Somatostatin inhibition of phospholipase G activity in isolated rat pancreatic islets

We have assessed the effect of somatostatin on the phospholipase C activity in isolated rat pancreatic islets. The phospholipase C activity was measured as the generation of inositol 1,4,5-trisphosphate and its metabolite inositol 1,3,4-trisphosphate from the hydrolysis of polyphosphoinositides. Inositol phosphates were measured using anion-exchange fast protein liquid chromatography analysis of extracts from islets prelabelled with myo-[³H]inositol. Somatostatin (1–1000 nmol l^-1) significantly inhibited the glucose-induced (12 mmol l^-1) phospholipase C activity in a concentration-dependent manner. The Ca²⁺ channel blocker verapamil (25 μmol l^-1) also inhibited the glucose-induced (12 mmol l^-1) phospholipase C, whereas the combination of somatostatin and verapamil did not induce any additional inhibition. At 3.3 mmol l^-1 glucose, the hypoglycaemic sulphonylurea, tolbutamide (1 mmol l^-1), increased the phospholipase C activity. This effect was reversed by somatostatin (100 nmol l^-1). Tolbutamide did not further increase the glucose-induced (12 mmol l^-1) phospholipase C activity. However, the somatostatin inhibition of glucose-induced (12 mmol l^-1) phospholipase C was reversed by tolbutamide. The activator of adenylyl cyclase, forskolin (20 μmol l^-1), did not exert any effect on the PLC-inhibition of somatostatin, whereas forskolin alone inhibited the phospholipase C activation at 12 mmol l^-1 glucose. Our study demonstrates that somatostatin inhibits the hydrolysis of polyphosphoinositides in pancreatic islets, apparently via a mechanism dependent on Ca²⁺ and not on cAMP.     

Somatostatin inhibition of glucose-induced electrical activity in cultured rat islet cells.

Electrophysiological studies of rat islet cells in monolayer culture were undertaken to determine the role of transmembranous ionic fluxes in the inhibitory action of somatostatin on insulin release. In the presence of somatotropin release inhibiting factor (SRIF) (2.5 nM), hyperpolarization occured with or without glucose (16.6 mM) in the medium. SRIF also inhibited the incidence of glucose-induced spike activity. The inhibitory action of SRIF occurred within 5 min and was readily reversible. An increase in extracellular K+ (5-13 mM) or Ca2+ (2.3-4.6 mM) prevented SRIF inhibition of glucose-induced electrical activity. The secretory response of cultured islets to glucose (16.6 mM) was completely inhibited by SRIF (2.5 nM). The presence of high [Ca2+]o or [k+]o enhanced insulin release in the presence of SRIF and glucose. Although phentolamine (5.0 microgram/ml) did not block the inhibition of glucose-induced electrical responses by SRIF, it prevented the inhibitory action of epinephrine (0.2 microgram/ml). It is concluded that the primary action of SRIF is to alter transmembranous cationic fluxes, as manifested by hyperpolarization and a decrease in the incidence of spike activity, which may prevent glucose from eliciting a normal secretory response.     

The interaction between cAMP-dependent and cAMP-independent mechanisms in mediating the somatostatin inhibition of insulin secretion in isolated rat pancreatic islets.

To characterize the intracellular mechanisms by which somatostatin modulates the insulin secretion, studies were performed with isolated rat pancreatic islets at 12 mmol l-1 glucose. Somatostatin (0.1-1000 nmol l-1) inhibited the glucose-induced insulin secretion concentration-dependently. Increasing intracellular cAMP concentration either with dibutyryl-cAMP (1 mmol l-1) or by the adenylate cyclase activator forskolin (20 mumol l-1) partly reversed the inhibition by somatostatin (100 nmol l-1). Neither somatostatin (100 nmol l-1) nor dibutyryl-cAMP (1 mmol l-1 were able to affect the low insulin secretion observed in the absence of extracellular Ca2+. To study cAMP-independent mechanisms of somatostatin, the experiments were performed with and without dibutyryl-cAMP (1 mmol l-1) present. Both somatostatin (100 nmol l-1) and the Ca(2+)-channel blocker verapamil (25 mumol l-1) inhibited the insulin secretion both with and without dibutyryl-cAMP present. An additional inhibition of the insulin secretion was observed when somatostatin was combined with verapamil in the absence, but not in the presence of dibutyryl-cAMP. We conclude that somatostatin inhibits the glucose-induced insulin secretion both by cAMP-dependent mechanism which requires extracellular Ca2+, and by cAMP-independent/verapamil-sensitive Ca(2+)-channel-dependent mechanism.     

Cytophysiological studies on isolated pancreatic islets in vitro.

Single, isolated pancreatic islets of mice and rats were incubated for varying time intervals (0.5-60) minutes with high (300 mg%) and low (50 mg%) levels of glucose. The structural integrity of islets decreased progressively with time regardless of glucose concentration. Degeneration of islets was greatest after 60 minutes of incubation. The total amount of insulin released from cytologically intact mouse islets incubated with high glucose levels was always greater than that with low glucose except following 30 seconds of incubation when no difference was observed. Peaks of insulin secretion noted after 2 and 15 minutes of incubation were correlated with light microscopic and fine structural changes indicative of active secretion in beta-cells, i.e., degranulation, granule margination. At 5 and 30 minutes of incubation many beta-cells contained enlarged Golgi zones and abundant profiles of swollen rough endoplasmic reticulum containing pale, amorphous granular material presumably indicating insulin synthesis. Emphasis is placed on the desirability of correlating physiological and biochemical studies of isolated pancreatic islets with cytologic examination.     

Cholecystokinin-stimulated insulin secretion and protein kinase C in rat pancreatic islets

In isolated rat pancreatic islets, the possible involvement of protein kinase C in cholecystokinin-8-stimulated insulin secretion was investigated. In islets exposed for 24 hours to the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (500 nmol l-1), a procedure known to down-regulate islet protein kinase C-activity, the insulinotropic effect of cholecystokinin-8 (10(-7) mol l-1) was partially reduced (by 34 +/- 8%, P less than 0.001). In contrast the insulinotropic response to acute exposure to 12-O-tetradecanoyl phorbol 13-acetate (10(-6) mol l-1) was totally abolished (P less than 0.001), whereas the insulin response to glucose (8.3 mmol l-1) was not affected. In normal islets, the protein kinase C-inhibitor, staurosporine, inhibited 12-O-tetradecanoyl phorbol 13-acetate- and glucose-stimulated insulin secretion (P less than 0.01), but was without effect on cholecystokinin-8-stimulated insulin release. Furthermore, in normal islets, cholecystokinin-8 had no effect on insulin release at a low glucose level (3.3 mmol l-1). However, at this low glucose level, cholecystokinin-8 clearly potentiated insulin release induced by acute exposure to 12-O-tetradecanoyl phorbol 13-acetate (10(-8) -10(-6) mol l-1, P less than 0.001). This potentiating effect was abolished by the removal of extracellular Ca2+. It is concluded that the insulinotropic effect of cholecystokinin-8 in rat islets is partially mediated by the protein kinase C pathway. Furthermore, the lack of effect of cholecystokinin-8 on insulin secretion at a low glucose level might be explained by an insufficient activation of protein kinase C under these conditions.     

A role of thyrotropin-releasing hormone in insulin secretion by isolated rat pancreatic islets

Insulin-secreting pancreatic cells also express thyrotropin-releasing hormone (TRH). Although the physiological role of TRH in this localization is unclear, its participation in glucoregulation has been implied. To test this hypothesis, we blocked the last step of post-translational maturation of the TRH molecule by disulfiram, which is an active inhibitor of peptide -amidation (PAM) within pancreatic islet cells. The treatment of male rats with 200 mg/kg/day of disulfiram during a 5-day period resulted in a low PAM activity, a high insulin content and its basal secretion from pancreatic islets, and the inability to release insulin in response to glucose (16.7 mM) or hypo-osmotic (30%) challenge in vitro. The addition of TRH (1 nM) to the medium during incubation restored the insulin content and both basal and glucose stimulated insulin secretions to control levels. Conclusion: TRH plays an important role in the mechanism of insulin secretion and its response to glucose stimulation.     

Glucose inhibition of 45Ca efflux from pancreatic islets.

Pancreatic islets were microdissected from ob/ob mice, loaded for 2 h with 45Ca and perfused with calcium-deficient medium. Irrespective of the glucose and calcium concentrations in the loading medium, increased glucose in the perfusion medium resulted in reduced amounts of radioactivity in the perfusate. A glucose inhibition of 45Ca washout was also evident when the specific radioactivity of the islets approached that of the labeling medium, indicating that the effect was not simply due to isotopic dilution. The depression of 45Ca washout diminished after culture of the islets in a serum-free medium and it was absent in islets taken from mice homozygous for the gene diabetes. The glucose effect became less pronounced when 50 micron D-600, an inhibitor of the calcium inward transport, was added to the calcium-deficient perfusion medium and abolished in the presence of 20 mM Ca-EGTA. The inhibition of the 45Ca washout observed is not necessarily due to a direct glucose interaction with the outward calcium transport but may also result from stimulation of the uptake and intracellular trapping of the cation.     

The fate of transplanted pancreatic islets in the rat.

Syngeneic pancreatic islets transplanted into the liver or the spleen reverse streptozotocin-induced diabetes in the rat, but allogeneic islets function only briefly and are rejected. Shortly after transplantation, thrombi often form around transplanted tissue, particularly around nonislet tissue that contaminates islet preparations. These thrombi are a source of transient liver injury in recipients of intrahepatic grafts. A few days after transplantation, syngeneic islets injected into the portal vein are found at the periphery of portal tracts in direct contact with periportal hepatocytes, some of which become hypertrophied. Isografts remain situated in the portal tracts for prolonged periods without adverse effect on the surrounding liver. In contrast, allogeneic islets injected into the portal vein are infiltrated by small lymphocytes within 2 days of transplantation and are rapidly destroyed by the host. Syngeneic islets injected into the splenic pulp localize in the sinusoids and, 1 month or more after transplantation, are often surrounded by connective tissue or local collections of hemosiderin-laden macrophages. Allogeneic islets injected into the spleen are rejected with the same intensity and at approximately the same rate as allogeneic islets injected into the portal vein. Transplant rejection leaves no significant lasting morphologic effect on the host liver or spleen.     

Effect of fatty acids on isolated ovine pancreatic islets.

A procedure was developed for the isolation of intact islets of Langerhans from sheep pancreas. The pancreas was disrupted by syringe injection of Hanks solution followed by collagenase incubation and islet separation by sedimentation. The islets were incubated in varying concentrations of glucose and butyrate. The rate of insulin release was approximately linear while the glucose and butyrate concentrations were increased. In additional studies at 2.5 and 5.0 mM levels of substrate concentration, the stimulation of insulin had the following pattern: octanoate greater than hexanoate greater than butyrate, whereas beta-hydroxybutyrate, lactate, acetate, and propionate had only slight stimulatory effects that were not statistically significant. Decanoate did not alter insulin release from isolated islets. These data confirm earlier in vivo reports that fatty acids stimulate pancreatic hormone release in sheep and that the stimulus is related to chain length of the fatty acid through C-8 but that C-10 has no effect. A hypothesis was suggested to explain these results based on chain length, solubility, and plasma membrane alterations.     

Cytophysiological studies on isolated pancreatic islets in vitro

Single, isolated pancreatic islets of mice and rats were incubated for varying time intervals (0.5-60) minutes with high (300 mg%) and low (50 mg%) levels of glucose. The structural integrity of islets decreased progressively with time regardless of glucose concentration. Degeneration of islets was greatest after 60 minutes of incubation. The total amount of insulin released from cytologically intact mouse islets incubated with high glucose levels was always greater than that with low glucose except following 30 seconds of incubation when no difference was observed. Peaks of insulin secretion noted after 2 and 15 minutes of incubation were correlated with light microscopic and fine structural changes indicative of active secretion in β-cells, i.e., degranulation, granule margination. At 5 and 30 minutes of incubation many β-cells contained enlarged Golgi zones and abundant profiles of swollen rough endoplasmic reticulum containing pale, amorphous granular material presumably indicating insulin synthesis. Emphasis is placed on the desirability of correlating physiological and biochemical studies of isolated pancreatic islets with cytologic examination.     

Regional distribution of phosphoinositide-specific phospholipase C activity in rat brain

The phosphatidylinositol (PI) and phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolytic activities of phosphoinositide-specific phospholipase C (PLC) were measured in membrane and cytosol fractions from 7 discrete areas of the rat brain. Both the PI-PLC and PIP₂-PLC specific activities were found to differ significantly among the 7 discrete brain areas. In the membrane fraction, the PIP₂-PLC activity was higher than that of PI-PLC in each region, suggesting that the PLC in membranes prefers PIP₂ to PI as substrate. The PIP₂-PLC activities in the membrane were high in prefrontal cortex and cerebellum, but rather low in medulla oblongata and hypothalamus. The PI-PLC specific activity in the cytosol was significantly higher than that in the membrane of all brain areas examined. The PI-PLC specific activity in membranes is inversely proportional to its activity in the cytosol. In the cytosol fraction, the distribution pattern of PI-PLC specific activity resembled that of PIP₂-PLC. These results indicate that PLCs are differently distributed in various regions of rat brain, and suggest the regional differences in neuronal transduction.     

Morphologic basis for loss of regulated insulin secretion by isolated rat pancreatic islets

Laboratories engaged in secretory studies of rat pancreatic islets often encounter high baseline insulin secretion with poor secretory response to secretagogues, such as glucose. The specific morphologic abnormalities that accompany this unregulated release have not been described. We isolated islets comparing two approaches. Both used stationary digestion with collagenase. In method I, we distended the biliary duct extracorporeally with collagenase and minced the pancreas after a 28 min digestion (37°C). In method II, we distended the pancreas intracorporeally and digested for 40 min without mincing. Both methods utilized a similar collagenase concentration (2 μg/ml in Hank's balanced salt solution (HBSS)). Both methods yielded over 300 islets/rat. Islets from both methods appeared intact, when viewed under the dissecting microscope. We found that adequate secretion from incubated islets was evoked with method I, i.e., low basal insulin levels at low glucose (3.3 mM), tripling at 11.0 mM glucose, and nearly quadrupling in response to higher glucose (16.7 mM). In contrast, method II was characterized by high basal levels without response to higher glucose. Ultramicroscopic examination of islet B cells in method I revealed normal cytological features, while B cells in method II showed marked degranulation, profiles of swollen endoplasmic reticulum, and swollen mitochondria. Morphometric analysis of B cells confirmed quantitatively a decrease in secretory granule density and mitochondrial enlargement in method II compared to method I. Anatomic changes, largely confined to the B cells of islets may account for functional alterations of responses. Defects cannot be predicted from gross appearance of islets. © 1993 Wiley-Liss, Inc.     

大鼠胰岛分离技术的建立及胰岛活性的影响因素

目的建立稳定有效的大鼠胰岛分离方法,探讨影响胰岛活性的因素。方法通过胆管注射胶原酶方法提取大鼠胰岛,通过水浴法比较不同条件下葡萄糖刺激的胰岛素分泌,用放免法(R IA)测定胰岛素。结果牛血清白蛋白(BSA)能明显提高葡萄糖刺激的胰岛素分泌水平,长时间培养(>7 d)的胰岛,其葡萄糖刺激的胰岛素分泌下降约25%,而新鲜分离的胰岛和经1~5 d培养的胰岛未见明显差异。RPM I1640培养液中葡萄糖浓度11.1 mmol/L组,其胰岛素分泌明显高于5.5 mmol/L和25 mmol/L这2组。结论BSA、RPM I1640培养液中葡萄糖浓度及培养时间均与分离胰岛活性有关。     

Ca-activated ATPase activity in subcellular fractions of mouse pancreatic islets.

Ca-stimulated ATPase activity has been demonstrated in homogenates of mouse pancreatic islets. On subcellular fractionation Ca-ATPase activity was found in secretory granules, mitochondria, and microsomes, but not in the postmicrosomal fractions. Highest specific activity was found in the granules. In all active subcellular fractions two Km(Ca) values for Ca-ATPase around 7.0 X 10(-6) and 1.8 X 10(-7) M were estimated. Assuming an ATP hydrolysis:Ca pumping ratio of 1:2, the highest capacity for active Ca transport was found in secretory granules and mitochondria. Concentrations of 40 mM or higher of Na and 10(-5) M cyclic AMP inhibited Ca-ATPase in all subfractions. Caffeine at a concentration of 10 mM inhibited Ca-ATPase significantly in secretory granules and microsomes. Also MG-ATPase activity was demonstrated in the various subfractions. This activity was compared with that of Ca-ATPase at identical concentrations of free metal ions and in the absence or presence of various inhibitors. It was concluded that high-affinity Ca-ATPase and Mg-ATPase are two different enzymic entities. Ca-ATPase may tentatively be assumed to participate in active transport of Ca between intracellular compartments and to constitute a Ca-accumulating system which returns the cytosolic free Ca concentration to the resting state after stimulation of the beta-cells by secretagogues. This enzyme may therefore play a significant role in regulation of insulin release.     

Function of rat pancreatic islets exposed to interleukin-18 in vitro.

The recently cloned cytokine interleukin-18 (IL-18) has been shown to promote a Th1-cell immune response, which may be a prerequisite for development of Type 1 diabetes. In this study we examined the effects of IL-18 on the function of isolated rat pancreatic islets. The islets were cultured in medium RPMI 1640 + 10% fetal calf serum and exposed for 48 h to recombinant human IL-18 (0, 0.1, 1 and 10 nM). In some experiments IL-18 (l0 nM) was combined with interleukin-12 (10 ng/ml), since these cytokines may act synergistically. IL-18 alone, or in combination, with IL-12 did not affect the islet DNA content suggesting absence of cytotoxicity. However, both cytokines induced an increased islet insulin content compared to non-cytokine exposed control islets. A slight increase in the medium insulin accumulation was observed when 1.0 nM IL-18 was added, but not in other experimental groups. Glucose-stimulated insulin release, glucose oxidation and (pro)insulin biosynthesis rates were not affected by the cytokines after culture. In acute experiments IL-18 had a small stimulatory effect on glucose-stimulated insulin secretion. It was also tested if IL-18 (10 nM) could affect IL-1beta (25 U/ml) induced suppression of the glucose oxidation rate, but this was not the case. We conclude that IL-18 has minor stimulatory effects on beta-cell function, and no clear synergistic effect is observed when IL-12 is added together with IL-18. If IL-18 is involved in beta-cell destruction in Type 1 diabetes, it is likely that this effect is secondary to an influence on the action of other cytokines.     

Morphological changes of isolated rat pancreatic islets: a structural, ultrastructural and morphometric study

Improved techniques for pancreatic islet extraction can yield a reasonable number of transplantable cells. However, the isolation and purification process may damage the islets and impair their physiological functions. The aim of this study was to determine the effect of the isolation procedure on the structure of isolated islets and to correlate this with their functionality. Islets were isolated from rat pancreata and purified by Eurocollins-Ficoll discontinuous density gradient processing, and then processed for light microscopy, and scanning and transmission electron microscopy. Morphometric analysis was also performed. Islet functionality was determined by reversal of streptozotocin-induced diabetes and the intraperitoneal glucose tolerance test in a syngeneic rat model of pancreatic islet transplantation. Fragments of variable size and shape comprised a relatively large proportion (26%) of the isolated endocrine tissue. Isolated islets showed slight alterations of cell ultrastructure. Major damage (including breakage of the plasma membrane) and loss of cells were observed in the peripheral cells of the isolated islets. An equal mass of islet equivalent (IEq, islets with an average diameter of 150 microm), but with a different islet equivalent/islet number ratio, was transplanted in diabetic animals. When larger and more complete islets were transplanted (higher ratio), better function of the graft was observed by reversal of hyperglycaemia and response to the glucose tolerance test as compared with the functionality and response of smaller (fragmented) islets transplanted (lower ratio). Digestion, trauma and hypoxia during isolation are responsible for qualitative and quantitative changes of isolated islets. Alterations in normal secretory function after the transplant were related to lower islet equivalent/islet number ratio. The incomplete integrity of the islets may explain the failure of the fine glycaemic metabolic regulation.