Elevated somatostatin in pancreatic islets of adrenalectomized dogs

We have observed both hyperglucagonemia and hypoinsulinemia in adrenalectomized (Adx) dogs. To determine whether these hormonal alterations are related to changes in distribution of islet hormones in the pancreas, we examined the concentration and total mass of insulin, glucagon, and somatostatin in the head, body, and tail of the pancreas by immunoassay and immunocytochemistry. We studied 6 normal dogs, 5 Adx dogs deprived of cortisol for 24 h (Adx I) and 5 for 48-72 h (Adx II). In normal dogs, single and double immunocytochemical staining showed that, in contrast to some other species, B (insulin) cells are mostly in the central region of islet, whereas A (glucagon) and D (somatostatin) cells are distributed randomly. This topographic distribution was not altered by adrenalectomy. In normal dogs, insulin concentration (micrograms per g) and total mass (micrograms) were higher in the tail (174 +/- 22, 2001 +/- 396) and body (165 +/- 22, 2850 +/- 600) than in the head (91 +/- 17, 668 +/- 156) of pancreas. Glucagon concentration (micrograms per g) and total mass (micrograms) were 17 +/- 2, 178 +/- 17 in the tail; 9.5 +/- 2, 158 +/- 32 in the body, and negligible (0.78 +/- 0.32, 7 +/- 3) in the head, whereas somatostatin concentration (micrograms per g) and total mass (micrograms) were 0.58 +/- 0.26, 4.20 +/- 1.5 in the T, 0.23 +/- 0.10, 3.9 +/- 1.6 in the B, and 0.22 +/- 0.05, 1.8 +/- 0.6 in the H. The striking finding was that adrenalectomy caused large increases in somatostatin in all three regions of pancreas in both Adx I and Adx II. The total mass of somatostatin in Adx I and Adx II increased 4-fold in the tail (P less than 0.02-0.005), 5-fold in the body (P less than 0.01-0.001), and 7-9-fold in the head (P less than 0.05-0.005) and concentration increased 6-fold in the body (P less than 0.005) and 7- to 8-fold in the head (P less than 0.01-0.001). There were no significant changes in the content of insulin and glucagon after adrenalectomy. Plasma concentration of glucagon increased by 50% in Adx I (P less than 0.005) and 70% in Adx II (P less than 0.02), insulin decreased by 39% (P less than 0.005), 23% (NS), respectively, and somatostatin increased by 258% (P less than 0.001) in Adx II. Thus the adrenal glands appear to play an important role in regulation of the content of somatostatin in pancreatic islets.     

Evidence for metabolic regulation of pancreatic glucagon secretion by L-glutamine

To investigate effects by L-glutamine on pancreatic A-cell secretion and intermediary metabolism, isolated pancreatic islets from normal and streptozotocin treated guinea pigs (A-cell rich islets) were incubated in the presence of glucose (5.5 mM) +/- L-glutamine (10 mM). Glutamine significantly enhanced glucagon release from 297 +/- 54 to 528 +/- 53 pg/micrograms DNA/h in normal islets and from 553 +/- 31 to 806 +/- 50 pg/micrograms DNA/h in A-cell rich islets. All results were expressed on the basis of islet DNA concentration, being 66 +/- 4 ng DNA per normal islet and 32 +/- 2 ng DNA per A-cell rich islet. Simultaneously, glutamine suppressed glucose oxidation to 64 per cent in normal islets and to 47 per cent of basal oxidation in A-cell rich islets. Islet content of ATP was also reduced by glutamine to about 60 per cent in A-cell rich islets, but not significantly changed in normal islets. Glutamine oxidation, at 5.5 mM-glucose, was considerably higher in A-cell rich islets (911 +/- 65 pmol/micrograms DNA/h) than in normal islets (313 +/- 52 pmol/micrograms DNA/h). Addition of porcine insulin (25 mU/ml) counteracted these effects by glutamine, i.e. suppressed glucagon release but increased glucose oxidation and ATP content of the A-cell rich islets. The present findings demonstrate that glutamine stimulates glucagon release and is readily metabolized by the A-cells. Furthermore, the regulation of glucagon secretion by glutamine appears to be reciprocally related to factors affecting glucose metabolism and ATP-levels in the A-cell.     

Changes in somatostatin concentration in pancreas and other tissues of streptozotocin diabetic rats.

Changes in immunoreactive somatostatin were examined in islets, whole pancreas, stomach, and hypothalamus of streptozotocin-diabetic rats. There was no change in islet somatostatin content at 2 days after the administration of streptozotocin, but thereafter, somatostatin progressively increased in the diabetic animals by 45% at 2 weeks, 230% at 6 weeks, and 500% by 6 months. By contrast, islet glucagon rose acutely and maintained a constant 2-fold elevation irrespective of the duration of the diabetes. Morphometric analysis of the somatostatin- and glucagon-producing cells in the islets revealed an apparent augmentation of both cell types. The concentration of somatostatin per total pancreas was also increased in the diabetic animals, suggesting that the islet increase was part of a true increase in pancreatic somatostatin. Pancreatic glucagon was unchanged despite the islet increase. The increase in pancreatic somatostatin was paralleled by an elevation in gastric somatostatin concentration, implying a common mechanism in response to streptozotocin for the somatostatin cells in these two sites. There was no change in hypothalamic somatostatin concentration. Islet somatostatin was also increased in alloxan-diabetic rats. suggesting that streptozotocin does not stimulate the D cells directly.     

Rapid depletion of somatostatin in isolated mouse pancreatic islets after treatment with cysteamine

Cysteamine (CSH; beta-mercaptoethylamine) is known to deplete pancreatic somatostatin without affecting the insulin or glucagon content. It may therefore be useful for studies of intra-islet regulation of hormone release. In the present study injection of CSH (60 mg/kg body weight) to mice decreased the somatostatin content of their isolated pancreatic islets to 50% in 1 h and 30% in 4 h as compared to islets of non-injected controls. Exposure of isolated mouse islets to CSH (100 micrograms/ml) for either 0.5 h followed by incubation in control medium for 3.5 h, or continuously for 4 h, decreased the somatostatin content to about 40% of the controls. There was no change in the islet content of insulin or glucagon. Islets pretreated with CSH (100 micrograms/ml) for 1 h in vitro showed a decreased glucose stimulation of both oxygen consumption and glucose oxidation. Measurements of insulin release after a similar preincubation of the islets indicated an increased basal release and an attenuated glucose stimulation. It is concluded that CSH rapidly decreases islet somatostatin both in vivo and in vitro. This depletion may lead to a loss of tonic inhibition by islet somatostatin on basal insulin release. It is, however, more plausible that the increased basal insulin release reflected a direct effect of CSH on the islet beta-cells.     

Insulin, glucagon, somatostatin, and thyrotropin-releasing hormone content and secretion by perifused fetal rat islets during culture

In the neonatal period of the rat, pancreatic thyrotropin-releasing hormone content decreases and the sensitivity of insulin secretion to glucose increases. In adult rat islets, TRH inhibits glucose-induced insulin release. The aim of this study was to investigate whether a high TRH content and release can be part of the explanation for the functional immaturity of neonatal islets. For that purpose, we have measured the tissue content and the secretion of immunoreactive insulin, glucagon, somatostatin and TRH in islets from 21.5-day-old rat fetuses cultured for up to one week. Insulin, glucagon and somatostatin content increased during one week of culture in the presence of 11.1 mmol/l glucose. The TRH content decreased during culture, but did not equal adult values. Insulin, glucagon and somatostatin responses to glucose were present after one week of culture. Glucose had no effect on TRH release in cultured fetal islets, but inhibited TRH release in adult islets. We conclude that glucose can stimulate insulin secretion without inhibiting TRH release, but that a decrease in islet TRH content and a sensitization of TRH secretion to glucose may be important in the full maturation of fetal pancreatic islets.     

The role of glucagon in the pathogenesis of the endogenous hyperglycemia of diabetes mellitus.

The effect of glucagon suppression by somatostatin upon endogenous hyperglycemia was studied in three forms of experimental insulin deficiency in dogs: alloxan diabetes, total pancreatectomy, and diazoxide administration. In six insulin-requiring alloxan-diabetic dogs deprived of insulin for 24 hr, mean plasma glucose declined to 77% +/- 6% of the baseline level of 350 +/- 41 mg/dl during 3 hr of glucagon suppression, significantly below the unsuppressed saline controls (p less than 0.01-0.05). When somatostatin was discontinued, glucagon rose and glucose increased 21% (p less than 0.05) in 30 min. Significant correlation between maximal changes in glucagon and glucose was observed (r = 0.81; p less than 0.001). Even during a 1-hr alanine infusion in such dogs, glucose declined an average of 36 +/- 9 mg/dl, instead of rising 51 +/- 7 mg/dl as in unsuppressed controls. Maximal changes in glucagon and glucose were correlated (r = 0.85; p less than 0.01). In eight depancreatized dogs pretreated intravenously with continuous insulin and glucose infusions, withdrawal of insulin was followed by a rise in extrapancreatic glucagon; mean plasma glucose rose from 212 +/- 43 to 415 +/- 80 mg/dl 270 min after the end of the insulin infusion. However, when glucagon was suppressed after insulin withdrawal, glucose remained below 240 mg/dl, significantly less than the controls (p less than 0.005); when somatostatin was stopped, glucagon rose and glucose increased 88 +/- 19 mg/dl within an hour. The rises in glucagon and glucose were significantly correlated (r = 0.68; p less than 0.05). Glucagon suppression by somatostatin during diazoxide-induced blockade of insulin secretion in four normal dogs reduced hyperglycemia significantly but did not prevent it. The results support the hypothesis that a relative or absolute excess of glucagon, as well as a relative or absolute deficiency of insulin, is etiologically important in the development of endogenous hyperglycemia in diabetes mellitus, the hyperglucagonemia probably mediating the glucose overproduction.     

Effect of glucagon or somatostatin on desensitized insulin secretion

In this study we have examined the role of glucagon and somatostatin in regulating glucose-induced desensitization of insulin secretion from rat islets. Measured in batch incubations with medium routinely used to induce three phases of insulin secretion, secreted glucagon levels fell off over 24 h to 20% of peak secretion levels. Although more responsive to various secretagogues, somatostatin secretion also declined to the same degree. Thus, the A- and D-cells desensitize to chronic stimulation as does the B cell. In other experiments, added glucagon (10(-6) M) enhanced glucose (11 X 10(-3) M)-stimulated insulin secretion 34% in the first 3 h; however, islets became insensitive to continuous glucagon by 4 h. The exogenous glucagon did not prevent or delay glucose-induced desensitization of insulin secretion. When glucagon was administered as acute 1-h tests over continuous glucose administration, the degree of B-cell response did not differ in the 1st, 3rd, or 6th hours and appeared to increase in the 21st hour. When islets were perifused continuously with glucose (22 X 10(-3) M) plus 3 X 10(-7) M somatostatin, glucose-induced insulin secretion was suppressed 50% in the first 3 h, but this inhibitory effect disappeared after 6 h. Desensitization was slightly delayed, but not prevented. When somatostatin was administered as acute 1-h tests over continuous glucose perifusion, the B-cell response was relatively constant in the 3rd, 6th, and 21st hours. Results show that 1) islet release of glucagon and somatostatin desensitizes during constant stimulation; and 2) islet release of insulin desensitizes to chronic potentiation or inhibition, respectively, by these hormones. Furthermore, 3) changing B-cell sensitivity to either glucagon or somatostatin cannot account for observed desensitization of insulin secretion with chronic glucose exposure.     

Long-term exposure of mouse pancreatic islets to oleate or palmitate results in reduced glucose-induced somatostatin and oversecretion of glucagon

AIMS/HYPOTHESIS: Long-term exposure to NEFAs leads to inhibition of glucose-induced insulin secretion. We tested whether the release of somatostatin and glucagon, the two other major islet hormones, is also affected. METHODS: Mouse pancreatic islets were cultured for 72 h at 4.5 or 15 mmol/l glucose with or without 0.5 mmol/l oleate or palmitate. The release of glucagon and somatostatin during subsequent 1 h incubations at 1 or 20 mmol/l glucose as well as the islet content of the two hormones were determined. Lipid-induced changes in islet cell ultrastructure were assessed by electron microscopy. RESULTS: Culture at 15 mmol/l glucose increased islet glucagon content by approximately 50% relative to that observed following culture at 4.5 mmol/l glucose. Inclusion of oleate or palmitate reduced islet glucagon content by 25% (at 4.5 mmol/l glucose) to 50% (at 15 mmol/l glucose). Long-term exposure to the NEFA increased glucagon secretion at 1 mmol/l glucose by 50% (when islets had been cultured at 15 mmol/l glucose) to 100% (with 4.5 mmol/l glucose in the culture medium) and abolished the inhibitory effect of 20 mmol/l glucose on glucagon secretion. Somatostatin content was unaffected by glucose and lipids, but glucose-induced somatostatin secretion was reduced by approximately 50% following long-term exposure to either of the NEFA, regardless of whether the culture medium contained 4.5 or 15 mmol/l glucose. Ultrastructural evidence of lipid deposition was seen in <10% of non-beta cells but in >80% of the beta cells. CONCLUSIONS/INTERPRETATION: Long-term exposure to high glucose and/or NEFA affects the release of somatostatin and glucagon. The effects on glucagon secretion are very pronounced and in type 2 diabetes in vivo may aggravate the hyperglycaemic effects due to lack of insulin.     

Neurotensin inhibits glucose but not glucagon-induced insulin and somatostatin release in isolated islets.

The effects of neurotensin on insulin and somatostatin release were examined in isolated pancreatic islets prepared from 3-4 days rats, and maintained in culture for 48 h before use. In the presence of 12 mM glucose, glucagon (50-2,000 ng/ml, i.e. 14-560 nM) caused a 2-fold increase in insulin and somatostatin release. Neurotensin (150 ng/ml, i.e., 100 nM) did not affect the glucagon-stimulated release, nor did it alter the release of either peptide measured at 12 mM glucose in the absence of glucagon. In contrast, neurotension markedly inhibited the release of both insulin and somatostatin that was induced by 23 mM glucose. These observations suggest that neurotensin may modulate the release of insulin and somatostatin evoked by high glucose concentrations, but not that resulting from the action of glucagon on pancreatic islets.     

Changes in the pancreatic A-, B- and D-cell populations during development of diabetes in spontaneously diabetic Chinese hamsters of the Asahikawa colony (CHAD)

We investigated the pathological changes in pancreatic islets during the development of diabetes in spontaneously diabetic Chinese hamsters of the Asahikawa colony (CHAD), using morphometric analysis and specific immunocytochemical methods. We also investigated the relationships between changes in islet cell composition and the hormonal changes in the plasma and pancreas. Plasma and pancreatic insulin levels were significantly lower in diabetic hamsters than in pre-diabetic hamsters. However, plasma insulin levels in the pre-diabetic hamsters were significantly higher than those in the hamsters from the non-diabetic control strain, although the pancreatic insulin content in the pre-diabetics was significantly lower than that in the non-diabetics. Since even a severely diabetic CHAD is alive for many months after the onset of the disease without injections of insulin, its clinical course seems to be close to that of type 2 human diabetes. In contrast, plasma and pancreatic glucagon levels were significantly higher in diabetic hamsters than in non-diabetics and pre-diabetics. There were significantly positive correlations between plasma and pancreatic insulin, and plasma and pancreatic glucagon levels in CHAD (P less than 0.01). On the other hand, no significant differences in the pancreatic somatostatin content were found among the non-diabetics, pre-diabetics, and severe diabetics. Significant correlations were found between plasma and pancreatic hormone levels (except for somatostatin) and the advance of diabetes in CHAD (P less than 0.01). Morphometric analysis by planimeter revealed that islets in the severe diabetics were 25% smaller than in the pre-diabetics. Significantly less B-cell area within the diabetic islets was found when compared with the non-diabetic and pre-diabetic islets. Significantly larger A- and D-cell areas within the diabetic islets were found compared with the non-diabetic and pre-diabetic islets. There was a significant correlation between the areas of the three types of cell within the islets and the severity of diabetes (P less than 0.01). It is suggested, therefore, that the pancreatic islet function in CHAD is closely associated with the morphologic changes in islet endocrine cells. The elevation of plasma and pancreatic glucagon levels and the marked increase of the A-cell area within the islets from severely diabetic CHAD may reveal an absolute increase of A-cell numbers.     

Immunolocalization of insulin, glucagon, pancreatic polypeptide, and somatostatin in the pancreatic islets of the possum, Trichosurus vulpecula

Antibodies to insulin, glucagon, pancreatic polypeptide (PP), and somatostatin were used in the immunofluorescence procedure to demonstrate localization of the four hormones in cells of the pancreatic islets of the brushtailed possum, Trichosurus vulpecula. Most pancreatic islets revealed some differences in the topographical distribution and cell number of each endocrine cell type. Insulin immunoreactive cells were observed in most islets where they occurred as groups of cells peripherally and within the islet. In several islets glucagon cells were the predominant cell population and were distributed peripherally as well as centrally. Pancreatic polypeptide cells were fewer in number and usually occurred as single cells within the islet. Cells immunoreactive to antisomatostatin serum were observed in varying numbers in the peripheral and central regions of the islet. The present immunofluorescence study demonstrates differences in the topographical distribution of the four major pancreatic hormones between a marsupial species and several of the eutherian mammals.     

Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice

Somatostatin (SST) potently inhibits insulin and glucagon release from pancreatic islets. Five distinct membrane receptors (SSTR1-5) for SST are known, and at least two (SSTR2 and SSTR5) have been proposed to regulate pancreatic endocrine function. Our current understanding of SST physiology is limited by the receptor subtype selectivity of peptidyl SST analogs, making it difficult to assign a physiological function to an identified SST receptor subtype. To better understand the physiology of SSTRs we studied the in vitro effects of potent subtype-selective nonpeptidyl SST analogs on the regulation of pancreatic glucagon and insulin secretion in wild-type (WT) and in somatostatin receptor 2 knockout (SSTR2KO) mice. There was no difference in basal glucagon and insulin secretion between islets isolated from SSTR2KO and WT mice; however, potassium/arginine-stimulated glucagon secretion was approximately 2-fold higher in islets isolated from SSTR2KO mice. Neither SST nor any SSTR-selective agonist inhibited basal glucagon or insulin release. SST-14 potently inhibited stimulated glucagon secretion in islets from WT mice and much less effectively in islets from SSTR2KO mice. The SSTR2 selective analog L-779,976 inhibited glucagon secretion in islets from WT, but was inactive in islets from SSTR2KO mice. L-817,818, an SSTR5 selective analog, slightly reduced glucagon release in both animal groups, whereas SSTR1, -3, and -4 selective analogs were inactive. SST and L-817,818 inhibited glucose stimulated insulin release in islets from WT and SSTR2KO mice. L-779,976 much less potently reduced insulin secretion from WT islets. In conclusion, our data demonstrate that SST inhibition of glucagon release in mouse islets is primarily mediated via SSTR2, whereas insulin secretion is regulated primarily via SSTR5.     

beta-Cell function relative to islet volume and hormone content in the isolated perfused mouse pancreas

The beta-cell function, total islet volume, and number were studied in 1- to 18-month-old mice, together with the extractable pancreatic insulin and glucagon. The beta-cell function, determined as the total amount of insulin released in response to glucose from the in vitro perfused pancreas showed an age-related increase, without any differences in the kinetics of insulin secretion between young and old mice. The total islet number and area in each individual pancreas was determined planimetrically after selective staining of the islets by perfusing the pancreas with dithizone. The islet area increased from 5.4 +/- 1.7 mm2 at 1 month to 16.3 +/- 2.1 mm2 at 18 months, whereas the number of islets remained virtually unchanged (1072 +/- 51). Pancreatic insulin increased with age by nearly 500%, in contrast to a 35% reduction in pancreatic glucagon. There was a strong relationship between body weight and total pancreatic DNA (P = 4.7 X 10(-8)), islet area (P = 3.2 X 10(-7)), insulin secretory capacity (P = 7 X 10(-4)), and total pancreatic insulin (P = 1.9 X 10(-5)), but no relationship between body weight and islet number. The insulin secretory capacity increased proportionally to the increase in islet area (P = 9.9 X 10(-3)). The islet area and total pancreatic insulin were closely related (P = 2.8 X 10(-12)), as were pancreatic insulin and the insulin secretory capacity (P = 3.3 X 10(-11)). There was a negative relationship between pancreatic glucagon and islet area (P = 0.005) and between pancreatic glucagon and insulin (P = 0.01). The close relationship between pancreatic insulin and islet area, shown to be an expression of islet volume, makes it possible to estimate the volume of the endocrine pancreas after standard RIA of pancreatic insulin. The combined morphometric and physiological analysis is unique in studying islet cell function relative to the volume of the endocrine pancreas.     

Paradoxical glucagon response in perifused islets of the diabetic Chinese hamster

Dynamic insulin and glucagon response to glucose was examined in the perifusion system to investigate the relationship between pancreatic hormone content and the pattern of hormone secretion in diabetic Chinese hamsters of the Asahikawa colony (CHA). Isolated islets of normals and diabetics from the CHA were perifused. When the medium was changed to high glucose (500 mg/dl), a low insulin response and paradoxical glucagon response were seen in diabetics compared with normals. Positive correlations were found between pancreatic insulin and the amount of perifusate insulin, and glucagon content and glucagon release, respectively. It is suggested, accordingly, that pancreatic hormone content is related to the amount of hormone release in CHA. A negative correlation between the amount of perifusate insulin and glucagon release was found. It is suggested, therefore, that an impaired suppression of glucagon release in the diabetic CHA animals could be attributed at least to insulin deficiency. These findings agree with the histological discovery of decreased B-cells and increased A-cells in the diabetic islets. Both decreased B-cells and islet numbers could be the cause of the low insulin response to glucose. Increased numbers of A-cells with hyperfunction resulting from local insulin deficiency could be the cause of the paradoxical glucagon response.     

MIN6 is not a pure beta cell line but a mixed cell line with other pancreatic endocrine hormones

MIN6 cells retains glucose-stimulated insulin secretion (GSIS) as isolated islets. We comprehensively evaluated the gene expression and production of other islet hormones in MIN6 cells. Islet hormones were demonstrated by immunohistochemical staining and measured by ELISA. The gene expression profiles of MIN6 cells were compared with those in the mouse islets obtained by the laser capture micro-dissection (LCM). MIN6 cells excreted insulin, glucagon, somatostatin and ghrelin. They expressed mRNAs of insulin I and II, proglucagon, somatostatin, pancreatic polypeptide (PP) and ghrelin which were shown in the mouse pancreatic islet core and periphery obtained by LCM. A variety of genes closely related to the islet hormone producing cells were expressed in MIN6. Confocal laser scanning microscopy revealed that MIN6 cells included not only insulin positive cells but also insulin and glucagon or somatostin double positive cells. Glucagon, somatostatin and ghrelin were detectable in the culture medium. The present study clearly demonstrated that MIN6 produce pancreatic endocrine cells. It would be possible to use this cell line as a model to research the development, cell differentiation and function of pancreatic islets.     

Lower blood glucose,hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic alpha cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr(-/-)). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr(-/-) mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to alpha cell, and to a lesser extent, delta cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr(-/-) mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and alpha and delta cell numbers. Furthermore, the lean phenotype of Gcgr(-/-) mice suggests glucagon action may be involved in the regulation of whole body composition.     

Effect of phentolamine on the somatostatin-induced inhibition of glucagon and insulin secretion.

Synthetic cyclic somatostatin was infused into either the cranial pancreaticoduodenal artery or the femoral vein of anesthetized dogs with or without previous administration of phentolamine. Somatostatin infused into the pancreatic artery at a dose of 50 ng/kg/min for 10 min caused significant decreases in blood flow and plasma basal concentrations of both glucagon and insulin in the cranial pancreaticoduodenal vein, resulting in a profound decline of bihormonal output during the infusion. Arterial plasma glucose was not reduced during the administration of somatostatin in the pancreatic artery. These somatostatin-induced decreases failed to be eliminated by a 0.2 mg/kg injection of phentolamine into the femoral vein followed by a 9-min infusion of this alpha-adrenergic blocker (0.02 mg/kg/min) into the pancreatic artery immediately prior to the somatostatin administration. An inhibition of glucagon and insulin output and a fall of plasma glucose caused by somatostatin (1.7 microgram/min) infused into the femoral vein for 30 min also were not abolished by a prolonged and simultaneous infusion of phentolamine (0.2 mg/min) into the femoral vein over a period of 2 hr. These results indicate that alpha-adrenergic receptor mechanisms do not play a major role in the inhibition of islet glucagon and insulin secretion by somatostatin.     

Effect of glucagon and cyclic adenosine 3',5'-monophosphate on protein phosphorylation in rat pancreatic islets

Isolated rat pancreatic islets, incubated in the presence of extracellular 32Pi to a state of steady 32P incorporation into cellular phosphopeptides, were exposed to glucagon, (Bu)2cAMP, or somatostatin for 10 min. In other experiments, homogenates of rat islets were phosphorylated using [gamma-32P]ATP with or without cAMP. Proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and phosphorylation of proteins was measured by liquid scintillation counting of gel slices. Glucagon (2.9 X 10(-7) M) stimulated the phosphorylation of 15 polypeptides (by approximately 20-50%) with major phosphorylation of proteins with mol wts of 138,000, 93,000, 53,000, 49,000, 35,000, 27,000 and 15,000 in intact rat islets and also stimulated insulin release by 202%. Somatostatin (6.6 X 10(-7) M) inhibited all the glucagon-stimulated phosphorylation by approximately 15-30% and also inhibited the glucagon-stimulated insulin release by 46%. (Bu)2cAMP (10(-3) M) stimulated 32P incorporation (by approximately 20-50%) into the same 15 peptides as did glucagon and also stimulated insulin release by 169%. When homogenates of rat islets were used. cAMP (10(-6) M) stimulated the phosphorylation of proteins (by approximately 25-60%) to an extent similar to that seen in the presence of glucagon or (Bu)2cAMP in intact islets. These findings indicate that the glucagon-stimulated phosphorylation of rat islet proteins may be mediated by cAMP-dependent protein kinase and that protein phosphorylation may be important in mediating the glucagon-stimulated insulin release.     

Insulin,glucagon, and somatostatin release from the prediabetic Chinese hamster

Summary Diabetes mellitus in the adult Chinese hamster is characterized by subnormal pancreatic insulin release in vitro, decreased insulin content, and lack of obesity. The cause of the islet B-cell failure is not clear. We measured insulin, glucagon, and somatostatin release from in vitro perfused pancreases of young (mean age 10 and 20 weeks), genetically diabetic animals (subline AC, mean plasma glucose 8.0 and 16.6mmol/l, respectively). Compared to age- and sex-matched normal hamsters (subline M, mean plasma glucose 5.3 mmol/l), the younger diabetic animals had a significantly elevated mean plasma glucose level, but net in vitro pancreatic release of insulin, glucagon, and somatostatin was normal. Pancreatic content of insulin and glucagon was also not significantly different from normal. At age 20 weeks, when the plasma glucose of the diabetic animals was even more elevated, pancreatic content and release of insulin were significantly subnormal, whereas glucagon and somatostatin release were normal, and pancreatic content of glucagon was normal. In a similar group of young (mean age 10 weeks) diabetic animals, non-fasting plasma insulin levels were within the normal range, but the corresponding glucose levels were excessive in most of the animals (13 out of 19). In conclusion, 10-week-old diabetic hamsters show mild hyperglycaemia which cannot be accounted for directly by decreased pancreatic release in response to a glucose plus arginine stimulus in vitro. Decreased ability of the B cell to respond in vivo to hyperglycaemia or peripheral resistance to insulin may contribute to later B-cell failure in the older diabetic hamster.     

An immunocytochemical study of endocrine cell development in the early fetal guinea pig pancreas.

The presence of insulin, glucagon, pancreatic polypeptide, and somatostatin containing cells and their ontogenic changes were investigated immunocytochemically in the early fetal pancreas of the guinea pig (Days 25-40). In the earliest tissues examined (Day 25 and Day 30) brightly staining glucagon cells were the most predominant endocrine cell population, followed by slightly fewer and weaker staining pancreatic polypeptide cells. Insulin and somatostatin immunoreactive cells were less numerous. At Day 25 all endocrine cells were located within the pancreatic tubules where some glucagon cells also coexpressed insulin. Similar dual immunoreactivity was present at Day 30. At Day 25 some of the pancreatic polypeptide cells also showed coexpression of somatostatin which persisted until Days 35-40. At these later stages insulin and somatostatin cells were increasingly frequent. Glucagon and pancreatic polypeptide cells were also conspicuous. The four endocrine cell types were found either in the pancreatic tubules or in the developing islets where they began to acquire an adult-like topographic distribution. These studies in the fetal guinea pig show that the four islet hormonal cells cytodifferentiate from an early stage. A small proportion of endocrine cells coexpress either insulin and glucagon or pancreatic polypeptide and somatostatin.