Biomedical subjects

M Schwanstecher

Publications and source records attributed to M Schwanstecher.

27 records · Page 2Linked to original sources

PubMed1994

Inhibition of K+ channels and stimulation of insulin secretion by the sulfonylurea, glimepiride, in relation to its membrane binding in pancreatic islets.

In isolated pancreatic islets of mice, the relationships between free glimepiride concentration and membrane binding or inhibition of ATP-sensitive K+ channels were examined. Microsomal membrane binding and K+ channel inhibition were half-maximal at 0.7 and 0.3 nmol/l glimepiride, respectively. The corresponding concentrations for glibenclamide were 0.4 and 0.6 nmol/l. Administration of glimepiride (10 nmol/l) or glibenclamide (10 nmol/l) to isolated mouse islets perifused with albumin-containing media induced a slow increase in insulin secretion. The kinetics of the secretory responses to glimepiride and glibenclamide were identical. Determination of albumin binding revealed that the free glimepiride and glibenclamide concentrations applied in our investigation were in the range of therapeutic serum concentrations of the free drugs. It is concluded that the effects of glimepiride and glibenclamide are very similar in mouse beta-cells.

Animals↗

PubMed1992

Pancreatic and extrapancreatic sulfonylurea receptors.

The hypoglycemic effect of sulfonylureas and their analogues results from their binding to a high affinity site in the B-cell plasma membrane. This site seems to be a structural component of the ATP-sensitive K(+)-channel and represents the pancreatic sulfonylurea receptor. Binding of sulfonylureas causes closure of the ATP-sensitive K(+)-channel and thereby initiates a chain of events eventually leading to the release of insulin. Diazoxide inhibits insulin secretion via opening of the ATP-sensitive K(+)-channel. Sulfonylurea receptors resembling the pancreatic receptor occur in nerve cells, cardiac muscle, skeletal muscle and smooth muscle. Neither these extrapancreatic receptors nor low affinity receptors for sulfonylureas in myocytes and adipocytes contribute to the therapeutic benefit of sulfonylureas.

ATP-Binding Cassette Transporters↗

PubMed1992

The binding properties of the particulate and solubilized sulfonylurea receptor from cerebral cortex are modulated by the Mg2+ complex of ATP.

Glibenclamide closes an ATP-sensitive K+ channel (K-ATP channel) by interaction with the sulfonylurea receptor in the plasma membrane of pancreatic B cells and thereby initiates insulin release. Previous studies demonstrated that the Mg2+ complex of ATP decreases glibenclamide binding to the sulfonylurea receptor from pancreatic islets. The aim of the present study was to examine the effect of adenine and guanine nucleotides on binding of sulfonyl-ureas to the cerebral sulfonylurea receptor. For this purpose, binding properties of the particulate and solubilized site from rat or pig cerebral cortex were analyzed. Maximum recovery of receptors in detergent extracts amounted to 40-50%. Specific binding of [3H]glibenclamide to the solubilized receptors corresponded well to specific binding to microsomes. In microsomes and detergent extracts, the Mg2+ complexes of ATP, ADP, GTP, and GDP inhibited binding of [3H]glibenclamide. These effects were not observed in the absence of Mg2+. In detergent extracts, Mg-ATP (300 microM) reduced the number of high-affinity sites for [3H]-glibenclamide by 52% and increased the dissociation constant for [3H]glibenclamide by eightfold; Mg-ATP was half-maximally effective at 41 microM. Alkaline phosphatase accelerated the reversal of Mg-ATP-induced inhibition of [3H]glibenclamide binding. The data suggest similar control of the sulfonylurea receptor from brain and pancreatic islets by protein phosphorylation.

ATP-Binding Cassette Transporters↗

PubMed1992

Adenine nucleotide-induced inhibition of binding of sulphonylureas to their receptor in pancreatic islets.

1. The effects of the Mg complex of adenosine 5'-triphosphate (MgATP) on binding of sulphonylureas to microsomes obtained from mouse pancreatic islets were examined. 2. MgATP inhibited the binding of both glibenclamide and tolbutamide to microsomes. 3. Binding of [3H]-glibenclamide inhibited by MgATP was not further diminished by Mg(2+)-bound adenosine 5'-(beta, gamma-imidotriphosphate) (AMP-PNP) or free adenosine 5'-diphosphate (ADP). Higher concentrations of MgAMP-PNP induced a partial reversal of the inhibitory effect of MgATP on [3H]-glibenclamide binding. 4. The apparent dissociation constant (K'D) for binding of [3H]- glibenclamide remained constant when 5. Extracellular ADP did not markedly stimulate insulin release from mouse pancreatic islets. 6. It is concluded that sulphonylureas and cytosolic nucleotides exert their inhibitory effects on the K-ATP-channels of beta-cells by binding to different sites. The binding properties of the sulphonylurea receptor seem to be modulated by protein phosphorylation.

Adenine Nucleotides↗

PubMed1992

Effect of MgATP on pinacidil-induced displacement of glibenclamide from the sulphonylurea receptor in a pancreatic beta-cell line and rat cerebral cortex.

1. The effects of blockers and openers of K+ channels on binding of [3H]-glibenclamide to microsomes obtained from a pancreatic beta-cell line (HIT-T15) or rat cerebral cortex were examined. 2. The blockers quinine, chlorpromazine and thiopentone and the openers cromakalim [(+/- ) 6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1- pyrrolidyl)-2H-benzo[b]pyran-3-ol] and minoxidil sulphate did not significantly interact with the sulphonylurea receptor of HIT-cells both at phosphorylating (presence of MgATP) and dephosphorylating (absence of MgATP) conditions. 3. In the absence of MgATP, pinacidil (200-500 microM) did not significantly displace [3H]-glibenclamide binding to microsomes from HIT-cells. The displacement of [3H]-glibenclamide binding was strongly enhanced by MgATP and was due to a decrease in the number of high affinity binding sites for glibenclamide. 4. MgATP enhanced pinacidil-induced inhibition of [3H]-glibenclamide binding to microsomes from rat cerebral cortex. 5. The effect of MgATP on pinacidil-induced inhibition of [3H]-glibenclamide binding was maintained after solubilization of the membranes from HIT-cells or rat cerebral cortex. 6. It is concluded that the sulphonylurea receptor is regulated not only by sulphonylureas but also by the K+ channel openers, diazoxide and pinacidil, and by protein phosphorylation. The binding sites for sulphonylureas and these K+ channel openers are not identical, but appear to be located at a single protein or at tightly associated proteins.

ATP-Binding Cassette Transporters↗

PubMed1992

The binding properties of the solubilized sulfonylurea receptor from a pancreatic B-cell line are modulated by the Mg(++)-complex of ATP.

Previous studies demonstrated that the Mg complex of ATP decreases glyburide- and increases diazoxide-binding to membranes from pancreatic islets. To examine further the mechanism of these effects, the sulfonylurea receptors in microsomes of the hamster B-cell line HIT-T15 were solubilized with detergents. Maximum recovery of receptors (40%) was obtained with Triton X-100. Specific binding of [3H]glyburide to the solubilized receptors (Kd = 0.35 nM, maximum number of binding sites = 170 fmol/mg of protein) corresponded well to specific binding to microsomes. In Triton X-100 extracts, MgATP (300 microM) reduced the number of high-affinity sites for [3H]glyburide by 50% and increased the dissociation constant for [3H]glyburide by 4-fold; MgATP was half-maximally effective at 20 microM. Development of MgATP-induced inhibition of [3H]glyburide binding to solubilized binding sites was not slower than dissociation of [3H]glyburide binding. Alkaline phosphatase accelerated the reversal of MgATP-induced inhibition of [3H]glyburide binding. In the presence of Mg++, not only ATP but also ADP, GTP and GDP inhibited [3H]glyburide binding to the solubilized receptor. However, MgADP did not inhibit [3H]glyburide binding when the MgATP concentration was kept low by the hexokinase reaction. MgATP significantly enhanced diazoxide-induced displacement of [3H]glyburide from the solubilized receptor. The MgATP-induced inhibition of binding was weakened by millimolar concentrations of free ATP. It is concluded that the binding sites for MgATP, glyburide and diazoxide are located at a single protein or at closely associated proteins which may include a protein kinase.

ATP-Binding Cassette Transporters↗

PubMed1991

Phosphate and thiophosphate group donating adenine and guanine nucleotides inhibit glibenclamide binding to membranes from pancreatic islets.

In microsomes obtained from mouse pancreatic islets, the Mg complex of adenosine 5'-triphosphate (MgATP) increased the dissociation constant (KD) for binding of [3H]glibenclamide by sixfold. In the presence of Mg2+, not only ATP but also adenosine 5'-0-(3-thiotriphosphate) (ATP gamma S), adenosine 5'-diphosphate (ADP), guanosine 5'-triphosphate (GTP), guanosine 5'-diphosphate (GDP), guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S) and guanosine 5'-0-(2-thiodiphosphate) (GDP beta S) inhibited binding of [3H]glibenclamide. These effects were not observed in the absence of Mg2+. Half maximally effective concentrations of the Mg complexes of ATP, ADP, ATP gamma S and GDP were 11.6, 19.0, 62.3 and 90.1 mumol/l, respectively. The non-hydrolyzable analogues adenosine 5'-(beta,gamma-imidotriphosphate) (AMP-PNP) and guanosine 5'-(beta,gamma-imidotriphosphate) (GMP-PNP) did not alter [3H]glibenclamide binding in the presence of Mg2+, MgADP acted much more slowly than MgATP and both MgADP and MgGDP did not inhibit [3H]glibenclamide binding when the concentrations of MgATP and MgGTP were kept low by the hexokinase reaction. Development of MgATP-induced inhibition of [3H]glibenclamide binding and dissociation of [3H]glibenclamide binding occurred at similar rates. However, the reversal of MgATP-induced inhibition of [3H]glibenclamide binding was slower than the association of [3H]glibenclamide with its binding site. Exogenous alkaline phosphatase accelerated the reversal of MgATP-induced inhibition of [3H]glibenclamide binding. MgATP enhanced displacement of [3H]glibenclamide binding by diazoxide. The data suggest that sulfonylureas and diazoxide exert their effects by interaction with the same binding site at the sulfonylurea receptor and that protein phosphorylation modulates the affinity of the receptor.

Adenine Nucleotides↗

PubMed1989

Control of insulin secretion by sulfonylureas, meglitinide and diazoxide in relation to their binding to the sulfonylurea receptor in pancreatic islets.

Sulfonylureas inhibit an ATP-dependent K+ channel in the B-cell plasma membrane and thereby initiate insulin release. Diazoxide opens this channel and inhibits insulin release. In mouse pancreatic islets, we have explored whether other targets for these drugs must be postulated to explain their hypo- or hyperglycaemic properties. At non-saturating drug concentrations the rates of increase in insulin secretion declined in the order tolbutamide = meglitinide greater than glipizide greater than glibenclamide. The same rank order was observed when comparing the rates of disappearance of insulin-releasing and K+ channel-blocking effects. The different kinetics of response depend on the lipid solubility of the drugs, which controls their penetration into the intracellular space. Allowing for the different kinetics, the same maximum secretory rates were caused by saturating concentrations of tolbutamide, meglitinide, glipizide and glibenclamide. A close correlation between insulin-releasing and K+ channel-blocking potencies of these drugs was observed. The relative potencies of tolbutamide, meglitinide, glipizide and glibenclamide corresponded well to their relative affinities for binding to islet-cell membranes, suggesting that the binding site represents the sulfonylurea receptor. The biphasic time-course of dissociation of glibenclamide binding indicates a complex receptor-drug interaction. For diazoxide there was no correlation between affinity of binding to the sulfonylurea receptor and potency of inhibition of insulin secretion. Thus, opening or closing of the ATP-dependent K+ channel by diazoxide or sulfonylureas, respectively, appears to be due to interaction with different binding sites in the B-cell plasma membrane. The free concentrations of tolbutamide, glipizide, glibenclamide and diazoxide which are effective on B-cells are in the range of therapeutic plasma concentrations of the free drugs. It is concluded that the hypo- and hyperglycaemic effects of these drugs result from changing the permeability of the ATP-dependent K+ channel in the B-cell plasma membrane.

ATP-Binding Cassette Transporters↗

PubMed1988

Glucose both inhibits and stimulates insulin secretion from isolated pancreatic islets exposed to maximally effective concentrations of sulfonylureas.

Isolated pancreatic islets from mice were perifused with media containing maximally effective concentrations of glibenclamide (0.1-10 mumol/l) or glipizide (1 mumol/l). In these islets an increase of the glucose concentration from 10 mmol/l to 40 mmol/l or addition of D-glyceraldehyde (20 mmol/l) caused a temporary decrease in insulin release which was followed by a sustained enhancement of release. alpha-Ketoisocaproate (3 or 20 mmol/l) did not inhibit insulin release; at high concentration it was an even stronger secretagogue than D-glucose or D-glyceraldehyde. It is concluded that high energy phosphates couple B-cell fuel metabolism and insulin release by acting both on the ATP-dependent K+ channel and on other targets not yet identified.

Animals↗