Streptozotocin diabetes: a glucoreceptor dysfunction affecting D cells as well as B and A cells

Pharmacologic inhibition of somatostatin receptor 2 to restore glucagon counterregulation in diabetes

Glucose homeostasis is primarily maintained by pancreatic hormones, insulin and glucagon, with an emerging role for a third islet hormone, somatostatin, in regulating insulin and glucagon responses. Under healthy conditions, somatostatin secreted from pancreatic islet δ-cells inhibits both insulin and glucagon release through somatostatin receptor- induced cAMP-mediated downregulation and paracrine inhibition of β- and α-cells, respectively. Since glucagon is the body's most important anti-hypoglycemic hormone, and because glucagon counterregulation to hypoglycemia is lost in diabetes, the study of somatostatin biology has led to new investigational medications now in development that may help to restore glucagon counterregulation in type 1 diabetes. This review highlights the normal regulatory role of pancreatic somatostatin signaling in healthy islet function and how the inhibition of somatostatin receptor signaling in pancreatic α-cells may restore normal glucagon counterregulation in diabetes mellitus.

Gap junction coupling and islet delta-cell function in health and disease

The pancreatic islets contain beta-cells and alpha-cells, which are responsible for secreting two principal gluco-regulatory hormones; insulin and glucagon, respectively. However, they also contain delta-cells, a relatively sparse cell type that secretes somatostatin (SST). These cells have a complex morphology allowing them to establish an extensive communication network throughout the islet, despite their scarcity. Delta-cells are electrically excitable cells, and SST secretion is released in a glucose- and K_ATP-dependent manner. SST hyperpolarises the alpha-cell membrane and suppresses exocytosis. In this way, islet SST potently inhibits glucagon release. Recent studies investigating the activity of delta-cells have revealed they are electrically coupled to beta-cells via gap junctions, suggesting the delta-cell is more than just a paracrine inhibitor. In this Review, we summarize delta-cell morphology, function, and the role of SST signalling for regulating islet hormonal output. A distinguishing feature of this Review is that we attempt to use the discovery of this gap junction pathway, together with what is already known about delta-cells, to reframe the role of these cells in both health and disease. In particular, we argue that the discovery of gap junction communication between delta-cells and beta-cells provides new insights into the contribution of delta-cells to the islet hormonal defects observed in both type 1 and type 2 diabetes. This reappraisal of the delta-cell is important as it may offer novel insights into how the physiology of this cell can be utilised to restore islet function in diabetes.

Free fatty acid receptor 4 inhibitory signaling in delta cells regulates islet hormone secretion in mice

OBJECTIVE

Maintenance of glucose homeostasis requires the precise regulation of hormone secretion from the endocrine pancreas. Free fatty acid receptor 4 (FFAR4/GPR120) is a G protein-coupled receptor whose activation in islets of Langerhans promotes insulin and glucagon secretion and inhibits somatostatin secretion. However, the contribution of individual islet cell types (α, β, and δ cells) to the insulinotropic and glucagonotropic effects of GPR120 remains unclear. As gpr120 mRNA is enriched in somatostatin-secreting δ cells, we hypothesized that GPR120 activation stimulates insulin and glucagon secretion via inhibition of somatostatin release.

METHODS

Glucose tolerance tests were performed in mice after administration of selective GPR120 agonist Compound A. Insulin, glucagon, and somatostatin secretion were measured in static incubations of isolated mouse islets in response to endogenous (ω-3 polyunsaturated fatty acids) and/or pharmacological (Compound A and AZ-13581837) GPR120 agonists. The effect of Compound A on hormone secretion was tested further in islets isolated from mice with global or somatostatin cell-specific knock-out of gpr120. Gpr120 expression was assessed in pancreatic sections by RNA in situ hybridization. Cyclic AMP (cAMP) and calcium dynamics in response to pharmacological GPR120 agonists were measured specifically in α, β, and δ cells in intact islets using cAMPER and GCaMP6 reporter mice, respectively.

RESULTS

Acute exposure to Compound A increased glucose tolerance, circulating insulin, and glucagon levels in vivo. Endogenous and/or pharmacological GPR120 agonists reduced somatostatin secretion in isolated islets and concomitantly demonstrated dose-dependent potentiation of glucose-stimulated insulin secretion and arginine-stimulated glucagon secretion. Gpr120 was enriched in δ cells. Pharmacological GPR120 agonists reduced cAMP and calcium levels in δ cells but increased these signals in α and β cells. Compound A-mediated inhibition of somatostatin secretion was insensitive to pertussis toxin. The effect of Compound A on hormone secretion was completely absent in islets from mice with either global or somatostatin cell-specific deletion of gpr120 and partially reduced upon blockade of somatostatin receptor signaling by cyclosomatostatin.

CONCLUSIONS

Inhibitory GPR120 signaling in δ cells contributes to both insulin and glucagon secretion in part by mitigating somatostatin release.

The somatostatin-secreting pancreatic δ-cell in health and disease

The somatostatin-secreting δ-cells comprise ~5% of the cells of the pancreatic islets. The δ-cells have complex morphology and might interact with many more islet cells than suggested by their low numbers. δ-Cells contain ATP-sensitive potassium channels, which open at low levels of glucose but close when glucose is elevated. This closure initiates membrane depolarization and electrical activity and increased somatostatin secretion. Factors released by neighbouring α-cells or β-cells amplify the glucose-induced effects on somatostatin secretion from δ-cells, which act locally within the islets as paracrine or autocrine inhibitors of insulin, glucagon and somatostatin secretion. The effects of somatostatin are mediated by activation of somatostatin receptors coupled to the inhibitory G protein, which culminates in suppression of the electrical activity and exocytosis in α-cells and β-cells. Somatostatin secretion is perturbed in animal models of diabetes mellitus, which might explain the loss of appropriate hypoglycaemia-induced glucagon secretion, a defect that could be mitigated by somatostatin receptor 2 antagonists. Somatostatin antagonists or agents that suppress somatostatin secretion have been proposed as an adjunct to insulin therapy. In this Review, we summarize the cell physiology of somatostatin secretion, what might go wrong in diabetes mellitus and the therapeutic potential of agents targeting somatostatin secretion or action.

The somatostatin-secreting pancreatic δ-cell in health and disease

The somatostatin-secreting δ-cells comprise ~5% of the cells of the pancreatic islets. The δ-cells have complex morphology and might interact with many more islet cells than suggested by their low numbers. δ-Cells contain ATP-sensitive potassium channels, which open at low levels of glucose but close when glucose is elevated. This closure initiates membrane depolarization and electrical activity and increased somatostatin secretion. Factors released by neighbouring α-cells or β-cells amplify the glucose-induced effects on somatostatin secretion from δ-cells, which act locally within the islets as paracrine or autocrine inhibitors of insulin, glucagon and somatostatin secretion. The effects of somatostatin are mediated by activation of somatostatin receptors coupled to the inhibitory G protein, which culminates in suppression of the electrical activity and exocytosis in α-cells and β-cells. Somatostatin secretion is perturbed in animal models of diabetes mellitus, which might explain the loss of appropriate hypoglycaemia-induced glucagon secretion, a defect that could be mitigated by somatostatin receptor 2 antagonists. Somatostatin antagonists or agents that suppress somatostatin secretion have been proposed as an adjunct to insulin therapy. In this Review, we summarize the cell physiology of somatostatin secretion, what might go wrong in diabetes mellitus and the therapeutic potential of agents targeting somatostatin secretion or action.

Cafestol, a Bioactive Substance in Coffee, Has Antidiabetic Properties in KKAy Mice

Daily coffee consumption is inversely associated with risk of type-2 diabetes (T2D). Cafestol, a bioactive substance in coffee, increases glucose-stimulated insulin secretion in vitro and increases glucose uptake in human skeletal muscle cells. We hypothesized that cafestol can postpone development of T2D in KKAy mice. Forty-seven male KKAy mice were randomized to consume chow supplemented daily with either 1.1 (high), 0.4 (low), or 0 (control) mg of cafestol for 10 weeks. We collected blood samples for fasting glucose, glucagon, and insulin as well as liver, muscle, and fat tissues for gene expression analysis. We isolated islets of Langerhans and measured insulin secretory capacity. After 10 weeks of intervention, fasting plasma glucose was 28-30% lower in cafestol groups compared with the control group (p < 0.01). Fasting glucagon was 20% lower and insulin sensitivity improved by 42% in the high-cafestol group (p < 0.05). Cafestol increased insulin secretion from isolated islets by 75-87% compared to the control group (p < 0.001). Our results show that cafestol possesses antidiabetic properties in KKAy mice. Consequently, cafestol may contribute to the reduced risk of developing T2D in coffee consumers and has a potential role as an antidiabetic drug.

Chronic Exposure to Proline Causes Aminoacidotoxicity and Impaired Beta-Cell Function: Studies In Vitro

BACKGROUND

Pancreatic islet-cell dysfunction is a hallmark in the development of diabetes, but the reasons for the primary β-cell defect are still elusive. Elevated circulating proline levels have been found in subjects with insulin resistance, obesity, and type 2 diabetes. Therefore, we assessed β-cell function, gene expressions, and cell death after long-term exposure of pancreatic β-cells to excess proline in vitro.

METHODS

Isolated mouse islets and INS-1E cells were incubated with and without excess proline. After 72 h, we examined: (1) β-cell function, including basal insulin secretion (BIS) and glucose-stimulated insulin secretion (GSIS), (2) transcription factors related to insulin gene expression and enzymes involved in the tricarboxylic acid cycle and cholesterol biogenesis, (3) cellular triglycerides (TG) and cholesterol content, (4) the death of INS-1E cells and 3H thymidine incorporation, and (5) protein expression of INS-1E cells in response to proline by proteomics.

RESULTS

We found that high doses of proline increased BIS and decreased GSIS in both isolated mouse islets and INS-1E cells. MafA, insulin 1, and the cytochrome c oxidase subunit VIa polypeptide 2 mRNA expressions were all downregulated, indicating that proline impaired insulin gene transcription and mitochondrial oxidative phosphorylation. In contrast, mevalonate decarboxylase gene expression was upregulated, and simultaneously, cholesterol content in INS-1E cells was enhanced. Protein profiling of INS-1E cells revealed that cytosolic non-specific dipeptidase and α enolase were differentially expressed.

CONCLUSIONS

Our results indicate that proline-induced insulin transcription and mitochondrial oxidative phosphorylation impairment may contribute to the β-cell dysfunction observed in type 2 diabetes. Caution should be applied in interpreting the pathophysiological role of proline since very high proline concentrations were used in the experiments.

Cafestol, a Bioactive Substance in Coffee, Stimulates Insulin Secretion and Increases Glucose Uptake in Muscle Cells: Studies in Vitro

Diet and exercise intervention can delay or prevent development of type-2-diabetes (T2D), and high habitual coffee consumption is associated with reduced risk of developing T2D. This study aimed to test whether selected bioactive substances in coffee acutely and/or chronically increase insulin secretion from β-cells and improve insulin sensitivity in skeletal muscle cells. Insulin secretion from INS-1E rat insulinoma cells was measured after acute (1-h) and long-term (72-h) incubation with bioactive substances from coffee. Additionally, we measured uptake of radioactive glucose in human skeletal muscle cells (SkMC) after incubation with cafestol. Cafestol at 10(-8) and 10(-6) M acutely increased insulin secretion by 12% (p < 0.05) and 16% (p < 0.001), respectively. Long-term exposure to 10(-10) and 10(-8) M cafestol increased insulin secretion by 34% (p < 0.001) and 68% (p < 0.001), respectively. Caffeic acid also increased insulin secretion acutely and chronically. Chlorogenic acid, trigonelline, oxokahweol, and secoisolariciresinol did not significantly alter insulin secretion acutely. Glucose uptake in SkMC was significantly enhanced by 8% (p < 0.001) in the presence of 10(-8) M cafestol. This newly demonstrated dual action of cafestol suggests that cafestol may contribute to the preventive effects on T2D in coffee drinkers and be of therapeutic interest.

The somatostatin receptor in human pancreatic β-cells

The peptide hormone somatostatin (SST) is produced in the brain, the gut, and in δ-cells in pancreatic islets of Langerhans. SST secretion from δ-cells is stimulated by glucose, amino acids, and glucagon-like peptide-1. Exogenous SST strongly inhibits the secretion of the blood glucose-regulating hormones insulin and glucagon from pancreatic β-cells and α-cells, respectively. Endogenous SST secreted from δ-cells is a paracrine regulator of insulin and glucagon secretion, although the exact physiological significance of this regulation is unclear. Secreted SST binds to specific receptors (SSTRs), which are coupled to Gi/o proteins. In both β- and α-cells, activation of SSTRs suppresses hormone secretion by reducing cAMP levels, inhibiting electrical activity, decreasing Ca²⁺ influx through voltage-gated Ca²⁺ channels and directly reducing exocytosis in a Ca²⁺ and cAMP-independent manner. In rodents, β-cells express predominantly SSTR5, whereas α-cells express SSTR2. In human islets, SSTR2 is the dominant receptor in both β- and α-cells, but other isoforms also contribute to the SST effects. Evidence from rodent models suggests that SST secretion from δ-cells is dysregulated in diabetes mellitus, which may contribute to the metabolic disturbances in this disease. SST analogues are currently used for the treatment of hyperinsulinism and other endocrine disorders, including acromegaly and Cushing's syndrome.

Abnormal alpha cell hypoglycemic recognition in children with insulin dependent diabetes mellitus (IDDM)

Children with IDDM have diminished glucagon responses to hypoglycemia. We evaluated possible mechanisms in 60 children and adolescents with IDDM (age 15.4 +/- 2.6 years, duration 7.8 +/- 3.5 years [mean +/- SD]) and without diabetic complications. These were: 1) suppression by hyperinsulinism, 2) autonomic neuropathy, 3) a pan-islet cell defect, and 4) a glucotoxic effect. Glucagon and pancreatic polypeptide responses to hypoglycemia (insulin bolus 0.15-0.75 U/kg) were studied after insulin withdrawal and 3 days of intensive insulin therapy. Responses to arginine and mixed meal were also studied. The control group consisted of children with non-growth hormone deficient short stature. IDDM children had lower glucagon responses to hypoglycemia than controls (p < 0.001), the response to arginine did not differ from controls, and was greater than the response to hypoglycemia (p < 0.001). Responses to hypoglycemia after insulin withdrawal and intensive therapy did not differ. Basal pancreatic polypeptide levels were lower in IDDM than in controls (p < 0.05) but responses to hypoglycemia did not differ between groups. Thus the diminished glucagon response to hypoglycemia reflects a defect in hypoglycemic recognition or response by the alpha cells.

Intra-islet insulin permits glucose to directly suppress pancreatic A cell function

Inhibition of pancreatic glucagon secretion during hyperglycemia could be mediated by (a) glucose, (b) insulin, (c) somatostatin, or (d) glucose in conjunction with insulin. To determine the role of these factors in the mediation of glucagon suppression, we injected alloxan while clamping the arterial supply of the pancreatic splenic lobe of dogs, thus inducing insulin deficiency localized to the ventral lobe and avoiding hyperglycemia. Ventral lobe insulin, glucagon, and somatostatin outputs were then measured in response to a stepped IV glucose infusion. In control dogs glucagon suppression occurred at a glucose level of 150 mg/dl and somatostatin output increased at glucose greater than 250 mg/dl. In alloxan-treated dogs glucagon output was not suppressed nor did somatostatin output increase. We concluded that insulin was required in the mediation of glucagon suppression and somatostatin stimulation. Subsequently, we infused insulin at high rates directly into the artery that supplied the beta cell-deficient lobe in six alloxan-treated dogs. Insulin infusion alone did not cause suppression of glucagon or stimulation of somatostatin; however, insulin repletion during glucose infusions did restore the ability of hyperglycemia to suppress glucagon and stimulate somatostatin. We conclude that intra-islet insulin permits glucose to suppress glucagon secretion and stimulate somatostatin during hyperglycemia.

Growth hormone and somatostatin in the plasma of transiently diabetic ducks: basal variation and response to glucose

In the duck, subtotal pancreatectomy induces a transient diabetes, with decreased insulin and glucagon basal levels as well as responses to glucose. At the same time, a transient increase in basal peripheral somatostatin occurs, followed by an increase in growth hormone in the postdiabetic state. Intravenous glucose induces a slight decrease in somatostatin secretion in normal, but not in diabetic animals, and no significant variation in growth hormone secretion at any state. An obvious role of growth hormone or somatostatin in the development of this transient diabetes in the duck could not be detected in this study.

Glucagon responses to intravenous arginine and oral glucose in insulin-dependent diabetic patients during six months conventional or continuous subcutaneous insulin infusion

To elucidate the impact of subcutaneous insulin infusion (CSII) treatment on the glucagon response to intravenous (IV) arginine and to oral glucose a 6-month prospective randomized study in insulin-dependent diabetics was carried out. The effects were investigated of CSII (7-patients) and conventional insulin treatment (UCT) (9 patients) on the changes in glucagon, growth hormone, glucose, lactate, glycerol, 3-hydroxybutyrate, and alanine to IV arginine and to oral glucose in insulin-dependent diabetics who were made euglycemic and isoinsulinemic using the artificial pancreas (Biostator, Miles, Elkhart, IN). HbA1c was significantly lower in the group treated by CSII. Despite the improved glycemic control no significant change in the responses of A-cell secretion to arginine or glucose challenges was found. In addition, there were no significant differences in hormone or metabolite values between the two groups at entry to the study or after 6 months of either therapy. Thus, normalization of the A-cell sensitivity to glucose in insulin-dependent diabetic subjects requires further amelioration of the intermediary metabolism than can be achieved with insulin pump treatment.

Retrograde perfusion as a model for testing the relative effects of glucose versus insulin on the A cell

In order to determine whether the A cell may be directly suppressed by glucose in the absence of insulin, canine pancreata were perfused in vitro, both antegrade, through the arterial system and retrograde, through the venous system. Studies of the islet microvasculature have suggested that blood flows from the B cell core to the mantle; thus, the A cell may be tonically inhibited by intra-islet insulin. Retrograde perfusion may then be expected to prevent insulin from reaching the A cell, releasing it from inhibition. Retrograde perfusion with 88 mg/dl glucose markedly increased both insulin and glucagon secretion relative to antegrade levels. In a series of experiments, glucose concentrations were changed from 88 to 200 mg/dl. An antegrade glucose change resulted in increased insulin (134+/-21%; P less than 0.0025) and decreased glucagon (-26+/-9%, P less than 0.025) secretion. A retrograde glucose increase resulted in increased secretion of both insulin (91+/-15%; P less than 0.0005) and glucagon (23+/-9%; P less than 0.0125). To confirm that retrograde perfusion deprived the A cell of endogenous core derived, vascularly delivered insulin, possibly resulting in increased insulin sensitivity, 0.3 mU/ml exogenous porcine insulin was infused. Antegrade, 0.3 mU/ml insulin, had no effect on glucagon secretion (P less than 0.250), while retrograde infusion of 0.3 mU/ml insulin significantly inhibited glucagon secretion (-31 + 8%; P less than 0.0005). The results of our study support the concept that the direction of blood flow and of flow-dependent intra-islet hormone interactions are from the islet B cell core to the mantle. It was further concluded that the normal A cell may not be suppressed by glucose in the absence of insulin.

Somatostatin, glucagon, and insulin secretion from perfused pancreas of BB rats

We reported previously that pancreatic somatostatin and glucagon content and D and A cells are reduced in spontaneously diabetic BB rats while portal plasma levels of these hormones are elevated but normalized by insulin therapy. Presently, we have characterized the basal and stimulated (glucose, theophylline, arginine) release of islet hormones from perfused pancreases of three groups: nondiabetic, untreated diabetic, and insulin-treated diabetic rats. Maximal glucagon and somatostatin release were significantly reduced in untreated diabetics. Treatment normalized glucagon but further reduced somatostatin secretion. Thus, hyperglucagonemia and hypersomatostatinemia cannot result from pancreatic hypersecretion but are of extrapancreatic (probably gut) origin. A theophylline-induced paradoxical inhibition of somatostatin secretion that was normalized by insulin was found in insulin-openic diabetic rats. This likely represents a secondary effect of insulin deficiency. One animal with the rare finding of spontaneous recovery from insulin-dependent diabetes was characterized as normoglycemic, hypoinsulinemic, hypersomatostatinemic, and normoglucagonemic. Normal basal and hyperglycemic insulin release was exhausted by more potent secretagogues. Somatostatin release was markedly exaggerated, whereas secretagogues had no significant effect on elevated basal glucagon output.

Abnormal D cell secretion in alloxan-diabetes: influence by drug and aberrant metabolism

Abnormalities of somatostatin secretion in diabetes may be secondary to B cell damage with resulting insulinopenia or other effects of diabetogenic agents, including toxicity toward the somatostatin-producing D cells. These possibilities were evaluated in isolated perfused pancreas from normal and alloxan-diabetic rats. In normal rats 3-isobutyl-1-methylxanthine (IBMX, 1 mM), alpha-ketoisocaproic acid (KIC, 5 mM), D-glucose (27 mM), and D-glyceraldehyde (5 mM) stimulated somatostatin release. In diabetic rats 3 days after alloxan, IBMX and KIC elicited somatostatin release, whereas glucose or glyceraldehyde were without effect. In diabetic rats 14 days after alloxan, an otherwise (in normal and 3-day diabetic rats) nonstimulatory concentration of IBMX (0.05 mM) markedly stimulated somatostatin release, whereas as in 3-day diabetic rats glucose was ineffective. Insulin treatment for 2 days did not affect the somatostatin response to glucose in normal rats, did not restore a somatostatin response to glucose 3 days after alloxan, but partially restored (P less than 0.01) a response to glucose (28% of normal) 14 days after alloxan. Insulin in vitro (1 mU/ml, 20 min) failed to restore a glucose effect. Administration of alloxan (1.0 mM) for 5 min to pancreases from normal rats inhibited glucose-induced somatostatin response from 1,562 +/- 401 to 206 +/- 83 pg/15 min (P less than 0.01), whereas the response to IBMX (1 mM) was not significantly decreased. Following different time courses, both an effect of alloxan and of metabolic derangement inhibit somatostatin responses to glucose in alloxan diabetes.

Loss of a priming effect of glucose on A and D cell secretion in perfused pancreases from alloxan-diabetic rats: role of insulin and alloxan

Under normal conditions, glucose acutely influences pancreatic islet B, A and D cell secretion. In addition, prior exposure to glucose modulates the secretory responsiveness of these cells (priming effect). We have tested whether alloxan diabetes influences priming effects of glucose on A and D cell secretion. Rat pancreases were perfused 72 h after alloxan treatment. A 20 min infusion of 27.7 mmol/l of glucose failed to induce priming effects, i.e. it did not inhibit the glucagon nor amplify the somatostatin response to a subsequent (15 min later) infusion of 8 mmol/l of arginine. Insulin treatment in vivo for 48 h restored a priming effect of glucose on glucagon secretion in the perfused pancreas, i.e. exposure to 27.7 mmol/l of glucose now inhibited subsequent arginine-induced glucagon secretion by 48% relative to a stimulation period with arginine preceding the glucose pulse (from 5.0 +/- 0.7 to 2.6 +/- 0.5 ng/min, p less than 0.01). Conversely, insulin treatment in vivo did not restore a priming effect of glucose on somatostatin secretion. Other effects noted were failure of 27.7 mmol/l glucose to stimulate, during its presence, the release of somatostatin from pancreases of the diabetic rats whether untreated or insulin-treated. Furthermore, insulin treatment abolished the arginine-induced somatostatin secretion observed in pancreases from untreated rats. It is concluded that short-term alloxan diabetes leads to loss of a priming effect of glucose on glucagon secretion and that this abnormality is secondary to direct or indirect effects of insulinopenia. Concomittant abnormalities of glucose regulation of somatostatin secretion may, in part, be secondary to a cytotoxic effect of alloxan on the D cell.

Acute effects of alloxan- and streptozotocin-induced insulin deficiency on somatostatin and glucagon secretion by the perfused isolated rat pancreatico-duodenal preparation

The secretion of somatostatin and glucagon by the perfused rat pancreatico-duodenal preparation was examined in situ under control conditions and after the induction of acute insulin deficiency by alloxan or streptozotocin. A 10 min 0.625 mmol/l alloxan perfusion resulted in an immediate and transient increase in basal insulin and glucagon release and a slightly delayed and persistent increase in basal somatostatin secretion. The insulin responses to 16.7 mmol/l glucose, 1 mmol/l theophylline, and 19 mmol/l arginine alone or in combination were virtually eliminated by alloxan treatment. Somatostatin secretion in response to the stimuli was completely inhibited or markedly attenuated. The glucagon-suppressive effect of glucose was unaltered by alloxan and the stimulatory effect of arginine was enhanced. Addition of 1 microgram/ml porcine insulin to the perfusion medium did not modify the alterations in somatostatin and glucagon responses to arginine. Streptozotocin treatment 90 min prior to the onset of perfusion resulted in changes in somatostatin, glucagon, and insulin responses to glucose and arginine similar to those of alloxan. The present results are consistent with an effect of alloxan and streptozotocin on the D cell similar to that on the B cells, namely, interference with a glucose-mediated effect on hormone secretion.

Increased somatostatin secretion from pancreatic islets of streptozotocin-diabetic rats in response to glucose

Glucose stimulates somatostatin release from perifused pancreatic islets of diabetic rats 42-47 days after the induction of diabetes, and 48 h after withdrawal of insulin replacement therapy. The glucose effect is augmented by theophylline or glucagon. Basal somatostatin release and glucose-induced secretion are significantly higher in diabetic islets than in controls. It is suggested that glucose promotes somatostatin release by directly interacting with islet D cells but not via indirect pathways. Glucose-induced stimulation appears to be modulated by a D-cell adenylate cyclase/phosphodiesterase system. Reasons responsible for increased somatostatin secretion by diabetic islets include reduction in B-cell mass, suggesting that B cells may normally suppress the secretory activity of D cells.