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A Sener

Publications and source records attributed to A Sener.

At least 163 records · Page 9Linked to original sources
PubMed1990

D-glucose and L-leucine metabolism in neonatal and adult cultured rat pancreatic islets.

Neonatal and adult rat islets, cultured for 7-9 days in the presence of 10.5 mM D-glucose, were incubated for 120 min with either D-glucose (2.8 and 16.7 mM) or L-leucine (1.0 and 20.0 mM). The total and anaerobic rates of glycolysis, as judged respectively through the generation of 3H2O from D-[5-3H]glucose and 14C-labelled lactate from D-[3,4-14C]glucose or D-[6-14C]glucose were higher in neonatal than adult islets, but increased to a lesser relative extent in neonatal than adult islets in response to a rise in hexose concentration. The flow through the pentose phosphate pathway, as judged from the difference between D-[1-14C]glucose and D-[6-14C]glucose oxidation was higher in neonatal than adult islets. The flow through the reaction catalyzed by pyruvate dehydrogenase, as judged from the oxidation of D-[3,4-14C]glucose, was lower in neonatal than adult islets incubated in the presence of 16.7 mM (but not 2.8 mM) D-glucose. The oxidation of acetyl residues relative to their generation rate, as judged from the ratio of D-[6-14C]glucose to D-[3,4-14C]glucose oxidation, was not affected by the hexose concentration whether in neonatal or adult islets, but was about twice higher in the latter than former islets. The rate of D-[6-14C]glucose oxidation was also higher in adult than neonatal islets, especially at the high concentration of D-glucose. In both neonatal and adult islets, a rise in hexose concentration stimulated preferentially the oxidation of D[3,4-14C]glucose or D-[6-14C]glucose relative to the utilization of D-[5-3H]glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗
PubMed1990

Hexose metabolism in pancreatic islets. Participation of Ca2(+)-sensitive 2-ketoglutarate dehydrogenase in the regulation of mitochondrial function.

A rise in extracellular D-glucose concentration results in a preferential and Ca2(+)-dependent stimulation of mitochondrial oxidative events in pancreatic islet cells. The possible participation of Ca2(+)-dependent mitochondrial dehydrogenases, especially 2-ketoglutarate dehydrogenase, in such an unusual metabolic situation was explored in intact islets, islet homogenates and isolated islet mitochondria. In intact islets exposed to a high concentration of D-glucose, the removal of extracellular Ca2+ impaired D-[6-14C]glucose oxidation whilst failing to affect the cytosolic or mitochondrial ATP/ADP ratios. In islet homogenates, the activity of 2-ketoglutarate dehydrogenase displayed exquisite Ca2(+)-dependency, the presence of Ca2+ causing a 10-fold increase in affinity for 2-ketoglutarate. In intact islet mitochondria, the oxidation of 2-[1-14C]ketoglutarate also increased as a function of extramitochondrial Ca2+ availability. Moreover, prior stimulation of intact islets by D-glucose resulted in an increased capacity of mitochondria to oxidize 2-[1-14C]ketoglutarate. The absence of extracellular Ca2+ during the initial stimulation of intact islets impaired but did not entirely suppress such a memory phenomenon. It is proposed that the mitochondrial accumulation of Ca2+ in nutrient-stimulated islets indeed accounts, in part at least, for the preferential stimulation of mitochondrial oxidative events in this fuel-sensor organ.

Adenosine Diphosphate↗
PubMed1990

Radioisotopic measurement of femtomolar amounts of NAD(P)H in the assay of enzymatic activity at a single cell level.

A radioisotopic method for the assay of reduced or oxidized pyridine nucleotides, based on the interconversion of 2-[U-14C]ketoglutarate or 2-keto[3,4-3H]glutarate and labelled L-glutamate in the reaction catalyzed by glutamate dehydrogenase, was applied to the measurement of lactate dehydrogenase activity in rat pancreatic islet homogenates. Using the tritiated tracer, the limit of sensitivity of the procedure for NAD(P)H assay was close to 1.0 fmol/sample, and lactate dehydrogenase activity could be measured in as little as 0.0005 islet/sample i.e., at a single cell level. This radioisotopic procedure, which can be used for the assay of various metabolites and enzymic activities, thus provides a tool for investigating the heterogeneity in metabolic behaviour of individual cells.

Animals↗
PubMed1990

A sensitive radioisotopic method for the measurement of NAD(P)H: its application to the assay of metabolites and enzymatic activities.

A radioisotopic method for the assay of NADH or NADPH is presented, which is based on the conversion of 2-[U-14C]ketoglutarate to 14C-labeled glutamate in the reaction catalyzed by glutamate dehydrogenase. The efficiency of the method is close to 75%, its precision (coefficient of variation) close to 5%, and its sensitivity close to 0.1 pmol/sample. This simple and rapid method can be applied to the measurement of several metabolites and enzymatic activities. In the present study, its application to the assay of sorbitol, 3-hydroxybutyrate, glutamate dehydrogenase, 3-hydroxybutyrate dehydrogenase, and glyceraldehyde-3-phosphate dehydrogenase is documented.

3-Hydroxybutyric Acid↗
PubMed1990

Calmodulin and calbindin in pancreatic islet cells.

The process of insulin release evoked by D-glucose and other nutrient secretagogues is triggered by an increase in cytosolic Ca2+ activity. However, some other insulinotropic agents may stimulate insulin release at a close-to-basal concentration of cytosolic ionized calcium. The control of cytosolic Ca2+ concentration depends not solely on the rate of Ca2+ entry into the cell through voltage-sensitive channels and Ca2+ exit via Na(+)-Ca2+ countertransport or active Ca2+ pumping, but also on the subcellular distribution of Ca2+, as dependent, for instance, on both Ca2(+)-ATPase activity and inositol 1,4,5-triphosphate-sensitive release in microsomes and calcium accumulation in mitochondria. Calmodulin and calbindin were both identified in pancreatic islet cells. Activation of adenylate cyclase by calcium-calmodulin may account for the increased production of cyclic AMP in islets stimulated by nutrient secretagogues. Calbindin is present in both normal and tumoral islet cells, and might participate to the alteration of islet function encountered in vitamin D-deprived or repleted rats. However, no target enzyme for calbindin was yet identified in islet cells. Independently of the role of calcium-binding regulatory proteins, the mitochondrial accumulation of calcium may account in part at least, for the preferential stimulation of mitochondrial oxidative events in the process of nutrient-stimulated insulin release.

Animals↗
PubMed1990

Impairment of the mitochondrial oxidative response to D-glucose in pancreatic islets from adult rats injected with streptozotocin during the neonatal period.

Pancreatic islets removed from adult rats injected with streptozotocin during the neonatal period display an impaired secretory response to D-glucose and, to a lesser extent, to L-leucine. Despite normal to elevated hexokinase and glucokinase activities in the islets of these glucose-intolerant animals and despite normal mitochondrial binding of the hexokinase isoenzymes, the metabolic response to a high concentration of D-glucose is severely affected, especially in terms of D-[6-14C]glucose oxidation. Thus, the ratio in D-[6-14C]glucose oxidation/D-[5-3H]glucose utilization is much less markedly increased in response to a rise in hexose concentration and, at a high concentration of D-glucose (16.7 mmol/l), less markedly decreased by the absence of Ca2+ and presence of cycloheximide in diabetic than control rats. This metabolic defect contrasts with (1) a close-to-normal or even increased capacity of the islets of diabetic rats to oxidize D-[6-14C]glucose, [2-14C]pyruvate, L-[U-14C]glutamine and L-[U-14C]leucine at low, non-insulinotropic, concentrations of these substrates; (2) a lesser impairment of the oxidation of L-[U-14C]leucine tested in high concentration (20 mmol/l), the effect of Ca2+ deprivation upon the latter variable being comparable in diabetic and control rats; (3) an unaltered transamination of either [2-14C]pyruvate or L-[U-14C]leucine; and (4) a modest perturbation of glycolysis. The most obvious alteration in glycolysis consists in a lesser increase of the glycolytic flux in response to a rise of D-glucose concentration in diabetic than control rats, this coinciding with an apparent decrease in affinity of glucokinase for the hexose.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Perturbation of pancreatic islet function in glucose-infused rats.

The secretory behavior of insulin- and glucagon-producing cells was found to be perturbed in isolated perfused pancreases removed from rats infused with hypertonic solutions of glucose for 48 hours. The anomalies included a high basal release of insulin and a paradoxical increase in insulin output and decrease in glucagon release in response to a fall in D-glucose concentration. Likewise, in isolated islets prepared from the glucose-infused rats, L-arginine or theophylline stimulated insulin release at a low ambient concentration of D-glucose, at variance with the situation found in islets removed from normal rats. These secretory perturbations could not be attributed to any obvious defect in either the transport of D-glucose into islet cells or its further utilization and oxidation, but coincided with the abnormal accumulation of glycogen in the B-cell. It is proposed that the latter anomaly may play a role in the altered dynamics of insulin release found in animals or patients with long-term hyperglycemia.

Animals↗
PubMed1990

Metabolic and secretory responses of parotid cells to cationic amino acids. Oxidation of the amino acids and interference with the oxidation of D-glucose or endogenous nutrients.

Cationic amino acids were recently found to stimulate amylase release from rat parotid cells. The possible relevance of their oxidative catabolism to such a secretory stimulation was investigated. D-Glucose, which was efficiently metabolized in parotid cells and which augmented O2 uptake above basal value, failed to affect basal or stimulated amylase release. L-Arginine, L-lysine and L-histidine failed to stimulate the oxidation of either exogenous D-[6-14C]glucose or endogenous nutrients in cells pre-labelled with [U-14C]palmitate or L-[U-14C]glutamine. The oxidation of L-[U-14C]arginine, L-[U-14C]ornithine, L-[U-14C]lysine and L-[U-14C]histidine, all tested at a 10 mM concentration, was much lower than that of D-[U-14C]glucose (5.6 mM). These findings argue against the view that the stimulation of amylase release by cationic amino acids would be related to their role as a source of energy in the parotid cells.

Amylases↗
PubMed1990

Hexose metabolism in pancreatic islet cells: the coupling between hexose phosphorylation and mitochondrial respiration.

The possible relevance of D-glucose phosphorylation by mitochondria-bound hexokinase to the control of respiration was examined in mitochondria prepared from either tumoral pancreatic islet cells (RINm5F line) or normal rat liver. In both systems, ATP generated by mitochondria exposed to ADP and succinate could serve as a substrate for the phosphorylation of D-glucose. However, after exposure to exogenous ADP in the presence of succinate, only mitochondria isolated from RINm5F cells displayed a sizeable increase in O2 consumption in response to a subsequent administration of D-glucose. In this respect, the discrepancy between mitochondria from islet cells and liver, respectively, was found to be attributable to the much lower hexokinase activity, relative to respiratory rate, in liver than in RINm5F cell mitochondria. It is speculated that the coupling between hexose phosphorylation and respiration in islet cells may prime the mitochondria to generate ATP during the early metabolic and secretory response to a rise in extracellular D-glucose concentration.

Adenosine Diphosphate↗
PubMed1990

Ketone bodies and islet function: 45Ca handling, insulin synthesis, and release.

D-(-)-beta-Hydroxybutyrate and acetoacetate cause a rapid, sustained, and rapidly reversible stimulation of insulin release from rat pancreatic islets incubated in the presence, but not absence, of D-glucose. This coincides with stimulation of both proinsulin biosynthesis and 45Ca net uptake. The ketone bodies also decrease 45Ca outflow from prelabeled islets perifused in the absence of Ca2+ and, in contrast, enhance effluent radioactivity in the presence of Ca2+. In the presence of D-glucose, the secretory response to D-(-)-beta-hydroxybutyrate is concentration related in the 2.5-20 mM range, abolished in the absence of Ca2+ or presence of KCN, and enhanced by theophylline and forskolin. It corresponds grossly to a shift to the left of the sigmoidal curve relating insulin output to the ambient concentration of D-glucose. The secretory, biosynthetic, and cationic response to acetoacetate is less marked than that evoked by an equimolar concentration of D-(-)-beta-hydroxybutyrate. These features are compatible with the view that the insulinotropic action of ketone bodies would be causally linked to their metabolism in islet cells.

3-Hydroxybutyric Acid↗
PubMed1990

Ketone bodies and islet function: 86Rb handling and metabolic data.

The metabolism of ketone bodies was investigated in rat pancreatic islets incubated in the absence or presence of D-glucose. The generation of 14CO2 from 3-14C-labeled ketone bodies, the interconversion of D-(-)-beta-hydroxybutyrate and acetoacetate (AcAc), the reciprocal effects of ketone bodies and D-glucose on their respective catabolism, and the influence of these exogenous nutrients on the output of 14CO2 from islets preincubated with either L-[U-14C]glutamine or [U-14C]palmitate provided an estimation of the nutrient-induced changes in O2 uptake that was in fair agreement with the observed modifications of islet respiration. There was a close correlation between such changes and the corresponding values for insulin output. Because the stimulation of insulin release by ketone bodies also coincided with a decrease in 86Rb outflow from prelabeled islets, these findings suggest that the insulinotropic action of ketone bodies is causally linked to their catabolism through an increase in ATP generation rate and a subsequent decrease in K+ conductance. A complementary participation of changes in mitochondrial redox state to stimulus-secretion coupling is considered, however, in the light of comparisons between the effects of D-(-)-beta-hydroxybutyrate and AcAc, respectively, on mitochondrial NADH generation, 45Ca net uptake, and D-[6-14C]glucose oxidation.

3-Hydroxybutyric Acid↗
PubMed1990

Stimulus-secretion coupling of arginine-induced insulin release: comparison with histidine-induced insulin release.

L-Histidine, when tested at a 10-mM concentration, caused a rapid and sustained stimulation of insulin release from rat islets exposed to either D-glucose (7.0 or 8.3 mM) or L-leucine (10.0 mM). The stimulation of insulin release could not be ascribed to an increase in oxygen uptake, to the generation of histamine from L-histidine, or to its participation in a transglutaminase-catalyzed reaction. Like other cationic amino acids, however, L-histidine rapidly accumulated in islet cells, increased 86Rb outflow from prelabeled islets perifused in the presence or absence of extracellular Ca2+, and stimulated the entry of Ca2+ into islet cells. Yet, the amount of exogenous L-histidine present in the islet cells with a positively charged side chain was estimated to be below the threshold value required for stimulation of insulin release by fully ionized cationic amino acids, such as L-arginine. Hence, the present findings argue against the view that the insulinotropic action of cationic amino acids is solely attributable to the accumulation of these positively charged molecules inside the islet B cell with subsequent depolarization of the plasma membrane.

Animals↗
PubMed1990

Metabolism and disorders of the endocrine pancreas.

The release of insulin evoked by D-glucose and other nutrient secretagogues in the pancreatic B-cell is causally linked to an increase in ATP generation rate. This fuel concept is documented by the higher glycolytic and secretory responses evoked by the alpha-anomer, as distinct from beta-anomer, of either D-glucose or D-mannose. Experimental models of B-cell dysfunction are characterized by several site-specific anomalies in hexose metabolism. For instance, alpha-stereospecificity of the functional response is apparently perturbed in spontaneously diabetic rats or human subjects. Because phosphoglucoisomerase participates in the anomeric specificity of D-glucose metabolism, the intrinsic properties of this enzyme are reevaluated, with emphasis on both the anomeric specificity and discrimination towards hydrogen isotopes as assessed by magnetic resonance imaging.

Animals↗
PubMed1990

Maturation of fetal rat islet cells in vitro during tissue culture is associated with increased mitochondrial function.

Tissue culture for one or seven days of pancreatic islets isolated from 21-day old fetal rats was found to be associated with a marked increase in the oxidation of L-(U-14C) glutamine by intact islets and in the activity of both alanine-glutamate and aspartate-glutamate transaminases as well as glutamate dehydrogenase in islet homogenates. This coincided with an increase in the relative amount of mitochondrial DNA. The activities of glucose-phosphorylating enzymes (hexokinase and glucokinase), glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase were less markedly increased during the culture period than those of enzymes involved in amino acid catabolism and located, in part at least, in mitochondria. The combined data suggest that the functional maturation of fetal islets during the culture period is associated with and may be attributable to a preferential maturation of their mitochondria.

Alanine Transaminase↗
PubMed1990

Uptake of calcium by pancreatic islet cell microsomes: inhibition by a monoclonal antibody to heart sarcoplasmic reticulum.

The uptake of Ca2+ by microsomes is thought to participate in the control of cytosolic Ca2+ activity in the insulin-producing pancreatic B-cell. In order to study such a phenomenon methods were developed to isolate microsomes from rat parotid cells, pancreatic islets and tumoral islet cells of the RINm5F line. In the latter case, a subcellular microsomal fraction was prepared in which the ratio of microsomal/mitochondrial enzyme markers, as well as that of ruthenium red-resistant/sensitive 45Ca2+ uptake was 20 times higher than in the corresponding mitochondrial subcellular fraction. The ATP-dependent net uptake of 45Ca2+ by RINm5F cell microsomes was inhibited at low temperature and by either vanadate or a monoclonal antibody to dog heart sarcoplasmic reticulum. Although the uptake of Ca2+ by microsomes may account for only a minor fraction of ATP consumption, its synarchistic regulation by ATP and Ca2+, at close-to-physiological concentrations, appeared well suited to play a major regulatory role in the control of cytosolic Ca2+ activity in intact islet cells.

Animals↗
PubMed1989

Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates; comparison with phosphomannoisomerase.

The isotopic discrimination, diastereotopic specificity and intramolecular hydrogen transfer characterizing the reaction catalyzed by phosphomannoisomerase are examined. During the monodirectional conversion of D-[2-3H]mannose 6-phosphate to D-fructose 6-phosphate and D-fructose 1,6-bisphosphate, the reaction velocity is one order of magnitude lower than with D-[U-14C]mannose 6-phosphate and little tritium (less than 6%) is transferred intramolecularly. Inorganic phosphate decreases the reaction velocity but favours the intramolecular transfer of tritium. Likewise, when D-[1-3H]fructose 6-phosphate prepared from D-[1-3H]glucose is exposed solely to phosphomannoisomerase, the generation of tritiated metabolites is virtually restricted to 3H2O and occurs at a much lower rate than the production of D-[U-14C]mannose 6-phosphate from D-[U-14C]fructose 6-phosphate. However, no 3H2O is formed when D-[1-3H]fructose 6-phosphate generated from D-[2-3H]glucose is exposed to phosphomannoisomerase, indicating that the diastereotopic specificity of the latter enzyme represents a mirror image of that of phosphoglucoisomerase. Advantage is taken of such a contrasting enzymic behaviour to assess the back-and-forth flow through the reaction catalyzed by phosphomannoisomerase in intact cells exposed to D-[1-3H]glucose, D-[5-3H]glucose or D-[6-3H]glucose. Relative to the rate of glycolysis, this back-and-forth flow amounted to approx. 4% in human erythrocytes and rat parotid cells, 9% in tumoral cells of the RINm5F line and 47% in rat pancreatic islets.

Animals↗
PubMed1989

Stimulus-secretion coupling of arginine-induced insulin release. Metabolism of L-arginine and L-ornithine in pancreatic islets.

Exogenous L-arginine and L-ornithine rapidly accumulate in rat pancreatic islets. L-Arginine is converted to L-ornithine and urea. Endogenous or exogenous L-ornithine generates di- and polyamines, the putrescine turnover being faster than that of spermidine and spermine. However, the major pathway for L-ornithine metabolism consists of its transamination to L-glutamaldehyde and further conversion to L-glutamate. The amines and L-glutamate derived from exogenous L-ornithine are incorporated into islet proteins at the intervention of transglutaminase and cycloheximide-sensitive biosynthetic processes, respectively. These findings suggest the hypothesis that the insulinotropic action of L-arginine and L-ornithine could somehow be related to the metabolism of these cationic amino acids in islet cells.

Animals↗