Glucose In Rat Pancreatic Islets Research Articles

Contribution of Na+/Ca2+ exchanger to glucose-induced [Ca2+]i increase in rat pancreatic islets.

The present study was carried out to elucidate the role of the reverse mode of the Na+/Ca2+ exchanger in an increase in intracellular Ca2+ concentration ([Ca2+]i) induced by a stimulatory concentration of glucose in rat pancreatic islets. The effects of KB-R7943, a selective inhibitor of reverse Na+/Ca2+ exchanger, on Na+o removal-induced [Ca2+]i changes were examined by a microfluorimetric method using fura-2 in perifused preparations of isolated rat pancreatic islets. Na+o removal induced a rapid increase in [Ca2+]i under 100 or 5 mM K+ conditions, respectively. The increases in [Ca2+]i induced by Na+o removal were inhibited by KB-R7943. The net amount of the [Ca2+]i increases during Na+o removal (Delta[Ca2+]i), obtained by subtracting the KB-R7943-independent Delta[Ca2+]i in the presence of KB-R7943 from Delta[Ca2+]i in the absence of KB-R7943, was significantly increased when extracellular K+ was raised. Increasing the external glucose concentration from 3 to 20 mM caused a biphasic increase in [Ca2+]i, which exhibited a transient increase (first phase) followed by a sustained increase (second phase) in [Ca2+]i. KB-R7943 (10 microM) partially inhibited the second phase of the [Ca2+]i increase rather than the first phase. These results suggest that the increase in [Ca2+]i induced by Na+o removal may be enhanced when plasma membrane is depolarized, and consequently, Ca2+ influx through the reverse Na+/Ca2+ exchanger may partially contribute to the glucose-induced [Ca2+]i dynamics in rat pancreatic islet cells.

Hexokinase shift to mitochondria is associated with an increased sensitivity to glucose in rat pancreatic islets

Hexokinase shift to mitochondria is associated with an increased sensitivity to glucose in rat pancreatic islets

Roles of GTP and phospholipase C in the potentiation of Ca(2+)-induced insulin secretion by glucose in rat pancreatic islets.

Glucose can augment insulin secretion independently of K+ channel closure, provided cytoplasmic free Ca2+ concentration is elevated. A role for phospholipase C (PLC) in this phenomenon has been both claimed and refuted. Recently, we have shown a role for GTP in the secretory effect of glucose as well as in glucose-induced PLC activation, using islets pre-treated with GTP synthesis inhibitors such as mycophenolic acid (MPA). Therefore, in the current studies, we examined first, whether glucose augments Ca(2+)-induced PLC activation and second, whether GTP is required for this effect, when K+(ATP) channels are kept open using diazoxide. Isolated rat islets pre-labeled with [3H]myo-inositol were studied with or without first priming with glucose. There was a 98% greater augmentation of insulin secretion by 16.7 mM glucose (in the presence of diazoxide and 40 mM K+) in primed islets; however, the ability of high glucose to augment PLC activity bore no relationship to the secretory response. MPA markedly inhibited PLC in both conditions; however, insulin secretion was only inhibited (by 46%) in primed islets. None of these differences were attributable to alterations in labeling of phosphoinositides or levels of GTP or ATP. These data indicate that an adequate level of GTP is critical for glucose's potentiation of Ca(2+)-induced insulin secretion in primed islets but that PLC activation can clearly be dissociated from insulin secretion and therefore cannot be the major cause of glucose's augmentation of Ca(2+)-induced insulin secretion.

Age-Related Impairment in the Short Term Regulation of Insulin Biosynthesis by Glucose in Rat Pancreatic Islets*

The pancreatic beta-cell is differentiated to express the insulin biosynthetic secretory pathway and to regulate this pathway principally by sensing the extracellular glucose concentration. With aging, glucose stimulation of both insulin synthesis and secretion is significantly impaired. The mechanism of the 40-50% decrease in the short term stimulation of insulin synthesis by glucose in rat pancreatic islets was studied in an effort to elucidate the specific stage at which the impairment occurs. In isolated islets from both young (4- to 5-month-old) and old (18- to 28-month-old) rats at normal (5.5 mM) and elevated (11 mM) glucose concentrations, insulin I was the predominant mature insulin species that accumulated during a 4-h labeling period. After preincubation of islets for 16 h in Minimum Essential Medium (with 5.5 mM glucose), total preproinsulin mRNA levels were similar in old and young islets; after preincubation in an enriched medium, RPMI-1640 (with 11 mM glucose), old and young levels were still similar, although preproinsulin mRNA levels were 6-fold higher. Proinsulin synthesis during a 30-min pulse label period was stimulated 10-fold in young islets and 7-fold in old islets at 16.7 mM glucose compared with synthesis at 2.8 mM glucose and was not affected by 5 micrograms/ml actinomycin-D. Furthermore, stimulation of the two major known intracellular second messenger systems in beta-cells, the cAMP system and the protein kinase-C system, with 8-bromo-cAMP, 3-isobutyl-1-methylxanthine, or a phorbol ester did not further increase proinsulin synthesis over a 30-min pulse label period in islets already stimulated by 16.7 mM glucose. Thus, the impairments in the short term effects of glucose on proinsulin synthesis in old islets are posttranscription of preproinsulin mRNA and likely to lie at the level of enhancement of translation of preexisting preproinsulin mRNA by factors activated or inhibited by the ambient glucose concentration. These age-impaired factors are not modulated through the cAMP or protein kinase-C system.

Possible involvement of diacylglycerol-activated, Ca2+-dependent protein kinase in glucose memory of the rat pancreatic B-cell.

Exposure to high concentrations of glucose potentiates insulin release from the pancreatic B-cell stimulated by various secretagogues after an interval under basal condition. We studied the role of diacylglycerol-activated, Ca2+-dependent protein kinase (protein kinase C) in this priming effect of glucose in rat pancreatic islets, using 12-O-tetradecanoyl phorbol-13-acetate (TPA), 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7),N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004) and forskolin. The priming effect of glucose was mimicked by 10 nmol/l of TPA, an activator of protein kinase C, but not by 5 mumol/l of forskolin, which increases cAMP via activating adenylate cyclase. When pancreatic islets were exposed to glucose (10 mmol/l) together with 50 mumol/l of H-7, an inhibitor of protein kinase C, the secretory response to glucose (10 mmol/l) after a 30-min interval was significantly reduced compared with that in the islets previously exposed to 10 mmol/l glucose alone. In contrast, this was not the case for HA-1004, its inhibitory activity against protein kinase C being less potent than H-7. These findings suggest that protein kinase C may play an important role in the priming effect of glucose on the pancreatic B-cell.

Interactions of Cholecystokinin and Glucose in Rat Pancreatic Islets

The effects of sulfated cholecystokinin (CCK-8S) and glucose on insulin secretion and polyphosphoinositide (PPI) metabolism were studied in isolated rat islets. Both agonists stimulate PPI hydrolysis, inositol phosphate accumulation, 3H efflux from [3H]inositol-prelabeled tissue, and 45Ca efflux from prelabeled cells. However, the effects of CCK-8S on PPI metabolism are considerably greater than those of glucose. Furthermore, the effects of CCK-8S on PPI and Ca2+ metabolism are observed whether islets are incubated in either 2.75 or 7 mM glucose, but CCK-8S only stimulates insulin secretion (a biphasic response) when the higher glucose concentration is present. Addition of 1 microM forskolin to islets incubated in media containing 2.75 mM glucose does not influence basal insulin secretion but sensitizes the islets to the action of CCK-8S. In the presence of forskolin, CCK-8S induces a very marked first phase but no second phase of insulin secretion. We postulate that CCK-8S acts in this tissue via receptor-linked PPI hydrolysis, leading to an inositol trisphosphate-induced Ca2+ efflux. These receptor-mediated effects of CCK-8S are not altered either by the ambient glucose concentration or the cAMP content of the islets, but these two factors determine the responsiveness of the islets (in terms of insulin secretion) to a given CCK-8S signal.

Interactions of cholecystokinin and glucose in rat pancreatic islets

Interactions of cholecystokinin and glucose in rat pancreatic islets

Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic, ionic, and secretory events

The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, 32P, 86Rb, 45Ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 m M was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in 86Rb and 45Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in 45Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in 45Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.

Multiple metabolic functions of glucose in rat pancreatic islets

Multiple metabolic functions of glucose in rat pancreatic islets

Multiple metabolic functions of glucose in rat pancreatic islets.

Metabolic interactions between glucose and amino acids were studied with isolated rat islets using glucose utilization and lactate formation as indicators. Certain amino acids (8-10 mM) are capable of greatly stimulating lactate formation from 5mM glucose. On a molar basis L-isoleucine is the most potent stimulator in a group of twenty-six amino acids. Aphysiological amino acid mixture (7.5-14 mM) or L-isoleucine (8 mM) profoundly altered the basic sigmoidal relation between glucose concentration in the medium and the rate of glucose utilization and lactate formation: with basal glucose (5 mM) both glucose utilization and lactate production were stimulated by the amino ACID MIXTURE and by L-isoleucine; at high glucose levels utilization was decreased by the amino acid mixture, but was unaffected by L-isoleucine, whereas lactate formation was decreased by both additions. The data indicate that amino acids may play a significant role in regulating the extent to which glucose serves as a fuel of pancreatic islet cells and in determining the pathways of glucose metabolism. In order to elucidate the mechanisms of the amino acid effect, studies with phloridzin, ouabain, iodoacetate, cytochalasin B, and Na+-deficiency were performed with the most effective amino acid, L-isoleucine. Each of these agents and Na+-deficiency substantially reduced or completely blocked the extra lactate formation induced by L-isoleucine (8-10 mM). The intracellular uptake of 14-CL-isoleucine by isolated islets was found to be Na+-independent, and uphill transport of this amino acid was not detectable, whether basal glucose was present in the medium or not. The action of iodoacetate in blocking glycolysis was reinvestigated. After forty-five minutes of exposure, 0.2mM iodoacetate completely blocks lactate formation as well as glucose utilization. Thisconfirms and extends earlier data for this laboratory and suggests that this SH-reagent indeed allows dissociation of the fuel and releasing functions of glucose.