Biomedical subjects

A Sener

Publications and source records attributed to A Sener.

At least 73 records · Page 4Linked to original sources

PubMed1998

Esterification of D-mannoheptulose confers to the heptose inhibitory action on D-glucose metabolism in parotid cells.

D-mannoheptulose, but not its hexaacetate ester, inhibits, in a competitive manner, D-glucose phosphorylation by either purified beef heart hexokinase or crude parotid gland homogenates. Yet, D-mannoheptulose hexaacetate, but neither the unesterified heptose nor acetate or its methyl ester, inhibits D-[5-3H]glucose utilization and D-[U-14C]glucose conversion to 14CO2 and 14C-labelled acidic metabolites and amino acids in intact isolated parotid cells. It is proposed, therefore, that D-mannoheptulose hexaacetate crosses efficiently the plasma membrane of parotid cells and, after intracellular hydrolysis, allows inhibition of D-glucose phosphorylation by the unesterified heptose. The ester of D-mannoheptulose could thus represent a useful tool to inhibit hexose phosphorylation and interfere with cell growth in cells otherwise resistant to the heptose.

Acetates↗

PubMed1998

Effects of D-glucose and starvation upon the cyclic ADP-ribose content of rat pancreatic islets.

Rat pancreatic islets were found to display a much lower content of immunoreactive CD38 and a much lower ADP-ribosyl cyclase activity than rat spleen or brain. Cyclic ADP-ribose was also measured by a radioimmunological procedure in rat pancreatic islets. In fed rats, the cyclic ADP-ribose content appeared higher after isolation of the islets in the presence of 2.8 mM D-glucose rather than in the absence of the hexose, progressively increased during incubation of the islets for 5-60 min at 37 degrees C, but failed to be affected by the concentration of D-glucose (zero to 20.0 mM) in the incubation medium. In rats fasted for 24 hours, the cyclic ADP-ribose islet content also increased during incubation, but again failed to be affected by the concentration of D-glucose in the incubation medium. Although these findings indicate that the islet cyclic ADP-ribose content is influenced by nutritional and environmental factors, they do not support the view that the insulinotropic action of D-glucose involves major change in the islet cell content of the cyclic nucleotide.

ADP-ribosyl Cyclase↗

PubMed1998

Insulinotropic action of beta-L-glucose pentaacetate.

The metabolism of beta-L-glucose pentaacetate and its interference with the catabolism of L-[U-14C]glutamine, [U-14C]palmitate, D-[U-14C]glucose, and D-[5-3H]glucose were examined in rat pancreatic islets. Likewise, attention was paid to the effects of this ester on the biosynthesis of islet peptides, the release of insulin from incubated or perifused islets, the functional behavior of individual B cells examined in a reverse hemolytic plaque assay of insulin secretion, adenylate cyclase activity in a membrane-enriched islet subcellular fraction, cAMP production by intact islets, tritiated inositol phosphate production by islets preincubated with myo-[2-3H]inositol, islet cell intracellular pH, 86Rb and 45Ca efflux from prelabeled perifused islets, and electrical activity in single isolated B cells. The results of these experiments were interpreted to indicate that the insulinotropic action of beta-L-glucose pentaacetate is not attributable to any nutritional value of the ester but, instead, appears to result from a direct effect of the ester itself on a yet unidentified receptor system, resulting in a decrease in K+ conductance, plasma membrane depolarization, and induction of electrical activity.

Adenylyl Cyclases↗

PubMed1998

99mTc-sesta-(2-methoxy-isobutyl-isonitrile) uptake by pancreatic islets, parotid cells, and mammary carcinoma cells.

99mTc-sesta-(2-methoxy-isobutyl-isonitrile)(Tc-MIBI) is currently used for imaging of several organs. In the present study, its uptake by rat pancreatic islets, rat parotid cells, and human breast adenocarcinoma cells (MCF-7 cells) was found to be grossly proportional to its concentration (up to 0.1 microM), time-related (with a fractional turnover rate close to 2-3 10(-2).min(-1)), and stimulated by D-glucose. Comparable values for the fractional turnover rate were found in prelabeled islets and MCF-7 cells, D-glucose failing to affect Tc-MIBI efflux from prelabeled islets. In the islets, the uptake of Tc-MIBI was decreased at low temperature, in the presence of mitochondrial poisons and at high extracellular K+ concentration, unaffected by the absence of extracellular Ca2+, and increased by nutrient secretagogs, such as 2-ketoisocaproate and the association of L-leucine and L-glutamine. These findings are consistent with the view that Tc-MIBI uptake is ruled by its extracellular concentration, and the polarization of both plasma and mitochondrial membranes. It is proposed that this lipophilic cation may be useful to detect alteration of nutrient metabolism in pancreatic islets deprived of any exogenous fuel.

Animals↗

PubMed1998

Insulinotropic action of the polyacetate esters of metabolized and non-metabolized monosaccharides in pancreatic islets from normal and diabetic rats.

The polyacetate esters of selected monosaccharides were recently found to either stimulate insulin release or inhibit glucose-stimulated insulin secretion in islets from normal rats. The present study extends such findings both to new combinations of either D-glucose or L-leucine and some polyacetate esters and to hereditarily diabetic, as distinct from normal, rats. In the normal animals, 2-deoxy-D-glucose tetraacetate (1.7 mM) increased both glucose- and leucine-stimulated insulin output. The secretory response to L-leucine was also increased by beta-D-glucose pentaacetate, but inhibited by alpha-D-galactose pentaacetate and D-mannoheptulose hexaacetate (1.7 mM) in the islets of normal rats. In the diabetic rats, the secretory response to D-glucose (8.3 mM) was increased by alpha- or beta-D-glucose pentaacetate and 2-deoxy-D-glucose tetraacetate (1.7 mM), inhibited by alpha-D-galactose pentaacetate and D-mannoheptulose hexaacetate, and unaffected by beta-L-glucose pentaacetate, all esters being tested at 1.7 mM concentration. L-Leucine-stimulated insulin release was also increased by alpha-D-glucose pentaacetate, but not significantly affected by beta-D-galactose pentaacetate, beta-L-glucose pentaacetate, 2-deoxy-D-glucose tetraacetate and D-mannoheptulose hexaacetate in the islets of diabetic animals. These findings suggest a dual mode of action of the esters in the pancreatic islet B-cell, involving both the metabolic response to their sugar moieties and a direct effect of the esters themselves upon a specific receptor system. It is proposed that selected esters could be used as insulinotropic tools in non-insulin-dependent diabetes mellitus.

Acetates↗

PubMed1998

Effect of repaglinide upon nutrient metabolism, biosynthetic activity, cationic fluxes and insulin release in rat pancreatic islets.

This study aims at gaining further insight into the mode of action of repaglinide in pancreatic islet B-cells. At a 1.0 mumol/L concentration, the meglitinide analog failed to affect the metabolism of exogenous D-glucose and that of endogenous nutrients in islets prelabeled with either L-[U-14C]glutamine or [U-14C]palmitate. Likewise, repaglinide (1.0 mumol/L) failed to modify significantly the incorporation of L-[4-3H]phenylalanine into TCA-precipitable material in islets exposed to a close-to-physiological concentration of D-glucose (7.0 mmol/L). The threshold concentration for the insulinotropic action of repaglinide was close to 0.1-1.0 mumol/L and a maximal response was reached at 10.0 mumol/L in islets incubated in the presence of 5.6-8.3 mmol/L D-glucose. At a higher hexose concentration (16.7 mmol/L), however, an enhancing action of repaglinide (10 mumol/L) upon glucose-stimulated insulin release was only observed over 25 min stimulation in perifused islets, no significant increase in insulin output being detected when islets were exposed to repaglinide (0.1 mumol/L to 0.1 mmol/L) over 90 min incubation at the high D-glucose level. The increase in insulin output evoked by repaglinide in the islets perifused at 16.7 mmol/L D-glucose coincided with a modest increase in 86Rb outflow and a marked stimulation of 45Ca efflux from prelabeled islets, suggesting stimulation of Ca2+ influx into the islet cells and subsequent activation of Ca(2+)-responsive K+ channels. When the administration of repaglinide was halted, the reversibility of its cationic and secretory effects was more pronounced in islets perifused at a high (16.7 mmol/L), rather than a low (6.0 mmol/L), D-glucose concentration. These findings support the view that the primary site of action of repaglinide consists in a remodeling of cationic fluxes, and document that this drug displays favorable attributes as an insulinotropic agent for the treatment of non-insulin-dependent diabetes, such as its lack of interference with nutrient metabolism and biosynthetic activity in isolated islets, the low threshold concentration for its insulin-releasing action and its capacity to augment, at least transiently, insulin release at a high concentration of D-glucose.

Animals↗

PubMed1997

Cyclic ADP-ribose measurements in rat pancreatic islets.

Cyclic ADP-ribose was measured by a radioimmunological procedure in rat pancreatic islets. The cyclic ADP-ribose content of the islets was much lower immediately after isolation (< or = 2.3 +/- 0.2 fmol/islet) than after a further incubation of 90 min in a salt-balanced medium (26.4 +/- 1.3 fmol/islet). Under the latter condition, the islet cyclic ADP-ribose content was not affected by the concentration of D-glucose and/or D-fructose in the incubation medium and, when expressed per micrograms protein, was 3- to 30-fold higher than that found in other biological samples such as liver, whole pancrease or tumoral islet cells. Although these findings might suggest a physiological role for cyclic ADP-ribose in islet cells, they argue against the view that the cyclic nucleotide acts as a coupling factor in the process of hexosestimulated insulin release.

Adenosine Diphosphate Ribose↗

PubMed1997

Mutation in the calcium-binding domain of the mitochondrial glycerophosphate dehydrogenase gene in a family of diabetic subjects.

The Ca(2+)-sensitive and mitochondrial enzyme FAD-linked glycerophosphate dehydrogenase (m-GDH) represents an essential component of the pancreatic B-cell glucose-sensing device. This report deals with the first identified case of mutation in the calcium-binding domain of the m-GDH gene in a patient with type-2 diabetes and his glucose-intolerant half sister. Single strand conformation polymorphism analysis indeed revealed an abnormal mobility of the 32P-labelled polymerase chain reaction product in these two subjects. The corresponding base pair mutations and amino acid changes were documented. In the diabetic proband, the relative extent of the Ca(2+)-induced activation of m-GDH in CD3+ T-lymphocytes was lower than in his brother with a normal m-GDH gene sequence.

Base Sequence↗

PubMed1997

Stimulation of insulin release by alpha-D-glucose pentaacetate.

Insulin release from rat pancreatic islets was stimulated by alpha-D-glucose pentaacetate (1.7 mM), but not by an equimolar concentration of beta-D-galactose pentaacetate. The secretory response to alpha-D-glucose pentaacetate was not reproduced by D-glucose and/or acetate, tested at concentrations equimolar to that of the hexose ester, and failed to be adversely affected by 3-O-methyl-D-glucose, even when used at a concentration sufficient to inhibit glucose-stimulated insulin release. These findings suggest that the insulinotropic action of alpha-D-glucose pentaacetate is attributable, in part at least, to the intracellular generation of D-glucose from the ester. The present work thus introduces selected esters of D-glucose as tools for the cellular supply of the hexose by a process that does apparently not involve its carrier-mediated transport across the plasma membrane.

3-O-Methylglucose↗

PubMed1997

Effects of dehydroepiandrosterone in rats injected with streptozotocin during the neonatal period.

Control rats and diabetic animals injected with streptozotocin during the neonatal period were either maintained on a standard diet or given access to food supplemented with dehydroepiandrosterone (DHEA, 0.2%) for 11 days before sacrifice. In both control and diabetic rats, DHEA feeding augmented the activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase and cytosolic NADP-linked malate dehydrogenase in liver, but not so in either the parotid gland or pancreatic islets. DHEA lowered, in both control and diabetic rats, the ratio between D-glucose oxidation and utilization and the rate of insulin release in pancreatic islets exposed to a high concentration of D-glucose, as well as the insulin concentration and insulin/glucose ratio in plasma. These findings support the view that, in diabetes, DHEA, by increasing sensitivity to insulin, may allow islet B-cells to avoid the otherwise unfavorable consequences of chronic hyperactivity.

Administration, Oral↗

PubMed1997

Synergistic insulinotropic action of succinate, acetate, and glucose esters in islets from normal and diabetic rats.

Succinic acid esters are currently under investigation as possible insulinotropic tools in the treatment of noninsulin-dependent diabetes mellitus. The present article introduces three novel nutrient esters and aims mainly to explore, in both normal and GK rats, the secretory response to such esters when tested alone or in combination. It documents that in pancreatic islets from normal rats, methyl acetate (10 mM), which fails to augment basal insulin output, potentiates the secretory response to succinate dimethyl ester (also 10 mM). It also reveals that alpha-D-glucose pentaacetate (alpha GPA) (1.7 mM) stimulates insulin release in the absence of any other exogenous nutrient and even more so in the presence of succinate methyl ester. Moreover, the methyl esters of succinic acid (10 mM), when used together with either methyl acetate or alpha GPA, provoked insulin secretion in islets from diabetic GK rats incubated in the absence of D-glucose, although no significant secretory response of such islets could be detected when each of these agents was tested separately. These findings thus draw attention to the insulinotropic potential in type 2 diabetes of selected combinations of nutrient esters, including a D-glucose ester presumably able to enter into islet cells without requiring the intervention of a hexose carrier.

Acetates↗

PubMed1997

Glucose-induced positive cooperativity of fructose phosphorylation by human B-cell glucokinase.

Human B-cell glucokinase displays sigmoidal kinetics towards D-glucose or D-mannose, but fails to do so towards D-fructose. Yet, D-glucose, D-mannose and 2-deoxy-D-glucose confer to the enzyme positive cooperativity towards D-fructose. For instance, in the presence of 5 mM D-[U-14C]fructose, its rate of phosphorylation is increased to 214.3 +/- 11.0%, 134.0 +/- 4.3% and 116.5 +/- 3.0% of paired control value by D-glucose, D-mannose and 2-deoxy-D-glucose (each 6 mM), respectively. D-glucose and, to a lesser extent, D-mannose also display reciprocal kinetic cooperativity. D-fructose, however, fails to affect D-glucose or D-mannose phosphorylation under conditions in which positive cooperativity is otherwise observed. These findings are relevant to the reciprocal effects of distinct hexoses upon their phosphorylation by B-cell glucokinase and, as such, to the metabolic and functional response evoked in pancreatic islet B-cells by these sugars, when tested either separately or in combination.

Cell Line↗

PubMed1997

Subcellular distribution of hexokinase isoenzymes in pancreatic islet cells exposed to digitonin after incubation at a low or high concentration of D-glucose.

It was recently proposed that stimulation of pancreatic islet by D-glucose results in the translocation of glucokinase from the perinuclear area to the cell periphery, where the enzyme might conceivably interact with either the glucose transporter GLUT-2 or some other proteins and, by doing so, become better able to express its full catalytic activity. To explore the possible interaction between glucokinase and the cell boundary, dispersed rat pancreatic islet cells were preincubated for 60 min at a low (2.8 mM) or high (16.7 mM) concentration of D-glucose, then exposed for 1 min to digitonin (0.5 mg/ml) and eventually centrifuged through a layer of oil for separation of the cell pellet from the supernatant fraction containing the material released by digitonin. Under these conditions, the bulk of lactate dehydrogenase and glutamate dehydrogenase activities were recovered in the supernatant fraction and cell pellet, respectively. The measurement of hexokinase isoenzyme activities in the two subcellular fractions, as conducted at low or high hexose concentrations and in either the absence or presence of exogenous hexose phosphates (3.0 mM glucose 6-phosphate and 1.0 mM fructose 1-phosphate) indicated a preferential location of the low-Km hexokinase in the cell pellet and of the high-Km glucokinase in the cytosolic fraction. Such a distribution pattern failed to be significantly affected by the concentration of D-glucose used during the initial incubation of the dispersed islet cells. These findings argue against the view that the glucose-induced translocation of glucokinase would result in any sizeable binding of the enzyme to a plasma membrane-associated protein.

Animals↗

PubMed1997

Potentiation by its esterification of the inhibitory action of 2-deoxy-D-glucose on D-glucose metabolism and insulinotropic action.

It was recently reported that selected esters of D-glucose are more efficiently metabolized and display a greater insulinotropic action in pancreatic islets than the unesterified hexose. Likewise, in the present study, the inhibitory action of 2-deoxy-D-glucose tetraacetate upon D-glucose metabolism in rat erythrocytes and pancreatic islets was found to be more pronounced than that of unesterified 2-deoxy-D-glucose. At a concentration of 10 mM, the ester also inhibited more severely than unesterified 2-deoxy-D-glucose the release of insulin evoked by D-glucose in isolated islets. It is proposed that 2-deoxy-D-glucose tetraacetate represents a useful tool to facilitate the intracellular delivery of the glucose analog and to increase its efficiency as a potential therapeutic agent.

Animals↗

PubMed1997

Insulinotropic action of alpha-D-glucose pentaacetate: functional aspects.

The functional determinants of the insulinotropic action of alpha-D-glucose pentaacetate were investigated in rat pancreatic islets. The ester mimicked the effect of nutrient secretagogues by recruiting individual B cells into an active secretory state, stimulating proinsulin biosynthesis, inhibiting 86Rb outflow, and augmenting 45Ca efflux from prelabeled islets. The secretory response to the ester was suppressed in the absence of Ca2+ and potentiated by theophylline or cytochalasin B. The generation of acetate from the ester apparently played a small role in its insulinotropic action. Thus acetate, methyl acetate, ethyl acetate, alpha-D-galactose pentaacetate, and beta-D-galactose pentaacetate all failed to stimulate insulin release. The secretory response to alpha-D-glucose pentaacetate was reproduced by beta-D-glucose pentaacetate and, to a lesser extent, by beta-L-glucose pentaacetate. It differed from that evoked by unesterified D-glucose by its resistance to 3-O-methyl-D-glucose, D-mannoheptulose, and 2-deoxy-D-glucose. It is concluded that the insulinotropic action of alpha-D-glucose pentaacetate, although linked to the generation of the hexose from its ester, entails a coupling mechanism that is not identical to that currently implied in the process of glucose-induced insulin release.

3-O-Methylglucose↗

PubMed1996

Do leptin receptors play a functional role in the endocrine pancreas?

It was recently speculated that leptin may exert a direct inhibitory effect upon insulin release from the pancreatic B-cell. This proposal meets, however, with two objections. First, although the message for leptin receptors is indeed detected in rat pancreatic islets, the short form of this receptor, for which no signalling function is known, represents the major species present in islet cells. Second, in the isolated perfused rat pancreas, leptin (1.0 nM) fails to affect the release of either insulin or glucagon.

Animals↗

PubMed1996

Insulinotropic action of methyl pyruvate: secretory, cationic, and biosynthetic aspects.

Methyl pyruvate was found to exert a dual effect on insulin release from isolated rat pancreatic islets. A positive insulinotropic action prevailed at low concentrations of D-glucose, in the 2.8 to 8.3 mM range, and at concentrations of the ester not exceeding 10.0 mM. It displayed features typical of a process of nutrient-stimulated insulin release, such as decreased K+ conductance, enhanced Ca2+ influx, and stimulation of proinsulin biosynthesis. A negative insulinotropic action of methyl pyruvate was also observed, however, at a high concentration of D-glucose (16.7 mM) and/or at a high concentration of the methyl ester (20.0 mM). It was apparently not attributable to any adverse effect of methyl pyruvate on ATP generation, but might be due to hyperpolarization of the plasma membrane. The ionic determinant(s) of the latter change was not identified. The dual effect of methyl pyruvate probably accounts for an unusual time course of the secretory response, including a dramatic and paradoxical stimulation of insulin release upon removal of the ester.

Animals↗

PubMed1996

Insulinotropic action of methyl pyruvate: enzymatic and metabolic aspects.

The metabolism of methyl pyruvate was compared to that of pyruvate in isolated rat pancreatic islets. Methyl pyruvate was found to be more efficient than pyruvate in supporting the intramitochondrial conversion of pyruvate metabolites to amino acids, inhibiting D-[5-3H]glucose utilization, maintaining a high ratio between D-[3,4-14C]glucose or D-[6-14C]glucose oxidation and D-[5-3H]glucose utilization, inhibiting the intramitochondrial conversion of glucose-derived 2-keto acids to their corresponding amino acids, and augmenting 14CO2 output from islets prelabeled with L-[U-14C]glutamine. Methyl pyruvate also apparently caused a more marked mitochondrial alkalinization than pyruvate, as judged from comparisons of pH measurements based on the use of either a fluorescein probe or 14C-labeled 5,5-dimethyl-oxazolidine-2,4-dione. Inversely, pyruvate was more efficient than methyl pyruvate in increasing lactate output and generating L-alanine. These converging findings indicate that, by comparison with exogenous pyruvate, its methyl ester is preferentially metabolized in the mitochondrial, rather than cytosolic, domain of islet cells. It is proposed that both the positive and the negative components of methyl pyruvate insulinotropic action are linked to changes in the net generation of reducing equivalents, ATP and H+.

Alanine Transaminase↗