Biomedical subjects

A Sener

Publications and source records attributed to A Sener.

At least 109 records · Page 6Linked to original sources

PubMed1995

The iodide channel of the thyroid. II. Selective iodide conductance inserted into liposomes.

An iodide channel has been previously identified in the plasma membrane of bovine throcytes [Golstein et al., Am. J. Physiol. 263 (Cell Physiol. 32): C590-C597, 1992]. The plasma membrane proteins were solubilized and ultrafiltered, and the protein fraction collected above 100 kDa was inserted in liposomes. Voltage-sensitive uptake of radiolabeled I- by these proteoliposomes was studied. To this end, an outward I- gradient was set up by loading the proteoliposomes with KI and removing extraliposomal I-. I- exit from the proteoliposome induces an inside positive membrane potential, which leads to the uptake of 125I- added to the incubation medium. This uptake was abolished by valinomycin, which in the presence of K+ short circuits the liposomal membrane potential, demonstrating the conductive nature of this uptake. A double reciprocal plot of I- influx over I- concentration suggests the existence of a single population of channels in these proteoliposomes with a Michaelis-Menten constant for I- of approximately 9 microM. When the proteoliposomes were loaded with KCl or KSCN instead of I-, no conductive uptake occurred anymore, suggesting that these anions are unable to diffuse through the I- conductance, hence do not generate a diffusion potential. I- uptake by KI-loaded proteoliposomes was not inhibited in the presence of a 1,000-fold excess of extraliposomal Cl- but was completely inhibited by a 1,000-fold excess of extraliposomal SCN-, indicating that Cl- does not permeate the I- channel, whereas SCN- inhibits it. SCN- and flufenamate were both shown to be competitive inhibitors of the I- channel with an inhibitor constant of approximately 10 and 750 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Impaired activity of rat pancreatic islet mitochondrial glycerophosphate dehydrogenase in protein malnutrition.

In rats that received a low protein isocaloric diet (protein content of the diet: 8 instead of 20%) during fetal life and thereafter up to the time of sacrifice at 12-13 weeks of age, a low plasma insulin concentration, a decreased insulin content of isolated pancreatic islets, and an impaired secretory response of the islets to either D-glucose or the association of L-leucine and L-glutamine coincided, in islet homogenates, with a low activity of the mitochondrial glycerophosphate dehydrogenase and an abnormally high ratio between glutamate-alanine and glutamate-aspartate transaminase activities. Opposite enzymatic changes were found in liver extracts of the same rats. No obvious change in these hormonal, secretory, and enzymatic variables were observed when the period of protein deficiency was restricted to fetal life. These findings support the view that, in protein malnutrition, an impaired activity of pancreatic B-cell mitochondrial glycerophosphate dehydrogenase contributes, possibly in association with other enzymatic anomalies, to the perturbation of islet function.

Animals↗

PubMed1994

Impaired FAD-glycerophosphate dehydrogenase activity in islet and liver homogenates of fa/fa rats.

The mitochondrial FAD-linked enzyme glycerophosphate dehydrogenase plays a key role in the pancreatic B-cell glucose sensing device. In the present study, the activity of this enzyme was examined in islets of fa/fa rats in which inherited diabetes mellitus is associated with obesity, hyperinsulinism and severe insulin resistance. The specific activity of both FAD-linked glycerophosphate dehydrogenase and glutamate dehydrogenase were decreased in islet and liver homogenates prepared from fa/fa, as compared to Fa/Fa, rats, this coinciding with a low ratio between glutamateoxalacetate and glutamate-pyruvate transaminase activity in both islet and liver extracts, islet hyperplasia, hyperinsulinemia and hepatic steatosis in the hyperglycemic fa/fa rats. It is speculated that a low activity of FAD-linked glycerophosphate dehydrogenase in the pancreatic B-cell may participate to the perturbation of glucose homeostasis in fa/fa rats, like in other animal models of non-insulin-dependent diabetes mellitus.

Alanine Transaminase↗

PubMed1994

Metabolic effects and fate of succinic acid methyl esters in rat hepatocytes.

The metabolic fate and metabolic effects of succinic acid methyl esters were examined in rat isolated hepatocytes. Both the monomethyl ester (SAM) and dimethyl ester (SAD) of succinic acid inhibited D-glucose metabolism. Such an inhibition affected, in order of increasing severity, the direct incorporation of D-glucose into glycogen and futile cycling between the hexose and its 6-phosphate ester, the phosphorylation of D-glucose, the generation of triose phosphates from the hexose, and the production of 14C-labeled lactate, pyruvate, and amino acids from D-[U-14C]glucose and its oxidation. The dimethyl ester of [1,4-14C]succinic acid ([1,4-14C]SAD) or [2,3-14C]-succinic acid ([2,3-14C]SAD) was efficiently converted to acidic metabolites. The oxidation of [1,4-14C]SAD largely exceeded that of [2,3-14C]SAD. Inversely the generation of newly formed radioactive D-glucose and glycogen appeared higher in cells exposed to [2,3-14C]SAD, rather than [1,4-14C]SAD. It is proposed that SAM and SAD are suitable nutrients both to cover the energy need of hepatocytes and to act as gluconeogenic precursors.

Amino Acids↗

PubMed1994

Secretory, biosynthetic, respiratory, cationic, and metabolic responses of pancreatic islets to palmitate and oleate.

Palmitate and oleate (0.5 to 1.0 mM) caused a time- and concentration-related augmentation of insulin release evoked by D-glucose (6.0 to 16.7 mM) in rat isolated pancreatic islets. This contrasted with an inhibitory action of the fatty acids upon L-[4-3H]phenylalanine incorporation into TCA-precipitable material, but coincided with an increased biosynthesis of proinsulin relative to that of other islet peptides. The failure of palmitate to cause an immediate increase in insulin output at a low glucose concentration (6.0 mM) coincided with an unchanged rate of O2 uptake over a 10- to 15-min exposure to this fatty acid. Over prolonged incubation (90 min), however, both palmitate and oleate (1.0 mM) stimulated 45Ca net uptake by islets exposed to 6.0 mM D-glucose. Like their insulinotropic effect, the time course for the oxidation of [U-14C]palmitate and [U-14C]oleate was characterized by a progressive buildup in 14CO2 production rate. Moreover, palmitate and oleate decreased D-[5-3H]glucose conversion to 3HOH and D-[U-14C]glucose conversion to radioactive acidic metabolites over short (30 min) but not prolonged (120 min) incubation periods. The two fatty acids also interfered with the generation of 14CO2 from islets prelabeled with [U-14C]palmitate, but not L-[U-14C]glutamine. It is concluded that, at least during prolonged exposure to either palmitate or oleate, the secretory, cationic, and metabolic response to these fatty acids displays features comparable to those usually found in islets stimulated by nutrient secretagogues.

Animals↗

PubMed1994

Insulinotropic action of formycin A.

The adenosine analogue formycin A is phosphorylated to its triphosphate ester in a sequence of reactions catalyzed by adenosine kinase and adenylate kinase. Formycin A triphosphate is an ATP analogue that is currently used to probe for ATP binding sites. Considering the key role ascribed to ATP in the coupling of metabolic to cationic events in the process of glucose-stimulated insulin release, we investigated whether formycin A displays insulinotropic action in rat pancreatic islets. Formycin A (10 microM to 1.0 mM) caused a concentration-related increase of insulin release evoked by 8.3 mM D-glucose and prevented the fall in insulin output otherwise observed over two successive incubations of 90 min each. Formycin A (1.0 mM) also augmented insulin secretion at low (5.6 mM) and high (16.7 mM) concentrations of D-glucose. At the low hexose concentration, the secretory response to formycin A was comparable to that evoked by either glibenclamide or glipizide. At higher concentrations of D-glucose, however, formycin A was more potent than the hypoglycemic sulfonylureas in enhancing insulin output. These findings support the role of ATP in glucose-stimulated insulin release and, therefore, suggest that ATP mimetics represent a new class of insulinotropic agents that have potential utility in the treatment of non-insulin-dependent diabetes mellitus.

Animals↗

PubMed1994

Protective action of succinic acid monomethyl ester against the impairment of glucose-stimulated insulin release caused by glucopenia or starvation: metabolic determinants.

The monomethyl ester of succinic acid (SME) was recently found to protect pancreatic islet B-cells against the impairment of glucose-stimulated insulin release caused by either glucopenia or starvation. The possible metabolic determinants of such a protective action are now scrutinized. After 180 min preincubation at 2.8 mM D-glucose in the presence of SME (10 mM), the oxidation of D-[U-14C]glucose, relative to either the utilization of D-[5-3H]glucose or the generation of 14C-labeled acidic metabolites, was higher than that after preincubation in the absence of SME and became close to that otherwise found after preincubation at 16.7 mM D-glucose. Likewise, after 3 days of culture at a low concentration of D-glucose (2.8 mM), the presence of SME in the culture medium tended to increase the subsequent oxidation of D-[6-14C]glucose and utilization of D-[5-3H]glucose. These two variables increased as a function of the concentration of D-glucose in the culture medium, this coinciding with a modest increase in hexokinase activity and a more pronounced increase in glucokinase activity. The presence of SME in the culture medium failed, however, to exert any obvious effect upon the respiration of the islets, suggesting that the protective action of the ester against glucopenia may also involve variables distinct from the metabolism of either endogenous or exogenous nutrients. Likewise, the fact that SME infusion to starved rats prevents the impairment of glucose-induced insulin release otherwise attributable to starvation may involve enzymatic determinants, such as a less severe decrease in glucokinase activity, metabolic variables, such as a greater relative increase in D-[U-14C]glucose oxidation relative to D-[5-3H]glucose utilization in response to a rise in extracellular D-glucose concentration, and other factors yet to be identified that participate in the secretory sequence at a site distal to those metabolic events triggered by D-glucose in the islet cells.

Animals↗

PubMed1994

13C NMR study of C2- and C3-deuterated lactic acid production by parotid cells exposed to 13C-labeled glucose in the presence of D2O.

The generation of C2- and C3-deuterated lactic acid produced by rat parotid cells exposed to [1-13C] glucose, [2-13C]glucose, and [6-13C]glucose in the presence of D2O was assessed by 13C NMR. The results indicated that the escape from deuteration amounted to about 46% at the phosphoglucoisomerase level, 100% at the phosphomannoisomerase level, 65% in the reactions catalyzed by phosphofructoaldolase and triose phosphate isomerase, and 58% at the level of glutamate pyruvate transaminase. Such high values are considered to support a possible enzyme-to-enzyme tunneling of metabolic intermediates at selected sites in the glycolytic pathway.

Animals↗

PubMed1994

Enzymatic and secretory activities in pancreatic islets of non-insulin-dependent diabetic rats after short-term infusion of succinic acid monomethyl ester.

The monomethyl ester of succinic acid (SME) was recently proposed as a novel tool for stimulation of proinsulin biosynthesis and insulin release in animal models of non-insulin-dependent diabetes mellitus. In the present study, either saline or SME (14 mmol/day) was infused for 3 days to control rats, animals injected with streptozotocin during the neonatal period, and Goto-Kakizaki rats with inherited diabetes. The infusion of SME failed to correct the anomalies found in the islets of diabetic rats, namely, a decreased activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase, a low insulin content, and an impaired secretory response to various nutrient secretagogues including D-glucose, 2-ketoisocaproate, and the combination of L-leucine and L-glutamine. These findings raise the question of whether a more prolonged administration of SME is required to raise the insulin store and improve the secretory potential of the endocrine pancreas in animals with type 2 diabetes.

Animals↗

PubMed1994

Enzymatic, metabolic and secretory patterns in human islets of type 2 (non-insulin-dependent) diabetic patients.

Islets were isolated by automatic digestion from non-diabetic cadaveric organ donors and from Type 2 (non-insulin-dependent) diabetic subjects. The activity of FAD-glycerophosphate dehydrogenase, but not that of either glutamate dehydrogenase, glutamate-oxalacetate transaminase or glutamate-pyruvate transaminase, was lower in Type 2 diabetic patients than control subjects. Hexokinase, glucokinase and glutamate decarboxylase activities were also measured in islets from control subjects. The utilization of D-[5-3H]glucose, oxidation of D-[6-14C]glucose and release of insulin evoked by D-glucose were all lower in Type 2 diabetic patients than control subjects. The secretory response to the combination of L-leucine and L-glutamine appeared less severely affected. Islets from Type 2 diabetic patients may thus display enzymatic, metabolic and secretory anomalies similar to those often observed in animal models of Type 2 diabetes, including a deficiency of beta-cell FAD-linked glycerophosphate dehydrogenase, the key enzyme of the glycerol phosphate shuttle.

Adolescent↗

PubMed1994

Insulinotropic action of glutamic acid dimethyl ester.

Glutamic acid dimethyl ester (GME; 3.0-10.0 mM) enhanced insulin release evoked by 6.0-8.3 mM D-glucose, 1.0-10.0 mM L-leucine, or 5.0-10.0 mM 2-amino-bicyclo(2,2,1)heptane-2-carboxylic acid, causing a shift to the left of the sigmoidal relationship between insulin output and D-glucose concentration. In the absence of D-glucose, GME also unmasked the insulinotropic potential of glibenclamide. In islets exposed to L-leucine, the insulinotropic action of GME coincided with an early fall and later increase in 86Rb outflow and augmentation of 45Ca outflow from prelabeled islets. The measurement of O2 uptake, NH4+ output, production of 14CO2 from islets prelabeled with [U-14C]palmitate, generation of 14C-labeled amino acids and 14CO2 from the dimethyl ester of either L-[1-14C]glutamic acid or L-[U-14C]glutamic acid, and D-[2-14C]glucose as well as D-[6-14C]glucose oxidation in the presence or absence of GME indicated that the latter ester was efficiently converted to L-glutamate and its further metabolites. The overall gain in O2 uptake represented the balance between GME oxidation and its sparing action on the catabolism of endogenous fatty acids and exogenous D-glucose. It is proposed that GME might represent a new tool to bypass beta-cell defects in D-glucose transport, phosphorylation, and further metabolism and, hence, to stimulate insulin release in experiments conducted in animal models of non-insulin-dependent diabetes mellitus.

Animals↗

PubMed1994

Pancreatic islet response to dicarboxylic acid esters in rats with type 2 diabetes: enzymatic, metabolic and secretory aspects.

This study aimed to compare the metabolic and secretory responses of pancreatic islets from animals with non-insulin-dependent diabetes to D-glucose with the effects of the methyl esters of succinic acid (SME) and glutamic acid (GME). The insulin secretory response to D-glucose was impaired in islets from rats with diabetes which was either inherited (Goto-Kakizaki (GK) rats) or acquired (streptozotocin-treated (STZ) rats). This coincided with a preferential alteration of oxidative relative to total glycolysis in intact islets and a selective defect of FAD-linked mitochondrial glycerophosphate dehydrogenase (m-GDH) in islet homogenates. This enzymatic defect was also found in purified B cells from STZ rats. It contrasted both with unaltered activities of glutamate dehydrogenase and succinate dehydrogenase in the islets of diabetic animals and with a normal or even increased activity of m-GDH in the livers of GK and STZ rats. The oxidation of [1,4-14C]SME and [U-14C]GME appeared decreased in islets of GK or STZ animals when compared with control rats, but no significant difference between control and diabetic rats was observed when the oxidative data were expressed relative to the rate of [U-14C]GME hydrolysis. Nevertheless, the absolute values for insulin release evoked by a non-metabolized analogue of L-leucine (BCH), by SME and by the association of BCH with either SME or GME were invariably lower in islets of GK and STZ rats than in those of control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗

PubMed1994

Insulinotropic action of AICA riboside. II. Secretory, metabolic and cationic aspects.

Preincubation of rat pancreatic islets with AICA riboside (0.1 to 1.0mM) caused a concentration-related stimulation of both 45Ca net uptake and insulin release evoked by 8.3 mM D-glucose, but failed to affect the conversion of D-[5-3H]glucose to 3HOH, the generation of 14CO2 and 14C-labelled amino acids or acidic metabolites from D-[U-14C]glucose, and the islet content in ATP, ADP or AMP. The secretory response to AICA riboside was not suppressed in islets preincubated with methotrexate. AICA riboside caused a progressive decrease in 86Rb outflow from prelabelled islets perifused at 2.8 or 6.0mM D-glucose. This effect faded out at a higher concentration of D-glucose (16.7 mM), in which case AICA riboside nevertheless provoked a delayed, progressive and not rapidly reversible enhancement of insulin output. At concentrations up to 0.4 mM, ZTP only exerted a modest effect on the activity of KATP-channels in inside-out patches of dispersed mouse islet cells. These findings raise the question whether the insulinotropic action of AICA riboside may be attributable to the sequential generation of ZMP, ZDP and ZTP from the nucleoside.

Adenine Nucleotides↗

PubMed1994

Insulinotropic action of the D-glucosyl and 3-O-methyl-D-glucosyl monomethyl esters of succinic acid.

Succinic acid methyl esters are currently under investigation as potential insulinotropic tools in animal models of non-insulin-dependent diabetes mellitus. The in vivo administration of these esters may result in the undesirable generation of methanol through their intracellular hydrolysis. As a first attempt to circumvert this drawback, we have now investigated whether the esterification of the carboxylic group of succinic acid monomethyl ester by D-glucose or 3-O-methyl-D-glucose affects its insulin-otropic action. Both the 6-O-D-glucosyl and 6-O-(3-O-methyl)-D-glucosyl esters were found to stimulate insulin release in pancreatic islets and the isolated perfused pancreas. The 6-O-D-glucosyl ester also stimulated insulin release after intravenous administration to anaesthetized rats. These findings suggest that the undersirable generation of methanol from the methyl esters of succinic acid could eventually be avoided by using other esters of this dicarboxylic acid, whilst keeping the benefit of their insulinotropic action.

Animals↗

PubMed1994

Hexose metabolism in pancreatic islets UDP-glucose pyrophosphorylase activity.

UDP-glucose pyrophosphorylase was measured in rat pancreatic islets, the generation of D-glucose 1-phosphate from UDP-glucose and PPI being eventually coupled to the generation of L-[U-14C]glutamate from 14C-labelled alpha-ketoglutarate. The activity of the enzyme was about one order of magnitude lower in islet than liver homogenates. The affinity of the enzyme for either UDP-glucose or PPi was comparable, however, in liver and islets. The activity of UDP-glucose pyrophosphorylase was somewhat lower in islets from animals with inherited or acquired diabetes mellitus than in those from control rats. These findings are considered in connection with the accumulation of glycogen in islets of hyperglycemic animals.

Animals↗

PubMed1993

Metabolism of endogenous nutrients in islets of Goto-Kakizaki (GK) rats.

The metabolism of endogenous nutrients was examined in pancreatic islets of control and Goto-Kakizaki (GK) rats. At the ultrastructural level, no glycogen was found in the islet cells of GK rats, a situation similar to that prevailing in normal islets. Likewise, by measuring the output of L-lactate from islets first incubated at 16.7 mM D-glucose and then at 2.8 mM D-glucose, no evidence of glycogenolysis was found in the islets of GK rats. The production of NH4+ and that of 14CO2 from islets prelabelled with either L-[U-14C]glutamine or [U-14C]palmitate were higher, however, in GK than in control rats. The changes in NH4+ and 14CO2 production evoked by D-glucose, by a non-metabolized analogue of L-leucine (2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid; BCH) and by 3-phenylpyruvate were qualitatively comparable in control and GK rats. The secretory response to these three secretagogues was severely decreased in the islets of GK rats. This coincided with an impaired enhancing action of D-glucose on the conversion of [2-3H]glycerol into 3HOH. It is concluded that the catabolism of endogenous amino and fatty acids in islets is greater in GK than in control rats, especially at low D-glucose concentration. This may account, in part at least, for the altered secretory response to BCH and 3-phenylpyruvate. For glucose-induced insulin release, however, an impaired acceleration of the glycerol phosphate shuttle apparently also participates in the secretory defect.

Ammonia↗

PubMed1993

Hexose metabolism in pancreatic islets: time-course of the oxidative response to D-glucose.

The time-course for the generation of 3HOH from D-[5-3H]glucose and for the production of 14CO2, 14C-labelled acidic metabolites and radioactive amino acids from D-[3,4-14C]glucose, D-[2-14C]glucose and D-[6-14C]glucose was monitored, over 60 to 120 min incubation, in both rat pancreatic islets and parotid cells exposed to a low or high concentration of the hexose. In islets stimulated by D-glucose, a progressive increase in the oxidation rate of glucose-derived acetyl residues was observed. Such a phenomenon was not observed in islets exposed to a low concentration of D-glucose, concerned specifically the oxidation of acetyl residues in the Krebs cycle as distinct from their generation in the reaction catalyzed by pyruvate dehydrogenase, and failed to occur in parotid cells. It is concluded that the increase in the oxidation rate of glucose-derived acetyl residues found in pancreatic islets represents an unusual phenomenon not encountered in other cell types and specifically regulated in terms of its time-course, concentration dependency and relationship to other oxidative events.

Amino Acids↗

PubMed1993

Modulation of the insulinotropic action of glibenclamide and glimepiride by nutrient secretagogues in pancreatic islets from normoglycemic and hyperglycemic rats.

In perifused pancreatic islets from euglycemic rats, the secretory response to either glibenclamide or glimepiride (1.0 microM each) increases as a function of the concentration of D-glucose (2.8-16.7 mM) present in the perifusion medium. On the contrary, the sulfonylurea-induced increment in 45Ca efflux from prelabeled islets decreases at increasing concentrations of the hexose. Neither glibenclamide nor glimepiride affect D-glucose metabolism in isolated islets, as judged from the production of 3HOH from D-[5-3H]glucose or the generation of 14CO2, as well as 14C-labeled amino acids and acidic metabolites, from D-[3,4-14C]glucose, D-[2-14C]glucose and D-[6-14C]glucose. The insulinotropic action of the hypoglycemic sulfonylureas is not impaired in islets prepared from rats infused for 48 hr with a hypertonic solution of D-glucose. The dimethyl ester of succinic acid is more efficient than D-glucose in supporting the insulin-releasing effect of glibenclamide or glimepiride. Thus, although the insulinotropic action of hypoglycemic sulfonylureas appears unaffected in a model of B-cell glucotoxicity, a potentiation of their secretory effects might be expected, in non-insulin-dependent diabetes, from the combined administration of succinic acid methyl ester.

Animals↗