1 secretion with seeding density was positive (r = 0.78, p < 0.001; r = 0.54, p < 0.001). Decreased viability corresponded with a significant increase in the Bax/Bcl-2 mRNA ratio at 300 and 600 IEQ/cm(2) and with a sigificantly reduced glucose-stimulated insulin secretion and insulin content compared to 75 or 150 IEQ/cm(2) (p < 0.01). The present study demonstrates that the seeding density is inversely correlated with islet viability and in vitro function. This is associated with a significant increase in VEGF and MCP-1 release suggesting a hypoxic and proinflammatory islet microenvironment.

Insulin contributes to fine-tuning of the pancreatic beta-cell response to glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) stimulates insulin secretion from pancreatic β-cells in a glucose-dependent manner. However, factors other than glucose that regulate the β-cell response to GLP-1 remain poorly understood. In this study, we examined the possible involvement of insulin and receptor tyrosine kinase signaling in regulation of the GLP-1 responsiveness of β-cells. Pretreatment of β-cells with HNMPA, an insulin receptor inhibitor, and AG1478, an epidermal growth factor receptor inhibitor, further increased the cAMP level and Erk phosphorylation in the presence of exendin-4 (exe-4), a GLP-1 agonist. When β-cells were exposed to a high concentration of glucose (25 mM), which stimulates insulin secretion, exe-4-induced cAMP formation declined gradually as exposure time was increased. This decreased cAMP formation was not observed in the presence of HNMPA. HNMPA was able to further increase the exe-4-induced insulin secretion when β-cells were exposed to high glucose for 18 h. Treatment of β-cells with insulin significantly decreased exe-4-induced cAMP formation in a dose-dependent manner. Lowering the phospho-Akt level by HNMPA or LY294002, a PI3K inhibitor, further augmented exe-4-induced cAMP formation and Erk phosphorylation. These results suggest that insulin contributes to fine-tuning of the β-cell response to GLP-1.

Stevioside does not cause increased basal insulin secretion or β-cell desensitization as does the sulphonylurea, glibenclamide: Studies in vitro

We have shown that stevioside (SVS) enhances insulin secretion and thus may have a potential role as antihyperglycemic agent in the treatment of type 2 diabetes mellitus. However, whether SVS stimulates basal insulin secretion (BIS) and/or cause desensitization of beta cells like sulphonylureas (SU), e.g. glibenclamide (GB), is not known. To explore and compare the effects of SVS pretreatment with those of GB and glucagon-like peptide-1 (GLP-1), we exposed isolated mouse islets to low or high glucose for 1 h after short-term (2 h) or long-term (24 h) pretreatment with SVS, GB or GLP-1, respectively. BIS at 3.3 or 5.5 mM glucose were not changed after short-term pretreatment with SVS (10(-7) M), while it increased about three folds after pretreatment with GB (10(-7) M). Glucose stimulated insulin secretion (GSIS) (16.7 mM) increased dose-dependently after long-term pretreatment with SVS at concentrations from 10(-7) to 10(-5) M. Pretreatment for 24 h with GB (10(-7) M) increased the subsequent BIS (3.3 mM glucose) (p < 0.001), but decreased GSIS (16.7 mM glucose) (p < 0.001). In contrast SVS (10(-7) M) and GLP-1 (10(-7) M) did not stimulate BIS but both enhanced the subsequent GSIS (16.7 mM glucose) (p < 0.05 and p < 0.05, respectively). While SVS pretreatment increased the intracellular insulin content, GB pretreatment decreased the insulin content. Our study suggests that SVS pretreatment does not cause a stimulation of BIS and does not desensitize beta-cells, i.e. SVS seems to have advantageous characteristics to GB as a potential treatment of type 2 diabetes.

Young, low-birth-weight men are not more susceptible to the diabetogenic effects of a prolonged free fatty acid exposure than matched controls

Low birth weight (LBW) is associated with increased risk of developing type 2 diabetes later in life. Progression from normal to impaired glucose tolerance and overt diabetes may depend, to some extent, on elevation of plasma free fatty acids (FFAs). We undertook this study to elucidate whether a prolonged physiological lipid load could unmask or augment existing metabolic defects in otherwise healthy young LBW subjects. Forty 19-year-old men (LBW [n = 20], controls [normal birth weight, NBW] [n = 20]) without a family history of diabetes underwent an intravenous glucose tolerance test (0.3 g kg(-1)), followed by 2-step hyperinsulinemic-euglycemic clamps (2 x 120 minutes: 10 and 40 mU m(-2) min(-1)) in combination with [3-3H]-glucose and indirect calorimetry. The tests were preceded, in randomized order, by a 24-hour continuous intralipid (20%, 0.4 mg mL(-1) h(-1)) or saline infusion. Estimates of cellular glucose metabolism were obtained and a disposition index calculated. Clamp FFA concentrations were 4- to ten-fold higher during lipid infusion. Both groups experienced a similar decrease in insulin-stimulated glucose disposal in response to lipid infusion (approximately 15%; P < .05), which was mainly accounted for by reduced glucose oxidation (approximately 30%; P < .001). Glycolysis, glucose storage, and glucose production were not significantly altered by lipid infusion. Nevertheless, the LBW group had significantly lower insulin-stimulated glycolysis during lipid infusion (approximately 27%; P < .05) than the NBW group. An appropriate increase in insulin secretion matched the decline in insulin sensitivity in both groups. A 24-hour low-grade intralipid infusion has similar effects on whole-body glucose metabolism and first-phase insulin secretion in 19-year-old, healthy, lean, LBW men with normal glucose tolerance and in NBW controls. We reproduced our previous finding of lower insulin-stimulated glycolysis in this population.

Insulin and secretagogues differentially regulate fluid-phase pinocytosis in insulin-secreting beta-cells

The physiological role of the beta-cell insulin receptor is unknown. To evaluate a candidate function, the insulin regulation of fluid-phase pinocytosis was investigated in a clonal insulinoma cell line (beta TC6-F7) and, for comparison, also in Chinese hamster ovary cells transfected with the human insulin receptor (CHO-T cells). In CHO-T cells, the net rate of fluid-phase pinocytosis was rapidly increased 3-4-fold over the basal rate by 100 nM insulin, with half-maximal stimulation at 2 nM insulin, as assayed by cellular uptake of horseradish peroxidase from the medium. Wortmannin, an inhibitor of phosphatidylinositol (PI)-3-kinase, blocked insulin-stimulated pinocytosis with an IC50 of 7.5 nM without affecting the basal rate of pinocytosis. In insulin-secreting beta TC6-F7 cells, the secretagogues glucose and carbachol (at maximally effective concentrations of 15 mM and 0.5 mM respectively) augmented fluid-phase pinocytosis 1.65-fold over the basal rate. Wortmannin also inhibited secretagogue-stimulated pinocytosis in these beta-cells with an IC50 of 7 nM but did not affect the basal rate of pinocytosis measured in the absence of secretagogues. Wortmannin did not influence either basal or secretagogue-induced insulin secretion. Although these beta TC6-F7 cells have cell-surface insulin receptors, adding exogenous insulin or insulin-like growth factor 1 did not affect their rate of fluid-phase pinocytosis, either in the absence or presence of secretagogues. From these observations, we conclude that: (1) in both insulin-secreting beta-cells and in conventional, insulin-responsive CHO-T cells, a common, wortmannin-sensitive reaction, which probably involves PI-3-kinase, regulates fluid-phase pinocytosis; (2) the insulin-receptor signal transduction pathway is dissociated from the regulation of fluid-phase pinocytosis in the insulin-secreting beta-cell line we studied; and (3) the enhancement of fluid-phase pinocytosis associated with secretagogue-induced insulin release in beta TC6-F7 cells is not attributable to autocrine activation of beta-cell surface insulin receptors.

Comparison of the properties of the ATP-sensitive K+ channels of pancreatic beta-cells of lean and obese (ob/ob) C57BL/6J mice

Cultures of pancreatic islet cells from obese and lean mice of the C57BL/6J strain were established and their secretory response to glucose stimulation was measured. Insulin secretion (as % of total cellular insulin content) from the cells of the obese mouse cultures was significantly higher than from lean mouse cells. The properties of the glucose- and ATP-sensitive potassium channels present in these cultured beta-cells were compared using the cell-attached and the inside-out configurations of the patch-clamp technique. The channels of both types of mouse were indistinguishable in terms of conductance, ionic selectivity, kinetic behavior, voltage dependence or sensitivity to glucose, ATP and ADP. It is concluded that the depolarized state and the hypersecretory response of obese mouse beta-cells are not related to an altered behavior of their ATP-sensitive potassium channels.

Direct effect of insulin and insulin-like growth factor-I on the secretory activity of rat pancreatic beta cells

Purified pancreatic Beta cells were labelled with 3H-tyrosine before studying their secretory activity in perifusion. At 1.4 mmol/l glucose, the cells released similar fractions (0.01% per min) of their contents in preformed and in newly formed insulin. At 20 mmol/l glucose plus 10(-8) mol/l glucagon, these fractional release rates increased by 16 and 40-fold respectively. The preferential release of newly synthesized as compared to stored insulin is attributable to a heterogeneity in individual cell responses. The secretory responsiveness to glucose plus glucagon was completely suppressed by 10(-7) mol/l clonidine. Insulin induced a 20% reduction at 10(-6) mol/l, but remained without effect at 10(-7) mol/l. Insulin-like growth factor-I provoked a 30% decrease at 5.10(-9) mol/l. It is concluded that the type-I insulin-like growth factor receptors on pancreatic Beta cells mediate a suppressive action on the insulin release process. Their high affinity for insulin-like growth factor-I allows physiologic levels of this peptide to participate in the regulation of insulin release. Their low affinity for insulin provides the basis for a minor feedback action by this hormone at concentrations exceeding the normal circulating levels.

Feedback inhibition by biosynthetic human insulin of insulin release in healthy human subjects

To determine the impact of biosynthetic human insulin (BHI) on endogenous insulin release, splanchnic output and arterial concentrations of C-peptide were measured in eight healthy men after intravenous administration of 0, 0.5, 1.25, U BHI . m-2 . h-1 for 70 min each. Euglycemia was maintained by a variable glucose infusion. Arterial levels of serum insulin were 48 +/- 6 pmol/liter before and 135 +/- 12, 265 +/- 18, and 593 +/- 47 pmol/liter after BHI infusion. Splanchnic C-peptide output was reduced by BHI infusion from 88 +/- 10 pmol/min before to 50 +/- 9, 28 +/- 10, and 18 +/- 16 pmol/min (P less than 0.0025). Simultaneously, arterial concentrations of C-peptide fell from 539 +/- 54 pmol/liter by 29 and 43% when 1.25 and 2.5 U . m-2 . h-1 of BHI were administered. Hepatic insulin uptake was directly related with BHI infusion rate (r = 0.88) and rose during BHI administration from a basal value of 58 +/- 7 to an uptake of 265 +/- 31 pmol/min when 2.5 U . m-2 . h-1 were infused (P less than 0.0005). Basal hepatic insulin clearance was 4.75 +/- 0.60 ml . kg-1 . min-1 and remained unchanged after BHI infusion as did hepatic fractional extraction of insulin, which was 61 +/- 4% in the basal state. Metabolic clearance rate of immunoreactive insulin (MCRi) was dose-dependently reduced by BHI infusion, whereas the relative share of hepatic insulin clearance in total MCRi rose simultaneously (P less than 0.01). We conclude that feedback inhibition of endogenous insulin release may play an important role in vivo. Furthermore, it appears that nonhepatic insulin degradation is a saturable phenomenon as total MCRi fell in the presence of its unchanged hepatic clearance rate after the infusion of large amounts of BHI.

The serum glucose response to glucagon suppression with somatostatin, insulin or antiglucagon serum in depancreatized rats

Total immunoreactive glucagon (IRG) and immunoreactive glucagon of A cell origin (IRGa) were measured in the serum of normal, sham-operated and depancreatized rats, after the administration of three glucagon antagonists: insulin (5--200 mU/rat/h), somatostatin (SRIF; 100 microgram/kg/h) and antiglucagon serum (AGS, enough to bind three times the calculated total amount of circulating IRG). Since no differences were noted between the responses of normal and sham-operated animals, the values were pooled and used as controls. Pancreatectomy caused a significant increase in serum glucose, IRGa and total IRG and a significant decrease in serum insulin. AGS and SRIF significantly decreased serum glucose in control, but not in depancreatized rats, even though SRIF caused a significant decrease of IRGa in all animals. SRIF significantly decreased plasma insulin in control rats, but did not modify total IRG secretion in either groups. In control rats the minimum effective hypoglycaemic dose of insulin (5 mU/rat/h) may have decreased serum IRGa, but not total IRG. At higher doses (20 mU/rat/h) insulin stimulated glucagon secretion. In depancreatized animals, higher doses of insulin (200 mU/rat/h) were needed to lower serum glucose. On the other hand, a dose of 100 muU/rat/h was sufficient to lower the serum IRG. We conclude that although hyperglucagonaemia may contribute to the hyperglycaemia of the untreated depancreatized rats, the excessive secretion of glucagon is secondary to insulin insufficiency and that, at least in this animal model, the hypoglycaemic action of insulin is only minimally dependent upon its ability to suppress glucagon secretion.