Defective early phase insulin release in perifused isolated pancreatic islets of spiny mice (acomys cahirinus)

In order to characterize pancreatic beta cell function in Geneva bred spiny mice (acomys cahirinus), the dynamics of immunoreactive insulin release were examined during perifusion of pancreatic islets isolated from normoglycemic acomys. The initial insulin response of acomys was slow: no clear-cut early (1 to 10 min) peak of insulin release was observed when glucose in the perifusion medium was abruptly raised from 2.8 mM to concentrations as high as 56 mM. This was true for islets of either young, or older more obese acomys. However, after 20 to 30 min of perifusion at the high glucose concentrations, the rate of insulin release from acomysislets became similar to that from islets of rats or mice. By contrast, glucose-induced insulin release responses observed with islets of Wistar-derived rats, Swiss albino mice, and inbred C57BL/6J lean or obese (ob/ob) mice, were clearly biphasic. Tolbutamide 1.5 mM, arginine 16 mM, and theophylline 10 mM were ineffective in stimulating insulin release from acomys islets in the presence of a substimulatory glucose concentration (2.8 mM), whereas these agents were effective in rat islets at the same substimulatory concentration of glucose. On the other hand, when these agents, as well as cyclic AMP 10 mM or cytochalasin B 10 mug/ml were applied in the presence of a stimulating concentration of glucose (16.8 mM), the glucose-stimulated insulin release from acomys islets was increased to the same or to a greater extent than from rat islets. It is suggested that the failure of all the agents tested to stimulate an early rapid phase of insulin release from acomys islets may be secondary to the observed initial insensitivity to glucose, which insensitivity may in turn reflect a selective impairment in the recognition of glucose as an insulinogenic signal in this species.     

Insulinotropic action of L-rhamnose in isolated rat islets

Summary L-Rhamnose at different concentrations stimulated incorporation of³H-leucine both into islet (pro)insulin and that released into the medium. Maximum isotope incorporation with either glucose or rhamnose was seen at a concentration of 16.7 mM, although the glucose-induced effect was significantly greater. Like glucose, rhamnose also enhanced the activities of acid phosphatase and cathepsin B in isolated rat islets.Communication No. 2759.     

Indirect two-site immunoradiometric assay of rat and mouse proinsulin

Summary An indirect two-site immunoradiometric assay for rat and mouse proinsulin using a rabbit antibody to synthetic rat C-peptide has been developed. The sensitivity of the assay is 0.006 pmol/ml. Proinsulin was 4.95% of the total proinsulin and insulin in extracts of rat pancreas and 5.45% in extracts of isolated rat islets. The mean fasting rat insulin and proinsulin concentrations were 0.13±0.09 pmol/ml (n=5) and 0.008±0.002 pmol/ml (n=5) respectively. The mean fasting mouse proinsulin concentration was 0.019±0.006 pmol/ml (n=8). In rats intravenous glucose produced a biphasic insulin response but proinsulin rose progressively to 0.021±0.011 pmol/ml at 45 min. In mouse oral glucose increased the proinsulin concentration to 0.13 pmol/ ml at 30 min. Proinsulin release from isolated rat islets was studied during intermittent or continuous high glucose (20 mmol/l) stimulation in static incubation. Significant increases in proinsulin release were only observed 90 min after initial exposure to high glucose whether glucose stimulation was continuous or intermittent. Both in vivo and in vitro glucose stimulation led initially to a fall in the proinsulin/ insulin molar ratio but later upon prolonged stimulation this progessively increased to above the basal value.     

The effect of glucose stimulation on 45calcium uptake of rat pancreatic islets and their total calcium content as measured by a fluorometric micro-method

Summary Glucose-stimulated ⁴⁵calcium uptake and total calcium content of rat pancreatic islets has been studied, using a new fluorometric micro-method to estimate total calcium. Extracellular calcium was separated from incubated tissue by a rapid micro-filtration procedure. Islets incubated up to 60 min with calcium chloride 2.5 mmol/l and glucose 2.5 mmol/l maintained the same calcium content (670±7.5 pmol/g DNA). When the glucose concentration was raised to 15 mmol/l no change in the total calcium content could be detected. On incubation with glucose 2.5 mmol/l in the absence of calcium, the calcium content decreased to 488±27 pmol/g DNA. On incubation with ⁴⁵calcium chloride 2.5 mmol/l for 5 or 30 min at 2.5 mmol/l glucose, islets exchanged 21 ±2 and 28±1% of their total calcium content and, at 15 mmol/l glucose, 30±3 and 45±2%, respectively. Thus, islet calcium has a high turn-over rate. Glucose stimulation results in an increase of the calcium uptake without enhancing the total calcium content and hence must increase the calcium-exchangeable pool.     

Altered proinsulin conversion in rat pancreatic islets exposed long-term to various glucose concentrations or interleukin-1beta

In order to elucidate a possible relationship between beta-cell function and conversion of proinsulin to insulin, isolated rat pancreatic islets were maintained in tissue culture for 1 week at various glucose concentrations (5 x 6-56 mM). Studies were also conducted on islets cultured for 48 h with interleukin-1beta (IL-1beta). By pulse-chase labelling and immunoprecipitation, the relative contents of newly synthesized proinsulin and insulin were determined. ELISA was used to analyse insulin and proinsulin content in medium and within islets. Using real-time PCR, the mRNA levels of proinsulin converting enzymes (PC1 and PC2) were studied. Islets cultured at 56 mM glucose had an increased proportion of newly synthesized proinsulin when compared with islets cultured at 5 x 6 mM glucose after a 90-min chase periods, however, no difference was observed after culture at 11 and 28 mM glucose. ELISA measurements revealed that culture at increased glucose concentrations as well as islet exposure to IL-1beta increased proinsulin accumulation in the culture media. The mRNA expression of PC1 was increased after culture at 11 and 28 mM glucose. Treatment for 48 h with IL-1beta increased the proportion of proinsulin both at 45 and 90 min when compared with control islets. These islets also displayed a decreased mRNA level of PC1 as well as PC2. Calculations of the half-time for proinsulin demonstrated a significant prolongation after treatment with IL-1beta. We conclude that a sustained functional stimulation by glucose of islets is coupled to a decreased conversion of proinsulin which is also true for islets treated with IL-1beta. This may contribute to the elevated levels of proinsulin found both at the onset of type 1 diabetes as well as in type 2 diabetes.     

Effects of leptin,acetylcholine and vasoactive intestinal polypeptide on insulin secretion in isolated <Emphasis Type="Italic">ob/ob</Emphasis> mouse pancreatic islets

Abstract. Obesity is often accompanied by hyperleptinemia, hyperinsulinemia, and an increased parasympathetic tone. Obese-hyperglycemic mice (Umeå ob/ob) have functional leptin receptors and a raised parasympathetic tone. We studied insulin release in islets isolated from 9-monthold severely obese ob/ob mice. Leptin (0.5–18 nM) did not affect insulin release together with 2.8–20 mM glucose. Leptin (18 µM) had no effect in the presence of low glucose (2.8–5.5 mM), but increased insulin secretion in islets challenged with 11.1 or 16.7 mM glucose. Leptin at 18 µM increased insulin secretion stimulated by the parasympathetic neurotransmitters acetylcholine (ACh; 10 µM) or vasoactive intestinal peptide (VIP; 10 nM), and by 5 mM theophylline or 2.5 µM forskolin. Overnight culture increased the effect of 18 µM leptin, but no effects were observed with 18 nM leptin. Pretreatment of islets with phorbol 12-myristate 13-acetate (PMA) did not suggest any involvement of protein kinase C. In summary, a high concentration of leptin stimulates insulin release in the presence of stimulatory concentrations of glucose alone and with parasympathetic neurotransmitters. Hyperleptinemia and increased parasympathetic stimulation may in part cause the hyperinsulinemia observed in obesity. This may aggravate insulin resistance and the abnormal metabolism in diabetes mellitus.     

Synthesis of proinsulin and large glucagon immunoreactivity in isolated langerhans islets from EMC-virus infected mice

The protein synthesis in normal and in EMC-virus infected mouse islets of Langerhans was investigated. Mouse large glucagon immunoreactivity was determined by an immunoassay after chromatographic separation. It was characterized as a peptide of 16 000 MW with in intracellular half-life of 35-45 min. The proportional reduction of void volume proteins, large glucagon immunoreactivity and proinsulin synthesis after infection shows, that both alpha- and beta-cells are damaged by the virus. A reduction in the synthesis of the three protein fractions was already found 6 hrs after inoculation of the virus and remained nearly constant for 48 hrs. An almost complete breakdown of protein synthesis occurred 60 to 70 hrs after infection and was paralleled by the first light microscopic changes in the islets. The stimulation of proinsulin synthesis by glucose was preserved for 48 hrs after EMC-virus infection.     

Effects of islet amyloid polypeptide (IAPP) on insulin biosynthesis or secretion in rat islets and mouse beta TC3 cells. Biosynthesis of IAPP in mouse beta TC3 cells.

Effects of rat islet amyloid polypeptide (IAPP) on insulin biosynthesis and secretion were examined in isolated rat islets and mouse beta TC3 cells. Culture of islets or mouse beta TC3 cells for 24 h in the presence of 10(-6) M IAPP and 5.5 mM glucose had no effect on insulin mRNA levels. The rates of proinsulin biosynthesis were not altered in islets incubated in 10(-4)-10(-9) M IAPP. In beta TC3 cells, proinsulin biosynthesis was stimulated by glucose, though no effects of IAPP were shown. Addition of 10(-5) M IAPP to islets incubated in 11 mM glucose decreased the fractional insulin secretion rates; however, the secretion of insulin from beta TC3 cells was not affected by 10(-5) M IAPP. On the other hand, mouse beta TC3 cells expressed the elevated level of IAPP mRNA. Metabolic labeling of beta TC3 cells revealed the synthesis of both proIAPP and mature IAPP. In pulse chase experiments, proIAPP was processed to IAPP in a manner similar to proinsulin. These data indicate that IAPP is a possible polypeptide hormone synthesized in pancreatic beta cells though it is unlikely that IAPP is a physiologically relevant modulator of insulin biosynthesis or secretion.     

Effects of short-term culturing on islet phosphoinositide and insulin secretory responses to glucose and carbachol

The ability of glucose and carbachol, alone or in combination, to stimulate islet cell phosphoinositide (PI) hydrolysis and insulin secretory responses in freshly isolated or in 20–24 h cultured rat islets was assessed. In freshly isolated,³H-inositol-prelabeled islets, 20 mM glucose alone or 1 mM carbachol alone stimulated significant increments in³H-inositol efflux and inositol phosphate (IP) accumulation. When stimulated with both agonists, a dramatic and synergistic effect on IP accumulation was noted. Carbachol (1 mM) alone had no sustained stimulatory effect on insulin secretion. Glucose (20 mM) alone induced a biphasic insulin secretory response. When compared to prestimulatory secretory rates of 18±4 pg/islet/min, peak first and second phase responses of freshly isolated islets to 20 mM glucose averaged 126±24 and 520±82 pg/islet/min, respectively. In the presence of both glucose (20 mM) and carbachol (1 mM), peak first and second phase responses now averaged 422±61 and 1016±156 pg/islet/min, respectively. In contrast to freshly studied islets, culturing islets for 20–24 h in CMRL-1066 medium attenuated all measured responses. The increases in³H-inositol efflux rates in response to glucose, carbachol, or their combination were significantly less than those observed with fresh islets. The IP responses were also attenuated. Second phase insulin secretory responses to 20 mM glucose alone (68±9 pg/islet/min) or the combination of 20 mM glucose plus 1 mM carbachol (358±85 pg/islet/min) were also significantly decreased when compared with fresh islets. We conclude from these studies that the process of culturing islets for one day in CMRL-1066 significantly decreases islet cell PI hydrolysis and insulin secretory responsiveness. These observations may help to explain the discordant conclusions reached concerning the involvement of PI hydrolysis and protein kinase C activation in the regulation of insulin release from freshly isolated versus cultured islets.     

Carbamylcholine and ouabain effects on Ca2+ handling and insulin release in islets from rats depleted in long-chain polyunsaturated omega 3 fatty acids

A number of metabolic, ionic and secretory variables were recently found to be affected in pancreatic islets obtained from second generation rats depleted in long-chain polyunsaturated ω3 fatty acids (ω3 rats). The present study further documents three sets of anomalies in such islets. First, after 90 min exposure to d-glucose (8.3 mM), the release of insulin from perifused islets, prelabelled with ⁴⁵Ca, is lower in ω3 rats than in control animals, despite comparable ⁴⁵Ca fractional outflow rate. Second, over 15 min exposure to carbamylcholine (0.1 mM), in the presence of d-glucose, the cytosolic concentration of Ca²⁺ is increased to a greater relative extent in dispersed islet cells from ω3 rats, as compared to control animals. This coincides with a greater relative increase in insulin output from perifused islets during the second phase of the secretory response to the cholinergic agent. Last, the increase provoked by ouabain (1.0 mM) in cytosolic Ca²⁺ concentration, ⁴⁵Ca fractional outflow rate and insulin release are all delayed in the ω3 rats. Taking into account the decreased activity of Na⁺, K⁺-ATPase in the islets of ω3 rats, these findings are interpreted as reflecting an impaired priming of insulin-producing cells when first exposed for 105 min to a physiological postprandial concentration of d-glucose.     

The role of the liver in the disposal of orally administered 14C-glucose in the normal rat

Summary 6-¹⁴C-glucose (specific activity 6.0 Ci/g) was administered by oesophageal tube to fasted normal rats; the load was 1.5 g/kg. Rats were killed, in groups, at 0, 10, 20, 30, 60, 120 and 180 min after administration of the load, and the blood and liver sampled. Serum insulin and serum glucose reached a peak at 20 min. The radioactive serum glucose measured by the radioactivity in the dimedone derivatives of 6-C of glucose reached a peak at 60 min. At this time, 92% of the serum glucose originated from the load. Liver glycogen started to rise after 10–20 min, reaching a maximum at 120 min. The incorporation of ¹⁴C-glucose into liver glycogen paralleled the changes in total glycogen. It is suggested that during the first hour following an oral glucose load there is a reduction in the hepatic release of glucose, accompanied by an increase in hepatic glycogen synthesis. During the second hour there is a further marked increase in hepatic glycogen. Between 120 and 180 min there is a decrease in hepatic glycogen, and hepatic glucose release increases. The results emphasize the important role of the liver in the disposal of an oral glucose load.Presented in part at the Autumn Meeting of the British Diabetic Association, Aberdeen, Scotland, 27th–28th September, 1968.     

Major species differences between humans and rodents in the susceptibility to pancreatic beta-cell injury.

The ability of beta cells to endure assaults may be relevant in the development of insulin-dependent diabetes mellitus. This study examines the susceptibility of human pancreatic islets to agents that are cytotoxic for rodent beta cells--i.e., sodium nitroprusside (NP, a nitric oxide donor), streptozotocin (SZ), or alloxan. After 5-8 days in tissue culture, human or rodent islets were exposed for 14 h to NP (50-200 microM) or for 30 min to SZ or alloxan (1-3 mM). Glucose oxidation by human islets was not reduced by NP, but there was a dose-dependent inhibition in rat (40-90% inhibition; P < 0.001) and mouse (10-60% inhibition; P < 0.05) islet glucose oxidation. Glucose (16.7 mM)-induced insulin release by human islets was not impaired after a 30-min exposure to SZ or alloxan, at concentrations that inhibited insulin release from rat (30-80% inhibition; P < 0.001) or mouse (10-70% inhibition; P < 0.05) islets. The viability of human beta cells purified by flow cytometry was not affected by SZ or alloxan (5 mM), as judged 1 or 4 days after a 10-min exposure and subsequent culture; these conditions were cytotoxic for rat beta cells, with 65-95% (P < 0.01) dead beta cells after 4 days. Human islets transplanted under the kidney capsule of nude mice were not affected by in vivo alloxan exposure, as suggested by preserved graft morphology and insulin content, whereas the endogenous beta cells of the transplanted mice were severely damage (80% decrease in pancreatic insulin content and morphological signs of beta-cell destruction). Thus human beta cells are resistant to NP, SZ, or alloxan at concentrations that decrease survival and function of rat or mouse beta cells. These marked interspecies differences emphasize the relevance of repair and/or defense mechanisms in beta-cell destruction and raise the possibility that such differences may also be present among individuals of the same species.     

Hypophysis and function of pancreatic islets

Summary Insulin secretion and biosynthesis of proinsulin and insulin were determined in isolated pancreatic islets of hypophysectomized rats. Control rats were of both same age and weight. Hypophysectomy was performed either 13 or 5 weeks prior to the investigation, the weight of the animals being either 80 or 170 g. Biosynthesis of insulin was estimated from the amounts of radioactivity incorporated into proinsulin and insulin after incubation of isolated islets at 50 or 300 mg% glucose in the presence of³H-leucine for 3 h. Islet proteins were separated on Sephadex G 50 fine. — Hypophysectomy resulted in a significant decrease of both glucose stimulated secretion and biosynthesis of insulin. It was found that this reduction was 1) more significant when compared with controls of same age 2) more marked in rats which had been hypophysectomized 13 weeks before than in rats after an interval of 5 weeks and 3) less in rats which had been hypophysectomized at a weight of 170 g than in rats in whom pituitary ablation was performed at a weight of 80 g. At basal glucose concentrations, no significant changes of both secretion and biosynthesis of insulin were apparent. The relation of radioactivity incorporated into proinsulin and insulin was unchanged under all conditions. Insulin content of the isolated islets used was found within about the same range in all rats, apart from the animals which had been hypophysectomized 13 weeks before. In islets of these rats, a reduction to 84% was observed. — Our findings may be explained by reduced sensitivity of the pancreatic B-cell to glucose and a slower rate of insulin biosynthesis after hypophysectomy.Alexander von Humboldt-Fellow 1970–1972.H.S. was on leave from the 2nd Medical Clinic, University of Vienna, Austria.Supported by Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, SFB Ulm 87, Proj. 11.     

Properties of isolated human islets of langerhans: insulin secretion,glucose oxidation and protein phosphorylation

Summary In the present study, human islets were isolated by collagenase digestion from the pancreases of three kidney donors. Maintainance of the islets in tissue culture enabled insulin release, glucose oxidation and Ca²⁺-calmodulin-dependent protein phosphorylation to be determined using the same islets. Increasing glucose over a range 0–20 mmol/l resulted in a sigmoidal stimulation of insulin release (28.8±5.2 to 118.4±25.8 U-islet^–-h^–, n=10; threshold <4 mmol/l). There was a marked correlation between the insulin secretory response of the islets to glucose and their rate of glucose oxidation (5.9±0.3 at glucose 2 mmol/l up to 25.8±1.8 pmol-islet^–.h^– at 20 mmol/l, r = 0.98). N-acetylglucosamine (20 mmol/l) failed to elicit a secretory response from the islets. Stimulation of insulin secretion by glucose was dependent upon the presence of extracellular Ca²⁺. Extracts of the islets contained a Ca²⁺-calmodulin-dependent protein kinase which phosphorylated a 48-kdalton endogenous polypeptide. Myosin light-chain kinase activity was demonstrated in the presence of exogenous myosin light chains. This report demonstrates for the first time the sigmoidal nature of glucose-stimulated insulin release from isolated human islets, and its correlation with enhanced glucose oxidation. Furthermore, this is the first report of the presence of Ca²⁺-dependent protein kinases in human islets.     

Effect of hyperthyroidization on (pro)insulin biosynthesis and secretion

Insulin production and secretion in simulated rat hyperthyrosis induced by L-thyroxin, injected intraperitoneally during 8 to 30 days, were studied on isolated Langerhans' islets, using collagenase fermentation. Insulin secretion in vitro was determined by radioimmunoassay, its biosynthesis being evaluated according to 3H-leucine incorporation into de novo formed islet proteins with insulin immunoreactivity. The dissociated effect of thyroxin on the secretory response of beta-cells and their hormone production was revealed. In hyperthyroidized animals a decrease in the islet insulin secretion was seen in the presence of a low glucose content in the incubating medium (5 mM), (pro-)insulin biosynthesis remaining unchanged. (Pro-)insulin concentration increased comparatively to the control following 8-day thyroxin injection under condition of the islet incubation with 15 mM of glucose. Insulin secretion returns to normal after augmentation of hexose content in the incubating medium up to 15 mM (hormone production being not inhibited), indicating the functional character of a decrease in beta-cell secretory response and a significant role in its genesis of the changed beta-cell sensitivity to glucose action, inducing insulin secretion.     

Metabolism of cold-stored pancreatic islets

Summary A previous study showed that the ability of glucose to stimulate insulin release was retained in islets stored at 8 °C for one week provided that glucose was present in a high concentration in the storage medium. The metabolic properties of islets stored in the cold have now been further explored in an attempt to clarify the protective effect of glucose. During storage in the cold the islet formation of ³H₂O from (5-³H) glucose and oxygen consumption were only a few per cent of that of fresh islets whereas the uptake of ⁸⁶Rb⁺ was 20–48%. Rewarming the cold-stored islets to 37 °C after one week of cold-storage restored the ⁸⁶Rb⁺ uptake, the formation of ³H₂O and ¹⁴CO₂ from labelled glucose and oxygen consumption to 75, 80, 60 and 40% respectively of fresh islet levels. The results emphasize the usefulness of cold-storage for preservation of functionally intact isolated islets.On leave from the Metabolic Research Unit, 1143 HSW, University of California, San Francisco, CA 94143, USA     

Altered glucose regulation of insulin biosynthesis in insulinoma cells: mouse beta TC3 cells secrete insulin-related peptides predominantly via a constitutive pathway.

The effects of glucose on insulin gene expression and proinsulin biosynthesis, processing, and secretion were studied in mouse beta TC3 cells, an established insulinoma cell line derived from transgenic mice carrying a hybrid insulin promotor-simian virus-40 tumor antigen. The level of insulin mRNA was maintained at high levels by culture in 11 mM glucose, but essentially disappeared after 48 h of culture without glucose. The rate of insulin biosynthesis in beta TC3 cells was also dependent on glucose concentration over periods of 24 or 48 h (but not during 3 h) of stimulation. Insulin biosynthesis decreased about 50% after 24 h and about 85% after 48 h of incubation without glucose. When beta TC3 cells were incubated without glucose for 48 h, the rate of conversion of proinsulin to insulin was decreased compared to that at 11 mM glucose. Insulin secretion was sustained by medium glucose and also exhibited a much lower threshold for maximal stimulation; 2-deoxyglucose uptake decreased about 50% after 48 h of incubation without glucose. Studies on the secretion of newly synthesized proinsulin/insulin revealed that up to 80% of the total cellular pool of labeled proinsulin was released during a 60-min chase compared to only 10% of labeled insulin. The release of immunoreactive insulin (IRI) during the chase period was stimulated by forskolin and phorbol-12-myristate-13-acetate 1.6- and 10-fold, respectively. However, the release of newly synthesized proinsulin was insensitive to these secretagogues. It is concluded that 1) as in normal islets, glucose influences the steady state levels of proinsulin mRNA in beta TC3 cells; 2) the rate of proinsulin biosynthesis reflects only the level of insulin mRNA; translational control is absent; 3) cellular conversion of proinsulin to insulin is up-regulated by glucose as in normal rat islets; 4) newly synthesized proinsulin is released predominantly via a constitutive, rather than a regulated pathway, in contrast to normal beta-cells.     

Time course studies of glucose-induced changes in glucose-6-phosphate and fructose-1, 6-diphosphate content of mouse and rat pancreatic islets

Summary The concentrations of glucose 6-phosphate (G6P) and fructose 1–6-diphosphate plus triose-phosphates (FDP + TPs) were measured in isolated islets of Langerhans from mice and rats after a sudden increase in extracellular glucose concentration from 0.5 to 3.4 mg/ml. In mouse islets, the contents of G6P and (FDP + TPs) were both raised after a two minute incubation at the high glucose concentration and remained elevated for at least 30 min. In rat islets, the G6P but not the (FDP + TPs) content was increased after a 5 min exposure to high glucose. After a 30 min incubation, both G6P and (FDP + TPs) contents were higher than at low glucose concentration. The (G6P)/(FDP + TPs) ratio was some tenfold higher in mouse islets than in rat islets. Increasing extracellular glucose concentration was associated with an increase in the (G6P)/(FDP + TPs) ratio. The results are consistent with the increased glycolytic rate in response to a raised extracellular glucose concentration arising primarily from an increase in the rate of phosphorylation of glucose, and with the hypothesis that the insulin secretory response to glucose may be mediated by a metabolite of the sugar.Abbreviations used G6P glucose-6-phosphate - FDP + TPs fructose-1, 6-diphosphate plus triose phosphates - 6PG 6-phosphogluconate     

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic β-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca²⁺]_c) in pancreatic β-cells stimulated by extracellular ATP. Extracellular ATP (50 μM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse β-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat β-cells than in mouse. In response to extracellular ATP, rat β-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse β-cells. Rat and mouse β-cells both responded to 9 mM glucose by increasing [Ca²⁺]_c. This increase, however, was pronounced only in the rat β-cells. In 9 mM glucose, extracellular ATP induced a pro-nounced calcium release above the increased level of [Ca²⁺]_c in rat β-cells. In mouse β-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca²⁺]_c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse β-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse β-cells but stimulates insulin secretion from rat β-cells, we suggest that store-operated Ca²⁺ entry may be related to exocytosis in pancreatic rat β-cells.