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Biomedical subjects

A Sener

Publications and source records attributed to A Sener.

At least 217 records · Page 12Linked to original sources
PubMed1987

Opposite effects of D-glucose and a nonmetabolized analogue of L-leucine on respiration and secretion in insulin-producing tumoral cells (RINm5F).

D-Glucose increased the cytosolic NADH/NAD+ ratio (but not the cytosolic NADPH/NADP+ ratio), augmented O2 uptake, raised the ATP/ADP ratio, decreased 86Rb outflow, and stimulated insulin release in tumoral insulin-producing cells of the RINm5F line. L-Leucine and 4-methyl-2-oxopentanoate also stimulated insulin secretion. In the RINm5F cells, as in normal islet cells, the nonmetabolized analogue of L-leucine, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH), activated glutamate dehydrogenase, augmented L-[U-14C]glutamine oxidation, and induced a more reduced state of cytosolic redox couples. However, in sharp contrast to either its effect in normal islet cells or that of D-glucose in the tumoral cells, BCH severely decreased O2 uptake, lowered the ATP/ADP ratio, increased 86Rb outflow, and inhibited insulin release in the RINm5F cells. These findings are interpreted to support the concept that the rate of ATP generation represents an essential determinant of the secretory response of insulin-producing cells to nutrient secretagogues.

Adenosine Diphosphate↗
PubMed1987

The coupling of metabolic to secretory events in pancreatic islets: comparison between insulin release and cytosolic redox state.

The cytosolic ratio of free NADH/NAD+ and NADPH/NADP+ was measured in rat pancreatic islets exposed to D-glucose, 2-ketoisocaproate or L-leucine. All nutrients augmented both cytosolic ratios above basal value. There was no parallelism between the magnitude of such changes and the rate of insulin release. It is proposed that the changes in cytosolic redox potential do not represent the essential, or at least sole, determinant of nutrient-induced insulin release.

Animals↗
PubMed1987

Interaction between D-glucose and Ca2+ in the priming of the pancreatic B-cell.

Prior exposure of the endocrine pancreas to a high as distinct from low concentration of D-glucose is thought to result in either an increased (priming action) or decreased (glucotoxic action) responsiveness of the B-cell to a subsequent stimulation by the hexose. In order to investigate these phenomena in vitro, rat pancreatic islets were preincubated for 180 min at increasing concentrations of D-glucose in the absence or presence of extracellular Ca2+, and then stimulated with the hexose (16.7 mM) in the absence or presence of theophylline (1.4 mM). Under these conditions, a dose-related priming action of D-glucose was observed without evidence of any glucotoxic effect. The priming effect persisted when the islets were preincubated in the absence of Ca2+, although such a prior Ca2+ deprivation itself decreased the magnitude of the further secretory response to D-glucose. The presence of theophylline in the final incubation medium minimized but failed to abolish the priming action of D-glucose. It is proposed that Ca2+ and cyclic AMP availability may interfere with the priming action of D-glucose. Moreover, the present results suggest that a more stringent or more prolonged pretreatment of the islet cells may be required in order to allow for the occurrence of the postulated cytotoxic action of D-glucose upon the pancreatic B-cell.

Animals↗
PubMed1986

Hexose metabolism in pancreatic islets: compartmentation of hexokinase in islet cells.

Hexokinase activity was found in both soluble (cytosolic) and particulate subcellular fractions prepared from rat pancreatic islet homogenates. The bound enzyme was associated with mitochondria rather than secretory granules. Relative to the total hexokinase activity, the amount of bound enzyme was higher in islet homogenates prepared at pH 6.0 (72 +/- 7%) than in islets homogenized at pH 7.4 (38 +/- 1%). The affinity of hexokinase for equilibrated D-glucose was not different in the cytosolic and mitochondrial fractions. In both fractions, hexokinase displayed a greater affinity for alpha- than beta-D-glucose, but a higher maximal velocity with the beta- than alpha-anomer. Glucose 6-phosphate inhibited to a greater extent cytosolic than mitochondrial hexokinase. A high Km glucokinase-like enzymic activity was also present in both subcellular fractions. It is proposed that the ambiguity of hexokinase plays a propitious role in the glucose-sensing function of pancreatic islet cells.

Animals↗
PubMed1986

The coupling of metabolic to secretory events in pancreatic islets: effects of 2-cyclohexene-1-one upon GSH content and secretory behaviour.

The GSH content and GSH/GSSG ratio were decreased in rat pancreatic islets exposed to 2-cyclohexene-1-one (CHX; 1.0 mM), but the drug failed to affect the cytosolic NADH/NAD+ and NADPH/NADP+ ratios. This coincided with inhibition of D-glucose oxidation, whilst the oxidation of L-leucine and L-glutamine was little affected by CHX (1.0 mM). The release of insulin evoked by either D-glucose or 2-ketoisocaproate was inhibited by CHX (1.0 mM), whereas such was not the case for insulin secretion induced by L-leucine, alone or in combination with L-glutamine. The latter amino acid protected the B-cell against the inhibitory action of CHX upon glucose-stimulated insulin release. CHX severely altered the normal relationship between nutrient oxidation, [45Ca] net uptake and insulin release. Since CHX also inhibited insulin release evoked by non-nutrient secretagogues, it is speculated that GSH may be involved in several cytophysiological processes including the control of glycolysis, intracellular calcium distribution, and responsiveness to this cation of Ca2+-sensitive targets.

Amino Acids↗
PubMed1986

The coupling of metabolic to secretory events in pancreatic islets: inhibition by 2-cyclohexene-1-one of the secretory response to cyclic AMP and cytochalasin B.

In rat pancreatic islets perifused in the presence of 2-cyclohexene-1-one (CHX; 1.0 mM), the secretory response to either D-glucose or 2-ketoisocaproate, but not that evoked by the association of L-leucine and L-glutamine, was severely decreased. This coincided with a decreased stimulation of [45Ca] efflux from prelabelled islets, whereas the inhibitory action of D-glucose or 2-ketoisocaproate upon both [86Rb] and [45Ca] efflux appeared little or not affected. In the presence of D-glucose, the islets exposed to CHX were virtually unresponsive to either forskolin, theophylline or cytochalasin B. A severe decrease in the secretory response to forskolin was also observed in CHX-treated islets exposed to L-leucine and L-glutamine. Except for a somewhat lower sensitivity to NaF, no major change in adenylate cyclase activity or cyclic AMP production was observed in CHX-treated islets. The activity of protein kinase A was decreased in such islets but its responsiveness to cyclic AMP appeared unaltered. Transglutaminase activity was severely decreased in homogenates derived from CHX-treated islets. These findings suggest that CHX, possibly by lowering the GSH content of islet cells, impairs the functional capacity of the effector system for insulin release, in addition to and independently of any effect that it may exert upon nutrient catabolism and cationic fluxes in the islet cells.

Adenylyl Cyclases↗
PubMed1986

Phosphorylation of 3-O-methyl-D-glucose by yeast and beef hexokinase.

Beef heart hexokinase and yeast hexokinase both catalyzed the phosphorylation of 3-O-[14C]methyl-D-glucose. The maximal velocity was 3 orders of magnitude lower and the Km for the glucose analogue 40-120-times higher than those observed with D-[U-14C]glucose. Hence, 3-O-methyl-D-glucose should not be considered as a truly nonmetabolized analogue of D-glucose.

3-O-Methylglucose↗
PubMed1986

Artefactual and true uptake of labelled sucrose by rat pancreatic islet cells.

Labelled sucrose is currently used as an extracellular marker in pancreatic islets or isolated islet cells. Contamination of [6,6'(n)-3H]sucrose by tritiated hexose results in severe overestimation of the extracellular space. The relative magnitude of this overestimation depends on such factors as the temperature and length of incubation and the presence or absence of metabolized unlabelled hexose. Even with uncontamined [U-14C]sucrose, a restricted amount of the disaccharide is apparently taken up, hydrolyzed and further metabolized in the islet cells. The uptake of sucrose by islet cells may reflect the phylogenic maintenance of a physiological attribute of these primitively intestinal epithelial cells.

Animals↗
PubMed1986

Sensitivity to pentamidine but resistance to streptozotocin of tumoral insulin-producing cells (RINm5F line).

Pancreatic islets removed from normal rats and tumoral insulin-producing cells of the RINm5F line were examined for their respective sensitivity to two distinct beta-cytotoxic agents, namely pentamidine and streptozotocin. After 20 h of culture in the presence of pentamidine (5-500 microM), a dose-related decrease in both insulin output and D-[5-3H] glucose utilization was observed in pancreatic islets. Under identical conditions, pentamidine caused a dose-related destruction of RINm5F cells and, in the remaining cells, also impaired D-[5-3H] glucose utilization. When pancreatic islets were exposed for 60 or 120 min to streptozotocin (3.77 mM), both the utilization of D-[5-3H] glucose and oxidation of D-[U-14C] glucose were impaired. However, under identical conditions, the RINm5F cells appeared resistant to the cytotoxic action of streptozotocin. Since streptozotocin, but not pentamidine, is thought to be transported into islet cells at the intervention of a hexose carrier, these findings support the concept that a low efficiency of this carrier-mediated process represents a fundamental defect of the RINm5F cells.

Amidines↗
PubMed1986

3-O-methyl-D-glucose transport in tumoral insulin-producing cells.

Tumoral insulin-producing cells of the RINm5F line were exposed at different temperatures, and for various lengths of time to increasing concentrations of 3-O-methyl-D-[U-14C]glucose. The uptake of the hexose represented a temperature-sensitive and saturable process, so that no rapid equilibration of hexose concentrations across the plasma membrane was reached, especially at low temperature and/or high concentrations of 3-O-methyl-D-glucose. The uptake of 3-O-methyl-D-[U-14C]glucose was not affected by a prior loading of the cells with the unlabeled hexose and its release from prelabeled cells was observed in the absence of any concentration gradient across the plasma membrane. The uptake of D-[U-14C]glucose and utilization of D-[5-3H]glucose was inhibited by 3-O-methyl-D-glucose, which failed, however, to affect D-[U-14C]glucose oxidation. At variance with the situation found in normal insulin-producing cells, the transport of D-glucose into the tumoral cells may thus play a regulatory role in its metabolism.

3-O-Methylglucose↗
PubMed1986

D-glucose transport and concentration in tumoral insulin-producing cells.

The uptake of D-[U-14C]glucose and D-[5-3H]glucose by tumoral insulin-producing cells of the RINm5F line was measured over 3-30 min of incubation at 7-37 degrees C. The apparent distribution space of the hexose ranged from values as low as the L-[1-14C]glucose space to values 10 times higher than the 3H2O space. Although a major fraction of the radioactivity recovered in the cellular pellet corresponded to the metabolites of D-glucose, the results suggested that the transport of D-glucose into the tumoral cells represents a saturable and temperature-dependent process. When D-glucose was measured by an enzymic procedure in cells incubated for 30 min at 37 degrees C in the presence of 50.0 mM D-glucose, the apparent distribution space of the hexose remained lower than the intracellular water space. These results indicate that the RINm5F cells have lost an essential attribute of the glucose-sensing device in normal insulin-producing cells, namely the ability to ensure the equilibration of D-glucose concentrations across the plasma membrane.

Adenoma, Islet Cell↗
PubMed1986

Influence of lactation upon pancreatic islet function.

The activity of adenylate cyclase, its responsiveness to NaF and forskolin, the activity of the protein kinases A and C, and the oxidation of exogenous D-glucose, L-leucine, and L-glutamine were all higher in pancreatic islets removed from lactating, as distinct from nonlactating, rats. Yet, the release of insulin evoked by D-glucose or the association of L-leucine and L-glutamine was lower in the islets obtained from lactating animals. The lactation-induced decrease in secretory activity was not attributable to a change in the insulin content of the islets, was not corrected by exposure of the islets to theophylline or forskolin, and was also observed in response to stimulation by Ba2+. The rapidly exchangeable islet Ca pool, as estimated from the basal value for 45Ca net uptake, was severely decreased in lactation. Moreover, a hypoglycemic sulfonylurea, which stimulated islet 45Ca net uptake much more markedly in lactating than nonlactating animals, provoked, in association with 12-O-tetradecanoylphorbol-13-acetate, a greater insulin output in lactating than nonlactating rats. It is speculated that the decreased secretory activity in islets removed from lactating rats may be accounted for, in part at least, by a decreased Ca content of the islet cells.

Adenylyl Cyclases↗
PubMed1986

Temperature dependency of the anomeric specificity of yeast and bovine hexokinases.

The phosphorylation of alpha- and beta-D-glucose by either yeast hexokinase or beef heart hexokinase was measured at both 10 and 30 degrees C. At 30 degrees C, the anomeric specificity of yeast hexokinase represented a mirror image of that of bovine hexokinase, in terms of both maximal velocity and affinity. A decrease in temperature apparently accentuated the anomeric difference in both maximal velocity and affinity of bovine hexokinase. Such a difference consisted in a higher maximal velocity with beta- than alpha-D-glucose, but a greater affinity for the alpha- than beta-anomer. In yeast hexokinase, however, the decrease in temperature suppressed the anomeric difference in maximal velocity and inversed the anomeric difference in affinity. In the case of both enzymes, the fall in temperature decreased more the maximal velocity recorded with alpha-D-glucose than that measured with beta-D-glucose, and severely lowered the Km for alpha-D-glucose, whilst failing to affect significantly the Km for beta-D-glucose. These findings, which allow to reconcile prior apparently conflicting data, reveal that the anomeric behaviour of hexokinase is affected by the ambient temperature. Our data also support the view that hexokinase underwent a phylogenic evolution in terms of its anomeric specificity.

Animals↗
PubMed1986

Reciprocal influence of glucose anomers upon their respective phosphorylation by hexokinase.

The phosphorylation of D-glucose (1.0mM) was measured in homogenates of tumoral islet cells incubated at 7 degrees C in the presence of labelled alpha- and/or beta-D-glucose, with or without exogenous glucose 6-phosphate. The close-to-maximal reaction velocity of hexokinase was higher with beta- than alpha-D-glucose. The latter anomer inhibited beta-D-glucose phosphorylation more than the beta-anomer decreased the phosphorylation of alpha-D-glucose. This behaviour was accounted for by the higher affinity of hexokinase for alpha- than for beta-D-glucose. These direct measurements of the relative contribution of each anomer to the overall rate of glucose phosphorylation in the presence of mixed populations of alpha- and beta-D-glucose validate the concept that the phosphorylation of D-glucose displays anomeric specificity even when the hexose is used at anomeric equilibrium. Glucose 6-phosphate inhibited the phosphorylation of the two anomers more severely when alpha-D-glucose rather than beta-D-glucose was the most abundant anomer.

Animals↗
PubMed1986

Anomeric specificity of mammalian hexokinase.

The anomeric specificity of hexokinase was examined in crude homogenates of rat parotid gland, erythrocytes and pancreatic islets. At 8 degrees C, the alpha/beta ratio in maximal velocity averaged 0.73, 0.66 and 0.75 in the parotid, erythrocytes and pancreatic islets, respectively. Hexokinase displayed a greater affinity for alpha- than beta-D-glucose as judged from three criteria: the Km value, the reaction velocity measured with mixtures of the two anomers and their effect upon the phosphorylation of D-[U-14C] glucose in anomeric equilibrium. The latter procedure yielded an alpha/beta ratio in Km close to 0.51, 0.49 and 0.39 in parotid, erythrocytes and pancreatic islets, respectively. Within the limits of this study, the anomeric specificity of mammalian hexokinase would appear to be a mirror image of that of yeast hexokinase.

Animals↗
PubMed1986

Glucose-induced activation of transglutaminase in pancreatic islets.

Transglutaminase activity in rat islet homogenates was increased after preincubation of the islets at high glucose concentration, and severely decreased after preincubation in the presence of either 1,2-bis(2-chloroethyl)-1-nitrosurea or 2-cyclohexene-1-one. The stimulatory action of glucose was still observed when the islets were preincubated in the absence or extracellular Ca2+. The enzymic activity was decreased by NAD+ or NADP+ but not NADH or NADPH, and inhibited by GSSG more than by GSH. These findings suggest that the glucose-induced activation of transglutaminase may be related to induction of a more reduced redox state with subsequent change in thiol-disulfide balance.

Animals↗